Really bad tracking

[steviemac500]

I was out trying to get the Dumbbell nebula tonight and was doing 75s frames iso800 25 shots. When I had finished, the nebula had an extremely obvious upward track in the frame despite tracking on my GOTO. I've never had this problem before, any ideas what could have caused it. Only thing I did different tonight was a 2 star align rather than 3.

[jambouk]

What mount? What scope? How do you polar align?

James

[StevieO]

Assuming you were using the equipment shown in your sig, that's 1000mm of focal length, so your polar alignment would have to be pretty good. I'm not familiar with the EQ5 - does it have a polar scope and what is your polar alignment routine (not described in your post). The star align only tells the mount where the scope is pointing, it doesn't polar align, which is a different thing altogether. This may be where your problem lies.

Regards

StevieO

[jambouk]

75 seconds is a pretty long exposure for an unguided image ar a focal length of 1000mm on an EQ5 even if polar alignment were absolutely spot on.

How do you polar align?

James

[Steve Ward]

You need to concentrate on getting your polar-alignment nailed .

Polar-alignment and Synscan-alignment are two completely different things , polar-alignment determines how accurately the mount will track a given object , accurate Synscan/Go-To alignment will merely determine how well the mount will find a given object .

[steviemac500]

Thanks guys. I understand the difference between polar aligning and the synscan set up. I do the same routine every time, I use Polar Align which gives the position of Polaris and I adjust the position of Polaris through the built in polar scope. My tracking has always been spot on using this method and I have had 75 second exposures with no problems. The tracking wasn't just slightly off, it was completely off. Byy the time the exposures had finished, the nebula had almost left the top of the frame. If I didn't know better, id have thought the motors hadn't been turning.

[dph1nm]

If the movement is in declination then it is probably polar alignment. If it is in RA then it is probably tracking related. So which way is "the top" on your image?

NIgelM

[Zakalwe]

Thanks guys. I understand the difference between polar aligning and the synscan set up. I do the same routine every time, I use Polar Align which gives the position of Polaris and I adjust the position of Polaris through the built in polar scope. My tracking has always been spot on using this method and I have had 75 second exposures with no problems. The tracking wasn't just slightly off, it was completely off. Byy the time the exposures had finished, the nebula had almost left the top of the frame. If I didn't know better, id have thought the motors hadn't been turning.

Does your handset firmware not have a polar Alignment routine? I would have thought that that would give superior results than the polar scope?

Autoguiding is your friend. It won't make up for gross polar alignment errors or really bad tracking, but it will open up a world of much longer subs, less subs lost and better quality data.

[steviemac500]

Thanks again guys. I'll have another look at the handset's polar align tonight. Auto guiding would be great but I'm also in the process of setting up a marine tank and I think my wife may not be too impressed! What would you say the longest exposure would be without guiding?

[ollypenrice]

Thanks again guys. I'll have another look at the handset's polar align tonight. Auto guiding would be great but I'm also in the process of setting up a marine tank and I think my wife may not be too impressed! What would you say the longest exposure would be without guiding?

Trial and error alone will tell you this. The variables are polar alignment precision, the position of the object in the sky, the luck of the draw in terms of periodic error mount to mount and, very importanly, the pixel scale at which you're imaging. (Briefly small pixels and long focal lengths are both more demanding.) Any statement like, 'I can do three minutes unguidied' needs all the variables above to be taken into account or it really tells you nothing.

Olly

[Astrosurf]

I had this problem with poor power feed from a rubbish cable (from Maplins). It'd be fine for a while then the tracking would stop. I changed the cable to a more expensive one and it sorted the problem.

Alexxx