Friday, February 29, 2008

Binoviewing III (final)

Eyepiece selection: As usual, we need to consider the eye relief, apparent field size, eye cup design, weight, and finally "nose relief".

Eye relief is obvious, just in mono mode, you see how important eye relief is, if the eye relief is very tight, I wonder if you can use it with two eyes, and one closely related thing is the size of the eye lens, in general, bigger means better.

Except eye relief, some eyepieces are very sensitive to eye positioning, and the field will blackout easily. These are less than ideal for binoviewing. Check before you pair up.

Apparent field size seems less important in terms of feeling in binoviewing. I have said that from the above section while 80 degree eyepiece pair does give a very good feeling about space walk, people also reported that even with narrow field eyepiece like Orthoscopics, the field is far more pleasing when used in a binoviewer! So, if you find orthoscopics less than happy in mono mode, it could be acceptable in a binoviewer. Again, try it out before going ahead. Finally, I found 80+ degree for both eyes is a bit hard to ingest at the same time, something like 70 degree would be enough. And maybe that's why Panoptics have been the most popular for binoviewing.

Concerning eye cup design, we rarely take care of that unless it is really really bad, but since you are likely to get closer to the eye lens with binoviewer, one should take care if the eye cup is well designed, effective to hide you from ambient light, if there is any, and also not to help dew formation. Also, watch out for the moisture from your eyes since we tend to get closer to the eyepiece when using the binoviewer. I found eyepieces with a well designed eye cup to be better for binoviewing, especially for solar observation.

For example, my Widescan Type III pair and my 20mm Tele Vue Plossl pair are both very easy to use on any object, but the 18mm J-Ortho pair is harder to use for solar observation, despite it works very well at night, mostly because the J-Orthos have no rubber eye cup.

The concern of the weight is about balancing, and also the capacity of your mount, I hope no one would use two Nagler Type I 13mm at the same time.

Nose relief is a new term, I guess you know what it is from its name. It's less a problem for Asian people, but I keep hearing problems in international discussion groups, check the size of nose to see if you have enough clearance between the eyepieces, eyepieces like Plossl will not be problematic but I am sure Pentax XL is another story.

Observing target: Well designed binoviewer works well for planets and also for the deep sky objects, but converted binoviewer like from microscope head, will not work for deep sky objects.

For the planets and the moon, binoviewer really gives more advantages than problems. But for deep sky objects, the situation is a bit more complex. At less than 12-14" aperture, we will realize the drop of light throughput with binoviewer, it is not trivial and it is readily noticeably. It translates to a reduction in limiting magnitude.

With the observational comfort brought by binoviewer, deep sky objects look better and more contrasty, but they are also dimmer. Deep sky objects tend to be dim and so dimmer really means dimmer. Planets are bright enough, the moon is even too bright so that a dimmer image is actually a good thing!

I found myself spending more time on a particular object than when I did in mono mode. So, it helps to offset part of the loss.

On planets, the result is very obvious. Binoviewer gives much better contrast than any eyepiece used in mono mode. The color saturation is also better. I can definitely see more detail with two eyes, and I can push the magnification higher than with one eye. Similar result for the moon, and I am sure any one will be shocked. My first look using two eyes, was at the moon and by then I was attracted and I made up my mind to buy one myself.

On deep sky objects, the result is not as simple. In one night, the transparency is good (6/10), and I tried to observe M42 inside my home which is located in the city center, Hong Kong. At low power, i.e. around 50x, the two eyes view (20mm widescan + 0.5 reducer) already beated the one eye view (40mm XL). Then, I push the power further up by removing the reducer, and the improvement in contrast is very obvious. Then, I kick in the 2.5x corrector and quite a lot of nebulosity popped up, and I can actually discern a bit of the structure within the nebulosity. I am very impressed.

On the same night, I swing to M41, but in that case, single eye view was better than two eyes. I concluded that the result was due to the light loss with two eyes. Reduction in limiting magnitude actually hurt the view of star cluster in a light polluted sky. I shall check again on darker sky.

In a summer night, I observed the M6 and M7 inside light polluted home again. I found that mono view is better and also noticeably brighter. However, when I scanned around, I could detect M19 using binoviewer, which was unseen when I was scanning using mono mode. Of course, switching back from binoviewer to mono mode again reveal that fuzzy patch. It means more contrast on binoviewing.

When observing the planets, color saturation becomes much higher with two eyes, I don't find similar effect on deep sky observation. Maybe light got splitted into two eyes, so that clone cells (responsible for color) does not get over their minimum threshold. I found that seeing color on some brighter deep sky objects with one eye easier. Of course, given large enough aperture, maybe it's not a problem?

From the experience I heard, as well as my own experience, I found that binoviewing will render marginally detectable deep sky objects to be invisible. However, it does make some brighter deep sky objects more striking. The amount of detail is similar, but the two eyes mode will simply make it easier and more comfortable to see.

Personally, I found binoviewing good to brighter deep sky objects but not for those dimmer ones. Note that it's not a matter of clear aperture of the prism, which affects the largest fully illuminated field rather than the overall brightness. It is more a question on the coating, transmission figure.

Verdict? I want binoviewing and mono mode, i.e. both, all depend on the target and the situation.

Observational comfort: While it is a second nature for experienced observers, observing with one eye is not comfortable at all. An eye patch for the non-observing eye would be nice, but it cannot beat two eyes observing at all. With two eyes, we tend to spend more time on a single object, so that we can pick out more detail.

I considered myself adapted to one eye observing, but I find it hard to stare into the eyepiece for more than 2-3 minutes continuously. But with a binoviewer, 30 minutes will not be a problem.

Illusion: With a binoviewer, you will have an impression that the image is actually larger. What I mean is that, you will see thing as bigger than that in mono mode at the same magnification. Also, when light is splitted by the binoviewer and re-combined by your brain, you have a false 3D feeling, but the false feeling is really very real in some situation. ;)

Imagine the Jupiter as a sphere floating in the vast dark space with its satellites, yes, it really feels like that.

False 3D feeling varies a lot from person to person, so no guarantee here and see your luck. Such feeling maximizes for the moon.

Notice the position where the dovetail is mounted

Balancing: We know premium eyepiece tends to be heavy to cause balance problem, it is even much more serious when used in pair plus the weight of a binoviewer. To give you some idea, my telescope is mounted at 99% near the end of the dovetail when used with a binoviewer.

For a mount without friction controls or locks, when you pick out the binoviewer to insert any corrector/reducer/amplifier, the rear side is going to be lighten suddenly. Please beware of that. Even when you switch eyepieces, you may face similar problem.

Mechanical issues, mount capacity: Like what I mentioned in the previous section, the extra weight can cause problem. If your mount is marginal for your setup originally, it *will* become overloaded when used with a binoviewer.

If your counter weight is already sitting near the end of the shaft, be prepared to buy one more counter weight.

Most binoviewer comes as 1.25" format, but to tell the truth, you will find it terrible to hold a binoviewer in a 1.25" diagonal. Consider going for a 2" diagonal just to have a more rigid support to your binoviewer plus your eyepiece pair. A 2" nose piece for the binoviewer would be nice as well, since one won't want to hold the binoviewer with two eyepieces with just a single small set screw originally designed for small 1.25" eyepiece.

I have conducted an experiment, I mounted my 1.25" diagonal on the visual back of my SCT, try hard to lock it with the two set screws. And then I put my binoviewer with two eyepieces on the diagonal. I can't fix the binoviewer at all, the diagonal will rotate to point my binoviewer to the floor. The poor little diagonal is trying to save its own life by getting rid of that heavy binoviewer at its best. (grin)

Finally, the set screw on my Tele Vue Ranger failes to lock down the focusing draw tube with the binoviewer. It's so heavy.

I've conducted an experiment. Ranger can reach focus without any corrector when used without a diagonal plus the 18mm ortho eyepieces, it can also reach focus with a 2.5x corrector before the diagonal. One can also use a 0.5x SCT corrector in the nosepiece of a binoviewer to lower the 2.5x magnification factor a bit and can still reach focus.

Small refractor with binoviewer?

Accessories: Check with the vendors, they have many different options, like on the correctors, the reducers, the individual focusers add-on, cases, etc. Beware of the compatibility of the accessories with your telescope type, brand and also the model in some cases.

Some accessories can be used among different binoviewers, some are specially designed, and there is no universal standard right now, so check before you buy.

Newtonian has a newtonian corrector, refractor has one for refractor, SCT has no need, but can still be used, check with the vendor.

For truss tube dobsonian users: We said that binoviewer should be used with a corrector in order to reach focus because of insufficient in-travel, so truss tube dobsonian users might want to have their truss cut shorter in order to accomodate a binoviewer without correcter?

It is a funny topic, indeed. To use a binoviewer without a corrector, cutting the truss tube is a solution, but there're some more factors to consider.

By cutting down the truss tubes, the secondary is now closer to the primary and with the original secondary mirror, it's going to be undersized, again, vignetting will result. You got wider field, but it is a vignetted field, on the other hand, like what I said, vignetting is not readily detectable. However, be aware of the problem. Also, undersized secondary in a newtonian means a reduction of effective aperture of the whole system.

Next, not only the secondary is undersized, the prism inside the binoviewer is going to be undersized, since it is now closer to the secondary and the primary. Alright, similar reasoning like the above.

So you see, a corrector does have its function to retain the maximum unvignetting field other than just correcting the focus position.

Lastly, with shorter truss, you decide yourself to stick with binoviewer. Maybe it is good to keep two sets of truss tubes.

An review in S & T mentioned that some binoviewer corrector will get into the optical path with low profile focusers which are common in many newtonians, since the whole corrector assembly is so thick, it will affect optical performance by giving a strange diffraction pattern.

Coating: Coating does not affect only light throughput or transmission, it also affects contrast. Good coating will pass the light more effectively, and at the same time, it reduces scattering.

We have metalic based coating and dielectric coating, the later is better and more durable but also more expensive. Metalic based coating on the beam splitting prism will give slightly different colors on each side of the binoviewer, but you won't notice that in actual use. Yes, you can see the difference if you do a side by side comparison.

Style and Origin: Earlier binoviewers are converted from microscopes and they are simply not suitable for astronomical usage, especially at low power. These units are usually 45 degree models, and having undersized prism. Stay away from them. There are some rare exceptions, but why take the risk?

Binoviewers designed for astronomy purpose are usually straight through, and they are to be used with a diagonal. Look no further, go ahead with this type.

Price: To me, price is a very critical factor. If price is no object, portable asides, we should really go for real binoculars, period. Consider the case if I shell out $1000 to buy a binoviewer, for example in my case, why not buy one more C8 OTA to create a pair of real binoculars?

One would say two C8 OTA will not be able to make a pair of binoculars, but to me, it can, at least on paper. Buy two diagonals plus extension tubes for each OTA, and then the eyepieces from each scope will be close enough to use for anybody. Since a moving primary will give a lot of focuser travel, by going this route, I believe the setup can still reach focus.

I have no money/time to verify this guess, but like a binoviewer, one would need to push the focus farther out like the above suggestion, adding one more diagonal will not take much more focuser travel than a binoviewer.

Support and Services: Binoviewers have been made very generic these days, but we must still pay attention to make sure it matches your existing equipments.

For examples, Celestron 9.25" and Meade 14" are reported to be some special cases where you will need to use 0.6x reducer instead of 0.5x. Some exceptionally lower power corrector may not work with all telescopes. Also, collimation of binoviewer is not quite user do-able currently.

All these mean good support and services are very essential.

Return Policy: Having said all these, maybe you take every precaution to avoid lemon, to avoid poor devices, there could be exceptions. So, a "no question asked" return policy would be the most important thing you should look for. Binoviewers are not cheap, if you got stuck, no one can help. Quality products should be guaranteed with a return policy, if one vendor cannot provide, better ignore it altogether.

Which Brand: People said, the refractor and newtonian debate will never end, the Celestron and Meade "discussion" also. But I tell you, these are not the worst topics in the field of amateur astronomy.

The package which I have selected. There is nothing called the best in the world.
Each unit is a unique compromise among performance, cost, etc.

When I started to find a good binoviewer for me, I go ahead to find in the Internet like usual. To my surprise, you see only one brand, yes, one only. No, to be exact, there are more than one, but in most places, you see only one single brand.

To me, Internet means freedom. Free(dom), open source software originated from the Internet, free discussions among so many people in the world are on the Internet, you see this article in the Internet. So, how come we can see only one single brand?

I don't exactly know, until one day, some good guys privately told me, one brand is prohibited, and there's a very long story behind, and they told me it's wise to stick with the other brand which is widely discussed in the Internet. Oh...

Yes, sometimes, we see the prohibited brand somewhere, but there will also be flame associated with it. The signal to noise ratio, is simply making no useful information available easily without going through a huge pile of trash.

So, I give no preference on which brand, I love good performance and good price. I keep one eye closed here.

(Remarks on 2008: there are far more players in the market now, and the amount of user reviews which you can find on the Internet is good enough, just search!)

Closing Remarks

Some of the information above were collected into my mind throughout all the days when I shop for a binoviewer. I tried to verify them myself before writing this article and it took me over one year to finalize everything. However, I know I will make mistake and some information here will soon before obsolete as technology moves on.

I've no special interest with any manufacturer or company. I am only interested in good products, something that can enhance my experience in this wonderful hobby.

No matter how, binoviewing is a wonderful experience, especially on brighter targets. Comfort, the enhancement of color saturation (give the light level is above the threshold of your cone cell), the improvement on contrast, are all hard to match with only one eye.

I tried to list every single consideration from my mind when I made my own decision here, just wish to save some effort for others to go through the huge pile of information over magazines, internet, your friends, etc. However, nothing replace your own experience. By all means find your club members, ask your local folks, try everything you can if possible.

Good luck and enjoy!

Wednesday, February 27, 2008

Binoviewing II

Comparing with binoculars

Two objectives against one

With a pair of binoculars, we have two objectives to collect light, and each eye will be using a separate telescope. With a binoviewer, we have only one objective to collect light, and each eye will be using one of the two splitted light cone from the same telescope.

Experiments tell us that, when observing with two eyes, the effect on light gathering will be like 1.4 times of each aperture, e.g. when you are using a 50mm binoculars, the brightness will be like using a ~59mm objective, but notice that the resolution is still in the class of 50mm. So, consider it's more like enhanced coating, i.e. it transmits more light, but it won't give more resolution.

On the other hand, with a pair of binoculars, you are using two telescopes, i.e. you will be seeing two different images with each eye. So, your brain is actually perceiving two images differ by a slightest bit of parallax (!!!), seeing, tube current. But with a binoviewer, the images are more or less identical for each eye, except otherwise at the very end of the optical chain like the floaters from your eyes. Contrast is improved as well, but the amount is again, hard to quantify.

With a 50mm telescope using binoviewer, assume 95% throughput, the brightness will be like a pair of binoculars with two ~41mm objective. Again, only brightness is concerned here, but the resolution of the system, remains like a 50mm telescope.

Actually, I am less interested in the numbers, but one thing we know is that, binoviewer does improve a lot on observational comfort and it really helps to bring a lot on contrast. We can see more on the planets and the moon with binoviewer, despite the fact that, yes, the image is dimmer.

Binoviewing ABC

"Cyclops" mode? People in the binoviewer community may use the word "cyclops" to refer to single eye viewing, however, I prefer to call it the mono mode instead. I found it insulting and not suitable, for example, people suffering from Amblyopia, viewing with two eyes does not make sense at all. But it's more like cultural issue than anything serious.

Images merging: It is easy to merge images with a pair of binoculars, but some people find it hard to do so with a binoviewer. This is a problem on the observer, the eyepieces used and also the binoviewer. Binoculars is an integrated system, but binoviewer is a after-work, so problem is expected.

When you find that you cannot merge the images of a pair of binoculars, you will know that they are probably out of collimation. The same logic applies to binoviewers.

A good binoviewer will hold its own collimation pretty well, but it is just part of the whole story. Binoviewer is not very much user collimateable, I think it is possible, but it's not easy either. Good vendors will check every binoviewer before shipping them.

From the user reports in the Internet, we are told that thumb screw type eyepiece holders will not work well for binoviewer, self centering holder in a compression ring style usually work the best. So, you see how critical the collimation should be, in order to have the images merged successfully.

Collimation problem can also be resulted in a pair of not exactly matched eyepieces. In that situation, rotating the eyepieces will help sometimes, after it is done, remember to mark it down their relative position, so that you don't have to re-do the process every time. Also, marking down which is left and which is right.

Lastly, it's about matching of eye balls. When individual focusing is required, and there's an accessory for this purpose. But if the difference is slight, you can loose the eyepiece holder and pull one side out a bit and then fix it at proper location again. A par-focal ring can serve the purpose to fix it in location.

One night I pointed my telescope at the moon, my father came in and he has no problem merging the image at ~50x, ~100x or even ~250x. One thing I should mention is that, he found problem observing with my Pentax 40mm SMC XL before, that eyepiece is one of the easiest eyepiece to use in the world to me. My mother also had a look that night, no problem as well. My parents rarely observe with my telescopes. They were amazed by the view like me.

Personally, I found that the key of merging is that, if you looked *for* the images in the eyepieces, you might have problem merging. However, if you looked *at* the images, you will see one image easily. I can tell my brain like this and I can see one single merged image or two separate images any time I wanted. Funny!

So, don't look into the eyepieces, but instead, look at the image. If it does not work, do a reset, i.e. close your eyes, step back, come back and try again. Pay attention to adjust the inter-pupilary distance, closer is better when you attempt to merge the images at first, once it is done, adjust it back to the most comfortable position.

The design of the eyepiece also play a part here. I found the pair of eyepieces with a nice eye cup and decent eye relief, work better than the pair without the eye cup in terms of merging.

An exact pair of eyepieces is required for binoviewing

Matching eyepieces: One of the reasons that makes binoviewing expensive is that, we need two identical eyepieces to work. If you have already gone premium, you will need to double the premium investment. Not only that, it could be hard to find exact pair, since different production runs of the same eyepiece model might result in slightly or even major difference in specifications. So, you might need to end up selling your existing ones at a lost, and buy a new matched pair in order to ensure that they are from the same production run.

Eyepieces having a longer history is going to be harder to match, for example, the Nagler 13mm. The original version and the new one differs by everything, except the focal length.

But a good news is that, there're different corrector available which can be used before the binoviewer to yield different magnification with a given eyepiece pair, just like a barlow lens. We can see them from 0.5x, 0.6x, 1.3x, 1.8x, 2x, 2.5x, 3x, 4x, 5x or even more... just to name a few of them. It helps to save some money back from having to invest in a many pairs of eyepieces.

Correctors are available to offer multiple power for a given pair of eyepieces

In mono mode, people tend to think a dedicated eyepiece for a given focal length will be better than an eyepiece + barlows combination, while it could be similar for binoviewing, expert suggests that it is actually wiser to magnify before the prism, since the prism inside the binoviewer, no matter how good, should contain some aberrations. So, if you magnify after the prism, i.e. with another pair of higher power eyepiece, you end up magnifying also the aberration of the prism, a chunk of glass with non-trivial size!

Corrector enables "magnification before prism" which is considered to be better than multiple pairs of eyepieces

I tend to agree with the above principle, but you also have to notice that, you won't love to use your deep sky eyepiece (lower power, wider field) to see the planets. So, you might want two pairs at least, even if they are of similar focal length.

If you are already spoiled by wide field eyepieces, give yourself a treat by staying with them. I did it, and I found the real space-walk experience which I never found so before in mono mode. The 40mm Pentax SMC XL is a bit "narrow" for my taste now.

Having said that, you might waste some field when using binoviewer. For example, when I use my 80+ degree eyepiece, I will move my eye ball or even my head a little bit to see the whole field, but with a binoviewer, you cannot easily do the same thing. People usually said around 70 degree can be seen at once, of course, YMMV.

Brightness and resolution: The beam of the telescope is splitted into two by the binoviewer, and therefore, a dimmer image is expected. Also, the more air-to-glass surface, the extra optical component, is going to reduce the overall light throughput.

So, even if the binoviewer works perfectly, you will see dimmer image. Of course, when the brain integrates back the splitted beam, the result will be brighter than either of the splitted beam, but it will still be dimmer than in mono mode.

My personal experience is that, the effect of adding two dimmer images from each eye, will not give up back even a 90% image as bright as in the mono mode. You may try it out with any binoculars, close one eye, evaluate the brightness and then open the other eye to see if the whole view would be brighten up when compared with the one eye view. If the above binoculars test won't brighten too much, it's going to be even less dramatic with a binoviewer, since we have two separate objectives in binoculars, but we got only one with a binoviewer. Personally, I found opening the other eye won't help too much.

While brightness is reduced, the contrast is improved significantly and the resolution is not affected. I don't exactly know why the contrast is improved, maybe the brain is stacking two images automagically to give more contrast. For resolution, it is a function of aperture, and less light throughput is not going to affect resolution of a telescope.

Other than contrast, the floater problem at small exit pupil (i.e. high power) is reduced by using binoviewer, and so the maximum useable power is higher than that in mono mode.

I never saw the color changing on the globe of Saturn very cleanly and easily myself in mono mode, but I found it very very easy to see the color on the individual banding on Saturn with two eyes. Magical! Dimmer but more detail. Now, it's a very clean gradual change of color across the globe instead of just some subtle changes.

Concerning the dimming effect, yes, at the lowest magnification, the image is quite a lot dimmer, but as you push the magnification, you won't see the drop very significantly. This effect is funny. I mean I don't find the view at 250x a lot dimmer than the 100x view. I found the 100x view of my 8" SCT very good with two eyes, but I begin to see it on the dim side at the same magnification when in mono mode. Another funny effect brought by binoviewer.

And one more point is that, viewing with two eyes will give an impression that the image is larger, and the field of view is wider, too. Frankly, I never got any space-walk feeling with a 80 degree eyepiece, but I do get a tunnel like feeling when I get back to a 50 degree eyepiece. However, I do really find the binoviewing view with two 80 degree eyepieces to be space-walk, out there but not from a telescope.

People reported that you won't even find the field with orthoscopics to be narrow with two eyes. I found it to be the case. Of course, do not try to compare side by side with a pair of 80 degrees here. The field really feels wider with two eyes.

Focuser travel requirements: Binoviewer is not small, it is much longer than a diagonal and it represents significantly more in-travel is required. Nearly no regular Newtonian or refractor will be able to focus, and compound telescope having a moving primary, however, will usually do without problem.

Remember that the amount of in-travel required is not just the physical length of the binoviewer, but it is the optical path length, when light travels inside a glass prism, the optical path length is actually longer than the length it travels inside a hollow tube.

Having said that, there is an accessories to fix the problem, we call it a corrector in general. Early correctors are just barlow lens, and since the length of the binoviewer is added between the "barlow" and the eyepieces, a 2x barlows when used as a corrector, will give effective magnification at more like 3x-4x. By then, the wide field usage of binoviewer is greatly reduced. That means a pair of 30mm eyepieces is acting like a pair of sub-10mm eyepiece, i.e. from low power wide field, to a medium to high power eyepiece.

Technology advancement due to the development effort from various vendors, we can now see correctors of much lower power, ranging from as low as ~1.0 something in some particular situation, to normally 1.2x, 1.3x to 1.8x, and 2x or even more. Some vendors are developing variable power corrector, multiple power corrector and even zoom corrector. Watch out for latest development.

Of course, there's no free lunch and they come with a (BIG) cost. Somehow not just an increase in price, but I have read about review such that the optical performance can get compromised, so it's another consideration than just a negotiation with your spouse, or just a bigger hole in your bank account.

For a moving primary compound telescope like an SCT, a corrector with focal reduction ability is available as well, like 0.5x to 0.6x, some particular models of SCT will not work with the 0.5x, so check with the vendor before your purchase. While those reducers are called 0.5x, when in use, they will be more like 0.6-0.7x. Due to the magnifying nature of a SCT secondary and the reduced distance between the primary and the secondary, the resultant focal length of the telescope when used with a binoviewer, is longer than the original value, so you see why a 0.5x works more like 0.6-0.7x when used in a SCT.

Notice that regular rear cell reducer/flattener for SCT will not work without special attention, because the reducer is much farther away from the eyepiece with a binoviewer in place. To make them work, the trick is to move the reducer/flattener to make it closer to the binoviewer. That is, from the normal configuration of SCT-reducer-diagonal, to SCT-diagonal-reducer-binoviewer. Binoviewer specific reducer is screwed in the nose piece of the binoviewer, so put your regular rear cell reducer/flattener more or less in the same place and you are going to make it work.

With those 0.5x corrector/reducer, it really transforms a SCT into giant medium power binocular. This is one of the most important catalysts for me to take the plunge to invest after all these years of speculating. Consider a 8" f/5-6 binocular, very attractive! isn't it!

Don't expect too much here however, I found with the 0.5x, the field is still smaller than that provided by a 2" wide field eyepiece. But it's really wide enough already.

Vignetting: It is a very complex issue. To make it simple, human eyes are not so sensitive about vignetting, so unless it is very serious, you won't see the problem.

This problem originated from several sources, like the clear aperture of the prism used in the binoviewer, and also the vignetting results from an extended light path outside the focuser, and the change of focus position. It could be from under sized optical corrector. Basically, anything smaller than the light cone in that particular part of the optical path, *might* vignet the final image.

With an SCT, the primary mirror has to move much closer to the secondary in order to accommodate that extra-length posed by the binoviewer. As a result, the secondary is somehow undersized in this situation, vignetting results. Another source of vignetting is the small rear opening of an SCT, which is less than 2" for those smaller models, the baffle will also limit the maximum illuminated field size, remember the baffles are designed for normal primary-secondary spacing.

Another source of the problem is the prism size, most models have a prism size smaller than the maximum field stop of a 1.25" eyepiece. In this situation, we see vignetting again with larger field stop eyepiece, like a standard 32mm Plossl. The prism is the most expensive component of a binoviewer, so it is the most effective place to cut cost.

Like I said in the first sentence, it is a complex issue. In theory, vignetting does occur in the above cases and there are actually more different situations as well. In practice, I found it hardly noticeable. Vignetting means dimmer or more fuzzy field near the edge of the field, but such dimming effect, may or may not be detectable at different exit pupil size. People tend to find that with larger exit pupil, such effect is more noticeable, and at a small exit pupil, the effect is less. So, a 32mm 50 degree eyepiece is going to be worse than a 16mm 80+ degree eyepiece.

Sometimes people wonder whether such undersized prism will only cause vignetting or it might also cut down the effect aperture size. I am no expert, but I would guess that it won't. Will you cut down the effective aperture by using a 1.25" diagonal rather than a 2"? I never heard of that. With smaller prism, I believe only vignetting will result, that means a dimmer, less illuminated field at the edge, but not reducing effective aperture (nor resolution therefore). Note that the same argument does not apply to Newtonian secondary.

Additional Remarks: The fully illuminated field delivered by a binoviewer depends not only to the aperture of the prism, but also the focal ratio of the telescope in use. According to a discussion in astromart forum, a typical binoviewer has an optical path length of around 120mm, of course, some are longer and some are shorter. For example, with a f/10 SCT working at around f/12 with a binoviewer, the beam from the eyepiece focus point will be about 120/12 = 10mm. For every 1mm more prism aperture, we can have 1mm more fully illuminated field. Suppose you have a binoviewer of 30mm prism, you will be able to use an eyepiece with 30-10 = 20mm field stop fully illuminated. You see even if you have a 30mm prism, you will end up very limited to get wide field! But like I mentioned above, it is simply not necessary to care too much about that, human eyes are not that sensitive in vignetting unless you look for them.

Also, it depends on your target. When I looked at the moon, the gradual dimming due to vignetting is not very detectable unless you look for them, but if you look at empty dark sky, you will see a fuzzy field stop very easily.

Recently, we see some new binoviewing with larger prism than normal 1.25" binoviewer. How can it be? They do the trick by employing a 2" interface at the telescope end, and 1.25" port on the eyepiece end. Of course, there are some models having 2" in the whole design.

(to be continued)

Monday, February 25, 2008

Binoviewing I

From today, I'll start a new series of article on binoviewing. This is not a new one from scratch, but it's based on the one which appeared in my website several years ago. I'm going to renew the content, and to make it here in my observation blog, and also to make it into smaller pieces so that it's hopefully easier to read and understand.

Binoculars and Binoviewer

Observation with two eyes has always been the most natural way to observe, but as the aperture size increases, it is expensive to keep since you need two sets of optics, and they're required to be well aligned.

Binoviewing and wide field eyepieces

To me, going for binoviewing and going for ultrawide eyepieces (80+ degree) are two surprisingly similar things. They are similar in a sense that it is not a must to go with either, and both of them have some downsides on paper, like more glasses, a more complicated optical chain, and also the price, etc.

Most people believe that to squeeze the most out of your equipments, binoviewers and ultrawide eyepieces should NOT be used at all. The reason is that most glasses in the optical chain is not very desirable, unless absolutely necessary, so you see why there are some simple eyepiece that costs a lot. While I agree with the facts behind, but I disagree with the causal relation derived since it's an oversimplification. Visual observation is NOT to squeeze all the detail from our equipments. If I am going to get the most out of my equipments, I should simply resort to photography and look no further.

No one can see more than a stacked and processed planet/lunar image produced by a webcam, and no one can see more than a long exposure deep sky photography produced by a camera. So, why we still use our eyes to observe? To me, visual observation has a strong emotional reason associated with it. Finally, to be precise, we are here to squeeze most detail or visual impact from our equipment visually. Other than emotional reason, our eyes are very good detector as well. For example, when you first see a larger globular cluster, you saw a faint fuzzy at first, but if the aperture is enough, after awhile, you will begin to see pinpoints of star popping into the view. Amazing experience! No photograph can really reproduce this effect.

Visual observation

It is a fantastic experience to have photons from so far aways to go through your eyeball and then hit your retina directly, those photons travel so long both in terms of time and distance in order to hit your eye! Good equipments bring a lot of visual impact and enjoyment and I believe it one of the fundamental reasons why we have so many amateur astronomers around the world.

Therefore, except what is inside those images formed by the eyepiece, the feeling is important at the same time. Feeling is something which is extremely hard to judge, or I should better say, we should not judge. Theoretically, going higher magnification than the resolution of the aperture would allow, does not make sense at all on paper, but practically, we usually push a bit higher since larger image is simply easier to see. So, it depends on the observer, the quality of the whole optical chain, plus the atmospheric condition at the time of observation.

Having said that, feeling is not the only reason why we go for wide field eyepieces and/or binoviewing, let's read on. Binoviewing does help to bring out more detail otherwise very hard to see with one eye.

Some wide field eyepieces

Wide field eyepieces

Wide field eyepieces improve contrast by giving more magnification, thus darkening the background sky, and at the same time keeping the same true field size of an eyepiece with a smaller apparent field at longer focal length.

For example, a 24mm Panoptics delivers the same true field as a 32mm Plossl, but at a higher magnification. To me, going lower power only mean two things, a wider field and a larger exit pupil. When the exit pupil is large enough, going lower power means only the need for a wider field. By this logic, you immediately know why we go for wield field design, for what we want is field size but not the lower power itself, for most of the time.

Other than the improved contrast, the feeling of openness provided by these eyepieces is great. Tele Vue names such feeling as "space walk". You feel like you're seeing things outside, and you are part of the scene instead of looking through a telescope, you feel like you're out in the universe, instead of standing on Earth, peeping through a small hole with a tube filled with a complex set of optical components.

The wider field can frame your target better, and it is particularly good for deep sky objects. You would love to use one in a telescope on an non-driven mount so that your object stays in the field for longer.

Having said that, I found the view provided by a cheap high quality orthoscopic to be better than my wide field eyepieces at high power especially for the moon and planets. So, there is no absolute answer to what eyepieces one should get. Maybe the only absolute is, we want more different eyepieces for different situations.


Entering binoviewers, which is why I write this article.

I believe most people started with a pair of binoculars in astronomy. Binoculars are handy, portable, and they provide a very wide true field of view, they are very comfortable to use, and your eyes are relaxed during the observation.

Somehow when we graduated with a telescope, we learn to use (and close) one eye to observe, we see a lot more than a pair of binoculars, but we missed the goodness of binoculars at the same time.

Can we get the best of both world? Yes, go ahead and invest in two telescopes, mount it together to form a pair of binoculars? Portability asides, collimation and price make it hard to realize. Or, invest in those giant binoculars? Again, price is a concern, also portability.

Binoviewer is the viable ticket.

(to be continued)

Thursday, February 21, 2008

20080221 Quiet Sun

We finally got sunny day, seeing is low at 3/10, transparency like 8/10, better than even last night. Exposure like 1/800s can be used for surface detail, and this is rare these days. Surface activity remains low, and there're a couple of interesting but small prominences around the solar limb, nice view.

1150 (GMT+8), surface detail:-

1151 (GMT+8), prominences:-

1153 (GMT+8), surface detail:-

Telescope, filter, cameras: Borg 45ED II, SM40/BF10, DMK31AF03.

Wednesday, February 20, 2008

20080220 M42

Seeing was no good at around 3/10 (rough estimate), transparency 6-7/10, with poor polar alignment indoor, I couldn't get longer than around 12s exposure per frame, even at around this point, some frames are bad and needed to be discarded.

Remarks on 20080221: when processing those AVIs, I found that a couple of color shots were captured in Y800 which means all the color information is lost, since I've chosen hardware debayering, next time I shall use RGB24 no matter for monochromatic camera or color camera, since it removes any risk of lossing data, and at the same time, it's more friendly to Registax V3 which I've to use for V4 does not support field derotation.

C5 at f/6.3, DMK31AF03.AS on a roughly aligned LXD55:-

Short exposure done at 2.4s each at 750 gain, longer exposure at 11.6s each at 800 gain.

2235 (GMT+8), this is the longer epxosure mentioned above, the shorter exposure one is not posted:-

Next was by the same setup, but with a DBK 21AF04.AS:-

Short exposure at 9.2s at 800 gain, longer exposure at 11.6s each at 800 gain.

2245 (GMT+8), again, this is the longer exposure one, the shorter one is ommited:-

Then, I switched to my Borg 45ED II, again using the Hirsch f/6.3 reducer (Exact configuration: Borg 45ED II + JMI NGF-S + 2" extension tube + 2"->SCT adapter + Hirsch reducer + SCT visual back + DMK 31AF03.AS), and the effective focal ratio should be like f/4.6, taken with DMK 31AF03.AS, this time the exposure could be longer at 16.4s exposure at 500 gain due to the short focal length:-

Next is 23.2s at 500 gain (600 gain will get plain white screen):-

Next is 27.6s at 600 gain with DBK 21AF04.AS:-

Equipment used: C5, Hirsch f/6.3 SCT reducer, Borg 45ED II, DMK 31AF03.AS, DBK 21AF04.AS, LXD55.

On choosing a narrow band solar telescope

Let's talk about how to choose a suitable telescope to use with H-alpha narrow band filters.

First of all, refractor should be a very nice choice. Objective H-alpha filter has the expensive etalon in the front, and therefore, you don't want to have any secondary mirror to block this valuable aperture. Of course, you can use your H-alpha as offset filter in a large aperture scope to get an unblocked objective, however, that would make the scope unnecessarily large.

Given a refractor is good, we shall see what kind of refractor is suitable. Normally, we will first look at the chromatic aberration, however, this is not the case for narrow band usage, since you're essentially looking at monochromatic light, and therefore, chromatic aberration is unimportant here.

Having said that, other forms of aberration should be well controlled, to name a few: spherical aberration, astigmatism, etc. A good achromat will do the job.

Since you will want to tune the etalon via some sort of tilting mechanism, you will want a short tube refractor so that you hand can access the tilting mechanism while looking through the eyepiece. So how long should be suitable depends on the length of your arm.

Finally it's the focal ratio of the telescope, the rear end blocking filter would perform better at slower focal ratio, and usually f/10 is around enough, I mean you won't get much better performance with even slower telescope. And f/8 should be fairly nice as well, so it's actually a balance between the point above. Notice that the focal ratio might not be the native telescope focal ratio. For example, a 70mm ~f/6.8 refractor, if used with a 40mm etalon in the objective, it will become a ~f/12 telescope since the aperture is stopped down.

Tuesday, February 19, 2008

My telescopes: past and present


1. Celestron C90: this is my first telescope, I never forget the view of Saturn and moon through it

2. Celestron C8: this is the cure for my aperture fever, I've used it extensively from inside my home for planets/moon with my Tech2000/Giro; I've also brought it with me for several trips in the darker areas in Hong Kong with my Unistar Lighter

3. Meade ETX 60: I bought it from ebay, in the hope to make it a remote observatory of the sun, yes, it did work basically, but with the lack of suitable space, and the need of a special design blocking filter, it never comes into real usage

4. Borg 45ED: this one is actually just a lens-in-cell with a custom made tube to connect to a focuser, another attempt to make a remote observatory, again it's not the best combination


1. Tele Vue Ranger: this is my second telescope and it's also the telescope which made me more involved in astronomy, this scope has been used extensively, and it brings back a lot of good memories. For this reason alone, I will never sell it.

2. Takahashi Sky90: this one is co-owned with my astronomy friend, it's still with me sometimes, and with my friend sometimes. It's a great scope afterall, very versatile.

3. Coronado PST: I bought it for disassembly, and it works very well, still some minor kinks to be ironed out for the scaling up project setup

4. Celestron C5: this one lives with me for many years and then I finally bought it from my friend, it's now my workhorse, if I want to shoot the planet/moon, or for some deep sky observation. This is not a C8 in any case, but this is the maximum aperture that we could have for what its size and weight can deliver, it's also one of the most important part of my PST scaling up project

5. Borg 45ED II: this is very successful, I would say despite it's my newest purchase, but I may have used it more often than any of my other scopes, with the Ranger as a close second, however, it will be used even more in the coming future, I even brought it along for aeroplane trip, it's simply compact and powerful, especially for the sun; it's also good for wide field observation where binoculars might not work if you want to share the view with others, this little gems on a small tripod will do this job very nicely

6. Pentax 10x50 PCF III: this made in Japan binoculars is hard to find these days when this kind of economic models are only made in China now, this is light and the image is crisp and bright, it's easy to be hand held as well, again I brought this one to aeroplane trip, and it's the one which reached the darkest sky which I've ever accessed.

7. Canon 10x30 IS: this one is actually replacing my Pentax, simply because it gives image as nice as my Pentax and at the same time, it also delivers a wider field of view at the same magnifications, the best thing is the IS which works extremely well. Frankly, after using this one for a couple of months, I'd say that I might eventually get myself a larger model, it's just a matter of time.

Sneak preview into the future:-

1. Another 5" fast achromat is coming, I planned to use it for solar work with a Herschel Wedge, I expect there are a lot of CA, but with a green filter, it's not going to be a major issue, and finally, it's going to shine in CaK, this one is cheap and with good quality, I believe it might win over the C5 for low power wide field deep sky work as well, let's see.

2. The 60mm CaK from Lunt's Solar: I want to see how good it performs for the above in CaK with the Baader CaK, my major concern will be bandwidth. I never worry about the resolution for the 5" scope above, but if the bandwidth is really limiting the whole game, the Lunt's might take over the above.

3. Canon 18x50 IS: this is my dream binoculars, I believe that it's really a killer for portable astronomy on wide field, however, the price tag is also a killer, so I'm waiting for any good chance. I won't get one in near future.

Projects on hand

1. C5/PST Project

- The mounting of the ERF: I shall measure the exact figures again, to make rings similar to the last one which I've measured correctly last time but written wrongly... This time I shall make two rings, held together by screws like 1-inch long to secure the whole thing better, last time it's only held by two very small screws protruded only 1-1.5mm into the ring, and it's not very secure indeed.

- A tilt-able mounting of the PST etalon: I shall design and make an adapter which connects the C5 to the PST etalon via SCT female thread and M50*1 male thread, time to go back to the drawing board to design the tilting mechanism.

- Non-tilt-able mounting of the PST etalon: it's already here, for double stacking with my Borg 45ED II, SM40/BF10 setup.

2. Takahashi TG-SP II Autoguiding Project

- Order one more 12V 5RPM Geared DC motor

- Design a mounting plate to host the two motors to the RA/DEC slow motion control shaft of the TG-SP II, this plate shall also host sockets for autoguiding and hand controller

- Shop for a suitable box for hand controller, just four SPDT switches together with a battery pack, 4AA i.e. 4.8V seems suitable for extremely slow guiding operation.

- This platform is to be used with camera lens and DSLR

Monday, February 18, 2008

Takahashi TG-SP II Autoguiding Update

The new motor has arrived, it's a 12V DC geared motor running at 5 RPM, it's around $150 HKD each shipped. It has a 6mm shaft which matches the shaft of the TG-SP II. A surplus inline coupling ordered from the UK works perfectly.

I hand-held the motor and apply 4.8V DC to the motor, let it run, it runs very slowly and it drives the DEC slow motion control shaft directly. The DEC moves extremely slowly, barely noticeable with naked eye if you put a mark there to see the mark drifts, otherwise, you simply cannot even detect the movement.

So this experiment works, it confirms that:-

1. inline shaft coupling works
2. the motor is powerful enough to drive the DEC slow motion control shaft directly
3. and the speed is about right

Now, the next step would be to order one more such motor, and then custom made a small box which fixes both motors in place, and to put all the electronics (Solarbotics L298 kit based circuit) inside.

Canon 10x30 IS, monocular, DC Geared motor

Yesterday it was cloudy essentially, I setup my telescope to wait for the sun anyway, while waiting, I took my Canon 10x30 IS binoculars to scan around the roof of some close by factory buildings, these buildings are paradise for birds, since they got some plant there, and with some water. I could at least identify four kinds of birds, they're all common in Hong Kong, but it's nice to see them at that close while not disturbing them a bit.

I also took my small monocular to have a look, the field of view is far smaller, and the view is also dimmer, even I set its zoom eyepiece to match the exit pupil size. Also image stabilizer is really a must have if you compare them size by size. Of course, given the 20x price difference, it's expected.

The 5 RPM 12V geared DC motor just arrived, the 6mm shaft looked very powerful, the gear box is larger than the motor itself. I plug it in a 12V DC and it moves very nicely, my finger cannot stop the shaft no matter how hard to try, I even placed it under the weight of a large PC tower case and it's still moving very smoothly. Three threaded mounting holes are available for easy attachment. It's rather heavy.

I shall try to attach it to my TG-SP II tonight with the inline shaft coupler which I already had at home to see if it's strong enough. If the proof of concept experiment works, I will order one more and then design the mounting hardware, to host both motors as well as the electronics inside. This is the last "unit testing" experiment. Next step would be integration testing by merging the mechanical components as well as the electronics.

Sunday, February 17, 2008

20080217 Cloudy day sun prominence

Very poor sky, next to impossible to do anything, exposure is 1/5s with near maximum gain, just got a peek of the prominence:-

Saturday, February 16, 2008

Power solution

My notebook requires 16V, if I have to use eneloop battery, it would mean 16 / 1.2V, i.e. around 13.3 pieces. By using 14 pieces, it would be 16.8V, should be fine I guess.

Then other devices like my DMK, can be powered by 8 pieces of eneloop, i.e. 9.6V.

Therefore, I will need 22 pieces, i.e. 6 packs. Charging them all requires 3 chargers working in parallel for two times. Not very well.

Maybe I need to buy a 12V Lithium battery pack? But I can find them locally?

20080216 Cloudy day quiet sun

Seeing 2/10, transparency 1/10, Borg 45ED II, SM40/BF10/PST, DMK31AF03. Sky condition too bad, not really suitable for imaging, but anyway:-

1312 (GMT+8), prominence only... this is easier under this kind of situation, since you can push the gain and lower the shutter to do an overexposure, it helps to cut through the cloud:-

1327 (GMT+8), stacking means averaging, so the moving cloudy could be averaged out sometimes:-

Finally a video taken to see how worse the situation is:-

Wednesday, February 13, 2008

Astronomy Budget of 2008

I never do this before, but this year is pretty special, since there're quite a number of items that could be planned.

1. Total solar eclipse trip

- it will cost both time and money, but this is one of the goals which I believe that I must accomplish in my life. :-)


- this item has been in my plan for several years. Due to the upcoming total solar eclipse trip, and the availability of live-view, I will want to implement it before maybe April or May so that I can train up myself on how to use it before the trip

3. Solar imaging upgrade

- Lunt's solar system has attractive offerings
- Upgrade and enhance my PST based system, solve internal reflection issue and the ERF mounting issue
- Improve the white light view (larger refractor, i.e.), and the CaK view (larger refractor or a better CaK filter altogether)

4. Anything else... this usually happens for me...

Tuesday, February 12, 2008

20080212 Last minute Sun

Seeing is low at 1/10 since the sun is very close to the top of a remote building, transparency is like 6/10. I just back home a while ago and found that the sun is very low, but still accessible from inside my home, so I setup my scope to catch the last minute.

Cloud moved in and out and that shorten the accessible moments significantly, anyway, I'm too addicted not to shoot.

Basically nothing as captured, except the last trace of a prominence.

I also included a video clip when the sun was "eclipsed" by the remote building.

Monday, February 11, 2008

Impulse purchase

This is an impulse purchase, to say it first, since it's driven by the offer and the decision is done within 5s.

It's a 5" well made achromat with focal length of 820mm, i.e. around f/6.6, it's a bit heavier than a C8 OTA, I tried it several years (yes, years) ago in a dark site, and I loved its color correction (of course, with the expectation of the correction of a fast achromat), and the field of view, the view of open clusters are very sharp and contrasty.

The first owner who shared the view with me years ago offered this scope to me at a very cheap price that no one will refuse, at that price, you would not even hesitate to give it as a gift to your friend. So, there must be some usage for this scope.

Tentatively, I will try to use it with a PST etalon, and if it's too fast, I'll try to add a barlow lens in front of it. I'll also try to use it with my APM Herschel Wedge to do some white light imaging, and CaK with the Baader.

The bottomline is that, I'll pass it to my friend as a gift, it will be a very nice deep sky instrument.

Sunday, February 10, 2008

20080210 Quiet Sun continues

Still very quiet, it's essentially cloudy today, but have a few minutes of break when I could see the outline of the solar disc through the cloud, I still setup my Borg, and at some moments, I could detect that there's no sunspot on the solar disc.

There seem to be no big prominences as well.


Experiment: I found that the blocking filter in the visual back of the PST is different from the BF10, not just for its size, but also for the transmission, I found that ERF + PST etalon + BF10 can show H-alpha detail but ERF + PST etalon + PST blocking filter cannot show H-alpha detail at all.

Saturday, February 09, 2008

20080209 Quiet Sun

Finally got some sunshine again, seeing is 1/10, transparency 8/10. Borg 45ED II, SM40/BF10, DMK 31AF03.

1140 (GMT+8), full disc shot:-

1143 (GMT+8), close up with 2x barlows, the filament/prominence on the upper left is processed to bring out the transit better in the insert at upper right:-

1146 (GMT+8), prominence shot at prime focus, close up 2x at 1149 (GMT+8):-

Monday, February 04, 2008

New motor ordered

Just ordered one 12V 5RPM reversible DC geared motor with 6mm shaft. Since it's slower than the ones which I had on hand, it should be even more suitable, and the diameter of its shaft matches the one from the TG-SP II exactly, it's going to give far less trouble. This one will be used on the DEC for motorized manual guiding first.

It has M3.5 mounting holes and should be easier to find than the M1.5 of the older motors.

Once it's proven to work, I'll buy one more for the RA.

20080203 Solar Mosaic

2 * 3 frames taken with Borg 45ED II, SM40/BF10 with 2x barlows, 1510-1517 (GMT+8), focus is quite far off, and thin cloud is floating around making very uneven illumination:-

Sunday, February 03, 2008

20080203 Sun again!

We had some many cloudy days! The sky finally cleared up and we got a view of the sun. Seeing 6/10, transparency 7/10. We have AR982 and the filaments near it, very nice indeed.

1503 (GMT+8), full disc, Borg 45ED II, SM40/BF10 at prime focus of DMK 31AF03:-

1504 (GMT+8), full disc, Borg 45ED II, SM40/BF10 at prime focus of DMK 31AF03:-

1505 (GMT+8), full disc, Borg 45ED II, SM40/BF10 at prime focus of DMK 31AF03, increased exposure to bring out the prominences:-

1508 (GMT+8), Borg 45ED II, SM40/BF10, with 2x barlows:-

1523 (GMT+8), with PST etalon added:-

1527 (GMT+8), same as above except with 3x barlows:-

False color version of the above:-


On the internal reflection issue of the PST etalon module and the blocking filter, I found that the problem even appears in a refractor, of course, with a refractor you cannot see the secondary shadow, but you can still see a circular patch of brightening, that also hurts the contrast as well.

So now I wonder if it's a good move to move up to the 5" refractor, since it won't help too much on the reflection issue, obviously I need something else to solve the internal reflection issue, like tilting of the PST etalon module or the BF or even both.