What went wrong?

[baileys2611]

I'm taking some 4200 second subs of Orion and this is the result from one of them. All the others turned out nice, this one looks weird :shrug:

I was thinking that if it was a tracking error then surely everything would be blurry. If it's comets then the trails shouldn't end at the stars themselves.

My scope is well balanced so I don't think it would have been anything causing the scope to slip, everything's on tight, looked at the set up and nothing has dropped off or is rattling around (well...more than normal at least).

This is the first time I've seen something that looks like this, so I'm wondering if anyone has any ideas on what happened?

[Andy01]

Looks like it may have been bumped part way through your exposure perhaps?

[mithrandir]

Simon, there are lots of geostationary satellites that track through Orion, but they would give straight tracks and would not explain the smeared stars. If the mount stopped tracking the lines would be straight too. It looks more like either right at the beginning or long way through the exposure you bumped the OTA.

[mg]

Hi.
My setup will do this if the gears bind during an exposure. Then the stars trails off like this with the trails directed east-west which I believe they are in your pic?
Random slipping off other components doesn't usually cause perfect east west trails.
Just a punt.

Edit. Actually not all your stars are trailed like I have seen so my guess is wrong...

[Jon]

Simon, where in the sequence was this? Was it the last sub you took?

I had something like this the other night when PhD started guiding on a hot pixel.

[skysurfer]

Tracking motor stopped because of flat battery?

[nebulosity.]

Yep, tracking has stopped, or either your mount has slewed of the target.
This has happened near the end of the exposure as you can see from the stars.

Jo

[Lee]

I'd agree.... the dimmer stars haven't trailed, probably as they aren't bright enough to leave one on a rapidly moving CCD, if you really stretch the frame harder you'll probably see more trails I'd think....

[scagman]

Why do some of the trails converge instead of running parallel to the trails next to them?

Cheers

[mg]

Possibly due to the field not being perfectly flat?

[baileys2611]

Ok, so there's a few possibilities. Thanks everyone for your help here :thumbsup:

I definitely didn't bump the mount, it's on a pier bolted down hard, the obs is a shed which I can close and lock, off which I can take the roof and observe. I set it to do 4 subs at 4200 seconds each, closed the door and went to sleep :D - but that doesn't preclude an animal deciding to 'investigate' it and somehow crawl up the side of the shed, we get a few possums locally and that could have happened. It hasn't before, but it could!

I like the idea that it decided to focus on a hot pixel instead of the guide star, that sounds likely here. Other possibility I've thought of thanks to this thread :thanx: is that it might also have hit a slew limit imposed by the software near the end of the exposure. That would have the same effect as the gears binding - but I've had the mount for a year now and the gears haven't bound yet, it was a warm-ish night so I'm guessing (hoping, discounting entirely 'cause it's a Paramount MX and that would give me a lot of grief if they were binding :sadeyes:) that they didn't bind. I've used the scope since this and it still slews and tracks smoothly.

This was the last sub in the sequence, would have been around 3:00am-ish. The others are nice, clear, no star trails.

Correct, the field is not flat. I'm using a focal reducer to get a 'flatter' field than without, but it's a C9.25 SCT and those can be notorious for mirror flop and curved fields towards the outside of the picture. Normally I'll take some subs, process them and crop the outside because I prefer my eggs to be NOT on my deep space pictures. :P

[Nico13]

Did you have it set to Park at the end of the imaging series.
It may have parked before the last exposure ended.