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Can coma direction help with determining spacing?


parallaxerr

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I am using a Baader MPCC as a flattener with my ST120 refractor and Nikon DSLR. Due to the large chip I have a lovely wide field but notice some coma/flaring towards the edges, pointing towards the centre of the image. Granted this is not a common setup, a cheap achro with known SA and a newtonian coma corrector, but it mostly does a good job, especially in Ha it would seem.

Question: Is it possible to work out from the direction of the coma which way I need to adjust the spacing?

Here's an example of the coma/flare from top right of image...

 

coma TR.jpg

And full image...

NewLRGBComposite.thumb.jpg.bb791fcaf7cef1f7d610820031c5b116.jpg

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Thanks AKB that's a good post. I searched for ages but was obviously using the wrong terminology. Having said that, would you call the effect seen in my image as star elongation?

If so, I'm in luck as the link you posted suggests the spacing is too close, which I can address with some delrin spacers, can't adjust inwards.

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I had something very like this with the wrong spacing (and used the diagram to good effect.)  I was way out initially, having omitted to count 10mm worth of m42 to m48 adapter, which I thought was integral to the coma corrector!  Having got it closer, I finessed with Delrin spacers too, although Baader do some very nice lockable variable spacers (costs a bit more than Delrin!)

I'd need to look closer, but since your chip is large this could also be tilt?  If you really want to spend £££ on this, then go for the CCDInspector program, which is made for issues just as this.

 

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I have had very mixed results from CCD Inspector.

To put it to the test I ran a series of subs through it all from the same batch and it variously reported curvature downwards and upwards and the tilt varied between different filters.

I am yet to be convinced.

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55 minutes ago, Skipper Billy said:

I have had very mixed results from CCD Inspector.

To put it to the test I ran a series of subs through it all from the same batch and it variously reported curvature downwards and upwards and the tilt varied between different filters.

I am yet to be convinced.

I've just done the very same thing and observed very different results, so I too am now a little dubious.

I think I'll stick to the mk1 eyeball for now...the centre of my images are good and it's only a little coma that caught my eye, with which I can play about with spacing.

The tilt, if actually there, may be due to sag. The camera is held in place by the short MPCC inserted in the draw tube so I've added a 30mm extension to the front of the MPCC and it now slides into the draw tube much easier and looks to be straight.

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