Hi Vikas. You are triggering my PTSD! I can help somewhat, although I went through so many iterations I may not have the exact answer and the scope is down in Chile so I cannot easily access it. So I apologize if there are some errors here.
First of all, I think I spot the error in the logic. I always calculated the MB distance from the back of the stock focuser. The annotation on your drawing has me questioning that. But, looking back at my calculations, I never subtracted the stock focuser from the 178. And there is no way that I would have made it to the focus point had I figured it that way.
Second, do not worry about focuser half travel/midpoint. You have a lot to play with there. The most you will need in the real world operating range is 5 or so mm. The movements are really tiny between focus points.
I will say, I am pretty sure that although PL says the adapter between the OTA and the ESATTO has the same depth as the stock focuser, I found that to be incorrect. There is a 5-10mm difference... shorter I believe and that was a curve ball in on of my setups. In the end it does not matter.
So, on the FSQ I use the PL4 inch with the requisite PL adapters on either end. I ran it for a bit without a reducer as you are contemplating. I choose to leave the manual rotator on board since the rig was in my backyard at the time.
From the back of the focuser with the adapter to get to M98
PL adapter M110/M98 5.5mm
Manual Rotator M98/M98 38.5mm
Tak 81S Aux Ring M98/M98 27.5mm
Tak CCA 250 coupling M98/M54f 32.2mm
Pegasus coupling M54m/M54m 2mm
Then the instrument pack. I use ZWO kit on that rig, my sensor+AFW+OAG+Focuser+tilt is 55mm. I was lazy and left the OAG in the mix. (That and it took me FOREVER to get to zero tilt and I didn't want to disturb the train) It also makes my instrument packs standardized so I can move them between the rigs. You can use a guide scope obviously and reclaim that space.
So in total 160.7. Add 1 for the filter and half focuser travel which is 17.5 and you come to 179.2. Pretty darn close to 178 and no problem pinching a bit from the focuser midpoint. The problem is not how little space you have, it is how much.
Now, someone else recently asked me for help with the 645 .72x Reducer spacing and the Esatto. That becomes a lot harder because your backfocus is reduced to 73.5mm (that is from the reducer). The reducer is attached to the back of the OTA and remains in a fixed position as the focuser and image train slide around it. I choose again to leave the manual rotator on. This setup took a bunch of work to figure out. With the E4 focuser + adapter + the manual rotator, the reducer still protrudes about 7mm out of the back end. This is where I figured the Esatto adapter from OTA to focuser is not the same as the stock setup. I didn't try it out before I took the stock focuser off, but I suspect that the reducer is flush with the stock focuser at midpoint which is not the case with the PL.
Either way I made it work. But I have a TINY amount of space with my 55mm instrument pack between the back of the reducer and the m98/m54 step-down. I have a custom adapter to fix that roughly 7mm protrusion issue, then the CCA250 stepdown. My focuser is cranked way in, but I reach focus just fine. And again, the required travel to make focus is tiny. Before people freak out, I did test to make sure I would not bottom out and damage the reducer or focuser. And the fix is to eliminate the OAG next time that I am down there which buys me more space. I intend make that change along with adding an ARCO rotator.
Current focus point with the reducer installed:
Rig currently as described withe the reducer:
BTW make sure you have a tiny allen wrench on hand to loosen the grub screw on the OTA which locks the threads on the stock focuser. Otherwise you can damage the OTA threads. I believe it is a .9mm size, but may be +- one size.
OH... and my other error... There are two different PL adapters from the OTA to the Esatto. One for the older FSQs and one for the new ones. Make sure you get the right one! I didn't.
I find that this website offers the widest array of PL gear with the most comprehensive info on MB distances of the kit. And they stock a lot more than others, I ordered quite a bit from there during the supply chain crunch and not bad on price even considering the shipping from Australia.
https://www.testar.com.au/collections/prima-luce-lab.CS and Good Luck!
