This discussion seems to have moved into a confusing area and I fear that some people who don't own this scope might get the wrong idea about these focusing issues. All I am going to write, below, applies only to the older version of this telescope which has a very slightly longer tube than the newer version. From what I understand, there are no focus/backfocus issues with the newer version of this scope.
When using any 2" eyepieces in a 2" diagonal, I have never encountered a problem. That doesn't mean that there isn't a 2" eyepiece out there that requires so much backfocus that it won't come to focus in this scope but it would require a lot of backfocus before there is a problem focusing and I have not come across that problem yet.
Using my Baader 1.25"/2" eyepieces configured as 2" eyepieces, you save upwards of 27mm in optical path length because more of the body then slips down inside the diagonal and there is no 2" to 1.25" adapter to add more optical path length. In short, none of my Baader eyepieces have any problems coming to focus when used in 2" mode with a 2" diagonal. It is not even close.
On the other hand, when using a 2" diagonal with a 2" to 1.25" adapter inserted, I did find I had problems with a few of my 1.25" eyepieces coming to focus. The problem eyepieces were the GSO Super Plossl 20mm, my Baader 36mm (in 1.25" configuration only) and my Baader 24mm (in 1.25" configuration only) was too close for comfort. Since we're using a 2" diagonal, it makes little sense to use the Baader eyepieces in 1.25" configuration when we're able to easily use them as 2" eyepieces and resolve the problem completely.
In fairness, I should also list the eyepieces that did work. The entire Orion Expanse lineup worked fine. My GSO Super Plossl 40mm and 32mm worked just fine although the 40mm vignettes but that is a different issue and completely expected. My Baader Morpheus 6.5mm, 9mm and 12.5mm focused just fine as 1.25" eyepieces. My Baader Hyperion 8mm and 17mm focused just fine as 1.25" eyepieces. I have a Celestron 25mm that came with a Celestron scope which, I think, is probably a Starguider rebrand and that worked just fine. Again, the only Baaders that I had a problem with were the 24mm and 36mm in 1.25" configuration only.
There are three things to point out about optical path length.
1. Different diagonals have different length optical paths. This can vary by a few mm's and upwards of 10mm or so when compared to prisms. This is a relatively fast scope, though, so I would not recommend a prism diagonal even though they have a shorter optical path. I have found that I see quite a bit of CA in this scope when using a prism diagonal. Some prisms probably work better than others but the general advice found online is to avoid prisms in fast scopes. (Good 2" prisms cost more than the cost of this entire scope anyway.) For my focusing tests today, I used a 2" High Point 99% Dielectric diagonal.
2. Purchasing a recessed 2" to 1.25" adapter will save you a few mm's for most 1.25" eyepieces. (See my photo, below.) I use one of these but I should point out that large eyepieces like the Baaders are not narrow enough to sit down inside this adapter so it still sits on top of this whole adapter. For most 1.25" eyepiece, though, this recessed adapter will save you a few mm's in optical path length.
3. Adding a compression ring to this ED72 focuser will eat up some of your valuable optical path length. My guess is that the one compression ring that I know is available for the SW 72ED eats up about 6mm of optical path. The rotator version will eat up even far more optical path than the compression ring. My advice is don't use these "upgrade" accessories with the older, longer version of this optical tube because they will cause bigger focus issues especially when using a 2" diagonal.
So, to sum up, you should not have any problems with any 2" eyepieces in a 2" diagonal on this scope if you don't add accessories that cut down on your optical path length. Also, focusing problems are minimal even when using 1.25" eyepieces with a 2" diagonal as long as you don't add accessories such as compression rings or rotators. Swapping out a standard 2" to 1.25" adapter for a recessed one will save you a few mm's too and will definitely help in most cases. Purchasing one of these is a very small price to pay to gain a few valuable mm's (some of my diagonals and Barlows came with these recessed adapters so I didn't need to purchase one). Understand that if you do add accessories to the focuser tube, such as compression rings or rotators, you will have even more problems when using a 2" diagonal. One last point is if you only plan to use 1.25" eyepieces, then it would make sense to use a 1.25" diagonal to avoid all potential problems.
Solar imaging is a bigger problem but really was not the topic in this thread but maybe I should quickly mention the issue. A 2" solar wedge has an optical path of 114mm so it is considerably longer than the standard nighttime 2" diagonal. I have found that I cannot get any of my 1.25" eyepieces to come to focus when using a 2" solar wedge. So, for solar imaging with this scope, I'd recommend the 1.25" solar wedge.
I hope this helps to clear up some confusion about the older version of this telescope.
Patrick
Edited by PatrickVt, 02 September 2020 - 09:56 AM.