Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Astigmatism in multiple Meade ACF scopes?

  • Please log in to reply
31 replies to this topic

#1 mruthjh

mruthjh

    Lift Off

  • -----
  • topic starter
  • Posts: 4
  • Joined: 20 Oct 2020

Posted 20 October 2020 - 05:46 PM

I work at a university and we're setting up equipment for use in astronomy classes. We've purchased several 10" F/8 Meade ACF OTAs. We're seeing what appears to be astigmatism to varying degrees when imaging (I don't think it can be seen through an eyepiece). Some OTAs are worse than others. I'm not an expert on this particular problem, so I'm not sure whether it is astigmatism. Can anyone help us figure out what may be causing this and whether there's a fix? I'd suspect a faulty OTA except that it's not limited to one scope. Screenshots of sections of an image from one of our cameras (ZWO ASI183MM pro) are here. When the camera is rotated, the shape stays with the telescope, so it doesn't seem to be an issue with the camera.


  • Jon Isaacs likes this

#2 TOMDEY

TOMDEY

    Cosmos

  • *****
  • Posts: 8,049
  • Joined: 10 Feb 2014
  • Loc: Springwater, NY

Posted 20 October 2020 - 06:11 PM

Yeah... it looks like astig. Would need a lot more info to come to any sort of conclusion. Most useful would be AC interferometry done on the OTAs in the lab.    Tom



#3 jmiller1001

jmiller1001

    Ranger 4

  • *****
  • Posts: 341
  • Joined: 21 Mar 2016
  • Loc: Bay Area, CA

Posted 20 October 2020 - 06:41 PM

Odd that it would be on multiple scopes...I think...are you using a reducer?



#4 mruthjh

mruthjh

    Lift Off

  • -----
  • topic starter
  • Posts: 4
  • Joined: 20 Oct 2020

Posted 20 October 2020 - 09:03 PM

Thanks. No reducer. We have the camera attached to a flip mirror, but this is out the back of the flip mirror so the mirror is flipped out of the way.



#5 luxo II

luxo II

    Mercury-Atlas

  • -----
  • Posts: 2,688
  • Joined: 13 Jan 2017
  • Loc: Sydney, Australia

Posted 20 October 2020 - 09:25 PM

Yup astigmatism. Looks like you have a batch of Friday afternoon scopes. 

 

If they are under warranty I'd return them. All.


  • Tom yates likes this

#6 jgraham

jgraham

    ISS

  • *****
  • Posts: 21,425
  • Joined: 02 Dec 2004
  • Loc: Miami Valley Astronomical Society

Posted 20 October 2020 - 09:45 PM

Before jumping to that conclusion I would carefully check the colimation and be sure that the scopes are thermally acclimated. I have lost count how many SCTs that I have fixed by zeroing in the colimation and running a cat-cooler for 30 minutes.

For precise work with a camera close is _not_ good enough.
  • Jon Isaacs and JMP like this

#7 mruthjh

mruthjh

    Lift Off

  • -----
  • topic starter
  • Posts: 4
  • Joined: 20 Oct 2020

Posted 20 October 2020 - 10:02 PM

The collimation seems at least close, though I've only collimated a telescope once before so I don't have a lot of experience. Temperature shouldn't be a problem since the telescopes stay outside with covers.



#8 Mitrovarr

Mitrovarr

    Gemini

  • *****
  • Posts: 3,226
  • Joined: 12 Sep 2004
  • Loc: Boise, Idaho

Posted 21 October 2020 - 12:35 AM

Any chance the correctors retaining rings are all screwed on too tight and deforming the correctors?



#9 TOMDEY

TOMDEY

    Cosmos

  • *****
  • Posts: 8,049
  • Joined: 10 Feb 2014
  • Loc: Springwater, NY

Posted 21 October 2020 - 12:53 AM

Any chance the correctors retaining rings are all screwed on too tight and deforming the correctors?

Actually, a pinched front Schmidt corrector plate shouldn't be problematic because it's dioptric and has zero Zernike power. That combination is theoretically (almost entirely) desensitized to pinching punch-through. In contrast, achromatic refractor objectives are hypersensitized to pinching, and stressed Primary Mirrors --- are the worst of all!   Tom


  • PETER DREW likes this

#10 Mitrovarr

Mitrovarr

    Gemini

  • *****
  • Posts: 3,226
  • Joined: 12 Sep 2004
  • Loc: Boise, Idaho

Posted 21 October 2020 - 01:19 AM

There are accounts of it being problematic, though. Maybe the effect is by something other than deformation of the primary figure?



#11 davidc135

davidc135

    Apollo

  • -----
  • Posts: 1,308
  • Joined: 28 May 2014
  • Loc: Wales, UK

Posted 21 October 2020 - 03:42 AM

Why can't the stig be seen visually? Sounds like a clue. Might some alignment be disturbed by the weight of the camera? 

 

Re. Corrector pinching. I experimented with stressing a corrector by tightening screws across diameters and no impact was seen on astigmatism in a DX-8 sct. Similarly, introducing corrector plate tip and tilt and changing the corrector's position laterally had no or little effect on stig. The scope was re-collimated after each alteration. Though it's just one set of experiments with non ACF optics David


Edited by davidc135, 21 October 2020 - 12:23 PM.


#12 jgraham

jgraham

    ISS

  • *****
  • Posts: 21,425
  • Joined: 02 Dec 2004
  • Loc: Miami Valley Astronomical Society

Posted 21 October 2020 - 07:01 AM

I'm a little puzzled by the star shapes. Astigmatism causes elongation is one direction and it switches by 90 degrees as you pass through the focus. On-axis in sharpest focus the stars should be roundish. Are these stars at the center of the field or at the edge? Astigmatism in the optics shows up on-axis when the elements are colimated. There's always going to be astigmatism of some form at the edge of the field (off-axis). These look a lot like images that I have seen at the edges of the field of my SN10 when I use a Baader Mk III MPCC.

Colimate (again, for imaging colimation must be spot-on), acclimate (no thermal plumes flowing off of the baffle), compare the center and the edge of the field. Astigmatism at the edge of the field is normal and usually doesn't show up very much once your I mbv ages are scaled. Images examined under a microscope almost always show fine defects that you won't notice on normal human scales.

One crisis at a time...

#13 JMP

JMP

    Viking 1

  • *****
  • Posts: 996
  • Joined: 31 Oct 2005

Posted 21 October 2020 - 10:32 AM

Although it doesn't directly answer your question, take a look at the Celestron Whitepaper on the Edge HD design. The off axis images look a lot like your images.

What can cause this? Jgraham suggests collimation and cooling can be the culprits. I agree. The temperature falling on a clear night is enough to cause problems. Wrapping reflectix around the scope might help. And collimation is critical. If collimation is off it's the same as looking at the off-axis image, there will be distortion.

There is an element of fine tuning involved in getting the best images.

#14 Dwight J

Dwight J

    Soyuz

  • *****
  • Posts: 3,640
  • Joined: 14 May 2009
  • Loc: Lethbridge, Alberta, Canada

Posted 21 October 2020 - 10:37 AM

If astigmatism is present it should appear visually too.  I suspect it is something in your imaging train instead.



#15 junomike

junomike

    ISS

  • *****
  • Moderators
  • Posts: 20,174
  • Joined: 07 Sep 2009
  • Loc: Ontario

Posted 21 October 2020 - 12:08 PM

Can't see it being multiple OTA's.  Further investigation is needed IMO.



#16 Mitrovarr

Mitrovarr

    Gemini

  • *****
  • Posts: 3,226
  • Joined: 12 Sep 2004
  • Loc: Boise, Idaho

Posted 21 October 2020 - 02:17 PM

Can't see it being multiple OTA's. Further investigation is needed IMO.


Yeah, one astigmatic SCT I would believe, but a whole bunch all having the same problem to the same degree? Doesn't make sense.

#17 Eddgie

Eddgie

    ISS

  • *****
  • Posts: 27,431
  • Joined: 01 Feb 2006

Posted 21 October 2020 - 04:50 PM

Why can't the stig be seen visually?

 

The human eye is not very good at seeing things. Camera's don't miss anything. 



#18 Eddgie

Eddgie

    ISS

  • *****
  • Posts: 27,431
  • Joined: 01 Feb 2006

Posted 21 October 2020 - 05:13 PM

Some of the worst SCT bench tests I have ever seen have been for Meade scopes. How some of those scopes got past Meade's quality control is beyond me but there have been some real barkers in the blue paint brand. 

 

The thing though is that I think it would be very rare to have any three or four random samples all have this kind of error so I would be thorough in eliminating other issues.

 

The best way to do this short of a lab is to do some star testing.  For astigmatism it is OK to use an artificial star and the star does not have to be maybe more than 20 focal lengths away.   If done horizontally, the scope should be collimated in this position too.

 

And I know the Meade telescope lovers will think badly of me for my opening remark, but I can easily substantiate that claim.

 

Here is one that shows textbook saddle astigmatism though that is not what I see in your image.. Just an example of some of the defects I have seen from Meade:

 

http://fidgor.ru/Obs...2/test_288.html

 

And look at this one. This is a pretty horrible telescope:

 

http://fidgor.ru/Obs...t/test_265.html

 

Now this one is not a terrible telescope but the astigmatism holds it back from being a very good telescope:

 

http://r2.astro-fore...500-u-astrofoto

 

I have seen worse than this from Meade though. Not specifically in the ACF model but some really horrible tests like this one which has to be in the top three lists of the worst telescope bench tests I have ever seen, but sadly one of the other two of the top three was also a Meade.  The last one was a Vixen 110.  Anyway, this is one of the worst of the blue brand I have seen.

 

http://fidgor.ru/Obs...2/test_459.html

 

So, Meade has made some really bad telescopes over the years, but I would still be rather surprised of all of the scopes in this batch were bad, so I encourage you to set up a star test to really see for sure. 


Edited by Eddgie, 21 October 2020 - 05:22 PM.


#19 Bill Barlow

Bill Barlow

    Aurora

  • *****
  • Posts: 4,776
  • Joined: 03 Dec 2007
  • Loc: Overland Park KS

Posted 22 October 2020 - 10:35 AM

I have owned more Meade SCT’s than Celestron and have found that the optics have been sharper in the Meade's with the ACF design.  Maybe I got lucky, I don’t know.  I have bought the last few Meade’s through Company 7 so that might explain some of the optical quality issues.  
 

For the original poster, I have never seen star images like what you got in the 10” f/8 models.  The 10” f/10 Meade ACF scopes I have owned (2) have been excellent.  Hope you find the cause of these issues.

 

Bill


  • thesubwaypusher likes this

#20 davidc135

davidc135

    Apollo

  • -----
  • Posts: 1,308
  • Joined: 28 May 2014
  • Loc: Wales, UK

Posted 22 October 2020 - 10:41 AM

The human eye is not very good at seeing things. Camera's don't miss anything. 

I meant that if astigmatism can't be seen visually, for instance in a star test, then there wouldn't be enough to worry about.  David

 

My aging non ACF Meade scts have pretty good optics.


Edited by davidc135, 22 October 2020 - 10:44 AM.


#21 Mitrovarr

Mitrovarr

    Gemini

  • *****
  • Posts: 3,226
  • Joined: 12 Sep 2004
  • Loc: Boise, Idaho

Posted 22 October 2020 - 10:44 AM

Shouldn't astigmatism be easier to detect visually than photographically? It seems like the kind of defect that would be easiest to see under high magnification.
  • Jon Isaacs, Dwight J and Peter Besenbruch like this

#22 mruthjh

mruthjh

    Lift Off

  • -----
  • topic starter
  • Posts: 4
  • Joined: 20 Oct 2020

Posted 22 October 2020 - 11:27 AM

Thanks, everyone. It looks like we'll have to do some further investigation. I do agree that it seems unlikely to be multiple problematic OTAs. When I get a clear night, I'll try attaching the camera directly to the visual back and eliminating the flip mirror altogether - the camera is fairly heavy so I could imagine that it may cause an issue.

 

And to answer the question about where in the field these stars were - they weren't quite at the center, but closer to the center than to the edge.


Edited by mruthjh, 22 October 2020 - 11:28 AM.


#23 Eddgie

Eddgie

    ISS

  • *****
  • Posts: 27,431
  • Joined: 01 Feb 2006

Posted 22 October 2020 - 01:29 PM

Shouldn't astigmatism be easier to detect visually than photographically? It seems like the kind of defect that would be easiest to see under high magnification.

The camera is far more sensitive to optical aberrations than the human eye is.  When you leave the shutter open for any length of time even minor amounts of astigmatism or coma will start to be easily seen.

 



#24 rmollise

rmollise

    ISS

  • *****
  • Posts: 23,271
  • Joined: 06 Jul 2007

Posted 22 October 2020 - 02:13 PM

Thanks, everyone. It looks like we'll have to do some further investigation. I do agree that it seems unlikely to be multiple problematic OTAs. When I get a clear night, I'll try attaching the camera directly to the visual back and eliminating the flip mirror altogether - the camera is fairly heavy so I could imagine that it may cause an issue.

 

And to answer the question about where in the field these stars were - they weren't quite at the center, but closer to the center than to the edge.

Bottom line? If the elongation of the diffraction rings does not change orientation 090 degrees on either side of focus, it is NOT astigmatism.

 

Also "close to the center" is not good enough. While these telescopes are corrected for coma, they still suffer from CONSIDERABLE field curvature. 



#25 Darren Drake

Darren Drake

    Skylab

  • *****
  • Posts: 4,151
  • Joined: 09 Oct 2002
  • Loc: Chicagoland

Posted 22 October 2020 - 02:20 PM

The camera is far more sensitive to optical aberrations than the human eye is.  When you leave the shutter open for any length of time even minor amounts of astigmatism or coma will start to be easily seen.

Any meaningful astigmatism would be easily detectable in a star test visually..




CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics