Help: why are my stars elongated at the edges of the frame...

[arad85]

OK, new to DSLR photography, and some images I took last night are really puzzling me as to what is happening.

Setup:

• CG-5GT, levelled, aligned with polaris (including making sure the polar scope is aligned)
  
• CG-5 GT two star aligned, with 4 extra stars - a total of 6 stars being used to compute alignment
  
• Scope: C-80ED attached to a skywatcher medium dovetail and 90mm rings (tight)
  
• Scope reasonably well balanced
  
• Camera - Canon 450D attached via max DSLR and revelation 50mm extension tube
  
• Focused by live-view
  
• ISO 400 at 30 second subs
  

My problem (shown by the attachments) is that the stars at the edge of the image look like they are expanding outwards from the centre point - as if I were starting to enter warp drive on the EWnterprise. I can't understand what is happening here - I was on solid ground so no chance of it being moved relative to the stars by a warp drive...

Anyone any ideas - the sub as a single image followed by the 4 full sized corners (they should be clockwise from the top left if the ordering has been preserved)

Thanks in advance

Andy

[blinky]

This is due to the ED80 not producing a flat field, it can be resolved (somewhat) by using a field flattner/reducer - The William Optics FFIII has quite a good reputation, I use the FFII model and it also works pretty well. The other option is to just crop the frame a little!

[blinky]

Oh and nice image as well BTW!!

[Psychobilly]

As Craig has said... A Field Flattner should sort it out... I use a WO FF III with my Megrez 72...

Peter...

[arad85]

Ahhhhhhhhhhh... thanks.. Yet another reason to spend money It really is a downward spiral isn't it

[SteveL]

Its not even a spiral... its just straight down...

[Psychobilly]

Its not even a spiral... its just straight down...

Indeed....

Anyone tried the Skywatcher Flattener yet ? looks a bargain...

Steve (FLO) have you had them yet?

Peter...

[arad85]

Its not even a spiral... its just straight down...

And to think I started out 4 months ago with a total budget of £400 for astronomy