Enjoying my Binotron 27, but the recent Mars occultation makes it clear it can flare significantly when a bright source is just on the edge/outside the clear aperture of the prisms. This is rarely a practical issue, except when viewing the limb of the full moon with highest power switch settings or a long focal length scope at the middle or highest setting.
I haven’t seen any posts about this, which makes me wonder is it something just with my particular Binotron? I’ve done a variety of tests and my Binotron seems free of any defects, so I suspect this is just the nature of it (and probably most any binoviewer really - I assume they all can flare at some angle).
Does anyone else encounter flare when a bright source is just outside the edge of the prisms in their Binotron?
It is almost certainly not the B-27 but I can tell you what it probably is.
The culprit is most likely tilt in the focuser and/or a slightly off center secondary mirror.
Here is the test for this. Mount the B-27 and with no eyepieces, look through the B-27 and see if the secondary mirror is exactly centered. My guess is that it will not be. Used this way, the binoviewer will act as a very long sight tube, and any tilt that is too small to be seen when using a typical eyepiece will be revealed. When you collimate using a laser, frequently, you will actually move the secondary mirror very slightly off of center, but generally it is close enough that in visual glance using your eye at a Cheshire, you might not see this. When you use a very long site tube though, this small difference can be revealed to you. If the secondary is slightly off center, then this means the rays are entering at a slight angle, and they can reflect off of the inside of the prism walls.
If it is a refractor, use a laser inserted into the eyepiece holder to check that your focuser is in perfect alignment to the optical axis.
Now it is possible that is off center in the OTA, but the more likely issue is that the focuser is not collimated to the optical axis.
I fought this problem for six months in my 12" dob and repeated collimation did nothing to improve it. One day, I sat down with a ruler and perfectly centered my secondary mirror both laterally, and with respect to the centerline of the focuser. It looked fine in the Cheshire, but just by chance, when I had the BV on it, I saw that the mirror was not exactly centered in the BV aperture. In fact, it was not even fully contained in the aperture, with part of it extending beyond the edge of the aperture.
I collimated the focuser and re-collimated the telescopes, and the internal reflections went away.
Sadly, not all focusers are collimateable, and on my 10" dob, I had to 3D print some shims to tame the tilt. It was necessary because of the same problem and the 3D printed shims fixed it.
I can't guarantee this is your problem, but I can guarantee it was my problem and I am sharing with you because I think it is your problem too. Maybe not, but I have a serious doubt that it is the B-27.
This condition will also show up in other binovewers as well so it is not a unique behavior of the B-27. If your binoviewer is tilted with respect to the optical axis, you are very likely to get internal reflections, regardless of model.
Edited by Eddgie, 23 January 2025 - 12:35 PM.