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pipnina

Chromatic abberation in a reflector? Maybe something to do with my camera/atmosphere?

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So looking at my image with sirius in it from a few night back, I notice that it is fairly blue on the top, and somewhat orange on the bottom. This looks a lot like mild CA and it occurs in other stars as well. Since a reflector in theory shouldn't suffer from CA, it must be something in my camera I presume? Or maybe atmospheric effects?

image.thumb.png.10d8fc88843589b22c714959ca04df42.png

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Just now, Steve Ward said:

Or possibly the Barlow , assuming you had one in the imaging train ... ?

Only mirrors and an imaging sensor in the train. I don't own a Barlow at the moment sadly.

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Was the star fairly low in the sky? Atmospheric refraction would then be to blame. Planetary imagers like me often use a tunable correction prism to counter that

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Chromatic aberration doesn't tend to look like that. Scroll down on this page to see how it normally looks.  https://starizona.com/acb/basics/using_startesting.aspx

Your star has a red side and a blue side, yes, but they are both at the same focus, I would say. CA arises from some wavelengths being out of focus and so producing a large, soft stellar image in the colours not focused. 

I've just read Michael's reply. I didn't know a prism could be used in this way.

Olly

Edited by ollypenrice
Typo
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Sirius and other bright stellar objects near the horizon will display colours due to atmospheric dispersion.
your telescope will see them too, and pass them to your eye, they are not caused by any anomaly in your mirror.
That's a large image of Sirius :icon_biggrin:.

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5 minutes ago, barkis said:

Sirius and other bright stellar objects near the horizon will display colours due to atmospheric dispersion.
your telescope will see them too, and pass them to your eye, they are not caused by any anomaly in your mirror.
That's a large image of Sirius :icon_biggrin:.

I suppose Sirius was rather low down compared to its highest point. Since Olly has mentioned that true CA would likely produce blurred orange and blue channels it would seem reasonable that it is the atmosphere separating the colours? Would this mean it is possible to shift the R and B channels into alignment with G digitally to fix the problem?

I've cropped it from an image just over 1.1 degrees wide and 0.75 degrees tall. I find it a bit weird really that all the stars (even closer to the zenith) are so bloated even in the center of the field. The theoretical limit of my scope should be 90% of the width of a pixel with this camera. (0.66as vs 0.68as) Even if we take atmosphere into account it seems a bit strange that even the dimmer stars make it to 8-10 pixels wide.

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I too would suspect atmospheric effects.

@ollypenrice you can get Atmospheric Dispersion Correctors which help overcome this effect. ZWO do a quite reasonably priced one.

I did try it but found for visual it was just too much faffing around, but for imaging I believe it is very effective.

https://www.firstlightoptics.com/zwo-accessories/zwo-125-atmospheric-dispersion-corrector-adc.html

Review here:

https://astronomyconnect.com/forums/articles/zwo-atmospheric-dispersion-corrector-product-review.34/

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If it is Atmospheric Refraction (looks like it to me), in Registax for example, there is a mechanism for re-aligning the red, green and blue images - maybe worth loading the image and trying this.

Chris

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........ctd. I think this sort of effect when imaging something as low in the sky as Sirius is a fact of life. Looking back at old images of mine when imaging Sirius A+B (the Pup which is next to Sirius A), the blue halo above and orange halo below are very obvious, eg in this image of Sirius A+B. North is up.

Chris

 

sirius_1_crop.jpg

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Atmospheric Dispersion can be a real issue!!

Atmospheric_Dispersion_spots.png

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