Off-Axis Guider query

[CSM]

I have a Celestron Off-axis guider and am trying to work out the correct spacing distances for the autoguider and the imaging camera.  When measuring for the imaging camera I would assume that you measure from the centre of the OAG to the imaging chip.  And for the Autoguider from the prism to the AG chip, or should it be from the centre of the OAG?

[Freddie]

Both should be taken from where the light path splits. So one should be from the prism to the image cam chip and the other from the prism to the guide cam chip.

[CSM]

Thanks Freddie

[Lonestar70]

Both should be taken from where the light path splits. So one should be from the prism to the image cam chip and the other from the prism to the guide cam chip.

Whilst I agree with Freddie in principle, you must also take into account the backfocus requirements of your reducer (if fitted) and adjust both camera sensors to be at the same required distance from the rear element of the focal reducer.

The Imaging camera is the more fussy, since any deviation from the focal reducers specified backfocus distance can make a huge difference to the image quality... the OAG Prism does not enter into the imaging camera's path so it is not really relevent in this measurement.

The guide camera is not quite so fussy and can have a little more leeway in location.

Hope you manage to get it sorted out... spacing can be a real pain sometimes.

Best regards.

Sandy.

[CSM]

Thanks Sandy.   Can't really see that the focal reducer will make any difference as the distance from it to the prism will be the same for both imaging and autoguider.  Or am I missing something?

[Lonestar70]

Thanks Sandy.   Can't really see that the focal reducer will make any difference as the distance from it to the prism will be the same for both imaging and autoguider.  Or am I missing something?

True, however, you may have to optimise the distance between the OAG prism and the reducer in order to get the best selection for guide stars when the Image sensor is at the correct backfocus distance from the reducer.

Once this optimised distance has been established then the spacing between the prism and the guide camera sensor can be established to achieve the same backfocus.

It is not always as simple as just making both sensors the same distance from the prism, as may seem convenient, and then adding enough spacers to satisfy the image camera backfocus, since this may not optimise the location of the prism for guide star selection.

I hope that makes some sense.

Keep Happy.

Sandy.

[CSM]

Hi Sandy - thanks but not really understanding that.  But leaving it aside, how far down should the prism be? I rather suspect that I need to move mine down, which can't at the moment as I haven't got the right allen key.

[Freddie]

The prism should be down so it is just beyond the edge of the imaging cam chip. You may then need to come out a touch further if you get any shadows on your images. Obviously that will have no impact on the guide cam focus position though so can be left once set.

[CSM]

Thanks Freddie.  This would seem to confirm what I suspect as the bottom edge of the prism is barely 3-4mm down.

[Lonestar70]

Prism protrusion will depend on your imaging camera sensor size.

A good way to set it is to run your imaging camera in live view or preview mode during daylight and focus on a very distant object... then move the scope to point at an evenly lit part of the sky... adjust the prism down until you can see it just starting to protrude into the edge of the frame... then carefully back it out until it just clears leaving no shadow.

Lock it at that and you should be OK.

Good luck and clear skies.

Sandy.

[D4N]

Focusing on a distant object in the day is a good way to get your guide cam par focal, it may need a little tweaking at night but at least you shouldn't need to adjust the spacers.

/Dan

Sent from my iPad using Tapatalk

[CSM]

Thanks Sandy and Dan.  Makes more sense now.  Just need to move that prism.

[BlueAstra]

Here's a few images showing my old setup with a C8 SCT and a Canon camera. In the first setup the FR was attached to the scope and the OAG attached to the scope by its nosepiece in a AE adapter. In the second image I moved the FR to the OAG assembly with the AE adapter fixed to the scope. I basically set up the imaging distances to the camera plane by calculation, then moved the guide camera to focus (during the day). I moved the prism to be just beyond the imaging chip boundary. The last image shows the guide camera FOV superimposed on the main camera FOV.

Graham

[CSM]

Thanks Graham.  I have also used a pylon for focusing though I am not sure I can do this now from within my observatory.