Thoughts please

[edarter]

Hi,

Having done a few astro photography sessions now I'm getting more confident with setting up, though I do still regularly have  'how the heck do you do that again?' moments. Processing the images is yet to be mastered but I'm keen to improve the quality of the subs I'm taking before they even get to the processing stage. Attached is the result of a session I did last night, straight out of DSS, only processing done by me was to align the channels before saving. I't's about 1 1/4hrs worth of data, 60 second exposures @ISO1600. Looking at the stars at the edge I can see there is tracking issues as they are elongated, but the larger ones in the middle seem to have their top left section missing. What could be causing this?

I'm using a SW 130PDS, NEQ6 unguided and an astro modified Canon D600. I plan / hope to get enough money for my birthday shortly to get a guiding setup but I'm not expecting that to be the answer to all the issues seen here so any advice / tips greatly appreciated.

 

Thanks
Ed

[Owmuchonomy]

Hi Ed. having had a similar set up to you in the past there are a few things to suggest some of which you may already have tried. Looks like you have field curvature which is an issue when using such a large chip. Which coma corrector do you use? 'Pacman' stars are caused on the 130 by the intrusion of the focuser tube into the ota which happens when trying to get in focus. Yes guiding will help a lot. Personally I avoid doing ANY processing after stacking in DSS. I would suggest using PS or similar.

[edarter]

Hi Chris,

I'm using the Baader MPC II CC

Cheers
Ed

[Waldemar]

Hi Ed,

To me this elongation does not look like a tracking issue, but like coma, because all star-elongation in the corners points to the middle of the image. You could try to change the distance of the MPCC in small steps by using delcrin rings or an adjustable spacer.
If it would have been caused by been tracking or rotation it would look totally different.
The little tracking issue shows in the evenly 'unroundness' all over the image
The 'packman' effect is caused by the focus tube as has been said before.

 

[edarter]

Hi Waldemar,

Thanks for the pointers, are these shims the ones you are suggesting:

https://www.firstlightoptics.com/adapters/baader-t2-delrin-spacer-ring-set.html

I'm not using the Baader t-ring, just a standard one, I assumed using the Baader on a t-ring would be a fit and forget thing :/

Thanks
Ed

[Waldemar]

Yes Ed, that's them !! 

The downside is that if your distance is too big already, they will be useless...
Your MPCC has a backfocus of 55mm, your canon takes 44 mm of that, so 11 mm left to fill up with spacers. Best would be to get a 9 or 10mm spacer, for that leaves you room to experiment with + or - distances.
Using a filter changes the backfocus distances, don't forget that. The pathway through the glass adds 1/3 of the thickness to the backfocus of your camera.

Hope this helps a bit

[edarter]

Thanks Waldemar, I've just posted in another thread where someone is having similar issues. Reading through that thread I'm even more confused! I'm using an M48 ring, so I think the back focus needs to be 57.5? My M48 ring is 10.9mm thick plus the 44mm gives me 54.9. So I need 2.6mm.
The Baader MPC III I have came fitted with a thin metal ring on its M48 thread, its 2.5mm thick but with it fitted on the CC I can't fit the CC to the M48 Ring! Remove it and everything fits together nicely, but with 55mm spacing, not 57.5.

To add further puzzlement, I can't find a 2.5mm spacer anywhere, the only option seems to be to buy the Baader Protective T-ring but thats £54 I could do without spending if at all possible. I'm also puzzled as to what this thin ring is for that came with the CC, it prevents fitment on the M48 thread but achieves nothing on the T2 thread (I have a separate T2 adaptor that I've tried it on but thought M48 was supposed to be the better option?)

Confused.com!

Ed