Correcting Coma in my F5 Fraunhofer Doublet.
Posted: Tue Sep 22, 2020 9:43 am
I have had this 150mm F5 Fraunhofer refractor for a year by now. I recognized a small amount of Coma on the optical axis. It was clearly visible from 200X off. A week ago I re-read an article by Wolfgang Rohr, in which he described how he corrected a similar telescope. Yesterday I decided it was time to do the same. This is the article. It is in German. You don't need to translate it, just have a look at the pictures.
http://r2.astro-foren.com/index.php/de/ ... fotografie.
Coma in doublets has several main causes.
1. The two lenses are not properly centered relatively to each other. The centering can be done by hand. The human fingers are quite sensitive to very small differences.
2. One of the two lenses is some hundredths of millimeters thinner or thicker at its own edges (uneven) and is therefore skewed. It is corrected by adding or subtracting very small amounts of spacing. Rohr in his article used 0.05mm thick alufoil. I used 0.01mm thick alufoil from the kitchen.
Example.
This is how severecoma looks like. One can easily see the coma - center and the coma -disc. The center is positioned at 270 degrees ( nine o'clock) and the disc at the opposite.
Coma of this size is even visible at low magnifications of 30X. It brings Strehl-ratio down at quite a fair amount. ( not as much as spherical aberration though)
According to Rohr spacing must be added at the edge of the lens where thecoma center occurs. So I did. No worries, I did it once before on a 4 inch f/ 10 Fraunhofer. I established where the center was by holding a pen in front of the lens while looking at an unsharp star like Altair. I marked its position on the outside with a marker. Unscrewed the holder. "Unpacked" the lens by removing all sorts of rings and lifted it up while it was resting on a cilinder. ( see the pics of Rohr). I the lifted the front lens and added some spacing in between at the right spot. (0,08mm, the first time I had to gamble) Remounted the lens like it came out of the cell , while taking care that the markers the manufacturer left on the side of the lens were in the same old position. I had to be sure it all went back in the same as it came out.
Back outside, remounted the cell and looked at Altair at high magnification.
This is what it looked like before:
0,3 wavelenghts out of line at 270 degrees.
Afterwards:
Oops, far too much!
I'll spare you the details of dismantling, correcting, remounting, 5 times in a row, getting very warm and it becoming later and later. Thanks to the weather Altair stayed my brave guide all evening.
In the end I came as far as this:
If I had wanted even more precise correction I should have had a spacer of 0,005 mm. Which I hadn't. So this was it.
The overall picture is like this:
So Strehl is at least 0,8 by now.
Quite relieved I was when I saw the results. I pointed at Mars. I saw the same details as on September the 20th! viewtopic.php?f=6&t=13479 There was a difference though, apart from the chromatic fringes, I saw that I had misplaced the dark area at 6 o'clock that evening. It should have been between 3 and 4 o' clock !
Warning: Never do this with a triplet. It takes professionals to do it.
http://r2.astro-foren.com/index.php/de/ ... fotografie.
1. The two lenses are not properly centered relatively to each other. The centering can be done by hand. The human fingers are quite sensitive to very small differences.
2. One of the two lenses is some hundredths of millimeters thinner or thicker at its own edges (uneven) and is therefore skewed. It is corrected by adding or subtracting very small amounts of spacing. Rohr in his article used 0.05mm thick alufoil. I used 0.01mm thick alufoil from the kitchen.
Example.
This is how severe
- 00- 1,5 wave 270 degrees.jpg (11.61 KiB) Viewed 4194 times
According to Rohr spacing must be added at the edge of the lens where the
Back outside, remounted the cell and looked at Altair at high magnification.
This is what it looked like before:
- 1- 0,3 wave 270 degrees.jpg (8.42 KiB) Viewed 4194 times
Afterwards:
- 2- 0,7 wave 90 degrees.jpg (8.1 KiB) Viewed 4194 times
I'll spare you the details of dismantling, correcting, remounting, 5 times in a row, getting very warm and it becoming later and later. Thanks to the weather Altair stayed my brave guide all evening.
In the end I came as far as this:
- 6- 0,07 wave 270 degrees.jpg (6.54 KiB) Viewed 4194 times
The overall picture is like this:
Quite relieved I was when I saw the results. I pointed at Mars. I saw the same details as on September the 20th! viewtopic.php?f=6&t=13479 There was a difference though, apart from the chromatic fringes, I saw that I had misplaced the dark area at 6 o'clock that evening. It should have been between 3 and 4 o' clock !
Warning: Never do this with a triplet. It takes professionals to do it.