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Help for star test results!

Started by T.R., 10/19/2006 01:13AM
Posted 10/19/2006 01:13AM Opening Post
I think I know what I have to do, but I dont want to jump the gun. Just took delivery of an import "apo" refractor. On star test with vega at zenith,160x, inside of focus nice pattern, outside rings obscured(more turbulence seen outside of focus?)similar to rough surface. Also inside of focus astigmatism obvious on opposite sides of view but none seen outside of focus??? Wouldn't astigmatism appear on both sides of focus and in the center of the view, not just at oppposite edges??? It appears that in a daylight terrestrial view there is vignetting all around the edge, more so where the astigmatism showed up on star test. What is the diagnosis? I had just a short time to do the star test as clouds moved in, I didnt get an overall general look at a field of stars. What level of quality should I accept from the "imports" toted as APO? I think I need to send this back...any help or advice greatly appreciated!
P.S. Televue eyepieces and everbright used!
Posted 10/19/2006 07:32AM #1
What level of quality should I accept from the "imports" toted as APO?

'Import' like TMB?

Dmitri
Posted 10/19/2006 08:16AM | Edited 10/19/2006 08:16AM #2
Are you using a green filter? Without filter, chromatic effects
can make patterns imbalanced inside / outside of focus. What
you see is typical without filter -- sharp patterns inside focus,
and fuzzy ones outside focus.

Not sure what you mean by "inside of focus astigmatism obvious on
opposite sides of view but none seen outside of focus." Are you
talking about the field edges? The field edges can have
astigmatism due eyepiece (probably more often than not), and it
can change inside vs. outside focus. (This is because
eyepiece off-axis astig interacts with scope off-axis astig
in complicated ways). You should be more concerned
about any astigmatism seen at the field center. Sounds like
you don't see any astigmatism at the field center. TeleVue
eyepieces vary in degree of field edge astigmatism; I would need
to know exactly which eyepiece and scope to possibly tell you if
it is normal.

Vignetting at the field edge is fairly normal in most scopes.
Rarely will you see a scope without some vignetting at the
field edge.

So far, if I understand your comments correctly, I do not see
any issues with the scope. If this is all you've found wrong,
it sounds like a good scope. What is this nice scope?
Posted 10/19/2006 03:03PM #3
I would suggest that you have a knowlegeable person test your scope. The descriptions that you gave leads me to think you are misinterpreting the star test. I believe you are getting confused by the different star patterns, some of which are due to eyepiece aberrations and some due to a person's eyes.

Rolando