Can I couple a focal reducer with an off-axis guider (C9.25" SCT)?

[dylan_odonnell]

Hi all,

I'd post some of my own pics if I wasn't so intimidated by the mind-bogglingly incredible photos that pass for "beginner" around here, so I'll bide my time.

I'm at a point where I'd like to start auto guiding, and have decided that an off-axis guider would be ideal. No flexure or extra mounting to worry about, keeps my rig portable and simple. I have access to dark skies and realise it may be more fiddly to find guide stars.

This means I will have two focus points, the OAG CCD and the DSLR that will hang off the end. Can I still add the focal reducer as below? Any caveats?

OTA -> OAG -----> FR ----> DSLR
|
| -----> CCD (Nexguide ideally)

Any advice appreciated.

Thanks in advance!

Dylan.

[Merlin66]

Dylan,
If you have an older C925 then the reducer (x0.63) needs to be at a fixed distance in front of the CCD chip - 110mm
Depending on the adaptors/ spacers you use (which OAG guider do you have??) you may be able to even fit the OAG between the reducer and the camera....otherwise in front (at f10) is still OK.

[Pinwheel]

I read these post but come on, not everyone knows these abbreviation. Can we please use words. Ok..:shrug: & What is a OAG!

[Merlin66]

Hmmm
OAG = Off axis guider
CCD= Charged coupled device
C925 = Celestron 9.25" Schmidt Cassegrainian Reflector...
x0.63 means a standard Celestron/ Meade lens system which is marked "x0.63" and reduces the effective focal length of the system by this amount when positioned at 110mm from the CCD chip.

Anything I've forgotten.

[orion69]

Placing OAG in front of FF/FR is not a good idea. OAG should always be between FF/FR and camera. If you have a filter wheel then image train should be: CCD->FW->OAG->FF/FR->scope.
110 mm is more than enough space for OAG and FW.

[Tony_]

I don't think the nexguide will work with an OAG. I have one and when I asked Celestron about using it with a 9.25SCT and an OAG they told me I would need a 4-5 magnitude star as a guide star.

There are not many DSOs with a 4-5 magnitude star in the same field. The nexguide is not sensitive enough to use with an OAG. I haven't tried it - but there are forums about it.

Regards,
Tony.

[blink138]

yes............ your manners ha ha! just joking merlin!
the operator could have been a little more humble i guess after all it is not his thread
pat

[dylan_odonnell]

Thank you Ken, Knez & Tony! That's good news apart from the Nexguide issue. I just realised that none of this would be compatible with fastar/hyperstar should I decide to go down that route later (My 9.25 is new enough to be compatible).

Maybe I will go down the guide scope route after all. On the other hand, OAG + PHD guiding seems to be a rock solid way to get sharp stars!

Thanks again,

Dylan.

[Merlin66]

Knez,
No problem with an OAG in front of the reducer....we used it that position many years ago for film astrophotography...in fact the Lumicon Giant Easy Guider was constructed that way...

[orion69]

There are at least 2 reasons why you should avoid placing OAG in front of a FF/FR.

1. It will degrade image quality because it will block some of the light which is not the case if positioned (correctly) behind FF/FR.

2. Guiding camera will be very far from prism which will make finding suitable star very difficult.

[traveller]

Hi Dylan,
Both OAG and guidescope have their advantages and disadvantages, and these have been covered in numerous posts.
I just piggy backed my ED80 on top of my C8 to try out, but the weather has been c-r-a-p in Melbourne of late.
Regardless of which setup you want to go down, the sensitivity of the guidecam is critical to pickup faint stars.
Lodestar has been a proven performer, but they are expensive. QHY5II-L (mono) is a very good camera according to reports and more affordable.
Cheers
Bo