Effect of excessive coma corrector distance

[Stub Mandrel]

When a field moves away from the sensor through the correct distance the stretching of stars goes from radial to tangential.

With a coma corrector too close (or absent) stars are smeared outwards into an egg shpe.

What shape do they become if the coma corrector is spaced too far? Do they get smeared inwards or do they get stretched tangentially like with a field flattener? Or something else?

I want to write some practical advice without wasting a night spent imaging with a deliberately wrong CC distance! This means I'm interested in 'real world' results not guesses ?

Thanks in anticipation!

[spillage]

The first image is the spacing is too close and the second it's too much.

Delrins are great - You put them between threads of connecting parts and so when you screw everything together, it will only screw down as far as the Delrin and so you gain some spacing.

 

Copied from an earlier post.

 

cheers,

[Stub Mandrel]

Thanks Mark,

That's the classic diagram for a field flattener but coma 'smears' stars asymmetrically rather than stretching them like a curved field - I'm looking for confirmation of the shape with a coma corrector.

[Stub Mandrel]

The only clue I can find is the statement that CCs work by adding 'reverse coma' which makes sense. In which case having it too far from the focal plane would cause stars to spread inwards, rather than outwards.

I suspect that, as a CC works by expanding the coma-free area, and we stop adding spacers once the image is coma free, that it may be quite difficult to add too much coma correction ina practical system, without running out of focus.

[spillage]

Sorry the info was wrong. I have never really thought about it,  I guess its just a case of concave/convex and the spacing allow the image from the mirror to be reflected inwards or outwards on the CC until the curves of images are equally matched which bends the original coma to appear flat. I can only guess that the curve on most modern day mass produced mirrors have a fairly standardized curve so one CC may work on several scopes or there are only a small number of companies producing mirrors. Although reading this back sounds kinda obvious. I have never really tried adding a huge amount of space when setting up a CC as its a job I hate and just want to get it correct asap.

[Stub Mandrel]

7 minutes ago, spillage said:

Sorry the info was wrong. I have never really thought about it,  I guess its just a case of concave/convex and the spacing allow the image from the mirror to be reflected inwards or outwards on the CC until the curves of images are equally matched which bends the original coma to appear flat. I can only guess that the curve on most modern day mass produced mirrors have a fairly standardized curve so one CC may work on several scopes or there are only a small number of companies producing mirrors. Although reading this back sounds kinda obvious. I have never really tried adding a huge amount of space when setting up a CC as its a job I hate and just want to get it correct asap.

It's weird, there is loads about 'diagnosing' field flattener spacing but I can't find a comparable guide for CC.