Help with this image please

[billyharris72]

Hi all:

Still very much trying to learn the ropes when it comes to imaging. An attempt from tonight on M71. Unguided, 30 second subs with an ASI1600MM, just to test my new set up. Shooting with a 130PDS on an HEQ5.

1. The stars are (or appear to be) trailing quite a bit, which is disappointing even for unguided at 30 second subs. I'm using an HEQ5, polar aligned through Sharpcap to an (alleged) accuracy of <10". Is this to be expected from the HEQ5?

2. I'm not 100% sure this is star trailing or PE though. I recently fitted a dew heater to my secondary, as the weather has been so damp here that my optics have been unusable within 20 minutes of set up. Could the distortion in the stars be due to the collar for the dew heater? I assume that is what is causing the misshaped stars (I checked and adjusted collimation just before shooting). If it is the dew heater, is there a way to combat dew without this problem?

3. What's going on in the bottom right hand corner (in fact right along the bottom now I look)? Any ideas what could be causing this?

Thanks for any help that might be on offer... 

Billy.

[geordie85]

Are you using a coma corrector? How tight have to got the clamps around your secondary and primary mirrors?

The outer edges appear to be suffering from slight coma and possible antagonism from pinched optics.

 

[billyharris72]

Thanks Geordie85. That's a fair point on coma - there probably is some, though the rather triangular stars produced by what I assume is the dew heater makes it harder to diagnose. I'm using the SW coma corrector and the spacing is close to the stated 55mm, but maybe not quite close enough. I've not had the primary mirror off the back yet to check for pinching - I've been putting that off.

What's bugging me more at the moment is the elongated stars close to the centre of the image (eccentricity is about 1.2, basically awful). I don't know if this is astigmatism or star trailing - could be the tripod has "settled" a bit after aligning, though I'd hope it wouldn't move so far as to rule out 30 second subs. That and the rather triangular stars (though given the problems I've been having with dew I might need to accept this as better than no stars). The whole business at the bottom and edges is worrying too - I'm wondering if this is amp glow, and how bad an impact this would have on attempts to use the camera for photometry (I've tried a few times and had okay results so far; maybe the trick will be to use the central 2/3 of the field only - having a decent size sensor certainly helps).

Might have to investigate the coma corrector spacing though - I can feel a purchase of one of those adjustable Baader t-tubes coming on.

Billy.

[AKB]

6 hours ago, billyharris72 said:

I can feel a purchase of one of those adjustable Baader t-tubes coming on.

I think that's the way to go... I did just that, but do remember to measure carefully: I got the 29-40mm version, but then discovered that I had not included a 42/48mm adapter in the calculation (worth another 10mm of length), so now I need to get the shorter one too. 

Oh well, you just can't have too many of these expensive bits of tubing.