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AT92 What kind of Abberation is this?

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20 replies to this topic

#1 DougSol

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Posted 13 September 2020 - 12:56 PM

Hello Refractor people,

I found that this AT92 woud not autofocus accurately, so I did a star test to see if the problem was the refractor. I'm attachinging the intra-focal and extra-focal images. I would welcome your opinions about the optics. Since the two images are different I suspect that something is wrong. Thanks

Doug

Attached Thumbnails

  • Extrafocal.jpg
  • Intrafocal.jpg


#2 PETER DREW

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Posted 13 September 2020 - 01:13 PM

The image looks nicely round but exhibits correction effects.  This is a triplet I believe and may not have been in thermal equilibrium.  Triplets are very sensitive to this.


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#3 db2005

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Posted 13 September 2020 - 01:14 PM

That looks like a severe case of spherical aberration to me. Have you tried to look at a star visually to check if the problem is also present visually?

 

EDIT: Make sure the scope is acclimatized. Many scopes show some SA while cooling.


Edited by db2005, 13 September 2020 - 01:15 PM.

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#4 DougSol

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Posted 13 September 2020 - 02:30 PM

To expand on my post for the sake of clarity,

I was using the AT92 in my observatory at around 9PM, and the temperature was stable. I noticed this problem all the way till 1AM when I was conducting the star test. I think that eliminates acclimation as a possibility.

I first noticed a problem when I was trying autofocus in NINA and noticed it kept failing as all the star HFRs outside of focus were off. I made sure that there wasn’t a problem with the focuser or my autofocuser by testing it with backlash, but that wasn’t the problem. I noticed that all of the stars inside of focus had sharp rings while the stars outside of focus were blurry while the camera was looping images, which was when I decided to do a star test. I did the star test as seen above with the AT92 and its field flattener and double-checked by repeating it without the field flattener, and got the same results. I’m suspecting the scope has severe spherical aberration but I’m not sure.

I haven’t put an eyepiece and looked at it visually, but I observed it by looping images with my camera.



#5 bobhen

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Posted 13 September 2020 - 02:48 PM

The image shows severe spherical aberration.

 

If this is a new purchase and if when you use an eyepiece it looks the same, I would ask for a refund or exchange.

 

Bob


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#6 peleuba

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Posted 13 September 2020 - 03:03 PM

The image shows severe spherical aberration.

 

If this is a new purchase and if when you use an eyepiece it looks the same, I would ask for a refund or exchange.

 

This.

 

 

=====

I have had two AT92’s and still have one - using it on my optical bench.   I’m currently on a 9 day observing trip so don’t have access to my home server where my optical bench tests are of my AT92’s to compare with your star test images...  

 

Your’s  is suffering from spherical aberration.


Edited by peleuba, 13 September 2020 - 03:03 PM.

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#7 TNmike

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Posted 13 September 2020 - 09:49 PM

DougSol, please keep us posted on what you find out. I have one on order and will need to check it out carefully when it comes in. But even the top tier vendors can have one get through, though rarely. Others somewhat more so. Hopefully this one is really an outlier, if indeed the optics are bad, and 99.9% are excellent.



#8 Star_Shooter

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Posted 13 September 2020 - 10:19 PM

Looks like 1/2 wave under correction.

#9 YAOG

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Posted 13 September 2020 - 11:08 PM

This is not a good lens, SA and pinched, give it back to whoever you bought it from. 



#10 Wildetelescope

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Posted 13 September 2020 - 11:39 PM

Hello Refractor people,

I found that this AT92 woud not autofocus accurately, so I did a star test to see if the problem was the refractor. I'm attachinging the intra-focal and extra-focal images. I would welcome your opinions about the optics. Since the two images are different I suspect that something is wrong. Thanks

Doug

 

This looks nothing like the star test of my friends AT 92.  Is it a new purchase?  Agree with SA assessments others.  

 

Jmd 



#11 Jon Isaacs

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Posted 14 September 2020 - 02:00 AM

That looks like a severe case of spherical aberration to me. Have you tried to look at a star visually to check if the problem is also present visually?

 

:waytogo:

 

What else in the optical train?  Anything?

 

Jon


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#12 DougSol

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Posted 14 September 2020 - 11:16 AM

A matched field flattener was in the optical train, but the star test was the same without it.



#13 DougSol

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Posted 14 September 2020 - 11:17 AM

I bought it from the original owner.



#14 YAOG

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Posted 14 September 2020 - 12:35 PM

I bought it from the original owner.

Give it back and get your money back or ask the original purchaser to send it back to Astronomics for correction or replacement. This is a poor star test, it may be correctable with spacers and recentering but as it sits it is a bad lens that I'm certain Astronomics would not let stand. I don't know what you paid for it but it is not good. 



#15 DougSol

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Posted 15 September 2020 - 09:29 AM

Thanks everyone for the excellent input. I'm returning it to the seller, who understands the issue, and he'll be contacting Astronomics.


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#16 Tyson M

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Posted 16 September 2020 - 12:37 AM

Thanks everyone for the excellent input. I'm returning it to the seller, who understands the issue, and he'll be contacting Astronomics.

Could've been caused by shipping the scope for resale. Id be surprised if they would assist in this cause for the seller, at least for free that is.

 

Thai's assuming they can even fix optical alignments like that.



#17 peleuba

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Posted Yesterday, 02:59 PM

Thanks everyone for the excellent input. I'm returning it to the seller, who understands the issue, and he'll be contacting Astronomics.

 

Sorry for the late reply to this...   That's great news...  And, what I post below is educational only.  You can cheerfully ignore it if it's not of interest.  

 

Below is a star test from the bench on a 110mm F/5.6 oiled triplet.  The AT92 should, on paper, be better.  Its a smaller aperture and the air gap gives the designer another degree of freedom in which to dial in correction.  The oiled triplet is F/5.6 all-spherical design which leaves a lot of high order SA in the star test.

 

You'll see some similarities between your AT92 and this 110.  You were correct to send it back to the original owner.

 

Images below are of the same 110mm APO at various point of defocus.  

Attached Thumbnails

  • 110-1.jpg
  • 110-2.jpg

Edited by peleuba, Yesterday, 03:01 PM.

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#18 YAOG

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Posted Yesterday, 05:46 PM

Sorry for the late reply to this...   That's great news...  And, what I post below is educational only.  You can cheerfully ignore it if it's not of interest.  

 

Below is a star test from the bench on a 110mm F/5.6 oiled triplet.  The AT92 should, on paper, be better.  Its a smaller aperture and the air gap gives the designer another degree of freedom in which to dial in correction.  The oiled triplet is F/5.6 all-spherical design which leaves a lot of high order SA in the star test.

 

You'll see some similarities between your AT92 and this 110.  You were correct to send it back to the original owner.

 

Images below are of the same 110mm APO at various point of defocus.  

Perfect examples for this thread. 


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#19 RajG

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Posted Yesterday, 06:20 PM

Sacek's website has an interesting discussion on how higher-order spherical aberration can result in asymmetric star test patterns in fast apo refractors and Mak-Casses while still being very well-corrected (diffraction limited).

 

Given how fast the AT92 is, it is likely that the balance of lower vs higher-order spherical aberration results in a star test pattern that is not perfectly symmetric & the results need to be interpreted carefully by an expert.

 

That said, if you were not completely satisfied with the scope or suspected an issue with it, you were right to return it.

 

https://www.telescop...eresting_aspect

 

 

https://www.telescop...or.htm#achromat



#20 peleuba

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Posted Yesterday, 07:01 PM

Perfect examples for this thread. 

 

Thanks!  The patterns seen in my photos are similar to what the OP showed in his.  My scope had a Strehl value of .860 or thereabouts.  This was back in 2012 and I would have to dig out the report I created on this.  The star test images were easier for me to post once I returned from a 9 night observing trip...      

 

 

 

Given how fast the AT92 is, it is likely that the balance of lower vs higher-order spherical aberration results in a star test pattern that is not perfectly symmetric & the results need to be interpreted carefully by an expert.

 

I have had two AT92's and still have one I use as a collimator for generating an artificial star on my optical bench.  Both Strehls of my samples were north of .980 and had a star test pattern that, while not perfect, look nothing like the OP's.  There is something amiss with this lens. 


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#21 RajG

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Posted Yesterday, 08:15 PM

 

I have had two AT92's and still have one I use as a collimator for generating an artificial star on my optical bench.  Both Strehls of my samples were north of .980 and had a star test pattern that, while not perfect, look nothing like the OP's.  There is something amiss with this lens. 

That's good to know. Thanks.




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