I had been wanting to try out my MBII binoviewer with my Lunt 50THa and had been aware of the various challenges associated with coming to a focus with the right magnification with that setup as this particular solar telescope has a reputation of not playing nice with binoviewers. My Lunt 50THa has a Feathertouch focuser, not sure if its a requirement to hold the weight of a binoviewer and eyepieces, but I cannot imagine the stock helical focuser being up to the task. I have never tried the stock focuser (I still have it) with a binoviewer and don't want to, but maybe it would work. I first considered trying a linear binoviewer and I almost bought the one Lunt sells but I have read enough cautionary tales about their use and optics that I was wary and since I already had an awesome MBII my plan was to make it work instead. Upon reading about others' experience with the Harry Siebert OCA, a low 1.2X power focus adapter a la Baader's own GPC, I decided to embark on that journey. For a frame of reference I had normally observed with my Lunt 50THa in cyclops mode usually with a Meade 6.5mm HD60 so before I did anything and while I was still contemplating the linear binoviewer I found a used second 6.5mm Meade HD60 thinking that that would be great.
After speaking with Harry Siebert on the phone a couple of times to pick his very helpful brain I ordered the 1.2X Universal OCA for H-Alpha scopes with 1.25" diagonals. His website was just a bit difficult for me to navigate, as others have similarly commented, but speaking to him made it perfectly clear what I had to order and within a few days it arrived. What arrived were two optical units, one to be inserted into the Lunt's diagonal nosepiece and the other into the nosepiece of the MBII which goes into the 1.25" eyepiece holder. The two optical units are different from one another (they look the same but must have different optics) and each one goes in its own place with a specific orientation which is marked on them.
The picture on the left shows one of the optics screwed into the end of the binoviewer's nosepiece and the one on the right shows the other optic screwed into the TOP of the diagonal's nosepiece (not the end). I bought an extra Lunt nosepiece from Lunt and just leave it assembled like this for binoviewing and use the original for cyclops mode instead of inserting and removing the optic. Yes it screws flush back onto the diagonal as there is plenty of space between that blue lens in the telescope end of the diagonal and the Siebert OCA optic.
This is a Baader 1.25" eyepiece holder which I have replaced the stock Lunt eyepiece holder with. It has T2 threads on the bottom and screws right on down on the T2 threads that are exposed when the stock Lunt eyepiece holder is removed. It is awesome and to hold a binoviewer I would say its desirable because it holds way better as you would expect given its a Clicklock. It is also deep enough to accommodate the MBII nosepiece with the Siebert optic in it. I removed the O ring from that optic and screwed in all the way to the now vacated space in the threads where the O ring was (it won't go farther and it protrudes slightly, more on that later) but when screwed in like that it fully inserts into the Baader eyepiece holder.
Now the eyepieces! Had to try the pair of 6.5mm Meade HD60's but they were obviously way too much power so I sold the used one I had found. I have some Morpheus pairs on hand for use with the MBII in other telescopes so I then tried a pair of Morpheus 9mm eyepieces which were still a little too much power (a dim soft focus) and it was hard to find and fuse the binocular image (maybe a larger than B600 blocking filter would have helped the fusion but it was still too much mag). I then tried a pair of 12.5mm Morpheus and they were OK but it was all kind of too big and heavy for the system and I wanted to try for a little more magnification via smaller size. I then ordered a second Lunt Zoom figuring that I would use it with the one I already had to dial in just the right power for the viewing conditions, and while that worked it was way too fiddly for me. I like zooms but two at once requiring coordination is not for me, but I imagine for others that would be OK and anyone who is a bilateral zoom fan would be just fine with a pair of Lunt zooms. I then ordered a pair of Svbony 10mm SV190 UFFs and they did not play nice at all, didn't focus sharply with too small a sweet spot where fusion could be had, and they are now for sale (nice eyepieces just not for this). I then finally ordered a pair of Lunt 12mm eyepieces and am happy to say that the Lunt 50THa MBII binoviewing experience is now a success! They are easy to fuse given their focal length and 60 degree FOV and provide a nice sharp evenly illuminated (big full sun sweetspot) image that's a pleasure to observe. The Siebert OCA's facilitate a nice easily obtainable focus within the focuser's limited range (the Feathertouch focuser for the Lunt is way more robust that the stock helical one but still its range is not great) and the image size it provides is similar to what the zoom delivers when it is set to slightly less than 10mm on its scale, probably 9.5mm equivalent if it were on the scale and probably close to if not the the maximum power I would want to use with a binoviewer in this telescope.
Few miscellaneous things...
I tried my LS50C double stack unit and whiile it fuses and is sharp the view is pretty dim and the sweet spot small. It has been said (by Eddgie I think) that binoviewing with an HA is like a stack-and-a-half and I would agree that is very accurate and I plan on using the double stack in cyclops only.
The Siebert optics each have a rubber O ring so that it can be more easily gripped I suppose and / or so that they cannot be threaded in so far that they would then be difficult to retrieve. I settled on using my MBII instead of my Televue Binovue because I like it better, but while experimenting I discovered that the MBII system comes to focus with the Siebert optic in the TOP of the Lunt diagonal nosepiece while the Binovue comes into focus with the Siebert optic in END of the Lunt Diagonal nosepiece. Upon speaking to Harry Siebert (again, and he is a pleasure to talk to and would say its necessary to do so in order to figure it all out) I learned that the OCA's power and focus ability is dependent upon the distance between the two Siebert optics. Given that I felt it was important to be able to thread the Siebert optic residing in the Lunt nosepiece into any position within the nosepiece depending upon circumstances I asked for a modified Lunt nosepiece optic fully threaded and without the O ring because the O ring sits in a cutout within the threads and removing the O ring still does not allow the optic to be threaded into the nosepiece all the way. I also had him put two small cutouts on the rim of the nosepiece optic to that it could be gripped there and threaded all the way inside the nosepiece and placed in any position inside it. The same logic could be applied to the Siebert optic in the MBII's nosepiece but since the system works fine for me and focuses with the appropriate magnification with that optic threaded all the way up to its O ring channel I did not want to bother Harry with modifying that one but others way want both optics fully threaded with the rim grips and no O rings for full control over placement of both optics. Having the ability to place the two optics at whatever distance from eachother is necessary might be important to some depending upon their specific binoviewer and eyepieces.
Here is the MBII on the telescope with the 12mm Lunt pair mounted on a Vixen Porta Mount II.
And here is a link to the 1.2X OCA page at Siebert Optics. I found it hard to follow especially before I knew anything about this stuff and calling Harry helped me a lot.
https://www.sieberto...cs-H alpha.html
Edited by swsantos, 13 November 2023 - 09:44 PM.
