William optics megrez 90mm APO?

Question

I just purchased a WO megrez 90mm APO. I want to do star testing so I looked up a few web sites on it because ive never done it before and I wanted to test the lenes. They said you should see concentric rings but all I saw was one two rings.The pics they showed had 5-7 rings or so. Also I am getting a very small amount of comet tails on the stars on the very outer edge ( when in focus) . I guess my question is... how much does the quality of the eyepiece effect testing of the scope? should I be using Neglers.......I dont have one

Answer 1

There's some good basic information on star testing here:

http://www.backyardastronomy.com/appendix/startesting.pdf
http://geogdata.csun.edu/~voltaire/tmb/tmb4.html

A few tips: for a 90mm use a first magnitude star and about 180x magnification. You must have good seeing (steady skies) to perform the test. A green filter may help, even on an apo. A wide field eyepiece like a Nagler is not required; a high quality Plössl or orthoscopic will be fine. Star tests are done in the middle of the telescope's field. Problems towards the edge of the eyepiece field should be discounted, since they're more likely to be eyepiece aberrations (lateral spherical aberration) than objective aberrations.

Answer 2

The multiple concentric rings show up on an out-of-focus star. The usual criterion is that the rings look the same on either side of focus. What you are seeing is correct for an in-focus star. There are a couple of articles on star testing below, from two master apo designers.

The distortion at the edge may very well be due to the eyepiece. Most eyepieces show a bit of off-axis astigmatism at the outer edge.

Answer 3

Hi. The rings, caused by diffraction, are small so a large magnification will help you see them. The coma is normal since no lens is perfect. A refractor will also show color changes in the image as you go through focus. Again this is normal. You do not need a Nagler. A good lens will show a set of rings in a steady sky that appear the same both in and out of focus.
Good luck and clear skies!

Answer 4

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