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Star test on an Orion GoScope 80mm Tabletop Refractor

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#1 Deepskyclusterstruck

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Posted 03 March 2021 - 09:05 PM

In focus stars seem almost points though not quite. When intentionally out of focus racked in and then out the circle of light was symmetrical but instead of rings single solid filled mass. Even on both sides of star and both sides of focus but no definable rings. When focused 2 tiny lines (one each side of some stars) but none on others. Some almost pinpoint with no such lines. Is this normal for this scope? I'm wondering what is likely to be my eyes, what might be from humidity, and what might indicate a problem with the telescope or even eyepiece. I used a 10mm eyepiece and weather report says 55 percent humidity. Also, could even small amount of artificial lighting behind me
contribute to seeing no rings around the stars but rather a solid filled circle instead?

#2 Jon Isaacs

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Posted 03 March 2021 - 09:15 PM

Hi:

 

The rule of thumb is that star tests are done with a 1mm exit pupil, the eyepiece is equal to the focal ratio of the scope.  Your scope has a focal ratio of F/4.4.  With a 10mm eyepiece, the exit pupil is about 2.3mm, that's too large to see diffraction rings etc.

 

Jon


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#3 Bean614

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Posted 03 March 2021 - 09:24 PM

What Jon said!!!!!!

   If you're going to do a star test, do it correctly. Otherwise,  it's absolutely meaningless, and shouldn't be used to indict ANY telescope. 



#4 eyespy

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Posted 03 March 2021 - 11:11 PM

Hi Deepsky,

 

It looks like that scope uses a 45 degree diagonal that I believe uses a type of modified roof prism with an ‘edge’ that is causing the lines. Usually, this type of prism diagonal will not take higher powers very well and is usually used for lower powered terrestrial daylight viewing. If you use a regular right angle prism diagonal you should see improved star images and without the lines.

 

Doug......


Edited by eyespy, 04 March 2021 - 02:32 PM.

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#5 Deepskyclusterstruck

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Posted 04 March 2021 - 05:29 AM

Jon-Thank you!

Bean614-In no way did I intend this as an indictment of the telescope. I figured the fault was in me. Glad it was. Now I know better.
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#6 Deepskyclusterstruck

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Posted 04 March 2021 - 05:35 AM

Eyespy- I had forgotten what diaganol I had in. It's not marked in any way. Thanks for the advice. I will abide by it.
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#7 LDW47

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Posted 04 March 2021 - 08:40 AM

Show a picture of which GoScope you speak of, there are at least 2 models !



#8 peleuba

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Posted 04 March 2021 - 09:45 AM

What Jon said!!!!!!

   If you're going to do a star test, do it correctly. Otherwise,  it's absolutely meaningless, and shouldn't be used to indict ANY telescope. 

 

Your broader point is well taken, but your statement is misleading.  

 

I am certain that Jon was referring to (judging) spherical correction of the lens where appropriate magnification is needed so the error can be more easily seen.  But, there are times where a low power star test is useful.  An example of this would be the examination of zonal errors.  These present as light and dark areas, sometimes global within a figure of revolution, and sometimes local within a small portion of the Fresnal pattern.  

 

The bottom line is that the Star Test is a terrific tool to use as it shows all errors including those that are environmental.  This strength is also its downfall and its often referred to as a "wobbly stack".   Meaning this:  because of the different variables involved, rarely does a given mirror or lens suffer from a single aberration.   Usually its multiple issues and these can be difficult to sort out when viewing the image in an eyepiece.  It takes a lot of practice to be able to isolate specific errors.  Performing the star test indoors really helps, but then you have the added headache of ensuring the telescope under test "sees" the artificial star at infinity.  If it does not spherical aberration that does not exist in the lens will be added to the result.  




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