Saturn processing problem

[cydonia]

Hi everyone,

I hope someone can help me with a processing problem I am currently having.

I'm imaging Saturn with a DMK21 camera, 3x barlow and RGB filters. When I do a RGB combine in Astra image, the red and green channels align correctly but the blue is always out. If I align the blue channel on the disk of Saturn, then on the left side on the rings the blue will be slightly out below the ring and on the right side it will be slightly out above the ring. The RGB channels were all then with in 5mins. Does anyone know why this is happening?

I've attached a screen dump, you can see the blue Channel is out on the each side of the rings but in opposite directions

Thanks in advance

[mswhin63]

It maybe the Barlow chromatic Aberration , mine does the same. Blue seems to be the one that requires it own focus during imaging. It has been a while imaging one day i hope to try it out again on Saturn in the near future to see if it a problem still.

[asimov]

Splitting the 3 channels, each one is slightly rotated anti-clockwise (from red to blue). So for some reason you are getting field rotation.

You only get field rotational problems by using an Alt-Az mount, or the camera/FW rotating on you somehow. Judging by the ghosting on each image, your collimation is out too. Might pay to check that.

[cydonia]

Thank you Malcolm and John for your replies,

My telescope is Alt-Az mount. I will try to shorten the time it takes me to make a RGB set and see if the image improves.

Thanks again

[asimov]

A pity to have to shorten your times - The other alternative is to derotate 2 of the images in software/an editing program...Unfortunately Astraimage does a lousy job at rotating. Maybe PS.

Registax does a good job at rotating, unfortunately you won't get a colour image after the RGB merge though.

[Shiraz]

I haven't used the feature, but I think WinJUPOS will allow you to do a rotate on each image before combining - be worth a try since your data looks pretty good. In any case, you shouldn't get that much rotation from an altaz over 5 mins. As John suggests, maybe something is slipping when you change filters. Regards Ray