A Better OAG

[barx1963]

I have used a couple of OAGs in my time. Never liked either of the particularly. Fiddly to focus is my main complaint. I know getting precise focus is not necessary, but I want to be able to control it. Is there a better OAG out there? Have used the ZWO and ATIK One OAG BTW.

Malcolm

[jwoody]

Hello Malcolm
I have a QHY one and it has a helical focuser for fine focus. I find it works really well.

Jeremy

[multiweb]

That's interesting. How do you orient your guider?

[RickS]

If it is like the helical focuser on the MOAG then the guide camera moves in and out but doesn't turn.

[jwoody]

The QHY OAG does rotate as it turns in or out.

[multiweb]

That's pretty cool. Do you have a pic or cross section?

[LewisM]

Or you could simply buy an add-on non-rotating helical focuser to the OAG - Baader makes one (focusing EP holder), several others do.

I use the SX mini-filterwheel with it's integral OAG. Absolute BREEZE to use.

[multiweb]

I need one to focus the lodestar on the SX OAG. At the moment I have to slide it up or down. It's a pita.

[barx1963]

I am guessing there are 2 issues. One is the clunkiness of the focus mechanism. I am not trying for pinpoint stars, but it is really difficult to adjust when it is sitting between the scope focuser and the filter wheel.
2, the camera has quite a large sensor.so the pick off prism is sitting well out from the centre line, so getting some distortion of star images from that.

Some thinking to do!!

[sharpiel]

Not sure what they're like to use nor what you want to spend but:

- There have recently been two ONAG's for sale in the classifieds. I'm not 100% sure that it uses a par focal arrangement but others can confirm. If so you'd be eliminating guide camera focusing once you focus the imaging chip.

- SBIG make self guiding cameras. I'd assume (not used one) that they'd be parfocal.

[Camelopardalis]

Malcolm, have you looked at the Celestron OAG? Might sound like an unlikely place, but it’s decently made, pretty heavy duty, and has a large prism. The helical focuser is of the non-rotating kind, from the point of view of the guide cam.

It’s a bit greedy with back focus, but is reasonably priced if you can accomodate it.

[barx1963]

Thanks Les
May research ONAGs. I actually have an older SBIG STT8300 with the selfguiding filter wheel. The guiding was a little problematic with PHD so haven't really used it. It is not parfocal with the imaging chip. The house has a focusing knob on the outside.

Cheers

Malcolm

[barx1963]

Thanks Dunk. Will do my backfocus calculation and think about that one!!

Cheers

Malcolm

[codemonkey]

+1 for the Celestron OAG. Loved that OAG, sadly I no longer have sufficient backfocus for mine :-(

[PRejto]

2 comments:


1. Just adding an ONAG does not eliminate the need to focus the guide camera. It needs to be focused just as precisely as an OAG, and especially so if you intend to use FocusLock. However, just like an OAG once focused you are pretty much done.


2. A very good option for a focuser that is very rigid, unlike many helical focusers (including the one that is supplied with the ONAG on more recent models) is Gary Jarrette's KISS Focuser that fits SX OAGs and QSI. It's well made and Gary is great to deal with. His focuser will also work very well on the ONAG with a special adapter that Gary knows how to make.




https://www.youtube.com/watch?v=MvLAQJsjsNY

Peter

[barx1963]

Thanks for all the comments. I grabbed a Celestron OAG. It is a beast and very well made. Focuser is great. It also is able to fully rotate at both ends (camera and scope) so you can ensure the prism is sitting on the long side of the sensor, thus reducing and instrusion into the field and basically set it up how it suits you. Yes it is a back focus hog. My scope with the 0.7 reducer needs 105mm of BF and the litecrawler focuser, OAG, Filter wheel and camera are using about 114!! Unfortunately I have to put a 6mm extension between the focuser and the OAG otherwise the OAG focuser is righ up against the main focuser body. If not for that it would be at 108mm. Have taken a few test shots. Stars appear reasonably tight but I think there is some curvature there. Oh well:question: