Image shift between LRGB filter changes? Flexure?

[Cirrus]

Novice imager here with all the gear and no idea.  😉

I have an Atik 383L mono, and an EFW2 with 7 x 36mm filters from Baader.  LRGB and NB.  Guided by a QHY5II on a 50mm guidescope.

With my first ever session with the mono Atik, after stacking my lights and combinng the RGB images together (Deep Sky Stacker) using a reference image for all the stacking, when I combined them in Gimp, I noticed that while the stars were all perfectly aligned, that my hot pixels (I didn't do darks or flats, I was just testing) were all in a line, in order.

I suspect this is flexure between the imaging scope and the guidescope, but I'm not sure.  Anyone have any input?

Thanks all.

 

 

[MarkAR]

Does seem a little weird, could be that the different colour filters focus at different lengths.

[The-MathMog]

I had something similar to this recently, and found out that it was an asteroid that slowly moved through the picture through the night. So when LRGB was taken it was in a slightly different spot. I don't have enough knowledge to say that this is certainly the case, but a possibility.

[Cirrus]

Its not an asteroid (the same effect is on all the hot pixels). The filters are supposed to be parfocal, although I didn't refocus between changes.

The more reading I do, the more I think it might be flexure. My guidescope is a 50mm one with a extendable tube and helical focuser, on a dovetail with two adjustable scope rings, all attached to the finderscope  mount on my imaging scope.

[steppenwolf]

The stars have remained in 'the same place' but the hot pixels have moved because you have some drift in your tracking.

 

[Cirrus]

10 minutes ago, steppenwolf said:

The stars have remained in 'the same place' but the hot pixels have moved because you have some drift in your tracking.

 

It was guided, albeit after a very tough polar alignment.

[steppenwolf]

9 minutes ago, Cirrus said:

It was guided, albeit after a very tough polar alignment.

Yes but that doesn't mean that you can't have drift, unfortunately. When you stack the images, the stars all align by adjusting the relative placement of each frame but in so doing, the hot pixels appear to move.

[Cirrus]

16 minutes ago, steppenwolf said:

Yes but that doesn't mean that you can't have drift, unfortunately. When you stack the images, the stars all align by adjusting the relative placement of each frame but in so doing, the hot pixels appear to move.

Thanks, that makes sense. With rough polar alignment, I shouldn't have expected too much.

There's a polar alignment feature in Ekos,.I ought to invest some time with it.

Thanks @steppenwolf

[geeklee]

Am I right in thinking these should calibrate out?  The hot pixels then wouldn't be present during stacking, or a particular stacking algorithm could assist too?  

I've seen a similar phenomenon previously (admittedly with an OSC camera) that I attributed to the issue described by @steppenwolf when just stacking a load of lights and nothing else.

Once the lights are calibrated, all the artefacts are gone.

Just curious on this one having seen what I thought was similar scenario.

[Pompey Monkey]

I concur with @steppenwolfthat this is a result of drift caused by imperfect polar alignment.

However, I'm a bit surprised that these pixels did not get calibrated out on the stacking. What pixel rejection did you use?

Also, dithering will soon be your best friend.

[Star101]

Looks like drift or dithering.  If one looks closer, there are several such white, red green blues all over the image. Calibrating out with Darks etc will remove them

 

Edited April 18, 2020 by Star101

[Cirrus]

32 minutes ago, Pompey Monkey said:

I concur with @steppenwolfthat this is a result of drift caused by imperfect polar alignment.

However, I'm a bit surprised that these pixels did not get calibrated out on the stacking. What pixel rejection did you use?

Also, dithering will soon be your best friend.

I didn't take any darks as I was just testing. They would of course have been removed had I done so.

I was just curious to see whether this was flexure or something else. I'll try again with some proper polar alignment and see if the problem remains.

Thanks all.