Help with OAG positioning please.

[michaelmorris]

I'm just starting to get into autoguiding using an off-axis guider. I'm a little unsure as to where is the best place to position my off axis guider in relation to my f6.3 focal reducer.

My normal set up with a focal reducer is:

DSLR - T adaptor - spacer - focal reducer - focuser - telescope

My normal set up with an OAG (but no focal reducer) would be:

DSLR - T adaptor - OAG - focuser - telescope

My proposed set up with the focal reducer and OAG would be:

DSLR - T adaptor - spacer - focal reducer - OAG - focuser - telescope.

Alternatively, would it be better to go for

DSLR - T adaptor - OAG - focal reducer - focuser - telescope?

[Psychobilly]

I dont have the focuser in my setup and my OAG is the TS-OAG9 so i have used a shortened spacer (46mm) to give 55 mm between the reducer and the T adaptor...

DSLR - > T-Adaptor -> OAG -> Spacer -> Reducer -> (Focuser) -> Telescope

I have bought one of the AE helical adjustable T extensions to dial in the spacing once the nights get longer...

Peter...

[Merlin66]

Depending on the collection of "adaptors" / spacers you have available, either will work.

The focal reducer needs 105-110mm distance between the reducer and the CCD chip; you already have 55mm with the DSLR.

Getting the guide camera into the par-focal distance is the key.

(On the Lumicon Cass OAG the pick-off prism is infront of the reducer lens.. this gives a smaller FOV but the longer effective focal length was claimed in the ol' days ( visual crosswire guiding) to help. With the sub pixel guide capabilities today - it doesn't matter)

Ken

[michaelmorris]

Thanks guys.

So would I be correct in summarising it thus:

You can have it either way, but try to maintain the correct focal reducer-to-imaging sensor plane distance.

[Psychobilly]

Yes... and getting the guide and imaging cams parafocal at the same time...

This is one time when you can never have too may adaptors in the box.. I have made a set of t extensions from 3-10mm and shims for intermediate lengths...

Peter...

[Merlin66]

Basically yes....

Target the 105mm spacing; do the fine focus tuning of the guide camera to the imaging camera during the day and "set" the guide camera using a par-focal ring or fixed spacer - that way the next time you set up it's all spaced correctly and ready to go.

If you use PHD for guiding consider adding "Al's reticle" this is an adjustable overlay grid which allows you to better position objects in the FOV.

Ken

AlsReticlev22.zip