What causes these aberrations? - Forums

Topic starter

Feb 20, 2023

I've been getting these asymmetric aberrations around bright stars on nearly all my images and struggling to find the cause.
They are pretty consistent irrespective of the target's position in the sky and filter type (present on each of L-Pro, L-Enhance, Antlia ALP-T, Askar D2).
I'm using Stellarvue SVX102T (0.994 Strehl) triplet with SFFR.74 flattener/reducer, 2" filters in EFW and ASI2600MC Pro camera.

I'm suspecting the flattener/reducer might be bad, but I'm quite new to astrophotography so it would be great to have some input from the more experienced folks here before I reach out to Stellarvue. The SFFR.74 should change the scope from 714mm f/7 to 535mm f/5.25 but plate solving says I'm around 518mm. I don't mind the halos, those are understood, but the asymmetry and purple rings seem like a coma problem.

I also have a SFFX1 pure flattener but have not tested if this aberration is also present with that, where I live I get literally just a few hours of clear skies a month for the last few months and don't have the flexibility to run a lot of tests.

The blown up images are 200-300%.

CS

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andrea tasselli

Frequent contributor

Feb 20, 2023

Hard to say but it might just be spherochromatism in action. I would test without reducer and report back. Try with and without the flattener. My bet is you won't find anything wrong, at least on axis.

Georg N. Nyman

Feb 20, 2023 · Edited: Feb 20, 2023

Looking at the shape and color of those aberrations, I can see that the longer wavelengths consistently are farer away from the center of the stars
than the shorter ones. This leads me to the conclusion that it is a chromatic aberration coming from the reducer. If it would come from the scope itself, it should be visible in the center of the stars in different color tints depending on the location in the image plane.
I would omit the reducer and take comparable images and come back with the results.
My experience with reducers was such, that I now use only the Riccardi reducer and Riccardi flattener for my refractor (Explore Scientific 127ED APO FCD) - they are horribly expensive ( for example the 1x one in Europe about 1000 Dollars) but they are excellent. Others I tried but returned to the vendor because of image quality problems.
That is my conclusion, I would be very much interested to see if I was right or wrong…

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Josh Jones

Feb 21, 2023

Does the reducer change backfocus on this setup? I would personally double check that is all in good order. And as other stated, try without reducer.

voigtm

Feb 21, 2023

Hi Maciej,
I am using the same reducer (the 0.74x for the SVX102T) on a SV105T upon the recommendation of Stellarvue. My stars look OK- I don't see the star 'halo' artifacts that you do. You can check my most recent Astrobin images to see what I'm getting.

I'm using 55mm of back focus with 2" Astronomik DeepSky filters in an EFW in front of a ZWO 294mm pro. My reduced focal length according to plate solving is 514mm- much shorter than the predicted 544mm (105 f/7, 0.74x). The next clear night I'm going to get some images using a full frame Canon without the reducer to verify the actual FL of the scope and will also try with both the reducer (to validate that it's giving a flat field) and a flattener that is supposed to work (SFF3).

You should try without the reducer and look at the stars in the center, but I would definitely contact Stellarvue and see what they have to say. They should provide help, since they really push these scopes (and accessories) as top notch.

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Maciej Karcz

Topic starter

Feb 21, 2023

Georg N. Nyman:
Looking at the shape and color of those aberrations, I can see that the longer wavelengths consistently are farer away from the center of the stars
than the shorter ones. This leads me to the conclusion that it is a chromatic aberration coming from the reducer. If it would come from the scope itself, it should be visible in the center of the stars in different color tints depending on the location in the image plane.
I would omit the reducer and take comparable images and come back with the results.
My experience with reducers was such, that I now use only the Riccardi reducer and Riccardi flattener for my refractor (Explore Scientific 127ED APO FCD) - they are horribly expensive ( for example the 1x one in Europe about 1000 Dollars) but they are excellent. Others I tried but returned to the vendor because of image quality problems.
That is my conclusion, I would be very much interested to see if I was right or wrong...

I always just assumed one was stuck with the flattener/reducer from the same manufacturer as the telescope. Thanks, I’ll check out the Riccardi F/Rs, this opens up bunch more possibilities.

Anything to know when trying to match a suitable F/R from another brand? Do they just need to be specified for a similar aperture/focal length?

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Georg N. Nyman

Feb 21, 2023

Maciej Karcz:
Georg N. Nyman:
Looking at the shape and color of those aberrations, I can see that the longer wavelengths consistently are farer away from the center of the stars
than the shorter ones. This leads me to the conclusion that it is a chromatic aberration coming from the reducer. If it would come from the scope itself, it should be visible in the center of the stars in different color tints depending on the location in the image plane.
I would omit the reducer and take comparable images and come back with the results.
My experience with reducers was such, that I now use only the Riccardi reducer and Riccardi flattener for my refractor (Explore Scientific 127ED APO FCD) - they are horribly expensive ( for example the 1x one in Europe about 1000 Dollars) but they are excellent. Others I tried but returned to the vendor because of image quality problems.
That is my conclusion, I would be very much interested to see if I was right or wrong...

I always just assumed one was stuck with the flattener/reducer from the same manufacturer as the telescope. Thanks, I’ll check out the Riccardi F/Rs, this opens up bunch more possibilities.

Anything to know when trying to match a suitable F/R from another brand? Do they just need to be specified for a similar aperture/focal length?

I was contacting my vendor in Germany for advise and we talked about other brands of reducers and flatteners. Out of that conversation came that I understood, that some other brands are very well suitable if the correction type fits. For my refractor and for several other brands those from Riccardi are a very good solution. The only drawback is their price - all Riccardi´s are very expensive, often two or three times the price of "standard" ones, but in my opinion, they are justified. The mechanical stability is outstanding and the optical performance excellent.

What kind/brand of refractor are you using?

And keep in mind that two/three wavelengths filters introduce some strange colors as wel - especially in the diffraction pattern...

You find more information about reducers and flatteners by Riccardi on their website http://www.teleskop-express.de

But please keep in mind, that is only my personal experience and point of view....

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