Bahtinov pattern issue with narrowband filter

Equipment Auto-Focusing Troubleshooting
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André Moleiro avatar
André Moleiro avatar
Hi everyone!I recently acquired a Radian Ultra Quad narrowband filter, and I'm finding it somewhat hard to get the right focus with it. Im using a RC6 and I never had problems finding the right focus with the Bahtinov mask, but now, when I'm trying to get the lines to align, they seem to be duplicated, which makes it much harder:https://ibb.co/hRktJYqIn this example I was using Alpha Centauri as my star. I eventually lost the data collected that night due to poor focus.

I have been using this RC6 for about six months now, never had this problem before.
 Any idea on how to get around this?
Thanks!
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jewzaam avatar
jewzaam avatar
When you take a picture do you see any halos around stars?  I wonder if the duplication is reflections somewhere in the imaging train.  I haven't seen this before though.  Have you tried focusing on a dimmer star?
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Rick Veregin avatar
Rick Veregin avatar
I haven't had this issue myself, but I have read of others with this problem. You seem to be getting interference fringes over much of the image, and seems like the separation of the rings is matching the separation of the repeat in the Bahtinov mask. This might be due to interference between your sensor and the NB filter. You don't give details of your setup, but if you have a field flattener or another filter UV/IR in line as well that could be the interference with the NB filter.  You may have a tilt between the filter and sensor–or something in your image train is not properly colliminated. Make sure everything is collimated and that you sensor is not tilted without the filter (focus in the centre, then see if all the edges are also in focus. If not there is a tilt of your camera sensor that needs to be removed, some cameras have adjustment screws for this.  Then put the filter in and see if the problem is better once everything is aligned.  Also, if possible, you might try moving the filter to a different location in your image train. 
Rick
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Björn Arnold avatar
Björn Arnold avatar
Hi André,

Have you turned the mask by 90 degrees and checked if the pattern changes? My bet would be reflection in the imaging train.
However, on my RC8 I once had a situation where the image looked quite similar since it wasn't cooled down sufficiently. By turning the mask, it turned out that the shift was along the Alt-direction.

CS,
Björn
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