C6 Collimation Question

[Barry]

Hi

I spent a fruitless evening on Monday due to a collimation issue with my C6.  I had recently fitted a Celestron F6.3 Reducer and so checked the collimation before starting.  It appeared to be way out and I spent a long time trying to correct it without success. The images were very poor, impossible to focus properly and with coma/flares in the corners and one weird loop artifact on a particularly bright star.

Today I set up a (very crude) artificial star to check how the reducer was affecting the scope. (I know that this is liable to be somewhat inaccurate but I could at least compare with and without the reducer). I think the following pictures show the reason for my confusion in the dark on Monday. With the "star" away from the centre of the FOV, even slightly, the side of the unfocussed disc which is nearest the edge of the FOV seems to be eroded, more so in the corners where the disc is down to one quarter, even though the central obstruction shadow would still be central if the whole disc was there (if you see what I mean).

I think I was trying to centre a distorted image in the dark on Monday, no wonder it only got worse!

Anyhow, my question is this: is this effect just due to the increased vignetting, due to the reducer or do I have another problem?

Many thanks for your help.

Barry

[Ben the Ignorant]

A star test must absolutely be done with the star in the exact center of the field, or the pattern will look distorted even if collimation is perfect. And the less you defocus (meaning the smaller the pattern), the more apparent miscollimation becomes. Look for the small bright dot (the Poisson dot) that appears in the middle area of the pattern; with a tiny bit of defocusing it will look clearly decentered if collimation is off. But when you defocus largely, the imperfection is harder to see.

These pictures make it clear:

http://www.astrophoto.fr/collim.html

[Barry]

Thanks Ben, if I have got it right, at least I can say from the linked article, that if the test had been a real star test it would show that the scope, with reducer, was not severely misaligned and would therefore form the basis for more accurate collimation when conditions allow, with a fainter star at higher magnification.

Would collimation using the imaging camera be OK if a DSLR is supposed to be the equivalent of a 6mm EP?

Barry

[Ben the Ignorant]

When the scope is collimated, it's collimated no matter what you attach to it.

[Peter Drew]

7 minutes ago, Ben the Ignorant said:

When the scope is collimated, it's collimated no matter what you attach to it.

As Ben says. Unfortunately, in many cases, the fixtures and fittings to the telescope result in small alignment errors giving the appearance of miscollimation. Best to address these rather than recollimate the telescope to suit. Diagonals are one of the worst culprits, I've made a diagonal test rig and it's surprising how many are a bit off.  

[newbie alert]

1 hour ago, Ben the Ignorant said:

A star test must absolutely be done with the star in the exact center of the field, or the pattern will look distorted even if collimation is perfect. And the less you defocus (meaning the smaller the pattern), the more apparent miscollimation becomes. Look for the small bright dot (the Poisson dot) that appears in the middle area of the pattern; with a tiny bit of defocusing it will look clearly decentered if collimation is off. But when you defocus largely, the imperfection is harder to see.

These pictures make it clear:

http://www.astrophoto.fr/collim.html

Great link Ben,thanks

[Ken82]

2 hours ago, Ben the Ignorant said:

When the scope is collimated, it's collimated no matter what you attach to it.

Not really . I've found when  using an 11" edgehd And after collimating I need to re collimate slightly when I attach the focal reducer . In theory if you add anything to the optical train the collimation may be out ever so slightly. Probably not enough to warrant a re collimation each time ! 

[Barry]

I do not own good high power eyepieces because I do not really do any visual astronomy; is it OK to collimate using the DSLR instead of the eyepiece? (I believe that it is the equivalent of a 6mm?)