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New C6 + 1st Star test results (am I doin' this correctly?)

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#1 AJK 547

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Posted 15 August 2019 - 09:53 AM

I just bought a new C6/Evo ( grab 'n' go) setup and wanted to perform a Star Test to determine the quality of the optics.  I waited until this morning when conditions were stable and with good transparency. 

 

*** I've been reading the excellent book 'Star Testing' by Suiter to give me insight.

 

Aug 15th, 4:00am

Star selected: Algol (2.12 mag) high in the sky.

C6/Baader Maxbright Star prism diag./ Baader BHZ MKIV @ 187X/cool down 2 hrs.

 

Initial results:

 

0) Collimation: VG - Excellent

 

1) Snap-test:  'excellent'... pin point star image (only a 'hint' of a surrounding defracation ring). This gave me GREAT happiness...

 

2) Inside / Outside focus:  nearly perfect same defracation pattern on BOTH sides of focus.  This result again gave me high hopes for this little SCT.  This result, performed 10-15 times, was consistent and supported the 'snap test' result.

 

3) Misslignment: None detected

 

4) 'Seeing': Steady/Stable

 

5) Pinched Optics: None detected

 

6) SA: Very slight undercorrection (between 1/4 wave and 1/8 wave images in Suiter's book). 

 

7) Roughness: Absolutely smooth defocused concentric image from CO to 'edge'.

 

8) Turned Edge: None detected

 

9) Astigmatism: None detected

 

Overall, based on Suiter''s images for a 'perfect optic' SCT it seems this new C6 is some were between 1/6 wave to 1/8 wave precision optically based on last night's star testing.  

 

Question, "Am I performing the Star Test correctly, and do you agree with my initial estimate of 1/6 to 1/8 wave optics?"

 

I'm contemplating getting a Ronchi EP to further test this and other scopes.

 

Any comments/insight are very welcomed...  and more testing will be conducted in near future.

 

 

 

 

 


Edited by AJK 547, 15 August 2019 - 09:59 AM.


#2 ngc7319_20

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Posted 15 August 2019 - 11:07 AM

Sounds like you are on the right track and thinking about the right tests.  I might have used more power -- closer to 300x or 400x.  One unknown is how close you were to focus when judging the patterns inside vs outside focus.  I like to keep it near focus - like 3 to 5 diffraction rings in the de-focused pattern - when looking for spherical aberration, astigmatism, etc..  As to whether it is 1/6 to 1/8 wave -- I would need to look for my self to tell.  But if at focus you are seeing a nice Airy disk with a very faint ring, there is not much to complain about.  Sounds like a very nice scope.


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#3 Pinbout

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Posted 15 August 2019 - 11:16 AM

 

Inside / Outside focus:  nearly perfect same defracation pattern on BOTH sides of focus.  This result again gave me high hopes for this little SCT.  This result, performed 10-15 times, was consistent and supported the 'snap test' result.

at 10 waves defocus, the central obstruction should be a similar size. - this will show SA quality.

 

larger defocus will present zones if any.

 

 

SA: Very slight undercorrection (between 1/4 wave and 1/8 wave images in Suiter's book).

what from his images are telling you this...

 

the size difference in CO will describe this, as well as difference in disc brightness as well as outer ring brightness.

 

remember this... from making mirror, bright rings inside focus - polish more (undercorrected), bright rings outside focus stay away (overcorrected)

 

check this out...

 

bench test comparing to star test.

https://www.youtube....l5m3ccdd2o&t=1s


Edited by Pinbout, 15 August 2019 - 11:31 AM.

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#4 AJK 547

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Posted 15 August 2019 - 12:04 PM

at 10 waves defocus, the central obstruction should be a similar size. - this will show SA quality.
 
larger defocus will present zones if any.
 
what from his images are telling you this...
 
the size difference in CO will describe this, as well as difference in disc brightness as well as outer ring brightness.
 
remember this... from making mirror, bright rings inside focus - polish more (undercorrected), bright rings outside focus stay away (overcorrected)


Thanks for the replies! The 'airy disk' test (performed numerous times) was truly a beautiful, 'blue-white' pinpoint in sharp focus... with only a single VERY SLIGHT diffraction ring surrounding the CO. Actually, the single diffraction ring never completely encircled the CO, but seemed to dissolve at certain areas around the CO, similar to the 'airy disk' that Suiter shows in a perfect optic 'scope.

The level of 'brightness' of the outside 'ring' (either side of focus) when compared to the inner 'ring' surrounding the CO was once again equal in luminosity. This led me to believe that the SA was nearly non-existent. There was no CO 'size' difference on either side of focus.

It was the 'snap test' that got me REALLY excited when Algol was presented as a beautiful 'pin-point' with no diffraction spikes, etc. Definitely more testing is in order when conditions are stable enough for Planetary and Lunar enjoyment.

Any additional insight please feel free to give!

Edited by AJK 547, 15 August 2019 - 12:13 PM.


#5 Pinbout

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Posted 15 August 2019 - 12:45 PM

 

There was no CO 'size' difference on either side of focus.

that's all you need to know its good. but the more out of focus the less sensitive the test is

 

but if you watched the test of the import 8inf6 the co size difference was extremely bad.


Edited by Pinbout, 15 August 2019 - 12:46 PM.


#6 AJK 547

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Posted 15 August 2019 - 01:45 PM

Yep, watched the YouTube on the 8"f6 and the C6 is in a totally different league.

 

Thanks for the input!

 

Now, if you had to put $$$ on if this C6 is 1/4 wave, 1/6th wave or 1/8th wave based on my observations last night...  any idea?

 

Thanks in advance.




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