|
Good question Paul. I think vignetting is not the primary focus or concern here as we are talking a 3" light path through the imaging train regardless of the rotators position.
The Optec 3" rotator weighs ~2kg without any adapters so we are talking a heavy unit. The Optec Inc website highlights this "This model is large and heavy and is intended for larger telescopes on observatory caliber mountings. The 2.4-inch thickness warrants telescopes that can accommodate this increase in the back-focus distance."
My suggestion would be to rotate the MOAG/STL only and not the focuser but it seems both topologies are plausible.
OTA -> Rotator -> Focuser -> Extension adapters -> MOAG/STL.
The 3" Pyxis has a 9kg max load capacity so there will be no problems there. 3" Feather touch weighs ~2kg and STL11k ~2kg with MOAG and guider at ~.5kg, so ~4.5kg. The downside with this imaging train configuration is that you need to handle additional power and data cables from the focuser (rotator cable wrap so need to ensure there is sufficient slack which is a minor point) and perhaps the more important point is how well the feather touch focuser works operating at different angles. Likely not an issue as the focuser mechanics such as roller bearing placement etc would be well thought through, but in any case if you rotate your imaging train as you see camera tilt or flexure, I'd be looking at the focuser. Perhaps replace it with an FLI Atlas.
OTA -> Focuser -> Rotator -> Extension adapters -> MOAG/STL
The alternative as you mention is the 3" Pyxis between the 3" feather touch focus and the MOAG/STL. That way you're not rotating the focuser thus minimising the amount of cords that need to wrap when the rotator moves to a different angle. This configuration however would place considerable stress on the focuser. 3" Pyxis ~2kg, MOAG/STL11k/Guider ~2.5kg, so ~4.5kg in total. The 3" FeatherTouch has a max capacity of ~6.5kg. Assuming the focuser is up to the task, this would be the preferred. The documentation does in fact reference that the rotator comprises of a fixed circular plate that attaches to the focuser mount and a rotating plate that attaches to the CCD.
I can't say I've really thought about this much as its different when you have a set up that uses the secondary mirror for focusing. The business end with the rotator, instrument adapters and cameras is simpler.
|