I may pass you my experience with a Sky90-II; not the same OTA, but same issue.
Once I upgraded the 2.5" original focuser to the FSQ-85 3", basically giving birth to a novel Sky90-III, I was also faced with a short backfocus concern, despite having a SV version. This because, obviously, the FSQ-85 focuser is larger/longer than the original Sky-90 one (see pix).
No problem, of course, with mono-view, or with a bino using the Ext-Q 1.6x or any OCS. But, using the bare (no-OCS) Mark-V, with the short Baader/Zeiss T-2 prism, for lowest possible magnification, I was just a tiny hair short of focus (we are talking less than a millimiter... but no focus is no focus...). Since this is the SV version, I didn't want to cut the OTA, which is perfectly tailored for the 95mm Tak clamshell (again see pix) and, after having tried the shortest adaptors on the market, decided to simply contact a machinist and have a tailor-made adapter ring which, after the removal of the original TKP37001 coupling, would screw into the focuser drawtube and, on the other side, has a mere T2 male so as to screw into the Baader/Zeiss prism. This gave plenty of focus to use, without any focus issue whatsoever and w/o OCS, TV Plossls 40, Zeiss Abbe 34, TV Panoptic 19, etc. Given that the FSQ-85 OTA is already obviously sized for its focuser size, such a tailor-made adapter should address any of your concern. You may see it in a separated slot in the middle left of the carrying case.
Hi Max, this is very helpful, thank you. Finally some concrete data points on this scope working with bino viewing. I'm a bit confused though about some of the parts involved in your process. If you remove the original Coupling (TKP37001), then does your custom part screw directly into the drawtube, and then your diagonal goes into the custom part? If so, then this sounds nearly identical to what Gavster posted earlier above, in post #6, only instead of a custom part, he used a Baader Ultra Short Clamp, although I am not sure if he removed the Coupling too. What exactly does the Coupling accomplish?
This being said -- and given that we are talking *your* telescope, with *your* money and, therefore, you are of course master of doing whatever you like --, and with all due respect to the excellent FSQ-85 optics, if your aim is planetary and lunar high mag observation AND you are not a frequent flyer with your equipment (...and you do not want to dismantle your equipment into parts/components...), ANY of the FC100 series (just to remain in the Takahashi realm) would give you *visibly* higher performances and satisfaction. Mainly the DZ, of course, but even simply the DF.
A "Flatfield Petzval Quadruplet" is clearly designed with astrophotopgraphy in mind. Telescopes tailored to your interests are usually called "superplanetary".
Hope this helps.
So I just had an FC100-DZ at my house for consideration. I ordered a TV-85 and an FC100-DZ together, to compare the two. While the FC100 did have amazing optics, it was just too long and big for my needs, so the TV-85 won. While I don't travel often, I just don't like big scopes very much, no matter how great their optics. 85 and 92mm scopes are my sweet spot, and it is the only aperature class I am interested in. I'm also looking for a "forever scope" in this class, and so my only two considerations right now are the TV-85 and the FSQ-85. Between these two scopes, I just need to know which one is the best, and then I can keep one, sell the other, and live in peace without every having to worry about telescopes again. I'm not like most other astronomers, I don't have the telescope bug, I'm happy to have just one scope and use it regularly and spend more money on accessories. I've had only one scope, a SV NHNG for 14 years, and it's finally time to replace it, with a scope which I will keep until I die.
All I need is a higher power to tell me definitively if that's the TV-85 or the FSQ-85. Sadly, there does not appear to be such a higher power, so am just going to order an FSQ-85 shortly and compare it with the TV-85 in person.
That being said, from what you have posted, it seems that an FSQ-85 should be able to come to focus with a bino viewer both with and without an OCS? If true, that's a great advantage over the TV-85, as it would allow bino viewing planets at high mag with OCS, but also enjoying the flat, wide starfields in bino without the extra magnification from the OCS. The TV-85 can only come to focus with an OCS in bino viewing.
One question though about your bino viewing with the FSQ-85 focuser: I know the FSQ-85 focuser only has 50mm of focus travel, as in, the drawtube only extends out a maximum of 50mm. This is really short by visual standards, as the TV-85 extends out 70mm. Did you find this was a problem in reaching focus with a binoviewer ever?
Edited by trias702, 22 May 2020 - 10:03 AM.