Correcting Coma in my F5 Fraunhofer Doublet.

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John Baars Netherlands
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Correcting Coma in my F5 Fraunhofer Doublet.

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Post by John Baars »


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 severe coma 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.
00-  1,5 wave  270  degrees.jpg
00- 1,5 wave 270 degrees.jpg (11.61 KiB) Viewed 3852 times
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 the coma 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:
1-  0,3 wave  270 degrees.jpg
1- 0,3 wave 270 degrees.jpg (8.42 KiB) Viewed 3852 times
0,3 wavelenghts out of line at 270 degrees.

Afterwards:
2-  0,7 wave  90  degrees.jpg
2- 0,7 wave 90 degrees.jpg (8.1 KiB) Viewed 3852 times
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:
6-  0,07 wave  270  degrees.jpg
6- 0,07 wave 270 degrees.jpg (6.54 KiB) Viewed 3852 times
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:
150  F5 na comacorrectie sept 2020.png
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.
Refractors in frequency of use : *SW Evostar 120ED F/7.5 (all round ), * Vixen 102ED F/9 (vintage), both on Vixen GPDX.
GrabnGo on Alt/AZ : *SW Startravel 102 F/5 refractor( widefield, Sun, push-to), *OMC140 Maksutov F/14.3 ( planets).
Most used Eyepieces: *Panoptic 24, *Morpheus 14, *Leica ASPH zoom, *Zeiss barlow, *Pentax XO5.
Commonly used bino's : *Jena 10X50 , * Canon 10X30 IS, *Swarovski Habicht 7X42, * Celestron 15X70, *Kasai 2.3X40
Rijswijk Public Observatory: * Astro-Physics Starfire 130 f/8, * 6 inch Newton, * C9.25, * Meade 14 inch LX600 ACF, *Lunt.
Amateur astronomer since 1970.
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Unitron48 United States of America
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Re: Correcting Coma in my F5 Fraunhofer Doublet.

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Post by Unitron48 »


Very informative report, John! Thanks for sharing!!

Dave
Unitron (60mm, 102mm), Brandon 94
Stellarvue SVX127D
http://www.unitronhistory.com

"Look deep into nature, and then you will understand everything better." Albert Einstein
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Makuser United States of America
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Re: Correcting Coma in my F5 Fraunhofer Doublet.

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Post by Makuser »


Hi John. I just found this link that you posted on another thread recently and as I have an Omni 150XLT f/5 Fraunhofer doublet refractor, I found this information most useful. Thanks John and the best of regards.
Marshall
Sky-Watcher 90mm f/13.8 Maksutov-Cassegrain on motorized Multimount
Orion Astroview 120ST f/5 Refractor on EQ3 mount
Celestron Comet Catcher 140mm f/3.64 Schmidt-Newtonian on alt-az mount
Celestron Omni XLT150R f/5 Refractor on CG4 mount with dual axis drives.
Orion 180mm f/15 Maksutov-Cassegrain on CG5-GT Goto mount.
Orion XT12i 12" f/4.9 Dobsonian Intelliscope.
Kamakura 7x35 Binoculars and Celestron SkyMaster 15x70 Binoculars. ZWO ASI 120MC camera.
>)))))*>
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