9 replies to this topic

[DVexile]

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?

[balcon3]

By flare do you mean a general brightening, or a specific intrusion of a point source of light into the field of view? The other night I saw a general brightening of the whole field in my Maxbright II when the full was just outside the field of view.  I’m not sure if this was flare or just that that area of sky was very bright because of the full moon.

[DVexile]

Sorry, good question, I should have been more specific about what I meant by "flare".

 

It isn't a point source, or reflected image.  It is more like a general brightening, but it is non-uniform.  It is very much like certain eyepieces that when you get a bright source just out of the field of view abruptly show a wash of light across the field of view.  Very common to see when hunting down a bright planet for example:

1. Far away the field is black/dark
2. Moving around you abruptly see edge brightening of the filed and a wash of light cover a good chunk of the field of view.
3. Move scope in that direction and often the field becomes black/dark again.
4. Move a bit further and the planet begins to appear in the field of view, nice and sharp with no flare.

I've got one or two eyepieces that do this (some of the ES82 series are sort of notorious for it).  In this case it appears to be the binotron doing it as it is independent of eyepiece and does it with eyepieces that do not normally have this behavior.  Many camera lenses can do the same.  It is really not that uncommon a thing and not necessarily a defect of the Binotron or any other binoviewer, but it was just really obvious in lunar observing.  Since lots of binoviewer enthusiasts are also big lunar observers I'm surprised I can't find any reports of it posted already, which was why I was asking for the experience of others.

 

I attempted to capture this with a phone camera handheld, so usual issues with that:

 

First, this my test source viewed through the eyepiece and Binotron.  It is a fair bit smaller than the moon actually.  With the source in the field of view it is sharp and there is really no flare/stray light issue of any kind.  There is of course a very slight brightening of the background just do to scatter in all the glass the light passes through, but overall a very high contrast and clear view.

 

 

Second, this is with the test source moved just outside the clear aperture of the Binotron, specifically it is outside the field of view on the right side of the image.  In this particular case the eyepiece used is the 24mm Panoptic, so its FoV is essentially identical to the clear aperture of the Binotron making it easy to tell where things lie.  Note the exposure is much higher in this shot to show the flare clearly compared to the previous image.  Naturally the flare is always dimmer than the source!  But it is visually obvious and dramatically reduces contrast in real observing of the moon when say the limb or terminator is at the center of the field while the rest of the illuminated moon crosses outside the clear aperture.  If one swaps in a different shorter focal length eyepiece at this point the flare remains.  It is very challenging to position the phone in the exit pupil in this particular case and the phone doesn't really want to focus, so this isn't 100% representative of what the eye sees (e.g. the eye when properly centered in the exit pupil doesn't see that reflected image of the source seen on the left edge of the frame image).

 

 

Finally, if when the flare appears in an eyepiece if I remove the eyepiece I can clearly see flaring occurring in the prism optical path near its edges:

 

 

When the source is either inside the clear aperture those flares on the right side of the prisms disappear.  Similarly, one far enough outside the clear aperture they disappear.  But in just the "right" place they are obvious.  And the moon is large enough that it can excite this flare over a fairly wide range of angles.

 

Also, I verified the exact same behavior with Jupiter, but of course Jupiter being small easily fits entirely within the clear aperture and so when actually pointed at Jupiter there is no issue.  But if I slew Jupiter outside the field of view eventually it too crosses the range of angles that provoke this and flare appears in the eyepiece.

 

Hope that's more clear!  Thanks so much for taking the time to post and ask an important clarifying question.

 

Did your MaxBright exhibit this kind of non-uniform flare?  Or did it just look like a uniform bright haze like from clouds or dew?

[balcon3]

I don't recall seeing anything like that in my Maxbright II, but I'll look specifically next time I am out. What I see is a general brightening when I get close to the moon or planet, and not different from what I see with some eyepieces in mono mode when I get close to a bright object. In fact sometimes I use the brightening to figure out which direction to move to. It doesn't interfere with viewing any objects in my opinion, including the moon at higher powers. 

[betacygni]

Every binoviewer I’ve ever used have glare/flaring next to an extremely bright object like the moon at a certain point as you near the object. Some binoviewers do this more or less than others, but as long as they perform well otherwise I’d consider it quite normal.

Edited by betacygni, 18 January 2025 - 07:38 PM.

[DVexile]

Every binoviewer I’ve ever used have glare/flaring next to an extremely bright object like the moon at a certain point as you near the object. Some binoviewers do this more or less than others, but as long as they perform well otherwise I’d consider it quite normal.

Thanks!  That was the conclusion I was trying to come to, because having played with the Binotron a fair bit in controlled testing it sure seems like it does everything it should and that it would be hard for any design to completely eliminate such off axis flare everywhere.

 

For what it's worth, I also tentatively believe that the higher magnification setting of the PowerSwitch may have a tendency to flare a bit more since it results in more diverging marginal rays entering the prisms.  Conversely, the lowest magnification setting (which is actually a converging reducer) appears to have a fair bit less.  I'm not a particularly deft hand at optics, so my thinking may be entirely erroneous!

 

Anyway, with my experiments I think I have a good understanding of when it occurs and thus can better plan to work around it.  I might end up with a slightly different configuration tailored for gibbous viewing that makes some compromises to eliminate the flare.  For everything else I'd ever look at this seems entirely a non-issue.

[betacygni]

I don’t do a lot of lunar observing so haven’t tested it much, but yes anecdotally I’ve also noticed different amplifiers, etc, can vary how much it happens. I suspect the issue is simply off axis light bouncing off various internal elements, so would make sense to me diverging light could make this a bit worse.

[Eddgie]

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.

[Eddgie]

Oh, and an important note with respect to the above. I did have this issue with another reflector in spite of the fact that I had collimated the focuser. In this case though, the weight of the binoviewer was causing the focuser tube to pull away from the eyepiece side bearings and once again, introducing tilt. The focuser was a Moonlight focuser and it was incapable of supporting the heavy weight of the binoviewer.  This was not a holding power issues, it was caused by the deflection of the focuser tube and the pinion shaft, making the focuser tube loose contact with the outer bearing pair, and causing the pinion support to deflect.  There was no fix for this. I replaced the focuser with Feathertouch, which has a much more advanced bearing and track design, and this eliminated the problem.

 

So, start with the sight tube to see if the secondary is not offset, but also make sure that if you are using a Crayford, that the focuser tube is not deflecting away from the bearings.  Again, there is no real fix for this. You can tighten the friction pad that supports the pinion shaft, but to get enough force on it to keep it from sagging, the slow speed knob might slip. For a reflector, asking Moonlight for a "Refractor" preload spring can help, but the focus will feel spongy and imprecise when you have the pinion support cranked down hard. 

Edited by Eddgie, 23 January 2025 - 12:35 PM.

[DVexile]

Thanks so much for taking the time to reply!  A bunch of your past posts on the forum have been super useful as I figure out the innards of the BT and how best to use it.  So I'm not surprised you've got some very apropos experience similar to mine, especially as you have also used a FlexTube with the Binotron.
 

It is almost certainly not the B-27 but I can tell you what it probably is.

Well, it happens in both my AT102EDL (with or without a diagonal) and in my FlexTube 250p.  Both flared badly during the Mars occultation.  It was just that FlexTube 250p would do it at both the medium and high magnification PowerSwitch settings, while the AT102EDL only had a problem at the high magnification setting.  

 

I've so far been attributing the difference to their different focal lengths (1200mm vs 700mm).  The medium power setting the FlexTube is enough to get the opposite limb of the full moon to cross the part of the field that causes the flare, but the AT102EDL needed the high magnification setting to get to that point.  And that theory matches the math for the 0.5 degree Moon and the and its size at the field stop between the two scopes.

 

BUT...

 

The culprit is most likely tilt in the focuser and/or a slightly off center secondary mirror.

I'd put really good odds on you being correct that this is very likely also happening in my 250p.  The 250p has definitely been more fussy with flare than the refractor in real lunar observing.  I was chalking that up to the increased focal length/magnification of the 250p.  But there's a good chance that's only a partial explanation, and the secondary/focuser issue you are describing is also at play creating additional problems and flare.  That would probably go a good way to explaining issues I've sometimes attributed to local lighting and such as an excuse.

 

All my controlled indoor testing has been with the AT102EDL so far.  So I've got way more stick time with the AT102EDL at this point and it is very likely I don't understand what all is going on with the 250p and the Binotron.

 

Thanks a bunch for taking the time to describe the problem so thoroughly.  Even if it turns out it isn't an issue at the moment, I do plan to change out the focuser and experiment with a variety of other aspects of the 250p and the Binotron, so there's a good chance I'd introduce the problem at a later date by accident!  Now that I know what to watch out for you will have likely saved me untold hours!

 

Cheers!