Stars have tails?

[drumsolo]

Hi all,

I took pics of m51 the other night and was guiding. Took 2 and 4 min exposures but all are showing star tails. 

Does this point to polar alignment or something else?

 

Many thanks

Stuart

[Davey-T]

If you know which way is north south, east west, in your image you can see the direction of the drift, don't think it's PA though.

Dave

[Anthonyexmouth]

Is that a crop? can you post the full image?

[almcl]

Almost all the elongation seems to be along the RA axis,  if so it's a tracking/guiding problem.  (Image below of your crop over laid on WWT map.)

What did your guiding program report as the RA oscillation?  If the reported error is small it might be caused by differential flex on your guide scope.  But if a largish error is reported then other aspects of the mount mechanics may need investigating.

Edited March 30, 2020 by almcl

[drumsolo]

Here is the full image.

I ae PHD guiding  didn't see any errors. Never had a problem before guding, Im not sure what you mean about RA oscillation or where i would find it?

[Anthonyexmouth]

4 minutes ago, drumsolo said:

Here is the full image.

I ae PHD guiding  didn't see any errors. Never had a problem before guding, Im not sure what you mean about RA oscillation or where i would find it?

[drumsolo]

I use PHD 1, is that info still available?

[Anthonyexmouth]

4 minutes ago, drumsolo said:

I use PHD 1, is that info still available?

Why are you still using that old version? 

[drumsolo]

Why not? I have just come back after 5 years so am not up to speed with latest software! What's the big difference? PHD1 always worked well in the past for me.

[Davey-T]

15 minutes ago, drumsolo said:

Why not? I have just come back after 5 years so am not up to speed with latest software! What's the big difference? PHD1 always worked well in the past for me.

Still using PHD1 and Win XP in my obs'y, works well enough, you can see the guiding corrections and guide graph that's enough to see what's going on.

Dave

[almcl]

Haven't used PHD (1) for a while now, so can't remember if it corrects for when imaging declination is different from calibration dec. 

PHD2 saves logs which can be analysed after the event to see what was going on using PHD logviewer which is handy, but unfortunately doesn't work with the PHD1 logs.

In addition there's an active PHD2 forum where the developers give advice and help on solving guiding problems.

[drumsolo]

Could the star trails be coma?

I am using a skywatcher coma corrector with LP filter on the top.

[Davey-T]

7 minutes ago, drumsolo said:

Could the star trails be coma?

I am using a skywatcher coma corrector with LP filter on the top.

Don't think so, try taking subs of a few seconds an progressively longer ones to see if the star trails get longer. with time.

Dave

[almcl]

Agree that doesn't look like coma.  I use the SW coma corrector and find it does a pretty good job at correcting it (albeit at the expense of some annoying reflections on very bright stars).

Another thing in passing, guessing from the equipment in your signature, you are imaging at around 0.9"/px but guiding at over 6"/px?  This may be  contributing to the issue.

[drumsolo]

11 hours ago, almcl said:

Agree that doesn't look like coma.  I use the SW coma corrector and find it does a pretty good job at correcting it (albeit at the expense of some annoying reflections on very bright stars).

Another thing in passing, guessing from the equipment in your signature, you are imaging at around 0.9"/px but guiding at over 6"/px?  This may be  contributing to the issue.

Sorry I don't know what you mean, can you explain?

Thanks

[geordie85]

I suspect it may be cause by tilt. The stars on the left are alot worse than your stars on the right. 

[almcl]

7 hours ago, drumsolo said:

Sorry I don't know what you mean, can you explain?

Thanks

It is suggested (in various places around the web)  that the resolution of the guiding set up shouldn't be more than 3 to 4 times that of the imaging train.  The reasoning being that  a sub pixel movement of the guide star, which PHD can usually detect, will translate into whole pixel movements on the image.  There's much debate about exactly what ratio does or doesn't work, (and PHD2 is said to be better at detecting such movement than PHD1) but when you add this to flexing between guidescope and imaging scope, mirror movement (if using a scope with a mirror) a ratio of more than 6 to one (which is what you have, if my guess about your equipment was correct) is probably going to contribute to ragged or elongated stars.