I have a new Sky-Watcher HEQ5 equatorial mount and EdgeHD 800 SCT OTA which includes a 40mm Celestron "Plossl" eyepiece with 43 degree FOV. I have just bought a range of B-H Hyperion eyepieces which fit a 1 1/4" diagonal but also have 2" barrels. Rookie mistake No. 1 - assume they would "just" work with existing diagonal!
The first issue: the screws on the diagonal have heads that sit higher than the barrel and the wider design of the B-H EP don't sit flush - they are sitting up by about 3mm (1/8").
The second issue: the screws will either get caught on the undercut (and probably "mark" the sides of the barrels which I would like to avoid) or just be a pain to operate.
One solution is to "junk" the new but entry level mirror diagonal - for the price it's probably OK but there's a feeling that it's probably going to "let the side down" so will be heading out the door before too long anyway. There's a million and one diagonals available but I'm seriously considering:
- 1 1/4" Celestron dielectric #93571 push-fit
- 2" Celestron dielectric (with 1 1/4" adapter) #93573 with SCT and push-fit adapters
Both have a twist-lock, but they obviously have different features:
- 1 1/4" uses visual back (two screws hold it in place)
- 1 1/4" has threads for 1 1/4" filters but 2" has "only" 2" threads (not on 1 1/4" adapter)
- 2" screw on to SCT back (replacing the visual back)
- 2" push-fit into a 2" focuser e.g. on many refractors
So my (dumb) question is what are the pros/cons for operating (living with) either type? The two screws in visual back would allow easy adjustment of the viewing angle? The SCT connection could come loose with a v. heavy 2" EP as the telescope slews? I have no experience of either type (current OTA has rotary focuser with nylon screws). Operation of both in a cold environment?
The extra weight of the diagonal for the mount wouldn't be an issue (extra 350g) and there's no problem with clearance.
I did read that the EdgeHD is "optimised" for 1 1/4" diagonals - get spherical distortion if focal point is moved too much. I estimated that the optical path is longer through the 2" back by about 6cm or 7cm, but as the visual back isn't needed 7cm is saved (so by design or accident) it's about/almost the same. So, it might be an issue with a 2" push fit used with a visual back e.g. longer optical path through the bigger diagonal plus length of a visual back?
FWIW I don't have immediate plans to buy the 0.7" focal reducer. It's not definite that I will go "long" because I might get a 2nd shorter FL OTA. I also have a Newtonian (which I will probably sell) so wider views are something that I would eventually want to reinstate.
Lastly, a question about the B-H Hyperion EP (unmodified) - can the 2" barrel straight in the 2" diagonal or will the 1 1/4" adapter always be needed?