I think I shall draw a short conclusion after about half a year of usage. The Tele Vue zoom becomes my eyepiece of choice if I have to pick one only. At 24mm, the real FoV provided is large enough for me to find my targets with the Ranger. And at 8mm, it provides enough magnification for me to observe even the planets. Of course, I usually bring the 2x Orion Shorty Barlows or the Tele Vue 5x Powermate along with the zoom. To compensate thing a little bit better, I will also bring the 32mm Sirius Plossl.
In a night with excellent seeing, I've the experience to clearly see the Cassini Division with the Tele Vue zoom at around 120x with the 5x Powermate, cloud bands on the Saturn is obvious as well. I've also see the GRS and many many (over 8!) cloud bands on the Jupiter. More importantly, all these were done with a 70mm Tele Vue Ranger! Averted vision is NOT required. However, I've done it in one and only one occasion. You will never know how your telescope performs until the sky is excellent. On deep sky objects, I've also seen the fan shape of the Orion Nebula on the same night, and that's also the first time.
Unluckily, I haven't brought my 20mm Tele Vue Plossl with me that night, or else I can tell how the zoom performs when compared with fixed focal length eyepieces. But I'm sure that the zoom will not be a disappointment.
And to me, it's an amazing piece of art.
5-Oct-2000: Night and 6-Oct-2000 morning
During the Chung-yeung festival, we want wild-camping at Chek-king. Tonight, I've brought with me only two eyepieces. They're the 32mm Sirius Plossl (to provide me with the largest possible FoV) and the new Tele Vue 8-24mm Zoom. I've also taken the 5x Tele Vue Powermate. After the whole night, I found that the 32mm was left alone in my eyepiece case.
The Pleaides and the Orion Nebula were superb in the Tele Vue zoom. I don't quite affected by the narrower field of view at low magnification of the zoom. Maybe it's because I has not been "spoiled" by the wider-field eyepieces, yet. The view of Jupiter and Saturn were very good as well, at least not a bit poorer than my 20mm Tele Vue Plossl. The zoom capability was really convenient and also useful. Once the target was found, I could zoom in-and-out until I found the optimal magnification. Since clouds moved in occassionally, I found I couldn't see the planets long enough to figure out the real performance of the zoom at higher power. I didn't get a chance to use the Powermate as well, but I've a feeling that the 2x Orion Shorty Barlow might be a better couple with the zoom.
The 5x Powermate should be a bit too high, at 100x even at the lowest (24mm) setting with the Ranger.
I also got a chance to watch the sun on the next afternoon using the zoom, the view was excellent. That was the first time I watch the sunspots with my scope, several sunspots (at least seven) were easily visible. I could also look the the detail of each of the spots, and they looked circular with smaller spots inside. So, there were actually several sunspot groups available. I had also taken some photographs with my friend's digital camera, and they can be found at my astrophotography page.
In conclusion, I love my zoom and my Ranger, as well as the new solar filter. Oh, yes... and also the new Gitzo G106 tripod, it was very light and compact, make it extremely portable. So, I sold my Manfrotto 055C in the following week. :)
This is not the first time I observed the Sun, the last time was a brief look by hand holding the Solar Filter on the binoculars, a few sun spots were detected last time. This time, I observed using the Ranger with the Solar Filter. Several sun spots were detected, with a cluster of sun spots somewhere near the edge of the solar disc. I was using the Tele Vue Zoom through my glass windows (!!!). I knew it's bad but it's an unplanned observation inside my apartment. Not bad, still.
13-July-2000: First Day and Night
- day -
I received my Tele Vue Zoom finally this morning. I bought this one from the Pocono Mount Optics, and I have got a Thousand Oaks Type 2+ Solar Filter for my Ranger as well in this order.
I tested the Tele Vue zoom in board day light, and the chromatic abbervation is obvious. This is due to the semi-apochromatic nature of the Ranger more than the Tele Vue zoom, but I've no way to confirm it since the Ranger is my only scope. The eye-relief is very comfortable throughout the whole focal range. The image is sharp edge to edge. The zoom eyepiece at different focal lengths are not parfocal, but they are closed to be so. Careful observation reveals that there's a piece of dust inside, cleaning on the exposed lens surfaces do not help. Too bad that it lies inside the eyepiece which cannot be disassembled easily.
I just hope that it won't be noticeable at night.
- night -
On the same night, I tested the Tele Vue zoom inside my apartment. It's rather cloudy tonight, but the cloud is moving very fast and so there're sometimes that I can observe the stars in-between the gaps. Stars are sharp to the extreme edge, and the dust inside the eyepiece is not detectable at night, luckily. Maybe it will only appear when I observe the moon. Anyway, I would rather leave it alone than to send it back, since it takes too much time. The apparent field of view is stated as 45 degree at 24mm and 55 degree at 8mm, and I would say it's accurate. To me, 45 degree apparent field of view is not restricting for my taste and the 55 degree at 8mm is nice as well, wider than my previous experience on observations. I only got a few plossls, and maybe that's the reason.
When compared with the 20mm Tele Vue Plossl, the field of view is smaller, as expected. The field stop size at its maximum for the zoom is still smaller than the 20mm Plossl. But the difference is small. I randomly look at the sky, and I can see exactly the same number of stars in both eyepieces, implying that advanced coating make the extra-elements not too bad on the zoom. I tried the zoom with the Tele Vue 5x Powermate and the Orion Shorty Barlow (2x), and they work very well.
On zooming, the apparent field of view expands but the actual field of view diminishes. Close stars become farther apart and the sky background becomes darker as the magnification increases. It's so nice that we can choose the optimal magnification without changing eyepieces. And that's the major reason for why I bought this eyepiece. I couldn't detect any real difference between my 20mm Tele Vue Plossl and the zoom. Maybe I should later try the zoom on the planets, the moon and other DSOs.