Stars Distortion Question

[pkinchington (Peter (Kanga))]

Hi - In this image of the eta carina nebula the stars are distorted in the corners particularly the lower LHS. What might have caused this? Any pointers or suggestions appreciated.
Cheers Kanga

[Dave882 (David)]

Hi Kanga
Might be helpful to know what gear you used for this shot?
Seems to my eye to be some star comets pointing inwards in all 4 corners which can be quite common without a field flattener (especially if you are using a larger imaging sensor).

[Craig_]

It looks like either a lack of a field flattener or a backspace issue if you do have one. Given the problem also seems more severe in some corners than others, this may also be compounded by some tilt in your image train.

[pkinchington (Peter (Kanga))]

Originally Posted by Dave882

Hi Kanga
Might be helpful to know what gear you used for this shot?
Seems to my eye to be some star comets pointing inwards in all 4 corners which can be quite common without a field flattener (especially if you are using a larger imaging sensor).

Hi Dave and Craig,
I used an 80ed f7 with qhy163M (4/3 sensor) and filter wheel - I used Ha,Oiii and G filters (the green filter was a visual one) and a 1X hercules field flattener ( I do not know what the back spacing is meant to be for this flattener).

Cheers Kanga

[pkinchington (Peter (Kanga))]

Originally Posted by Craig_

It looks like either a lack of a field flattener or a backspace issue if you do have one. Given the problem also seems more severe in some corners than others, this may also be compounded by some tilt in your image train.

Hi Dave and Craig,
I used an 80ed f7 with qhy163M (4/3 sensor) and filter wheel - I used Ha,Oiii and G filters (the green filter was a visual one) and a 1X hercules field flattener ( I do not know what the back spacing is meant to be for this flattener). The apparent tilt may be due to the way I cropped the image.
Cheers Kanga

[Craig_]

If you're using a field flattener that suits your scope (no idea if that one does or does not), it's likely a backspace issue you need to work through. Someone else may be better placed to comment on whether you need to add or remove backspace to improve things though, I believe the distortion pattern can tell you this but I am not sure.

[pkinchington (Peter (Kanga))]

Originally Posted by Craig_

If you're using a field flattener that suits your scope (no idea if that one does or does not), it's likely a backspace issue you need to work through. Someone else may be better placed to comment on whether you need to add or remove backspace to improve things though, I believe the distortion pattern can tell you this but I am not sure.

Hi Craig,
I'll investigate the backspacing issue further. I think I may have to add a couple of mm of extension. Can you exceed the recommended backfocus extension is exceeding the recommended extension allowable?
Cheers Kanga