Saturday, January 20, 2007

Focus test and brief eyeiece test

Cloudy today, but in order to test whether the Sky90 can reach focus with the APM Herschel Wedge or not, I still take it out to aim at terrestial targets. Herschel Wedge cuts 95% of light and it's still enough to let you see something.

With a low profile 2"->1.25" visual back other than the original Tak one, it reaches focus without problem, even still have some more rooms like 1cm or so. Therefore, it's now just a matter of clear sky and big sunspot! Let's hope.

I've also conducted a brief test of my existing eyepieces, including a 7mm J-ortho, a 13mm Nagler Type 6, a Japanese 20mm Widescan Type III, and a 20mm Tele Vue Plossl.

The Nagler first, I could detect color with both my Sky90 and the Borg 45ED on high contrast object, green and purple. It's not very obvious, but it's certainly there if you look for them. Straight lines become curved when it comes to the field edge, not a problem for night time observation. Eye-relief is very comfortable without glass, you can see all the way to the field stop, the rubber eye guard is very useful. However, it's a little bit difficult to hold the view in day light. It's the most expensive eyepiece of the batch.

The Widescan next, I could detect false color too, to a similar degree with the Nagler. Also, this eyepiece gives a warmer tone overall. Straight line become curved like the Nagler, but it's lesser, maybe due to the lower power. Eye-releif is very comfortable without glass, and you can see all the way to the field stop, the rubber eye guard is also effective. It's easy to hold the view in day light making it a fine eyepiece for solar observation. It's around 70% cost of the Nagler, but the performance is 90% at least. Image only become a little bit fuzzy near the field edge.

The Ortho follows, no false color for this eyepiece despite it's the highest power of the group. Color fidelity is excellent, straight lines remain straight all the way within the field fo view. Eye relief is a little bit short but still very useable. No rubber eye guard here, and you will need to cup your hand around to block ambient light. It's around 10% of the cost of the Nagler, the performance excels with just far smaller field of view. But it's my planet eyepiece so I have no problem with it, I still got plenty of background sky with this tiny field. Image is sharp and contrasty all the way, excellent value.

Finally we have the Tele Vue Plossl, in short, it delivers every bits of performance of the Ortho at around 40% of the cost, field of view is a bit bigger than the Ortho, the rubber eye guard is superb and serve its purpose well. Eye relief is great without eye glass. Color fidelity is excellent, very sharp and contrasty throughout the whole field of view, straight line is not distorted in any sense all the way across the field stop. I've two identical sample for this one for binoviewing, I even keep them attached in the binoviewer for storage, I seldom take them out.

In short, the cheapest eyepieces are the best performers, they just give a smaller field of view. Having said that, I love wide field eyepieces for deep sky observation, since the feeling of being out there, is unbeatable. Well made Ortho and Plossl simply deliver excellent value, clinical and scientific might be best describe them.

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