Optec 3" rotator positioning

[Paul Haese]

Ordered a 3" Optec rotator today for the RC12. I am not sure about the position this should be located. Should it be positioned between the focusor (which is a feather touch with rotation adapter)and the MOAG/STL11000 or should it be between the RC12 extension tube and the 3" feather touch focusor? The images on the web suggest the former, and getting an adapter from rotator to the focusor is not going to be easy.

Looking for some advise or thoughts. I am thinking that the rotator needs to be as close to the camera as possible to prevent vignetting issues.

[jase (Jason)]

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.

[Paul Haese]

Yes it is a puzzle.

I can get either done with adapters made up by Optec, they have both lots from what I am told. The question is which one. I am sort of leaning towards the scope - focusor - rotator - moag - STL at this stage. I am using a similar configuration on the TSA, but that is only lifting the rotator and QSI 683-8 WSG.

The feather touch is a big arse focusor and does not seem fussed by the STL on board, so I am thinking this might be the best way to go with another 2 kg added. There is still plenty of leeway with 2 kg spare.

I have raised the question with Optec and I am sure they will get back to me with the way they would prefer.

[Bassnut (Fred)]

I went through exactly this and ended up with the Optec 3" nearer the cam. Its suited the adaptors I had, but also I noticed the slightest movement in the Optec bearings, so I figured nearer the cam was best as it puts less stress on the Optec. The spacers on the OTA are rock solid and easily take the stress where as the the Optec near the OTA and then spacers with the cam on the end would seem to me to be a huge side load stress on the Optec bearings.

[frolinmod]

For mostly convenience sake I tend to think it's best to minimize what gets rotated. Fewer cables (cables that need loops) and stuff (stuff that isn't symmetrical and that can change the rotational weight distribution that is) moving in 180s the better.

[Paul Haese]

Thanks guys, certainly thinking that from the focusor to rotator is good.

[Bassnut (Fred)]

Oh, and checkout AAG rotator app http://www.aagware.eu/aag_software.htm if you dont know already, its syncs with the sky6/X FOV indicator. Just click and drag on the FOV indicator in sky and the Pyxus will follow it, very cool !.

[Paul Haese]

Actually I didn't know about that program. I have just been rotating in the sky and select the framing I like then using the get function in CCDAP. When I launch that it rotates to the desired angle and then as you know it rotates 180 at the meridian flip.

Will look into this program a bit more.

[frolinmod]

Originally Posted by Bassnut

Oh, and checkout AAG rotator app http://www.aagware.eu/aag_software.htm if you dont know already, its syncs with the sky6/X FOV indicator. Just click and drag on the FOV indicator in sky and the Pyxus will follow it, very cool !.

TheSkyX now has those features built in. You need only link your field of view indicator to your rotator. You can grab the FOVI and rotate it. You can also sync the rotator to the FOVI. In image link you can do both as well. The rotator control also permits one to have a rotator offset (physical vs. apparent rotator positions). AAG rotator is not required.