Different colours either side of stars

[codemonkey]

I recently took 3x600 of each R, G and B for NGC 55, figuring that I'd capture small parts of each colour every night and that way I could see it improve as I went, rather than capturing all the R on one or two nights and having to wait until I did everything before I could see the results.

Anyway, when I did this I noticed that the stars were green on one side and red on the other. I thought maybe this was because I only had a small number of subs so they didn't average out slight guiding errors, so I did an experiment last night.

This is NGC 253, obviously, but there's 15-18 subs of each R, G and B here. The subs are short and the moon is obviously about 80% full; I wasn't trying to capture a good image, just test this theory.

I thought that 15 was a good enough sample size that it should average out any small guiding errors. Unfortunately I still have the problem, although it's blue and green this time.

Something to note here is that it seems as though the star shape is different between the colours, not just bigger / smaller or more / less defined, which is why I was thinking it was a guiding error.

Does anyone have some suggestions on what this is? For all the other alternative causes I can think of I would expect to see different size stars, not different shaped stars.

Unless the stars aren't actually a different shape, but a different size and the registration process is getting the center wrong and offsetting them... hmm...

[Merlin66]

Lee,
Sounds very much like atmospheric chromatic aberration....
What was the altitude of the FOV when imaging - obviously the higher the less atmospheric effects.
http://commons.wikimedia.org/wiki/File:Atmospheric_Dispersion_spots.p ng

[LightningNZ]

NGC 253 is pretty high overhead for the much of the night. Unless these were taken not long before dawn, I'd not have thought the atmosphere would have been too bad?

[Amaranthus]

Seems mostly likely due to a slight misalignment of the chrominance channels. How are you stacking them Lee?

[Rex]

Hey Lee, just a thought and it may be irrelevant, but are you refocusing after you change to the filter for each channel? If not that could be your problem because not all filters are parfocal. Like I said just the first thing that jumped into my head when I read your original post.

[codemonkey]

huh... that's very interesting. I wonder if that could be it. Images were captured with NGC 253 being between about ~45 and ~65 degrees.



Seeing was pretty rough, but I'm not sure if that's the same thing?



I'm registering them all against the same original B frame. All images are registered using PixInsight's default StarAlignment registration settings. Each colour was stacked separately using PixInsight's ImageIntegration module: Average combination, Additive with Scaling normalisation, Noise Evaluation weighting and Winsorized Sigma Clipping.



Good thinking Rex. I haven't been and this is one of the things I thought as well, but I was thinking the stars would be bigger / more diffuse whereas in the RGB integrated image it just looks like the green stars are longer. What I should (and will) check is the difference between a star in the red and green subs using pixel math as that should give a clearer idea.

[Amaranthus]

Try doing a blink comparison of the 3 final colour stacks and see if you can notice any shift in the stars across the R, G, & B channels (e.g., convert each to a JPG and then do a rapid cycle through them using e.g. Windows Photo Viewer). It doesn't matter if you are registering against the same frame if something is going wrong with the registration.

[codemonkey]

Thanks Barry :-)

I've done a diff of the frames now which confirms my earlier thoughts; the stars are a different shape, which I think I can resolve to guiding errors culminating in slighting different shaped stars.

The blue and green also look out of focus compared to the red but even if I got the focus right the stars would be a different shape giving rise to these irritating halos (which of course would be having the same effect on everything, it's just easier to see on the stars).

[multiweb]

What scope did you use?

[codemonkey]

Skywatcher Black Diamond ED80 refractor (F7.5 @ 600mm). Being a refractor it's likely to cause focus issues between the colours but it shouldn't cause a different shape to the stars I wouldn't think...?

[multiweb]

Yes that would be it. Unless it's an apochromat your colors will have different focus points and it'll get worse off axis. If you shoot with an osc, not much you can do. If shooting with a mono and filters, refocus for each filter, then use deconvolution on the channels that have the fatter stars before combining the color.

[Amaranthus]

The ED80 is technically an apo, just a doublet rather than a fully corrected triplet (like the ED80T, which is what I use). But given that you should really be refocusing every 45 min to 1.5 hours (depending on temp gradients), my advise is to ALWAYS refocus when changing filters, as good standard practice :)

My feeling is that your alignment is based on star centroids, and when they are somewhat bloated or out of focus, the centroids can shift (slightly). Thus, your 3 colour stacks are slighly misaligned. I had something like this problem in a recent experimental image I did of an Open Cluster using an achromat at f/4 (where the colours are most definitely NOT parfocal!!): http://www.iceinspace.com.au/forum/showthread.php?t=126868

[multiweb]

My understanding is that you need at least 3 lenses (apo) to correct for chromatic aberrations. Isn't your ED a doublet?

[Amaranthus]

My ED (Orion ED80T CF) is a triplet with FPL-53 in the front lens. http://www.telescope.com/Telescopes/Refractor-Telescopes/Refractor-Optical-Tube-Assemblies/Orion-ED80T-CF-Triplet-Apochromatic-Refractor-Telescope/pc/1/c/10/sc/346/p/101422.uts

Lee is using a SW BD ED80, which is a doublet. It's still considered an apo, although the colour correction isn't as exact as a triplet (in general): http://www.optcorp.com/telescopes/refractor-telescopes/sw-s11100-pro-80-ed-apo-refractor-telescope.html

[LightningNZ]

An ED can be good, but it'll never be an APO. That's why I love my little AT65-EDQ - almost no false colour. It does need fairly frequent refocusing though. I tend to refocus 2 or 3 times in a night.

[Amaranthus]

Every scope needs regular refocusing if the temperature is changing, although this is mitigated to an extent by carbon fibre bodies.

[RickS]

It is possible to make an apochromatic scope with a doublet but not a fast one (the Telescopes, Eyepieces, Astrographs book has an example of a f/15 air-spaced doublet apochromat.) I expect that the ED80 is not a true apo.



Rolando from Astro-Physics claims that a metal body works better in a refractor because dimensional change in the scope body tends to counteract the change in focal length of the lens with variation in temperature. I haven't looked into this in any depth but I'm prepared to take his word for it :)

Sorry for hijacking your thread, Lee. I do see effects like this sometimes but it's normally a symmetric halo where stars in one or more colours are bigger but much the same shape.

Cheers,
Rick.