Chromatic effects on the image of the Moon

[Mr Giovanni Di Giovanni]

Dear colleagues,

Good morning, everyone.

I kindly ask for your help in understanding a feature of my telescope. It is a small MAK90 f1250 catadioptric telescope, which I enjoy using to observe the brightest objects, mainly the Moon from my balcony at home. I also take some shots with my Sony Alpha 6000 mirrorless camera using direct focus. The resolution is low and the results are mediocre, but I am satisfied nonetheless. However, I am puzzled to see that chromatic aberrations always appear on the lunar disc. As can be seen in the enlarged images, one edge is outlined in red and the opposite edge is outlined in blue.

What causes this?

Thank you and best regards to all.

Translated with DeepL.com

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[Dr Paul Leyland]

At what altitude was the moon at the time?

The atmosphere acts as a prism and deviates red and blue light differently at different altitudes. The effect is most apparent where the air is thickest — at low altitude.

https://en.wikipedia.org/wiki/Atmospheric_refraction covers the phenomenon but only slightly and could be greatly improved in my opinion. Searching on “atmospheric dispersion corrector” will produce more information and a fair description appears at https://astronomytechnologytoday.com/2017/07/06/atmospheric-dispersion-corrector/ but the available information is far from satisfactory in my opinion.

Paul

[Mr Giovanni Di Giovanni]

Thank you, Paul. I was not familiar with the topic discussed in the link you provided. To be honest, I was thinking about some defect in the corrective plate. I had not considered the effect of atmospheric refraction. So, I understand that I probably never get a good focus precisely because of atmospheric dispersion.
I took that image with the Moon about 30° above the (astronomical) horizon and about 20° above the profile of the mountains surrounding my location. The table and graph in the link are very interesting. As for the atmospheric dispersion corrector, I don’t think my little one deserves it.
Ciao

[Robin Leadbeater]

A test would be to take images with the telescope rotated to different angles (but with the camera orientated the same relative to the horizon). If it is atmospheric refraction the red and blue areas will always be top and bottom relative to the horizon. If it is the telescope, they will move to different angles relative to the horizon

Cheers
Robin

[Mr Giovanni Di Giovanni]

Yes, this is a test I haven’t done, but it absolutely must be done. On the first clear evening, I will carefully carry out this procedure. Then I will update the post.
Thanks, Bill.

[Dr Paul Leyland]

Robin Leadbeater wrote:

<p class=”wp-dark-mode-bg-image”>A test would be to take images with the telescope rotated to different angles (but with the camera orientated the same relative to the horizon). If it is atmospheric refraction the red and blue areas will always be top and bottom relative to the horizon. If it is the telescope, they will move to different angles relative to the horizon

<p class=”wp-dark-mode-bg-image”>Cheers<br class=”wp-dark-mode-bg-image”>
Robin

Good point. I wish I had thought of that.

[Steve Knight]

According to my university notes angular seperation of red & blue images from atmospheric chromatic abberation is:

¬4 x 10^-6 x tan angle from zenith. Angles are in radians.

Based on a 450nm refractive index of 1.0002796 and a 650nm refractive index of 1.0002758.

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