17 replies to this topic

[CADSAS]

The Cranbrook School based Astronomy club (CADSAS.org) operates a very large reflecting telescope, built in the 1970s by Alan Young and for many years used in an observatory at Burwash, Sussex.  It has a mirror of diameter 572mm.

 

After Alan's death the telescope was left abandoned until it was rescued and given to Canbrook School. It was rebuilt here in a purpose built observatory. We have used this successfully but it has never produced pin sharp images due to the fact that it is not accurately collimated.

 

Repeated efforts to perfect the collimation have gone nowhere so we feel we need the help of someone who knows how to tackle such a large instrument.

 

Any tips or suggestions will be gratefully received

 

Rod Smith

Edited by CADSAS, 15 March 2025 - 05:39 PM.

[Redbetter]

Without knowing how it is configured, it will be difficult for anyone to help.  If you have links to images of the scope and information on the design folks will probably be able to give some advice.

[kgb]

The Cranbrook School based Astronomy club (CADSAS.org.uk) operates a very large reflecting telescope, built in the 1970s by Alan Young and for many years used in an observatory at Burwash, Sussex.  It has a mirror of diameter 572mm.

 

After Alan's death the telescope was left abandoned until it was rescued and given to Canbrook School. It was rebuilt here in a purpose built observatory. We have used this successfully but it has never produced pin sharp images due to the fact that it is not accurately collimated.

 

Repeated efforts to perfect the collimation have gone nowhere so we feel we need the help of someone who knows how to tackle such a large instrument.

 

Any tips or suggestions will be gratefully received

 

Rod Smith

Hello Rod,

 

Could not get that specific link to work, but was able to get CADSAS.org to work. I am including a couple of pictures for others to help in evaluating the best way to assist. Looking at the 15sec exposure, there is definitely something going on. At first glance there appears to be some coma and possibly some tracking issue. Do you happen to have a full uncropped image that you could provide? Also, do you know the focal length of this scope and confirm that this is a Newtonian deaign?

Regards,
Karl

[ngc7319_20]

The image looks vaguely astigmatic.  The primary mirror is large enough that it will need excellent floatation cell and edge support.  Potentially the design of the rear cell might need to be re-worked.  The secondary mirror might also be suspect and need replacement.  There might also be questions about the mirror's figure, or whether the glass was annealed properly.  This will be difficult to sort remotely.  I think this project will need a dedicated on-site expert and perhaps several days / weeks to sort out.  Hopefully it might just be a little collimation adjustment, but potentially could be much bigger project.

Edited by ngc7319_20, 15 March 2025 - 03:21 PM.

[TOMDEY]

Looks like a nice telescope!

 

It's obviously a Newtonian... which is the simplest telescope type to align/collimate. Size doesn't enter into the technique. If, despite time and effort,  you can't find an axial image free of coma (or other aberrations) --- then you are dealing with imperfect mirror(s). Axial coma can be aligned out; axial astigmatism can be bent out by applying a bending force to the primary mirror. I've done this on my 29-inch telescope successfully --- easy, cheap, emperical. Spherical and higher order axial Zernike aberrations cannot be stressed out. My white-paper on this can be found in Amateur Astronomy Magazine... couple excerpts from that attached... I believe back-issues are available from the publisher.    Tom

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[TOMDEY]

PS: Picture of my ~Tweaker Clock~ behind my 29-inch mirror. The axial Zernike Astigmatism was quite objectionable. The clock immediately reduces it to inconsequential.     Tom

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[CADSAS]

Thanks everyone.  I will consult with colleagues who know more about the scope than I do.  What I can say is that when the mirror was resurfaced a couple of years ago the report said it was of very high quality.  I'll dig the report out.

Thanks Karl and Tom and others

 

During the next week I'll take some pictures of the set-up and post them.  Maybe someone can see something which needs fixing.

Meanwhile I'll have a read.

 

Mind you the idea of trying to bend the large thick glass mirror sounds a bit dodgy to me.

 

Rod

[Vic Menard]

From what I could find online, the scope is a 0.57m f/4.7 ‘Alan Young’ Newtonian.

The imaging camera in 2019 was a ZWO ASI174MM.

Is the scope equipped with a coma corrector?

Do you have access to any collimation tools?

 

Mind you the idea of trying to bend the large thick glass mirror sounds a bit dodgy to me.

It's actually very easy to bend a large glass mirror. Usually, we try to ensure that it isn't being bent (especially with an equatorial mount).

Edited by Vic Menard, 15 March 2025 - 06:17 PM.

[TOMDEY]

The needed pull-forces are typically small... a few ounces typical. It's a one-time adjustment that holds for months or more on my telescope. It took me only a few minutes to tune out the astigmatism looking at a bright star with my son turning the tensioning knobs from behind the PM as I "talked him in". It was magical and satisfying. I've been doing optical testing and assembly on space optics at work for decades --- I guess it might not seem trivial to others. Like driving a car or playing tennis to someone who has never done it before?    Tom

 

PS: The insight to differential wavefront stress-tuning is to use ~force actuators~ not ~displacement actuators~.

Edited by TOMDEY, 15 March 2025 - 07:21 PM.

[Sean Cunneen]

A Newtonian is not difficult to collimate compared to other designs, but the size and age might bring along some gremlins to tend with. If possible, can you find out the thickness of the primary and secondary? Large thin mirrors need a lot of help to keep astigmatism at bay, especially pointing low. Given that the mirror was tested and given a good bill of health, the next thing to check is the mirror cell to make sure all the bits are freed up and moving the way they should. After the cell, check the secondary, make sure none of those mechanics are binding and that the secondary is secure. If there is glue involved anywhere, that would be a huge red flag.

Finally, the tube assembly construction needs to be very stiff. Things can get loose through the decades. A good test is to take images at different extreme orientations to determine if any flexure is taking place. Those huge mount forks could be applying flexture to the ota at some orientations but not others.

Nothing a few clear nights can't decode!

Sean

[DAVIDG]

 I would look at the mechanics of the tube first before suspecting astigmatism in the primary. With the tube being a homemade truss, and split into two truss sections, the mechanical axis of the bottom section needs to be align up with the top section. If not the primary would be off center to the top. So when you collimate it will look aligned but the mirror is tilted to incoming light and that will give astigmatism in the image.

 

                    - Dave 

[CADSAS]

Thanks everybody.  Every thing written helps us understand our problem and hopefully will lead to a fix.

 

Information gleaned by various people is all correct.  Answering your questions, the telescope is a Newtonian of focal length 2682mm with a mirror 1570mm in diameter.  It is not fitted with a coma corrector.  The secondary glass in ¹/₂" thick, the main mirror 1¹/₂" thick.  The telescope was It was built and used by Alan Young an accomplished amateur astronomer.  He was a specialist engineer who made racing car chassis but building telescopes was a lifelong hobby.

We were given the telescope as a pile of bits and have adapted it and got it working although it has never produced sharp pictures, we think due to our inability to collimate and focus it adequately.  We have decided now that we really must attempt to solve the problem.

 

Reading your suggestions we decided that the first thing we should do is check carefully the main mirror housing.  Sure enough it wasn't quite right so the mirror was at a slight tilt.  We corrected that and checked all the other things we could, confirming that the telescope axis is correctly aligned and that there is no obvious flexing in the structure.  As far as we can tell, all is good.

We then checked the position of the focuser and decided that it was not quite true.  Adjustment of that was followed by a careful process of collimation using a Cheshire collimator.  As far as we can tell we now have all this correct but are waiting for a clear night (probably Thursday) to try it out.  Hopefully I will be able to report success or at least great improvement.

[CADSAS]

Karl,

You asked me for an uncropped image. The problem is I can't find out how to post images here. What is the procedure please.

Rod

[Kitfox]

Karl,

You asked me for an uncropped image. The problem is I can't find out how to post images here. What is the procedure please.

Rod

 

The key is to get the image below 500 kb; lots of image scale manipulators for free out there, but I just email them to myself and Outlook asks what size you want.  Voila!

[Vic Menard]

...the main mirror is 1¹/₂" thick.

My f/4 22-inch Dobsonian has a 1.75-inch "thin" mirror*. At 1.5-inch, you'll need to correctly support the edge of your 22.5-inch primary mirror or it will warp under its own weight. Suporting a Dobsonian mirror is much simpler than an equatorial Newtonian as the bottom edge of the mirror is always the bottom edge. https://www.cruxis.c...ecalculator.htm

 

 

* Thin compared to the old 1 to 6 thickness ratio I used to deal with in my mirror making days! 

Edited by Vic Menard, 18 March 2025 - 06:23 PM.

[kgb]

Karl,

You asked me for an uncropped image. The problem is I can't find out how to post images here. What is the procedure please.

Rod

As an alternative to Kitfox’s suggestion. You could upload the full size image (assuming it doesn’t exceed 50MB) into a new “Album” which is located in the “My Gallery” tab accessed from the user drop-down window. Then link to the new album by copying the hyperlink and pasting it into a new reply to this thread.

[Mike W]

Center dot the primary (I use page protectors), shoot a laser in the focuser and see where it hits the primary in different orientations of the scope to start. Also see where it hits the diagonal?

[Vic Menard]

Center dot the primary (I use page protectors), shoot a laser in the focuser and see where it hits the primary in different orientations of the scope to start. Also see where it hits the diagonal?

A collimation reference center marker is pretty much a requirement for routine assessment and correction of the axial alignments (the center marker reference should also be verified with a diligent star alignment). The coma "free" field diameter* at f/4.7 is 2.3mm, so any residual error of the center marker should be less than 1/2 the coma free field diameter, and ideally ~1/4 the coma free field diameter.

 

The coma "free" field diameter (sometimes called "the sweet spot") defines the diameter where coma contributes no more than 0.2 Strehl (1/4-wave) to the image. Coma continues to increase linearly outside this field diameter.