3 replies to this topic

[dbernst]

I am about to purchase a new refractor, and am considering either the WO Cat 91 or the Askar SQA85. I currently image with an ASI2600-MC Pro but like the idea of having a scope capable of handling full frame if I decide to upgrade my camera in the future. My understanding is that these two refractors can be really high quality, at least in theory (amazing spot diagrams) but because of lax QC, you can wind up with a clunker. As a beginner to intermediate-level astrophotographer, I am interested in knowing what specific tests people recommend that you always do when you buy a new scope. I would want to use the first 30 days before the return window closes to determine whether I received a winner or loser. Based on what I see posted, here is what I see so far:

(1) Aberration inspector of the image corners in a starry area of the sky in Pixinsight on a minimally processed image.
(2) Inspecting for tilt in ASTAP (although shouldn't that be adjustable, or can some tilt be too much to fix?).
(3) Imaging a bright star looking for diffraction spikes (If present, how much is acceptable?)

What other tests do you recommend?

Thanks,
Dan

[gnowellsct]

You are thinking entirely about quality for imaging. Those tests are probably the most rigorous.

But you can do certain things that tell you about the quality of the scope. Choose a distant branch against a bright blue sky and push the magnification up to 3x per millimeter of aperture or higher to see if there's any chromatism. And look around and practice focusing on different objects at different distances. Getting to know the Scope's daytime behavior makes it easier to use it at night.

Put your heaviest eyepiece or imaging setup in the focuser and point the scope straight up and see if you get any run out.

Put your widest field eye piece in and check the overall view all the way up to the edges. This is best done with a super premium eyepiece.

Point the scope at Jupiter and Saturn to see how much planetary detail you can get out of it.

Point the scope of a variety of different angles including the ground to see if the lens cell is holding collimation.

Test the mechanics of the dew shield in terms of sliding in and out. Assess it with and without the dust cap on. Sometimes The dust cap and dew shield can get a a little bit too finely crafted and you end up vacuum sealed.

Rack the focuser all the way in and all the way out several times. It should be smooth from beginning to end.

Assess the degree of vibration during your focus. You have a new scope. It is not a done deal that it is well matched with your mount.

Study the rings and make sure you know how they work. There are a surprising number of design permutations.

The general idea here is that you want to get to know your scope and how it feels and responds and how to work with it even in the dark.

Etc.

Edited by gnowellsct, 07 April 2025 - 02:12 PM.

[Jeff B]

Focuser alignment with the middle of the objective

 

Cheshire check

 

If it passes those two:

 

Double pass test for spherical and color correction

 

High power visual assessment for:

 

Coma

 

Astigmatism 

[dbernst]

Thanks. Any others for astrophotography?