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

Unexpected Astigmatism (SkyQuest XX14g)

  • Please log in to reply
20 replies to this topic

#1 austin.grant

austin.grant

    Surveyor 1

  • *****
  • topic starter
  • Posts: 1,517
  • Joined: 18 Oct 2010
  • Loc: Albuquerque, New Mexico

Posted 26 November 2020 - 10:41 AM

Greetings all!

 

I've owned this Orion SkyQuest XX14g for a couple of weeks now, and it's really been a great performer. Optics star tested well. I posted in a previous thread that I needed to modify it to work with a MaxBright II Binoviewer / 2" 1.7X Glass Path Corrector / Coma Corrector. I ended up getting longer collimation bolts and just moving the mirror 15mm up the tube. All of that went well, and reassembly was a breeze. The problem I'm having is that now that it's reassembled, the views last night were wildly inferior to everything I've seen with it so far. The optics exhibited a strong astigmatism, clearly changing direction 90-degrees inside and outside focus, and stars at any magnitude above 200X did not come to sharp pinpoint focus. I realize there are several possibilities for what the cause is, but I'm hoping for more input from everyone!

 

Here are my theories:

 

1. Collimation: I believe that collimation, if poor enough, can cause a reflector to exhibit astigmatism. The problem is, I really don't believe the collimation was bad at all. I've been using passive tools and a laser long enough to be able to get everything pretty close, and that was no different last night. 

 

2. Cooling: I know that last night was significantly cooler than the last two weeks around here, so it was a larger temperature drop than anything I've had since owning the scope. Still, the mirror had been outside acclimating for 2+ hours before I decided to star test it. I did not have any cooling fans on it. 

 

3. Mirror Cell: This scope has the conical-back mirror that attaches to the cell via a fastened threaded central cylinder. There is a small amount of play between the attachment cylinder and the mirror cell, and before I removed the mirror it was loose enough that the mirror could move laterally by up to 1-2mm. When I reinstalled the mirror, I snugged the system up enough to remove that play. There's no correlation between the torque on that attachment system and the mirror itself, as it's just fastened to the top of the attachment cylinder and not impacted by how tight you put it in the cell. I'll try to get some pictures of this, but I don't think it's the culprit. In fact, once I tried other remedies, I loosened this retaining ring so that the mirror again was able to move, slightly, and it had no impact on the astigmatism. 

 

I'm leaning towards insufficient mirror cooling, and then maybe collimation errors, as the culprit here. The only thing I forgot to try while I was out was to rotate the primary mirror and see if the astigmatism rotated with it. If the astigmatism rotated, I assume this would have helped eliminate collimation as the main problem. 

 

Also, because of that small 1-2mm play in the mirror to cell attachment, it was really hard to try to get the mirror perfectly centered. I stressed about that awhile, but then remembered that it was loose enough to move around before I ever touched it and had star tested very well. I don't know how important that 1-2mm play is, and I also don't have a great plan for perfectly centering the mirror anyway. 

 

What have I done? smile.gif I appreciate any input on helping me get the optical performance back to what it was 3 days ago.

 

Thanks,

 

Austin


Edited by austin.grant, 26 November 2020 - 10:42 AM.


#2 J A VOLK

J A VOLK

    Apollo

  • *****
  • Posts: 1,253
  • Joined: 24 May 2006
  • Loc: Flagstaff

Posted 26 November 2020 - 11:17 AM

Sounds like somehow you introduced some stress into the primary - it doesn't take much to cause astigmatism...


  • Jon Isaacs and litesong like this

#3 Stefano Delmonte

Stefano Delmonte

    Vostok 1

  • -----
  • Posts: 148
  • Joined: 17 Aug 2020
  • Loc: Salou (Spain) & Broni (Italy)

Posted 26 November 2020 - 11:26 AM

I'd exclude cooling, I had a similar situation with a different dob, 12" from GSO, from pinpoint to light astigmatism, in my opinion has been sinsce I've dismount primery and secundary in order to flock the tube, flocking has been succes and advisable, but I suppose for a bad collimation I've never been able to see again pinoint stars.

 

Another possibility but very unlikely is your eyes has changed in a couple of week, this could happen if you are over 40.

 

My final suggestion is to go back to original point and see what happen, then step by step repeat all the upgrades.

 

Good luck

 

Ste



#4 Darren Drake

Darren Drake

    Skylab

  • *****
  • Posts: 4,168
  • Joined: 09 Oct 2002
  • Loc: Chicagoland

Posted 26 November 2020 - 12:07 PM

Try experimenting by rotating the primary in 60 degree increments and see if the astig rotates with it and take it from there..


  • Asbytec likes this

#5 austin.grant

austin.grant

    Surveyor 1

  • *****
  • topic starter
  • Posts: 1,517
  • Joined: 18 Oct 2010
  • Loc: Albuquerque, New Mexico

Posted 26 November 2020 - 12:12 PM

Try experimenting by rotating the primary in 60 degree increments and see if the astig rotates with it and take it from there..

Thanks!  That's the test I forgot to try out last night. Should be clear this evening, so maybe I can take another shot at it.



#6 austin.grant

austin.grant

    Surveyor 1

  • *****
  • topic starter
  • Posts: 1,517
  • Joined: 18 Oct 2010
  • Loc: Albuquerque, New Mexico

Posted 26 November 2020 - 12:30 PM

Here's how the mirror attaches to the cell. From what I can see, snugging up the bottom retaining ring (not pictured, but exactly like the top retaining ring in the image) would only apply more pressure to the attachment system, and have no effect on the mirror at all. 

IMG-1924.jpg


Edited by austin.grant, 26 November 2020 - 12:30 PM.


#7 Starman1

Starman1

    Vendor (EyepiecesEtc.com)

  • *****
  • Vendors
  • Posts: 48,925
  • Joined: 23 Jun 2003
  • Loc: Los Angeles

Posted 26 November 2020 - 01:34 PM

If the astigmatism is only at the edge of the field, it's the eyepiece.  Fast scopes induce astigmatism in many inexpensive eyepieces.  Cure: eyepieces without astigmatism at your f/ratio.

If the astigmatism is across the field, but only at lower powers, it's your eye.  You may need to wear glasses with larger exit pupils.  Pop your glasses on.  Problem gone?  Then you need long eye relief eyepieces.

If the astigmatism is across the field at all powers, it is in the scope.  It is often the secondary (most common) but can also be the primary.  A center-only support doesn't really work with larger apertures.

It takes only a few nanometers of flexure to display astigmatism.

That's a big piece of glass, and needs fans.  If the temperature is dropping, you might have cooling issues the entire night without some form of active cooling.

When you said you moved the mirror up the tube 15mm with longer collimation bolts, what exactly do they press on?


  • Sol Robbins, Jon Isaacs, SteveG and 1 other like this

#8 austin.grant

austin.grant

    Surveyor 1

  • *****
  • topic starter
  • Posts: 1,517
  • Joined: 18 Oct 2010
  • Loc: Albuquerque, New Mexico

Posted 26 November 2020 - 01:59 PM

If the astigmatism is only at the edge of the field, it's the eyepiece.  Fast scopes induce astigmatism in many inexpensive eyepieces.  Cure: eyepieces without astigmatism at your f/ratio.

If the astigmatism is across the field, but only at lower powers, it's your eye.  You may need to wear glasses with larger exit pupils.  Pop your glasses on.  Problem gone?  Then you need long eye relief eyepieces.

If the astigmatism is across the field at all powers, it is in the scope.  It is often the secondary (most common) but can also be the primary.  A center-only support doesn't really work with larger apertures.

It takes only a few nanometers of flexure to display astigmatism.

That's a big piece of glass, and needs fans.  If the temperature is dropping, you might have cooling issues the entire night without some form of active cooling.

When you said you moved the mirror up the tube 15mm with longer collimation bolts, what exactly do they press on?

Thanks for all the feedback, Don! 

 

I'll need to get the scope back out and try to address the prevalence of the astigmatism relative to the field. My main observations from last night were specifically regarding an 8mm eyepiece putting me just north of 200x, and the astigmatism was present in slightly-defocused stars in the center of the field. 

 

I've had the scope out 9 times in the last two weeks, with all the same eyepieces, and this is the first incident with astigmatism. It's not my eyes, as I have astigmatism-correcting contacts that work very well. 

 

I'm skeptical that the mirror cell isn't sufficient only because it's been sufficient on the other 8 nights. 

 

The two changes are that I moved the mirror up the tube, and that the temperature was colder than I'd had it out in before. The mirror cell is a 2-piece design, and the mirror fastens in the center to the top piece while the collimation bolts push/pull the top piece relative to the bottom piece. They make no contact with any part of the mirror, and I don't see any way that they would have caused this. I'll attach pictures below. 

 

I'm leaning more and more towards it being a thermal issue. I'm going to get a fan to keep on it for the next observing session, and also rig up a 3-fan system similar to what Orion sells for $70, but at a lower and more reasonable $15 in fans from Amazon.

 

Again, thank you for the input, and I'll try to come back with more information after this evening.

 

Austin

Attached Thumbnails

  • IMG-1926.jpg
  • IMG-1927.jpg
  • IMG-1928.jpg

  • CounterWeight likes this

#9 Darren Drake

Darren Drake

    Skylab

  • *****
  • Posts: 4,168
  • Joined: 09 Oct 2002
  • Loc: Chicagoland

Posted 27 November 2020 - 09:07 AM

When you reassembled after getting the new bolts did you put it back in the exact same orientation?  If not then that may be the cause.  If you're not sure then I'd experiment with the other possible orientations..



#10 austin.grant

austin.grant

    Surveyor 1

  • *****
  • topic starter
  • Posts: 1,517
  • Joined: 18 Oct 2010
  • Loc: Albuquerque, New Mexico

Posted 27 November 2020 - 10:50 AM

Hi Darren,

 

I did not put the primary back in the same orientation. I did make a mark on the edge of the mirror and the edge of the mirror cell before removal so that I could orient them the same, but decided that it probably wasn't that crucial. If this mirror is bad enough to show aberrations just based on primary rotation, I'd be shocked. Well, not as shocked as if you'd told me that someone at the assembly location actually tests the system performance based on primary rotation. That's a step that seems beyond the price range of this level of gear! :)

 

But I'll orient it as it was previously and report back. No astronomy for a day or two, we have 30mph sustained winds and 55mph gusts from now through the evening. No thanks!


  • SteveG likes this

#11 Glory Eye

Glory Eye

    Vostok 1

  • -----
  • Posts: 157
  • Joined: 17 Jan 2019
  • Loc: Southern Utah

Posted 28 November 2020 - 11:43 AM

Mathematically speaking, any rays that are parallel to the symmetry axis of a parabola that strike the curve will all reflect to a focal point called a focus. In a telescope, the ideal placement of the secondary mirror is at a distance from the primary such that the focal plane of the secondary exactly intersects the focus of the light rays reflected from the primary. By moving the primary closer or further from the primary, you're moving the focus of the parabola by the same amount. In your current configuration, your focus is now 15mm behind the secondary so that the rays that hit the secondary are not all the way converged. What you're seeing is the result. If you move your primary back to it's original position, the astigmatism will probably disappear.



#12 Starman1

Starman1

    Vendor (EyepiecesEtc.com)

  • *****
  • Vendors
  • Posts: 48,925
  • Joined: 23 Jun 2003
  • Loc: Los Angeles

Posted 28 November 2020 - 01:20 PM

Mathematically speaking, any rays that are parallel to the symmetry axis of a parabola that strike the curve will all reflect to a focal point called a focus. In a telescope, the ideal placement of the secondary mirror is at a distance from the primary such that the focal plane of the secondary exactly intersects the focus of the light rays reflected from the primary. By moving the primary closer or further from the primary secondary, you're moving the focus of the parabola [away from the secondary] by the same amount. In your current configuration, your focus is now 15mm behind the secondary so that the rays that hit the secondary are not all the way converged. What you're seeing is the result. If you move your primary back to it's original position, the astigmatism will probably disappear.

You have a few typos, which I correct above.

 

The red sentences make no sense.

First, the focus of his parabaloidal primary is a lot more than 15mm behind the secondary.

Second, the rays hitting the secondary are not completely converged to focus at the secondary.  They should not be or nothing will be at the focal point.

Third, the secondary is flat--moving the primary or secondary relative to one another does not induce astigmatism or any other aberration, it merely moves the focal plane position relative to the secondary.

His moving the primary forward has nothing to do with astigmatism unless the primary or secondary mirror is mechanically distorted by something in the scope.


  • Jon Isaacs and CounterWeight like this

#13 Glory Eye

Glory Eye

    Vostok 1

  • -----
  • Posts: 157
  • Joined: 17 Jan 2019
  • Loc: Southern Utah

Posted 28 November 2020 - 08:37 PM

You have a few typos, which I correct above.

 

The red sentences make no sense.

First, the focus of his parabaloidal primary is a lot more than 15mm behind the secondary.

Second, the rays hitting the secondary are not completely converged to focus at the secondary.  They should not be or nothing will be at the focal point.

Third, the secondary is flat--moving the primary or secondary relative to one another does not induce astigmatism or any other aberration, it merely moves the focal plane position relative to the secondary.

His moving the primary forward has nothing to do with astigmatism unless the primary or secondary mirror is mechanically distorted by something in the scope.

Starman1,

Sorry for my grammar. I had a misunderstanding about where the focus is actually placed relative to the secondary. I looked at some diagrams of reflectors that cleared things up.

Thanks.


  • Jon Isaacs likes this

#14 Cames

Cames

    Apollo

  • -----
  • Posts: 1,015
  • Joined: 04 Aug 2008

Posted 28 November 2020 - 09:23 PM

Just in case this may have been overlooked, the most common cause of newly acquired astigmatism is the mirror clips contacting the coated surface and/or the circumference of the mirror.  No amount of contact is safe.

 

Also, while you have the mirror cell out, apply pressure to the bevel.  Ensure there is  no rocking or teeter-totter.

----------

C

 

ps The push-pull arrangement of the cell support screws may 'potato-chip' the desired flatness of the cell structure if they are torqued too tightly.


Edited by Cames, 28 November 2020 - 09:40 PM.


#15 SteveG

SteveG

    Cosmos

  • *****
  • Posts: 9,525
  • Joined: 27 Sep 2006
  • Loc: Seattle, WA

Posted 28 November 2020 - 10:58 PM

The only thing that makes sense to me is thermal. The OP says "slightly defocused stars". If it was astigmatism, stars tend to look like little crosses that cannot be focused out. Defocused stars are just bigger looking.

 

To the OP, take your scope outside, and put a household box-fan on it for an hour or more before viewing.



#16 austin.grant

austin.grant

    Surveyor 1

  • *****
  • topic starter
  • Posts: 1,517
  • Joined: 18 Oct 2010
  • Loc: Albuquerque, New Mexico

Posted 28 November 2020 - 11:42 PM

It’s absolutely astigmatism, but I agree that the cause is likely thermal. I’ve got fans ordered for the cell, and if I get out before they arrive I’ll point a box fan at the back of the mirror.

Lockwood discusses thermally-induced astigmatism here, along with some ways to confirm it.

http://www.loptics.c.../starshape.html

 

I’ll share results as soon as I have some.


Edited by austin.grant, 28 November 2020 - 11:46 PM.

  • SteveG likes this

#17 austin.grant

austin.grant

    Surveyor 1

  • *****
  • topic starter
  • Posts: 1,517
  • Joined: 18 Oct 2010
  • Loc: Albuquerque, New Mexico

Posted 05 December 2020 - 11:10 AM

Okay, after more testing, I've determined that the cause was definitely thermal. Before adding fans or anything, I took the setup out one more night where it got pretty cold and did some testing. Rotating the primary mirror caused the astigmatism to rotate, so that eliminated most other causes. I knew that the mirror had done fine on more moderate nights, so I didn't anticipate any major inherent flaw in the optics. Turns out that when you live in the high desert and the winter temps fall off a cliff right at sunset, large mirrors don't like that! smile.gif

 

I ordered some Arctic case fans from Amazon (don't recommend, by the way. 3/5 arrived functional) and after a quick install and parallel wiring, we have cooling! Nice way to repurpose my TalentCell 12v pack. The last three nights I've had the scope out, I can watch the astigmatism vanish as the mirror cools. So far, it's taken about 2-hours of cooling with the fans for the mirror to stabilize. Doesn't seem to matter whether that's with setting the scope up after sunset or an hour or so before. 

 

Thanks for all of the help, I really appreciate everyone working on helping me solve this. 

IMG-2058.jpg

 


  • Jon Isaacs, Bill Jensen, SteveG and 1 other like this

#18 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 89,402
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 07 December 2020 - 07:25 AM

Austin:

 

Are those three disks, factory counterweights?

 

Jon



#19 austin.grant

austin.grant

    Surveyor 1

  • *****
  • topic starter
  • Posts: 1,517
  • Joined: 18 Oct 2010
  • Loc: Albuquerque, New Mexico

Posted 07 December 2020 - 08:41 AM

Yep, factory counterweights. There are actually six of them, 2.2-pounds each. Just enough to balance the tube...as long as you are using a 1.25” Plossl. I’ve got a pair of Harbor Freight lifting magnets that I use to fine tune the balance.

#20 Asbytec

Asbytec

    Guy in a furry hat

  • *****
  • Posts: 17,949
  • Joined: 08 Aug 2007
  • Loc: Pampanga, PI

Posted 07 December 2020 - 08:55 AM

I wonder if the mass of the counterweights will pose a thermal issue. 



#21 austin.grant

austin.grant

    Surveyor 1

  • *****
  • topic starter
  • Posts: 1,517
  • Joined: 18 Oct 2010
  • Loc: Albuquerque, New Mexico

Posted 07 December 2020 - 09:11 AM

Potentially, but I’m not interested in finding a different way to add 13+ pounds of counterweight in some different fashion. The fans have sufficiently mitigated the problem for my uses.

The largest thermal issue I have now is simply me standing at the eyepiece. I may look at making a half-shield or some sort of kydex shroud similar to another post to keep my body heat from impacting the views. This Orion shroud is wonderful for blocking stray light, but I’m not sure how it’s performing at keeping heat out.
  • Asbytec likes this


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