Coma corrector and distance from backplate

[earth titan]

Tried this in another section but it may have been the wrong one, so will try here.

How crucial is the distance between the coma corrector and the camera back on my f4 imaging newt?

New to the use of coma correctors etc. and it only just dawned, I might need spacers or similar. Been persevering for a long time now to correct the problems it has, as I still get weird shaped stars (distorted rings (not spheres)).

If so, how do I sort the spacing?

[Stephen]

It's extremely critical, you haven't got much more than 1mm either way tolerance.

T2 extension tubes needed.

[earth titan]

Aha....

How do I know what the distance is, or is this just trial and error?

[steppenwolf]

Which coma corrector do you have?

Most (but not all!) require a spacing of 55mm from the mounting face of the corrector to the sensor itself. You need to know how far in the sensor is from the front of the camera.

[earth titan]

From what I can find, it needs 75mm spacing. It is an Altair Astro 2" corrector.

This may explain my problem.

What a dummy....

[steppenwolf]

Indeed, the A/A one does require 75mm rather than 55mm. So, now you need to determine the sensor to camera face distance for your camera and then make up a suitable spacer. If you are using a DSLR camera, this will nominally be 44.5 to 45.0mm but a CCD camera could be anything from about 9.0mm to 20.0mm. A DSLR camera would require a 'T' adapter which adds a nominal 10mm.

[earth titan]

Looks like I need about 35mm.

Thanks everyone.

This has been bugging me for months.......

Can't believe I didn't twig before.

[earth titan]

Bits ordered.

Once again, thanks to everyone.

Just goes to show once you think you know what you're doing, something comes along you hadn't considered.

[Earl]

Dont forget to get some of the Baader Spacers.

Very very handy!