Diagnosing star elongation issues

[codemonkey]

Now that I've spent a few nights gathering data after months of, well, not... I'm more inclined to try and get to the bottom of my recent elongation issues. Looks like there might be some clear sky time tonight but with a moon phase of 57% I'm thinking I should give imaging a miss and try and get to the bottom of my most recent star shape issues.

I expected that I was going to find the primary was stuck to the cork pads on the mirror cell again and freeing it up would fix it, but that turned out not to be the case.

Can someone confirm my plans for diagnosing make sense:


Confirm problem exists in short unguided exposures; if not it's tracking/guiding related
Assuming not tracking/guiding related, rotate the camera. If the orientation of the elongation stays fixed with relation to the image produced by the camera, it suggests collimation and/or tilt
Assuming it's not collimation/tilt, rotate the primary mirror. If the orientation of the elongation changes, it suggests primary mirror astigmatism, if not, it's the secondary


Not sure if I'll need to recollimate after rotating the primary, which I can do without removing from its cell. Probably will, but it'd be better if I didn't have to, because that way less things are being changed and I'm more likely to find the cause...

[Atmos]

Your plans sound good Lee.
What you could do is rotate the primary to see if it changes on-axis astigmatism and if it doesn’t then recollimate just to make sure that collimation isn’t the problem.

If all else fails then I’d imagine it’s either the secondary or possibly still somehow the primary mirror cell.

[Startrek]

I can only let you know what I experienced with my 6” f6 newt 6 months ago
My stars were tight ( not eggy or oval ) but flared out a bit like a torch on the right hand side and it was worse towards the right hand side off axis area of the image
Long story short it was collimation, secondary was out and primary cork pads had compressed on 2 sides
Stripped it down , replaced the pads with thicker ones and re collimated using a Cheshire and 2 laser collimators
Problem fixed , although the scope did suffer a bit from coma which I knew about
If your image has oval or eggy star across the whole image is guessing it’s your mounts tracking even at short subs
Don’t know if this helps in any way
Cheers

[codemonkey]

Thanks Colin! :-) Yeah, good idea, I'll test it after rotating the primary without recollimating and then collimate and test again, assuming I get that far.

I'm pretty suspicious of focuser axis alignment by way of the secondary... I don't trust my skills with the auto collimator and I don't know if I have the spacing on it right which could be throwing me off, so hopefully I won't get to the point that I have to rotate the primary anyway.

[codemonkey]

Thanks Martin :-) I use (all catseye) a cheshire for primary collimation and the off-axis pupil of an auto-collimator for secondary. I'm pretty confident of primary collimation but suspicious of the secondary.

I've been thinking about replacing the cork pads with something else after I found the mirror stuck to them once which (I think) was causing astigmatism, but I've never gotten around to it. Might just replace the entire cell at some point.

I doubt it'll be tracking with 1-5sec exposure on a very well aligned, pier-mounted Mach 1 so I think I'll be able to cross that off the list pretty easily. I think I'll also see the elongation switch direction as I shift focus, so I'll test that up front.

[glend]

Lee if your elongation shifts when you rack the focuser, i would say that is your problem.

[codemonkey]

Cheers, Glen. I think that's standard behaviour of astigmatism. I guess if your focuser was shonky it might cause a similar thing (focuser axis alignment shifting, as it racks causing astigmatism), but I'd be pretty surprised if that was the case on a lite crawler... these things are built very strong

[codemonkey]

Finally got around to running some tests last night in a short gap between the clouds.

Results were:

Rotating the primary made negligible difference to the orientation of the elongation.

Using the Lite Crawler to rotate the drawtube (with camera attached) caused the elongation to rotate with respect to the image, implying whatever is causing the elongation remained fixed while the camera moved.

So, based on the above it looks like it's the secondary or its mounting. Seems to be mounted ok, with three small dobs of some kind of adhesive, which I believe is the recommended method. From memory it is, however, adhered directly to the aluminium stalk, so perhaps that's the problem... maybe what I should do is re glue the mirror but put something that more closely matches the CTE of the glass between the mirror and the stalk.

Thoughts?

[xelasnave]

Keep seeking a solution but startools has a feature "repair" which I find excellent on most occassions to fix star problems. You may find you can use data that might be thrown out.
Alex

[Startrek]

I concur with Alex, Startools does a great job repairing eggy or deformed stars
It has a limitation though , it can’t repair stars with medium to large diffraction spikes , they end up looking a bit deformed , so I just mask them out.
I have a Baader coma corrector now so I haven’t use the repair module since as my images are 95% coma free.

[Imme]

Do you use a field flattener?

I spent months looking for the cause of elongated stars and it ended up being a lens in my flattener

[Joshua Bunn]

Hi Lee,
This also suggests it could be CCD tilt, in my experience. What do you think?


Josh

[codemonkey]

Thanks Alex and Martin, that's a good idea; better than throwing out data!




Thanks Jon. I don't use a flattener as such, but I do have a coma corrector so the same thing applies really. I think if it was the coma corrector the orientation would have stayed the same with respect to the image when I did the rotation test, because the camera and coma corrector retained their relative positions.



Thanks Josh. I had thought that tilt would manifest in an aberration that remains fixed relative to the image when I rotate the camera, no?

I have noticed another difference between the images I took during the rotation tests... I notice that at one orientation the focus position is different to another, which suggests a collimation/tilt issue. I've been very particular with the autocollimator lately and ensuring it's being used as close to the focal plane as I can get it in an effort to ensure that there's no focuser axis misalignment, which I've read can cause similar issues.

My only thought now is that the focuser is not square to the tube (which I have read does not matter, thus why I haven't considered it before)... maybe that means that at one orientation the image will be better than at other orientations?

I've now been in contact with Teleskop Service in Germany who were very quick to respond. Based off my tests they think it's astigmatism in the secondary or its mounting (though they're perplexed because they tested the mirror on their optical bench... suggests mounting issues to me). They're intending to send me a new secondary.

I'm going to try squaring the focuser before I accept their replacement and if that gives me no joy then I'll go down the replacement route. Either way, I think I've narrowed it down to two potential causes and have solutions for both so I feel this might finally be coming to an end...

[Joshua Bunn]

Yes you are right Lee, I mis-interpreted your original post :)

[Paul Haese]

Hi Lee,

don't you just love Newtonians? ;)

So this is some of the ideas I have used to find solutions to problems.

Check the rotation of the secondary to focuser. That is certainly one cause of tilt or elongation. You don't need to be far off to have this problem.
Next check the collimation thoroughly. Both the above and collimation have huge effects on the appearance of tilt when combined with a coma corrector.
Then check the actual corrector (threads etc). These suckers are prone to tilt and paying a lot does not guarantee you don't get tilt. Use a laser through the corrector to see if it is centred. I found it was not the case on the OOUK corrector.


Just remember this is going to be something very minor, only fractions of a millimeter that is causing this.

[codemonkey]

Thanks for the advice Paul, it's much appreciated... gotta say, life was a lot simpler when I used refractors ;)

I squared the focuser to the tube the other day... or at least I did so to the best of my ability. Looks like it was fine going down the tube, but off a bit across the tube.

After squaring the focuser I recollimated using Catseye kit. This time I did something different; I didn't use any of the thumbscrews on the focuser adapter whilst using the AC, but held it very tightly against the focuser adapter at all times. Doing this I was then able to rotate the AC with maybe the barest hint of "destacking" visible via the offset pupil. I've never been able to do that before, it usually unstacks significantly as I rotate it. This also confirms the AC was correctly placed at the focal plane, otherwise it would have unstacked as I rotated it. The unstacking I saw as I rotated it was at most 0.5mm and if I recall correctly the offset pupil magnifies errors 8x, which should mean the error was approx 0.06mm which I imagine is as close to perfect as you can get.

Given the above I'm pretty confident my collimation, both primary and secondary.

Now, please correct me if I'm wrong, but when I did the rotation test, I kept the relationship between the camera and the coma corrector unchanged, so if it were an issue with the coma corrector, wouldn't it have stayed fixed with respect to the image?

The one thing I can think of at the moment is tilt introduced by tightening the thumbscrews (from the focuser adapter) onto the coma corrector...

I'm not entirely sure how to check the coma corrector with the laser... can you elaborate on that? Note also that my coma corrector has no eyepiece adapter or anything, it's just got an (M42?) thread on it