12 replies to this topic

[Lazerguy2010]

Hi fellow CN'ers!

 

I just bought back from a friend a pristine SW Classic 250P 10'' Dobsonian.  I sold it to him a year ago, thinking at the time that I wanted a 12''...   In the meantime I've had -and sold- a 12'' Flextube 300P. Good scope... but oh man the weight!!!

 

So, back with my beautiful 250, (and an OTA that weighs 28lbs instead of the 300's 50lbs!),

 

I was doing a resolution test yesterday (with a DIY artificial star) and found that I couldn't get anything close to round star images ;o(

The star image never gets to a nice round Airy disc. Dead-on focus yields a scrambled egg not even a roundish shape.

Moving away from focus by a little, I get an elongated ''line'' image in 2 different directions that are 90 degrees from each other for in-travel vs out-travel of the focuser tube.

Moving out of focus by a lot, I get a very well-centered and uniform donut, it's perfect... except it's elongated by, say 20%.

Outer circle of the donut is very regular, no pinching of the primary that I can see.

 

So I figure I have a pretty obvious case of astigmatism, and from what I read on CN, my first suspect would be the secondary mirror.

I understand that before suspecting the secondary itself I should investigate the way the sec. is mounted (possible pinching?). I have also checked squareness of the focuser vs tube, it's all good.

 

The thing is, the way the sec. mirror is mounted (looks a lot like a simple 1'' piece of 2-face white foam tape) I can hardly imagine any pinching going on there...  please see attached picture.

 

So, should I order a better secondary right away, or is it even worth trying to re-mount the existing one with silicone?

 

if you needed a good secondary, who would you order it from?

 

Thanks

 

 

[Asbytec]

It depends. If you want to chase down potential causes of astigmatism, read Lockwood's article. But, if you want a better diagonal, anyway, then you can order from Antares or other vendors. Either way, I would mount it with silicone. People have complained about the double sided tape. 

[CrazyPanda]

I was doing a resolution test yesterday (with a DIY artificial star) and found that I couldn't get anything close to round star images ;o(

The star image never gets to a nice round Airy disc. Dead-on focus yields a scrambled egg not even a roundish shape.

 

While it does sound like astigmatism is present, what size is the artificial star and what is the distance at which you tested? Are you 100% sure its apparent angular size would put it below the scope's resolving limit? What you describe of the star at best focus sounds like it's potentially too large and/or too close to be used as an artificial star.

[Asbytec]

Yea, a "scrambled egg" appearance may be from a large artificial star source, but the 90 degree flip is telling. A real star test should firm up any astigmatism. 

[Lazerguy2010]

My DIY artificial star is 8 microns in diameter, (160 micron pinhole projected 20:1 through a 10mm eyepiece) viewed from a distance of 12,4 meters, which gives an angular size roughly 0.13 arc-sec. So it's well below Dawes' limit of approx. 0.5 arc-sec for the 10in scope.

I have used it to test other scopes (Celestron C80, AT125EDL, SW Classic 200P) with those it gives nice round Airy discs, so I think it's ok.

 

Incredibly valuable article from Lockwood!!! Thanks for the link!  I'm going to read n' learn and do more testing to ascertain the things I have guessed.

[Dan Watt]

Note the direction of the astigmatism and then rotate the primary mirror 90deg and check again. This can help determine if it's an issue with the primary or secondary.

[Lazerguy2010]

Follow-up:

So yesterday I read the first part of Lockwood's article, which convinced me to begin from the start and check for primary mirror pinching. Well...guess what? Four of the six mirror clamps were awful tight!

When I bought the scope last year the mirror was so clean that I hadn't taken out the cell to clean it, so I never checked the mirror clamps. 

Loosened all six clamps to the point where I could just rotate the mirror by hand, reinstalled and tested:  astigmatism gone!

And to think that I was unjustly suspecting that poor diagonal.

The Airy disc is a lot better now.

Tonight I will re-collimate using Lockwood's method and see what I get.

[Asbytec]

Yea, it's a great article. I chased down astigmatism by, as you did, making sure the clips were not too tight. They weren't, but it is a common source of astigmatism. I also reglued my diagonal mirror and then rotated the mirror cell so the primary was not resting on one edge support. It rests on two. So, one of those two fixed it. Probably regluing the diagonal after cutting off that sticky tape did the trick. 

Edited by Asbytec, 23 April 2025 - 04:08 PM.

[Jon Isaacs]

With commercial Dobs, when the image is astigmatic, "Are the mirror clips touching the mirror?" should be the first question.  What happens is that someone cleans the mirror and doesn't realize that the clips should not touch the mirror..  

 

Jon
 

[RodgerDodger008]

Exactly that Jon, when I read OP had the scope and sold it, clearly was not a issue before can only be the clips touching or maybe let it cool and do a real star test outside.

[Lazerguy2010]

The first time I had the 10in, I didn't test it indoors. I only did real star-tests to check collimation, and at the time I wasn't quite aware of the impact of seeing conditions, so I didn't pay attention to the elevation of an object vs horizon and the seeing varying from night to night. Whatever astigmatism was there at the time probably would have gone unnoticed among carelessly chosen viewing conditions.

 

I told my friend about the tight clamps and astigmatism. He said he did remove them to clean the mirror (it was awful clean already, but he's real picky) and took care to re-install them with the screws just finger-tight.

From the astigmatism, now I know that even finger-tight is too tight. Reading Lockwood's article, the cue of ''being able to slightly move the mirror'' made me realize that the clamps must not be tight at all, they really need to be a little loose.

And the tests before/after loosening confirm that without a doubt.

I feared that the mirror being a little loose might allow it to wander in its cell and lose collimation, but after a few nights out I find that this is not a problem.

[Asbytec]

Yea, in a Dob mounted scope, the mirror simply rests on the edge supports and shouldn't flop around. The clips are not meant to clamp the mirror in place. They just prevent it from falling out of the cell. The mirror's center of mass causes it to rest against the back support, so it won't lean into the clips. Under normal use it's not going anywhere, but the clips just make sure it won't. In a GEM mounted Newt slewing to different angles, the mirror can slide in its cell slightly. But a Dob not so much if at all. 

Edited by Asbytec, 29 April 2025 - 05:44 PM.

[RodgerDodger008]

The first time I had the 10in, I didn't test it indoors. I only did real star-tests to check collimation, and at the time I wasn't quite aware of the impact of seeing conditions, so I didn't pay attention to the elevation of an object vs horizon and the seeing varying from night to night. Whatever astigmatism was there at the time probably would have gone unnoticed among carelessly chosen viewing conditions.

I told my friend about the tight clamps and astigmatism. He said he did remove them to clean the mirror (it was awful clean already, but he's real picky) and took care to re-install them with the screws just finger-tight.
From the astigmatism, now I know that even finger-tight is too tight. Reading Lockwood's article, the cue of ''being able to slightly move the mirror'' made me realize that the clamps must not be tight at all, they really need to be a little loose.
And the tests before/after loosening confirm that without a doubt.
I feared that the mirror being a little loose might allow it to wander in its cell and lose collimation, but after a few nights out I find that this is not a problem.

The thickness of a business card should be able to fit between the clamps and the mirror. Finger tight can mean many things.

For me I obviously dont put a business card there because i dont want to scratch the mirror. I wiggle the clamps to make sure they dont touch. It does mean sometime my mirror is more prone to move when moving the scope from inside to outside but I do collimation touch ups anyway.

I would recommend removing the mirror cell and loosening the clamps slightly either it will confirm or eliminate that as a problem or not

Edited by RodgerDodger008, 29 April 2025 - 11:54 PM.