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So I was doing some research and realized I was doing my back focus wrong. I'll have to buy a new less profile t-ring adaptor. But I was wondering, what is the difference in coma for a Coma Corrector(Paracorr Type-2)? Like it says the optimal back focus is 55mm, but I cant go lower than 63mm. Will there be a big difference between that and 55mm? Any photo examples? I wont have the money to go and buy a new one for a while but want to go out... |
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In my experience, the Paracorr is pretty flexible and not too fussy about sub-mm spacing accuracy. That being said, 8mm is a lot of difference on back focus distance. I have never tested anywhere close to that configuration. I have only gone by nominal length and tried to match 55-56mm requirement. It will definitely not be ideal but whether it works at all depends on all the detail, especially sensor size. I use my Paracorr Type 2 with an APS-C sensor and coma is corrected all the way to the corners of the sensor. It's possible the for a smaller sensor, an error in back focus like you are describing may lead to bloated (spherical aberration) but still somewhat round stars. If you have the gear already, you may as well try it. I do have to ask though, why do you end up with a 63mm imaging train? Is there a way to reduce that length? Luca |
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Well with the t adaptor I'm using 63mm is just the smallest distance I can get my camera sensor to the Paracorr. I'm using a Sony A7III as a camera. Its a great camera but its sensor depth is 19mm, add on to the t ring its too much... It seams I'm going to have to save up for a t-minus ring since the standard t-ring is to large... Thankfully I just got a job, XD |
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The corrector distance has to be correct for about a tenth of a millimeter for a fast system in addition a filter correction is also neccessary when used. In this case you have to add the third part of the filter thickness. 8mm is in no way acceptable. You will not be happy with this very large difference. Cheers, Christoph |
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Christoph Lichtblau: Thanks, thats good to know. I'm saving now for a t-minus ring. With that I can space it correct... |
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I use a Paracorr type 2 with my ONTC 10" native f4 and a APS-C mono sensor with filters with about 3mm thickness. I've just been through a test series as my first approach aiming for 55mm gave me slight coma in the corners (I'm very picky about star shapes). I tested 55, 55.5, 56.5 and 57 mm. I got perfect results with 56.5 mm, although 57mm wasn't bad either. My recommendation is to aim for 56.5 +/- 0.5 mm CS Robert |
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Robert Shepherd: Vary interesting... Cant wait wait till I get my t-minus ring to test mine out. I'm using an Orion 10" f/3.9 Newtonian Astrograph and a Full Frame Sony A7III camera. |
