Tringaular stars - advice please

[pluto]

Last night I finally had an opportunity to try out my new STT-8300M with my newish Tak TSA120.

Before setting up for imaging I had a look at the moon and Jupiter through the Tak and the views were awesome, super sharp and great colour.

But when I started imaging I noticed my stars were coming out triangular.
There was probably some flex in the focuser and the extension tubes as the backfocus is huge on this scope.

Reading around the net it sounds like it could be a collimation issue or pinched optics, both of which I'm assuming are hard/expensive to fix with a Tak..?

This image is a 1:1 crop from just off the centre of a 10min Ha exposure. Guiding looked pretty stable and focus was spot on according to my bahtinov mask. The stars are a pretty uniform shape accross the whole image too.

Any advice would be greatly appreciated!

[allan gould]

Hugh
It looks as if it's due to flexture in your imaging train, especially if visual gives no sign of the triangular stars.
Are all your connections screw or are they the usual clamp types with tightening screws. One way to get over this is to use a longer extension plate to fix your scope to the mount head and then use a clamp at the camera end to rigidly fix the camera to the end of the extension plate. This makes it tight and takes out the flex. Hope this is clear enough.
Allan

[pluto]

Hi Allan, thanks for your reply.

There is a ~50mm extension screwed into the focuser, then a ~100mm extension with the tightening screws and then the camera is held in that extension with tightening screws.

Flexure was my first thought and I'd always planned to buy some better threaded extensions. I'll look into a longer plate, that's a good idea, thanks!

[Ajemsa]

Hugh,
I 'm having similar problems but I know with certainty that it is my tracking. I know you stated that the guiding looked pretty stable but are you sure that the guiding resolution (and fluctuations) is suitable for your imaging resolution?
You could probably rule out a few things by taking short exposures and progressively getting longer until you notice the problem.
Andrew

[gregbradley]

yeah take a short 10 sec focus exposure and see if the stars are round. That way you know its either tracking or flex. It would be pretty bad flex to do that though.

Was it just one image that looked like that or many?

Greg.

[pluto]

The guiding looked ok but it was a fairly rough polar alignment and there was some movement on the guide star. However I've had much worse guiding and seen plenty of blobby/eggy stars (with my previous scope/camera) but never a triangle..?

I'll definitely try next time with a good polar alignment, and that's a good idea to try different exposure lengths.

Thanks :)

[pluto]

Thanks Greg,
It was many, I took about 10 and they all look similar.

Actually I've just gone through them and it does look like there is some quite noticeable tracking errors on most of them but they all have roughly triangle shaped stars.

When I was doing 1 second focus exposures the stars looked perfectly round... maybe it is just bad guiding...? :shrug:

[gregbradley]

There you go. If you had pinched optics you should be able to see this effect visually and it would appear on every image.

If its a guiding issue it looks like there must be a large periodic error spike at some point in the worm cycle.

Have you sat and watched a whole exposure's worth of 1 exposure with these stars? Was there a sudden spike in errors at one point?

A triangular shape like that is unusual in my experience with tracking. Normally if I get a bad spike it looks like double stars everywhere or a bad elongation in one direction but a triangle.

It does look a bit like coma as well. Are you using a corrector/reducer? Perhaps its at the wrong spacing. Usually Tak is either 87mm or 120 odd mm. There is a list of metal back distances for Tak scopes at Anacortes.

Greg.

[pluto]

I watched the guide star for a little while and there was a bit of moving around but it looked fairly uniform. It was also a bit windy and I think that accounts for some of the more pronounced streaks on some of the other exposures.

I have no corrector/reducer yet, I'm planned on getting a corrector but I want to make sure everything is working without it first.

I hope it is just guiding, next clear night I'll take my time with a good polar alignment and make sure balance etc is spot on, at least then I can either fix or rule out guiding as the problem.

Thanks for your help Greg.

[gregbradley]

If its flexure guide errors won't show that. It could be guiding the guide scope just fine but the difference between it and the imaging scope could be widening as the flexure occurs. The proof is more the image.

Greg.

[Exfso]

I don't suppose you have done a star test have you? My first Toa130 had stars like that and it turned out to be astigmatism. The whole scope was replaced by Takahashi.:question:

[pluto]

It's guiding with the OAG that's part of the self guiding filter wheel so no issue with flexure there.
I think there is a bit of flex in the extension tubes attaching the camera to the focuser.



I haven't done a star test. I bought the scope secondhand from a member of this forum who is very experienced with Taks so if there was a manufacturing issue I'm sure he would have sorted that out long ago.


Thanks for the advice guys.

[gregbradley]

I would do a star test. Defocus a bright star visually and look at both sides of focus. As I said I have never seen that type of star pattern in any of my scopes. Closest was an 80mm triplet APO and the focuser was way out of centre with the lens.

Greg.

[pluto]

I'll definitely do a visual star test next time I set up.
However while taking focus exposures I did see a bright star on both sides of focus and it looked perfectly round at all focus positions.

[Shiraz]

FWIW is there any possibility that you could have vibration induced by the cooling fan(s) in the camera? The fan(s) should have low vibration, but if it just happens to be at the right frequency to induce a resonance, you could get this sort of image. Saw similar when I tried to fit a Barlow between my scope and QHY8 - worked fine with no Barlow, or with an MPCC, but the Barlow produced just the right combination of mass and flexure and the little stellar points turned into little lines (not triangles). In my case, just carefully placing my hand on the camera was sufficient to damp the resonance - not a solution, but a useful diagnostic.

[pluto]

Interesting Ray. I'll check that out next time I set up but I would have thought that if it was caused by a vibration the effect would show up even on short (1 second) focus exposures...?

[Shiraz]

depends on the frequency I guess - I have no idea what mine was. The SBIG is a relatively heavy camera, so you might expect fairly long period resonances (if there are any) - 1 second exposures might possibly be OK in that case. Of course you should check for tracking errors, but it is probably still worth checking for resonances - bit of a long shot, but I was surprised at how distinct and persistent the resonance was in one configuration of my system whereas it was otherwise quite stable.

[Barrykgerdes]

I am no authority on astrophotography but I looked closely at your photo and the triangles are regular 45 degree Rt angle triangles. This suggests movement in both axes that is beyond the guiding control. The guider is trying to correct for a large tracking error because the polar alignment is a fair way off. I think PEC may be the major cause.

If you can get your polar alignment closer it will reduce the need for correction in both planes and if this gives an improvement I think that will be the proof.

It does not look like any sort of problem with the optics

Barry

[pdalek]

Big triangles with an equatorial mount is usually caused by wind. The dec correction hunts about through the backlash. The ra just corrects through rate change. If windy, increase bias weight on both axes.

[pluto]

Thank you all for your help, my stars are round!! :D

It was guiding, I set up tonight, balanced everything well, and did a reasonable drift align and now my stars are round, maybe a little soft... but round!!

[strongmanmike]

Well investigated Hugh, you must be relieved?...and a nice start there to an image of the Rosette too :thumbsup:

Mike

[Ajemsa]

Well done Hugh, I was pretty confident that it was a guiding issue as I have seen it too many times with my own setup. Another thing it could have been is slippage on your axes. Just make sure all the knobs are tight before you start guiding.
Andrew

[pluto]

Thanks guys, very relieved!
Got a couple of hours of the Rosette and the setup looks like it's working pretty well. This is without a flattener too and it looks good.

Now I just need to take it somewhere dark!