Parfocality issues with LRGB set ?

Filters Focuser Acquisition LRGB Deep Sky
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Andreas Zeinert avatar
Andreas Zeinert avatar
Hello,
I recently bought an LRGB Pro set from Antlia, 1.25" mounted in a ZWO wheel on a new APO 100mm f/5.8 refractor. When making focus on the Luminance filter every thing was fine with pinpoint stars. I then launched a test series of LRGB images and two hours later I had a big suprise : the green and blue images were slightly out of focus and the red ones even strongly (you start to see "donuts" on the star disks…). This is not a question about temperature or problems with a loose focuser train as the L after the RGB were normal again and there is a brutal defocusing observed from the last L to the first R in the series. It looks like these filters are not parfocal which would be the first ever I used (I had Astronomik, Astrodon, Optolong). They are supposed to be parfocal even if we know that some times a very slight adjustment might be necessary. Does anyone here has faced a similar problem ?

Thank you very much in advance,

Andreas
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jewzaam avatar
jewzaam avatar
If they are supposed to be parfocal then I'd contact the seller about it as an issue. If not parfocal then handle with filter offsets so you only need to focus with one of them.
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Andreas Zeinert avatar
Andreas Zeinert avatar
Thank you very much Jewzaam.
In the meantime I could carefully check this issue with automatic focus routines under SGP and the conclusion is : B and G have the same focus position, L has a quite different one and R is very different. And yes, they are supposed to be parfocal…So there is an issue and Antlia unfortunately did not reply after four days. I am bit disapointed as it will complicate acquisition. A part from that the quality of these filters concerning halos is very good.
Cheers,
Andreas
Reg Pratt avatar
Reg Pratt avatar
Edit: Never mind, missed that filter offsets were already mentioned.
Jonny Bravo avatar
Jonny Bravo avatar
The problem you're facing is one of optical design of your scope and the glass between where the light enters the system and where it strikes your sensor. The light is refracted differently based on wavelength, and as such will come to focus at different points.
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Mau_Bard avatar
Mau_Bard avatar
I might be wrong, but parfocality and filter offset are two different concepts.
In my understanding two filters are parfocal when they have the same optical thickness, therefore the optical train length does not change when exchanging the filters.
The case here seems to refer instead, as already remarked, to filter offsets, that are produced by the fact that different optical colors may focus in slightly different positions. This is easier to manage programming focuser offsets. With parfocal filters one can get offsets anyway, that are not due to a filter defect.
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jewzaam avatar
jewzaam avatar
If you do not have parfocal filters you need to focus for each filter.  If they are parfocal you'd need to focus if the focal point of the optics were not consistent across filters.  It's stated the scope is an APO so it should not suffer from chromatic aberration that would lead to different focal planes for different wavelengths of light.

parfocal: of or relating to different eyepieces (of telescopes or microscopes) that all focus their images in the same plane, so that they can be interchanged without readjusting the instrument.
https://www.dictionary.com/browse/parfocal
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Bob Lockwood avatar
Bob Lockwood avatar
If you do not have parfocal filters you need to focus for each filter.  If they are parfocal you'd need to focus if the focal point of the optics were not consistent across filters.  It's stated the scope is an APO so it should not suffer from chromatic aberration that would lead to different focal planes for different wavelengths of light.

Just because it says it's an APO doesn't mean it's color correct. Not all APO's are created equal, Glass, coatings, all make a difference.
Maybe the scope is the issue, not the filters.
Jonny Bravo avatar
Jonny Bravo avatar
If you do not have parfocal filters you need to focus for each filter.  If they are parfocal you'd need to focus if the focal point of the optics were not consistent across filters.  It's stated the scope is an APO so it should not suffer from chromatic aberration that would lead to different focal planes for different wavelengths of light.

parfocal: of or relating to different eyepieces (of telescopes or microscopes) that all focus their images in the same plane, so that they can be interchanged without readjusting the instrument.
https://www.dictionary.com/browse/parfocal

Notice the definition you quoted. That would define parfocal to mean if he had camera X on the scope and swapped it out for camera Y, where both sensors were at the same position, he would not have to refocus.

No glass can perfectly correct the entire visible spectrum. Some glass does better than others. Some scopes are manufactured to higher and tighter tolerances than others. Is there a reducer and/or flattener also in that light path? Plenty of things like seeing changes and temperature changes can have an impact. 

My own Chroma filters are parfocal. They most certainly do not have the same focuser position on either my GT81 or my EdgeHD. Looking at some recent AF runs for LRGB data I'm collecting on M81 with the Edge and 0.7x reducer using the ZWO EAF...

L: 49580
R: 49610
G: 49509
B: 49537
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Andreas Zeinert avatar
Andreas Zeinert avatar
Dear All, thank you very much for your inputs. The scope is an 4 inch Imaging Star TS100Q refractor with a twin dual element system, quite similar to the Petzval design of the FSQ106. I agree that no optical system is perfect but if the scope were not apochromatic I would not get pinsharp stars with the luminance filter which lets pass all the wavelengths. All the more because in Antlia filters the Luminance spectrum is even broader including the violet and yellow part of the spectrum which is intentionnally not covered by the RGB set (to reduce halos and light pollution).
The system is at f/5.8, giving a CFZ of about 0.04 mm. Antlia claims +/-0.05 mm tolerance of filter thickness leading to .33*0.05mm = 0.0165 mm of optical path difference. Assuming + for one and - for another one you can double it but I expected to be in the tolerance range. I will work with filter offsets.
@ Jonny Bravo : thank you for your values. Mine are L = 29437, G=B= 29474 and R = 29528. I have to check what this means in µm on my setup but the results is when you are focusing on L, B and G turn a bit soft and R is out of focus.
All the best Andreas
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