There are two ways to attach a Denk II to a T2 diagonal. The first is to have a custom made dovetail that attaches to the Denk, then use the Baader Quick Connector. I had such a dovetail made for my Binotron. Denkmeier made it for me but they no longer make this part. You could probably get the dimensions from a Mark V. That is what I did.
The second would be to buy a T2 adapter from RAF Camera. This would screw on to the bottom of the binoviewer and then thread to the top of the T2 diagonal).
Now, here is why I would not recommend it if you have a Denk II with power switch (which you did not say you had, but I am just making sure that I cover your question.
First, with the power switch, even with the T2 diagonal, it is doubtful that you will reach focus in straight through mode. The power switch and binoviewer have a light path that is in excess of 140mm and even with the short light path T2 prism, you are talking about 39mm of light path, so by the time you add the adapter, you are over 180mm, and even binoviewer ready refractors will struggle to accommodate that much inward travel.
Next, even if you are lucky enough to reach focus in straight through, you won't have enough travel to reach focus in low power mode. It is just impossible. The amount of inward focus travel is extreme. This means that you would be limited to two powers and again, that assumes you would be able to reach focus in straight through mode.
So, if you are using a power switch you won't reach focus in low power mode, and unless you have a binoviewer ready telescope, you might not reach focus in straight through mode.
In these cases, it just seems to me (but maybe not to you) that you would be better off simply using the OCS with a standard 2" diagonal. Yes, you throw away the ability to work at native focal length, but you gain the ability to have three useful powers.
If you are only working with the Denk II and no power switch, measure flange to focal plane distance carefully (distance from the back of the fully racked in focuser to the focal plane). Again, just putting your BV on a T2 diagonal does not guarantee that you will reach focus. The light path for the T2 prism is 38mm or 39mm (I forget) and the light path through the Denk II is as I recall, 116mm, but I would measure that too just to be sure, or search the forum for people that have measured it, or ask, but as I recall, it is 116mm. The light path for the RAF Camera T2 adapter is about 3mm, assuming that it screws all the way in.
So, the total would be about 158mm.
The easy way to estimate the flange to focal plane distance is to put a 2" diagonal into the focuser, and using an eyepiece that has the field stop of the eyepiece near the top of the eyepiece holder (junction of the eyepiece barrel and lens housing) and focus at infinity. Now, measure the amount of focuser tube that is exposed between the rear of the focuser and the front of the diagonal. Since most 2" diagonals have a light path of around 100mm (you can measure this too to be exact, but I have never seen one shorter than 100mm) you would have need to have 58mm of focuser tube exposed for the Denk II to just reach focus.
If my number for the light path of the Denk II is correct though, you now know how to determine if you can reach focus with a T2 diagonal and the RAF Connector, but no power switch.
Since I was never able to get the low power arm to work in any refractor configuration without the OCS, I had to make the decision as to whether to work at native focal length and change eyepieces, or to accept the 1.3x low power mode (using the Denk diagonal because standard diagonals only give 1.4x for low power). I chose to go native focal length (no power switch) an use zooms on smaller scopes that were intended on wide field, and use the OCS on larger scopes that were intended for smaller DSOs (which lets be fair, is mostly what we use larger scopes for) so I would have three useful powers with wide field at each setting.
So, I have shared how you can do it, but I have also cautioned you on things that you should consider before spending any money on this, and things you can measure to see what configuration options would be best for you. Until you know your flange to focal plane distance, you really don't know what options you have.
Edited by Eddgie, 10 October 2019 - 08:29 AM.