RGB Alignment Issues - CCD Inspector

[Tommohawk]

Hi All.

Having some issues with my refractor RGB alignment - hadn't noticed  anything amiss before now, but by chance I've recently done a few images of the Beehive cluster with runs partly before and partly after a meridian flip. The colour alignment looked pretty good generally in the past - by which I mean that combining RGP images stacked in APP looks good. There is a slight eccentricity of the star centres but not really noticeable. But if I mix ante and post meridian flip subs, the displacement becomes quite obvious.  Hope that makes sense!

I started a thread on this here, but because this bit involves more detailed analysis with CCD inspector I thought I'd start another thread. So I considered three possibilities - flop in the focuser, tilt due to the optics, tilt due to the camera.

I then did a more organised set of subs - a run of RGB before the flip, and then a run after the flip, to check for focuser flop. Then I put the results through CCD inspector - see chart below. 

The results are puzzling! The before flip (AM) RED1 GREEN1 and BLUE1 images from show differing amounts and directions of tilt - not huge differences I suppose. And the R and G tilt looks very different after the flip - BUT the Blue shows minimal change. This is baffling - isn't it? I'd have expected the tilt to be the same for all filters in the AM run, and then if any focuser flop for them all to differ on the second run by the same amount. 

I was expecting this to either prove or disprove focuser flop, and then I could go on to check whether the tilt was in the optics or the camera - but I seem to have fallen at the first. BTW, I did multiple images and the tilts varied only fractionally, so none of the figures in the chart are flukey. Also each filter was focused separately, and always using a star dead centre in the image. (I also checked the subs from the previous image run a couple of weeks back and the results seem similar although there were fewer subs - so pretty sure this wasnt just something weird on the day)

Hopefully I've explained this clearly enough - anyone have any ideas??? 

		AM/PM	CURVATURE %	TILT %	AXIS	COLLIMATION
	RED1	AM	12	7	-117	2.9
	GREEN1	AM	20.6	28	-129	12.1
	BLUE1	AM	28.6	44	-138	22.3
	RED2	PM	19.3	50	41	14.3
	GREEN2	PM	13.5	25	27	8.1
	BLUE2	PM	31.7	47	-137	26.3

 

[GyCx]

Hi

I don't have any answers but I am new to mono imaging and in my first attempt of imaging the  fireworks galaxy i have the similar issue of I cannot get a good colour from my combination of RGB. There was a flip after the LUM & red channels were captured and then the Blue, Green & Ha were captured after the flip.  I will test by ensuring my next target does not require a flip. and post the result. by any chance did you get anywhere with this issue?

[Tommohawk]

Did I get anywhere? ... not really!

Well, I concluded the offset/misalignment is likely due to sensor tilt, but didn't actually get to resolve it. That said I found an alternative processing method which makes things look better. I found that combining RGB images and doing star calibration in astro pixel processor seemed to improve things but not sure why.

The other solution I found was to ditch widefield and do galaxies and planetary for the time being! 

I'm hoping that a tilt adjuster will solve the problem but I have put this on hold for the moment.