I have been attempting to build up a low profile diagonal for a scope which does not normally have enough back focus for a 2" diagonal. Here are some of my adventures that may help someone looking for a similar solution.
What makes this possible is the near-infinite number of adapter rings possible from places like Hutech/Borg, Alpineastro/Baader, and others. These allow you to assemble a custom diagonal just about any way you want it using standard threads, such as:
31mm, the standard thread for most 1.25" diags.
36.4mm Vixen and Takahashi standard, used by many others
43mm Vixen and Takashi standards
48mm filter thread
50.8mm 2" UNC standard SCT thread
57mm Borg standard
60mm Vixen standard
Careful attention to the pitch of these is important (0.75mm or 1.0mm), as well as whether each thread of the adapter is internal or external, as you peruse the literature.
I have taken several approaches:
1) Use a 1.25" diagonal with adaptors increasing its capacity to 2".
Borg sells a 1.25" prism diagonal set up with a 36.4mm inlet with locking ring, and 36.4mm outlet. The clear apertures are 30mm and 28mm respectively, which are typical for a standard size diagonal. Adapter rings and ocular holders can be combined with this standard thread to build up a 2" diagonal with close to the short standard path.
Alpineastro sells the Baader T2 prism diagonal using the 35mm Zeiss prism, based on standard T-thread interfaces (42mm with a 0.75mm pitch), infinitely configurable with various attachments. The clear aperture is 34mm on both input and output sides, a significant increase over a standard diagonal. Use of the mirror version with a 2" ocular holder demonstrated that the edge of field of the Nagler 31mm shows about 70% illumination, which is not obviously vignetted on terrestrial images.
Takahashi's LDP large prism diagonal, used in the Quintuple turret, but available separately.
At one time, Celestron sold Baader's 2" prism diagonal, but this is no longer produced by Baader. Kasai in Japan shows one on their price list.
AP sells a nice T-thread to 2" ocular holder with compression ring. Nice, but about 50mm tall.
As the rear adapter will be larger in diameter than the prism housing, you may have to worry about clearances with your focuser.
The final distance from the bottom to the top of the 2" ocular holder must be 37mm minimum to accept modern 2" oculars.
2) Use a prism diagonal, which moves the focus point about 1/3 of the prism path further out (due to the geometrical laws of optics).
The path of a converging beam (your telescope), through a prism of thickness = T and index = N, is increased D by:
D = T * (N - 1)/N
Since the glass used in a prism diagonal is typically BK7, which has an index N of about 1.5178, the path increase D is about 1/3 of the thickness.
Along with the path increase come disturbances of the color correction and spherical abberation. This may be used to advantage in the rare case where the color shift and spherical abberation compensate the objective correction allowing benefit from the adjustment. I found just such a situation at the same time I needed the backfocus increase, but my experience is that this is rare.
At F5, the spherical overcorrection is about 1/10 wave, calculated, but the star test results tend to agree. The color shift pulls the red end of the spectrum slightly closer to focus. These shifts would go unnoticed on a typical achromat, but a fine ED scope might be ruined by these changes.
Beyond F5 the changes decrease dramatically. By F10 they would certainly be in the noise. Takahashi and others routinely include prism diagonals with F6 and F8 scopes. So unless you have an F6 scope or less, don't worry about it.
3) Find the largest prism possible, to take adavantage of the largest clear aperture.
With the death of the Baader 2" Zenith prism (the mirror version is the famous Baader 2" Maxbright dielectric), options are limited. Bare 2" prisms are available from Edmund Scientific if you want to roll-your-own and try directly replacing a mirror with a prism in the original housing. For a more of an off-the-shelf solution, I found the Takahashi Large Prism Diagonal.
It contains an advertised 45mm prism, has a Takahashi standard 43mm/0.75mm adapter at the front end, but adapter removed, the threaded front face is stepped down to the 42mm T2 standard allowing much flexibility.
Note that even a 2" SCT mirror diagonal is stopped down to about 42mm at the front end, so we are almost in the 2" world with this prism.
The exit face is a male thread of about 37mm/0.75mm pitch that is not removable, but there is a way around this as we shall see. I used a standard 2" nosepiece to T-adapter for the front face (one could of course use a 1.25" nosepiece to T-adapter for smaller scopes). For the rear, I used Borg parts: a T-thread to 60mm ring, one or two 12mm long 60 to 57mm rings, and a 57mm to 2" ocular holder. The holder has two large steel thumbscrews with teflon tips to protect my oculars.
Finally, as the bonded-in exit thread of the LDP is not removable nor compatible with any adapter I can find (intended to mate with the Turret and the 32mm Erfle), the T thread adapter above slips down and around it, centered perfectly. The LDP visual coupling ring then threads down from above and captively locks it in place. Though this last adapter eats up about 10mm back focus, rememeber I gained about 15mm by using a prism in the first place.
So I ended up with a fine, essentially 2" prism diagonal, whose optical characteristics complemented my scope, with a low profile, and the extended back focus I was looking for.
Some of the Baader and Borg parts, as well as the Takahshi LDP are available from Anacortes. Other info sources:
follow the links to the components and the Baader drawing of all the adapters layed out.
Lots of details about path lengths of the adapters, etc.
The Borg 36.4mm diagonal.
Lots of photos of various adapters