A torrid evening...what's causing these star shapes?

[parallaxerr]

It's been a while...a LONG while actually since I last imaged, but tonight I got the bug again and fancied having a go at M27, the Dumbell nebula. Since last time though, my scope has changed from the ST120 to an Equinox 120, all other parts of my setup have remained the same, so: HEQ5, Nexguide, Nikon D3200, Equinox 120 w/.85 reducer/flattener.

I got everything setup and aligned quite early using Altair and Vega, alignment errors were in the order of 20-30 arcseconds per axis, previously this has been fine when guiding. Anyway, once it was a bit darker I decided to re-familiarise myself with my gear, specifically the nexguide. That didn't really take long, I had a star (Altair) focused, locked, calibrated and guiding in no time. I set a 10min exposure as a test. Unfortunately the result was terrible!...

I decided to check the alignment of the mount by the drift method, which returned good results, confirming the alignment was OK...

Then I locked the guider back on to Altair and as it was going through it's calibration, I could hear quite a loud clicking noise on the dec axis, it could be felt through the mount and scope too so I thought that's what must have caused the poor image. I took the motor cover off and could see the dec axis was jerking at low speed, i.e. during cal or with motor speed set to 1. I removed the belt and noted the motor was fine on it's own but there were stiff spots on the worm drive when turning the spur gear by hand. So, a quick visit to Astrobaby's website to remind myself how and I adjusted the worm drive to be smooth again.

Started everything fresh and during calibration and subsequent guiding - no clicking. Result....so I thought.

Back to Altair and this time a quick 2 minute exposure to test and I got this....

Not bad at first, I thought, until I noticed the teardrop shaped stars. I tried a 30s exposure and got the same result on the brigther stars (except Altair). I don't think guiding would even come in to the equation at 30s so is this an issue elsewhere? What causes teardop shaped stars? Are they even teardop shaped? Looks almost like diffraction spikes???

Finally, I was determined to see if I could at least get a glimpse of M27 and managed one sub before the clouds rolled in. As if the clouds weren't bad enough, the sub was out of focus because apparently the Equinox focuser is struggling with the weight of my DSLR. What a pants evening.

Any guidance re: star shapes gratefully received,

cheers

 

[wimvb]

They are not diffraction spikes, but rather small jumps in dec. How is your scope's balance? Try to find out how guiding behaves with the best balance you can achieve; no east-, west-, front-, or back heavy. You can then also try with a slight polar misalignment and no or one sided dec guiding. Does the nexguide provide any form of feedback? (Eg, phd always saves a guide log.)

[parallaxerr]

34 minutes ago, wimvb said:

They are not diffraction spikes, but rather small jumps in dec. How is your scope's balance? Try to find out how guiding behaves with the best balance you can achieve; no east-, west-, front-, or back heavy. You can then also try with a slight polar misalignment and no or one sided dec guiding. Does the nexguide provide any form of feedback? (Eg, phd always saves a guide log.)

I did some more thinking about this, this morning. I agree it's a jump, but it's in RA because the camera was at 90° during the subs. I checked the RA axis of the mount and there was a fair bit of backlash, which I have now removed.

Interestingly, last night was probably the best aligned I've been during an imaging session, so you may be on to something. I did note that the Nexguide was rapidly correcting in RA and DEC in both directions so I may well have been too well aligned and balanced, silly as it sounds. I did shift the weights slightly East heavy later on but the clouds rolled in. Next time I try, I'll misalign slightly as I think previously the Nexguide was mostly correcting in one direction only.

[wimvb]

5 hours ago, parallaxerr said:

I did note that the Nexguide was rapidly correcting in RA and DEC in both directions

You may be chasing the seeing. Keep the exposure time of the guide camera long enough.

[parallaxerr]

2 hours ago, wimvb said:

You may be chasing the seeing. Keep the exposure time of the guide camera long enough.

Good point, never considered that. The exposure was very short due to being locked on to Altair. Thanks

[parallaxerr]

Update on this issue:

After adjusting both RA and DEC belts I tried imaging again and got the same results, only to then discover the focus on the guidescope was miles off! I hadn't noticed because the nexguide goes into zoom mode when guiding so the guide star always looks bloated on the screen, but when I manually zoomed out it was the same. Refocusing and then increasing exposure time sorted the issue and I got 600s subs OK again.

I was chasing a very bloated star which in poor seeing conditions was jumping all over the shop and with a short exposure it was causing the guidescope to over-correct