Different colours appear differently focused

[Alexandros]

Hello everyone,

I just yesterday received the equipment I was missing in order to start taking short exposure prime focus astrophotographs and I set it up last night in order to get some photos of the moon. After a while of figuring out how everything fit, I tried to focus up the image using my cameras live view and zooming in the display to make sure the focus is as good as it can get. Focusing was not as hard, as I had read that with my scope, an 130/900 skywatcher explorer newtonian scope, a barlow lens is necessary as there is not enough inward travel on the focuser.

After verifying image was focused I took the picture and to my surprise the colours seemed to have been focused differently, or somehow refracted after taking the photograph. This was not visible in the live view feed on my camera beforehand however. Specifically the blue colour seems to have been the most unfocused as you can see it fringing on the edge of the moon, as well as if you zoom in the ridges and edges of the craters, in the photograph I took yesterday, you can see all the spectrum of colours appearing there. I am using the barlow included in my scope which might be of bad quality so I think the issue is that, but I would like your opinion as well. What do you think the problem is?

[TerryMcK]

What is your scope? Sorry I've just read it again and can see what your scope is.

It doesn't look focused to my eyes. The fringing may be due to bad collimation. I used to get colour fringes on my reflector before I bought a laser collimater.

It could also be the Barlow as you have mentioned.

Edited January 9, 2020 by TerryMcK

[Pompey Monkey]

Your scope is a reflector = no chromatic aberration (CA).

The (cheap) barlow has at least one refracting lens = possibility of CA.

99.5% it's the barlow.

 

[Alexandros]

55 minutes ago, TerryMcK said:

What is your scope? Sorry I've just read it again and can see what your scope is.

It doesn't look focused to my eyes. The fringing may be due to bad collimation. I used to get colour fringes on my reflector before I bought a laser collimater.

It could also be the Barlow as you have mentioned.

Hi Terry, thanks a lot for the answer. I agree, the photo does not look focused, I will try next time to play with the focus a little back or front from the point I was last time, just to be sure. The eclipse should be fun to try and image as well. However, one thing to mention is that my mount is not tracking and does not have any motor to correct for the rotation so I attributed the lack of focus to the motion of the sky. I tried to compensate for it by using a fast shutter speed, 1/250 and high ISO. Should I got for a faster shutter speed? Nevertheless, I will try again tonight and report my findings.

Collimation on the other hand is something I did not consider. I am not very proficient in that matter, so that could be the issue. Could it be sensor bloom as well?

Edited January 9, 2020 by Alexandros
Added collimation paragraph

[Alexandros]

15 minutes ago, Pompey Monkey said:

Your scope is a reflector = no chromatic aberration (CA).

The (cheap) barlow has at least one refracting lens = possibility of CA.

99.5% it's the barlow.

 

Hi Pompey Monkey, thanks for your reply. Yes I was thinking the same thing, however my scope has a spherical mirror so I don't know the effects of that in astrophotograph. I have tried cleaning my barlow today, in order to make sure there was nothing blurring my view. I will try again tonight with the eclipse and see what results I can achieve.

[Yawning Angel]

Solutions aside, it's possible to clean up:

You can tweek the rgb alignment in Registax, but any fix for the edges tends to introduce CA in the lower parts. I've brought the colours together in a compromise, which has sharpened it up a little. Then converted to B&W to drop the remaining distracting fringing

For a single shot, I'd keep the ISO fairly low to keep the noise under control and manage the exposure with shutter speed. You will find a single shot softer than a stacked video, as you're at the mercy of seeing

[Alexandros]

11 minutes ago, Yawning Angel said:

Solutions aside, it's possible to clean up:

You can tweek the rgb alignment in Registax, but any fix for the edges tends to introduce CA in the lower parts. I've brought the colours together in a compromise, which has sharpened it up a little. Then converted to B&W to drop the remaining distracting fringing

For a single shot, I'd keep the ISO fairly low to keep the noise under control and manage the exposure with shutter speed. You will find a single shot softer than a stacked video, as you're at the mercy of seeing

Wow that looks great! I will try to learn that software and try to do it on my own next time! I want to try stacking but as I don't have a tracking mount I don't know if it will be easy to do. Also finally, the sun just went down so I'm off to set up my scope and try again!

[Yawning Angel]

1 minute ago, Alexandros said:

the sun just went down so I'm off to set up my scope and try again

Good luck!

[Alexandros]

Quick session for astrophotography, it appears that the culprit was the barlow. I can see the abberation getting worse in the edges when I was pointing at venus and moving the scope slightly. Also I could see it going from red to blue when I was defocusing to one side and then the other. So it is definitely a refractive issue there, which must be the barlow. So now I am stuck with a three way dilemma. Should I

a) Buy an adapter for afocal photography?

b) Get a better barlow?

c) Get a shorter focuser?

What do you suggest as the best course for right now?

[alacant]

Hi

Easiest: better Barlow

Best: move the primary mirror toward the secondary. Either cut the tube or fit longer bolts to the mirror cell.

HYH

 

[Alexandros]

Thanks for your suggestions Alacant.

I think I will go with the better Barlow for now. I don't think I am ready to modify my scope as of yet, maybe when I get a new one I will get back on that 😛. Wouldn't that also lead to a decrease in focal length as well as the aperture of my scope?

[alacant]

2 hours ago, Alexandros said:

a decrease in focal length as well as the aperture

Hi

No. The focal length remains the same but the point of focus is now further from the telescope tube. This enables you to bring your dslr to focus with or without the Barlow. Aperture: you may lose a little light from the edge of the secondary but you'll not notice it.

HTH

[Alexandros]

7 hours ago, alacant said:

Hi

No. The focal length remains the same but the point of focus is now further from the telescope tube. This enables you to bring your dslr to focus with or without the Barlow. Aperture: you may lose a little light from the edge of the secondary but you'll not notice it.

HTH

I think you just broke my brain with the focal length 😛 . I have to think about that! 

[carastro]

Colour fringing with planets is a common problem.  I just fix it with processing techniques.  Quite a while since I did planetary imaging so have forgotten what I did now.

Carole 

 

[symmetal]

The main cause of the colour fringing would be atmospheric dispersion where the atmosphere behaves like a prism and the blue light is bent more than the red as it has a shorter wavelength. The lower the altitude in the sky of the target the worse it gets as the light travels through more atmosphere. 

Here's an article from the BAA explaining it.

Here's an article from Ian Morrison which also gives some methods of correcting it with various software  programs.

Alan

[carastro]

Quote

The main cause of the colour fringing would be atmospheric dispersion where the atmosphere behaves like a prism and the blue light is bent more than the red as it has a shorter wavelength.

Ah yes I remember now.  I think there is an asmospheric Dispersion corrector you can buy for this problem it's about £124.

Carole 

[Merlin66]

The attached graph shows the impact of atmospheric dispersion.

 

[symmetal]

I find this graph from the BAA article I mentioned above is a bit easier to interpret. At first glance Merlin66' graph gives the impression that red and blue are dispersed in opposite directions.

Alan

[Alexandros]

Hi all, thanks for your replies! I do not think my problem is atmospheric dispersion, I think I am a little too inexperienced to even hit that limitation with my current cheap equipment. It looks like lateral chromatic aberration and generally bad quality of the Barlow that is limiting me right now. When I mentioned Venus, I noticed that the colour fringing changed when I was looking at Venus from different angles, which is the symptom of a simple lens if I am not mistaken. If it was atmospheric dispersion, should the colour fringing be the same no matter where in my field of view I had Venus?

[symmetal]

1 hour ago, Alexandros said:

Hi all, thanks for your replies! I do not think my problem is atmospheric dispersion, I think I am a little too inexperienced to even hit that limitation with my current cheap equipment. It looks like lateral chromatic aberration and generally bad quality of the Barlow that is limiting me right now. When I mentioned Venus, I noticed that the colour fringing changed when I was looking at Venus from different angles, which is the symptom of a simple lens if I am not mistaken. If it was atmospheric dispersion, should the colour fringing be the same no matter where in my field of view I had Venus?

I missed your post about the colour shifting when looking at Venus. You're right that atmospheric dispersion would appear the same at all positions when using the barlow (if it was a good barlow ) Also in your initial moon image the fringing is too large for the size of the image, to be just dispersion. Didn't mean to lead you down the wrong path  , but it does give you something to be on the look out for in future when you've obtained higher quality equipment. 

Alan

Edited January 12, 2020 by symmetal

[alacant]

 

11 hours ago, Alexandros said:

I do not think my problem is atmospheric dispersion

I've been around for around for a few years and must say that with the standard of equipment to which I have access that that would be a nice problem to have!

This may allow you to see atmospheric dispersion.

Cheers and clear skies.

Edited January 12, 2020 by alacant

[Alexandros]

OK, so for now my objective is to get a new barlow, possibly something a bit cheaper than the one suggested by alacant 😛 . I was thinking of getting this one from skywatcher https://www.planitario.gr/barlow-2x-759.html (there is an english version of the site on the top right that you can access if it is not already), which appears to be better from the one I already have. Do you think that this will reduce the abberation to imperceptible levels for my scope, or should I go for something else? I would certainly like to shorten the barrel of my scope but I am sure that would void the warranty, wouldn't it?

 

Also Alan, when I reach that point I will most certainly ask you for help. I will probably have some good photos to share by that time