thanks Barry/Dunk.
So this morning I got on to Televue, didnt quite catch the fella's name, but he was more than happy to provide advice...ended up being a 25 minute conversation
.
He brought up the EdgeHD white paper on his computer and we had a chat.
He advised that it is always better with an SCT to have the camera further back than the specified prime focus distance as opposed to too close to the rear cell, but otherwise there is plenty of range at which focus can be achieved.
Lets look at prime focus photography first:
As stated by the white paper (page 13), T mount to sensor distance for DSLRs is usually 55mm. This is the same for the CWT-2070. The RAD-1074 adds about 3mm to the overall length (it has a very thin shoulder). Now on page 13/14 of the white paper, it can be seen that the Large T Adapter adds 91mm of extension from the rear cell, so with the 55mm standard T-sensor distance you get guess what 146mm. The Baader visual back provides a lot less (not sure how much) extension, so the Televue gentleman recommended the use of a X3C-0009 3.5" (88.9mm) 2" extension tube, thus getting me very close to the 146mm mark.
It sounds to me like Celestron want you to get it to the 146mm mark to get you in the middle of the forward/back focus range of the mirror.
The gent also recommended the use of their ACC-0003 2" visual back, where apparently the ring from the celestron diagonal threads onto so that you dont need a 3.29" to 2" thread adapter. However this is 2.1/4" long, so i dont think the 3" extension would be required, he then suggested the X2C-0008 2" extension, which again gets me close but slight further back.
So for prime focus full frame DSLR, his recommendation and my preferred option is:
Baader 3.25 Visual Back --> X3C-0009 --> RAD-1074 -->CWT-2070
Now his comments about the powermate 2" sizes were interesting. Apparently there may be an issue with inserting the powermate in place of the X3C-0009 above (or in between the X3C and RAD), because the glass element of the powermate would be too far physically from the camera sensor. There may also possibly be a problem with internal reflection. The gentleman recommended this instead:
Baader 3.25 Visual Back --> X3C-0009 --> Powermate 4x --> PTR-4201 --> standard Canon T ring.
Although the T ring is 42mm, because it is much closer to the glass element of the powermate physically, apparently there will be minimal vignetting. If I try the CWT-2070 and RAD-1074 combo with the powermate, then I would not need the extension tube (and ill have the CWT-2070 and RAD-1074 anyway), so I have a number of options to test out. I still feel ill get vignetting with the PTR-4201 though
