Astigmatism in Maksutov-Cassegrain system or what?

[bojan]

Because my MTO-1000A suffered from what I believed was slight astigmatism, I decided to do something about it.
So I went googling and found many web-pages on the subject, suggesting this was because the glass was stressed by over-tightening retaining rings...
Now, after dismantling and relaxing the stress to the optical parts, the problem remained the same in nature, however significantly worse in magnitude.
Now I am wandering if the original problem was caused by pinching/stressing of glass at all....

The brief description of star image as the focus is moved in and out is as follows:
Out of focus: slightly elliptical ring is visible.
Closer to the focus: elliptical ring becomes sharper, and very narrow.
Focus: Cross-like shape is visible
Out of focus, other side: Elliptical ring is again visible, however orientation of axis is 90° compared to other side of the focus (what one would expect from astigmatism).

I suspect the scope went out of collimation somehow (but how, it is beyond me.. unless I bent something while dismantling.. which is not easy considering the heavy mechanical design of the scope).

I tried to find suitable optical simulation software to determine what may have gone wrong, but it seems all the packages available to me (Oslo, Modas, Linos, Atmos) can not deal with lateral misalignments and/or tilt of optical components of the system.

I am determined to fix this problem this or that way.. at least I want to have the same situation as before dismantling. It is a matter of pride now !!! ;)

Anyone has any ideas or similar experiences ?


Edit: image attached (Mu Cruci, focal with 400D), to illustrate the problem better. This was the situation before dismantling.. now it is 3x worse...

[DeanoNZL]

Hi Bojan,
I have one of these as well, but have not used it for many moons.
Have you tried "CCDInspector", from ccdware? 30 day trial available.
This will give live results while adjusting.
Also, have you tried adjusting wjth the flattener/reducer removed from the t-ring attachment point? This may have moved, or could be slightly loose.
Sorry I couldn't be more help.

Adrian

[bojan]

Hi Adrian,
Thanks for this, however I knew about this s/w.

My problem here is that there are no adjustments for collimation for this lens.
I even had a theory/explanation at some stage that the spacer ring between two front correction plates was bent, therefore of un-even thickness, thus causing the astigmatism of the system after relaxing (because it expanded), but after replacement with new one, there was no improvement.
That is why I wanted to do some simulation of similar system to see if a problem like this could be caused by misalignment at all, and what that misplacement should be to produce the outcome of such magnitude...

BTW,
have you tried to use yours as astrocamera? I would be interested to see the test pics :-)

[DeanoNZL]

I will see what I can do for pics.
It was found in a second hand shop for $100 a long time ago.
The mirror has seen better days, but stars were round from memory.
I will get back to you.:thumbsup:

[Merlin66]

http://markishky.hit.bg/Mto.htm

You've probably found this link already. Your description is certainly astigmatism. Looking at the optical layout my attention would be on the positioning/ alignment of the element #5 this seems to be possible "guilty peson"

[bojan]

Thanks..
Yes, I found it already...
I have not touch the field lens, though.. unless it moved somehow in the process of dismantling. I will definitelly have a good look at it.
Also, I suspect now that maybe something went in between main mirror (element #3 ) and its seat, so the mirror is now slightly tilted?
The worst thing is, I had so-so performance, but I tried to improve (because the lens was not good enough for a-photography as it was) ... and now I ended up with pile of glass and metal trash.

[Merlin66]

Put the lens cover on and check the optics with a Cheshire collimating eyepiece, this should show any gross mis-alignment. As the front elements are pretty spherical to each other I wouldn't think they would be as sensitive to a small mis-aligment and give you the images you see....
Do you have a copy of Suiter's book " Star testing of Astronomical telescopes"? A really good reference.

[bojan]

Now I feel pretty stupid...
It seems I had a problem with tube mounting.. I used a spring-loaded belt around the MTO tube (close to where main mirror is located) that holds it in the cradle... and this belt was too tight !!!
When I released it almost completely, the astigmatism virtually vanished in 12mm eyepiece.
So I was struggling with ghosts..
Thank you all for your replies.. they definitely made me think.
And the end result is, now I do have much better MTO-1000A, it is better than it ever were (because the relaxing of optic contributed as well).
Despite its looks like it is build like a tank, it is still not rigid enough and it is easy to "pinch" the main mirror from the outside!
And now I remember I was reading about this on one German website, but I thought this can not possibly apply to my older and heavier model. :screwy:

[bojan]

Adrian,
I would still be interested in test photos taken with your MTO.
Just make sure it is fully temperature stabilised (1 hour or more) before use otherwise you will be disappointed too..
Despite removing one problem (tight bracket around the tube) I am still not happy with performance - astigmatism is still there and visible, only smaller (however it is OK for visual) .

[Kal]

Glad to see you got it all sorted, and don't feel too hard on yourself, I'm sure most of us have missed much simpler things when trying to fix something. I was fixing a doorlock just this last weekend and for the life of me I could not understand why the door handle would still move when I "locked" the door. I re-read the instructions a dozen times, before realising I had left out a piece that sits between the handles two steps prior, it was still in the box. :screwy:

[bojan]

Finally I found some time to better document and conclude this astigmatism saga:
MTO-1000A glassware is pretty good but the assembly of the lens is not.
It is obvious on the attached images, how the tight retention ring that holds the mirror in place deforms the glass to such extend that the image becomes unusable for astro-photography.
Now, I have mirror which is actually quite free (not good, I am afraid: it slightly rattles in its cell and I will have to think of some way to keep it in place (and prevent damage from accidental external shock), while at the same time no pressure is applied which may deform the glass (silicon rubber pads, perhaps?).
There is still some residual astigmatism left (perhaps because now mirror "hangs" of axis slightly?), but nothing compared to what it has been before I started to play with it. I will try to remove this later, if it can be done, I will do it.
My biggest problem was in understanding what other people said about how they solved this problem. I thought that they were using too colorful language when writing that "mirror should be relaxed such that it almost rattles".. This had to be taken as it was written.. and so on.
Anyway... this lens is now much better than it ever was....
The images are 30 sec exposures with Canon 400D, ISO1600, 200% crop from central part of image, displayed with DPP. There is no visible difference of star images from corners, which means the focal plane is pretty flat.

EDIT:
To illustrate further the effect of too tight bracket around the tube, here I added a central part of two shots, taken when I was not aware I was squeezing the tube (and mirror with it) too much. They are deliberately taken slightly out of focus to emphasize the astigmatism.

[Merlin66]

It it possible to rotate the mirror in its cell? That way you could confirm if the mirror is astigmatic or it's gravity tilting the mirror.
I'd still recommend putting a Cheshire eyepiece on it; see what the collimation is really like.
How much clearance is there is the cell? What about wrapping a thin strip of masking tape around the edge?
The mirror, like any optics don't like to be in direct contact with metal; there's a chipping hazard and expansion problems. The mirror should not be tightly held in place; just enough to "feel" it being able to move ( doesn't have to rattle, just "feel" loose).

[bojan]

Thanks for your comments, I really appreciate them :-)
There is not much lateral clearance for mirror in the tube (perhaps 0.05mm, but not much more than that). So there is no space for anything as thick as masking tape I am afraid. . Maybe aluminium foil, or Teflon tape like the one used for plumbing? I will have a look...
The retention mechanism consists of three rings: one in contact with the back of the mirror, a spring (it is a tube ~110mm in dia, 0.5mm thick walls and 7-8 mm long. It is splinted by 3 long cuts, so that it can be compressed (not much. maybe 0.2mm by reasonable force) in length, and there are 3 pads that are in contact with the previous ring.
And the third ring has thread at the outer edge, it screws into the main tube and it is fixed in place with one grabscrew 1mm dia at the side of the main tube.
I suspect that the seat for mirror (I haven't removed it yet, to keep dust out) is not flat, this is the only explanation why even slight pressure from this mechanism causes astigmatism, as demonstrated with those pictures. Or mirror edge itself is not flat. The thickness of the mirror is ~10mm in centre (so it must be pretty flexible)
The mirror can be rotated easily now, but clouds are back over Melbourne so this experiment will have to wait..
And yes, I will try cheshire eyepiece soonest..

EDIT:
There is one more aspect of this mirror mount design that I noticed and did not like: When grabscrew is tightened, it forces the threaded ring in the opposite side (there is considerable clearance for this, 0.1 - 0.2mm or so)... And because of the slope of the thread (metric, 60°) this ring is therefore forced towards the mirror.
So I had to check the pressure on the mirror after tightening the grabscrew, and adjust the clearance in couple of iterations.
Anyway, this is not well designed. I think I will remove the grabscrew and apply silicon rubber to the retention ring on couple of spots, to prevent it from moving on its own. This way I will still be able to dismantle the mirror if the necessity arises in the future.