New design - Annular ring OAG???

[Merlin66]

It just dawned on me that the new reflective guide slit I developed for the spectroscope could be easily adapted to provide an annular reflective "ring" for an OAG application.
Think of it as a flip mirror in front of your camera with a clear aperture of say 25mm diameter( or whatever was "optimum" for the CCD chip - it could even be a rectangular aperture to better match the CCD....)
The outer area beyond the clear aperture, out to say, 35-40mm diameter would be a highly reflective "ring" - this would provide a very large guide field of view - any star image found in the annulus could be used for guiding...
Comment/ questions/ concerns???

[alistairsam]

any pics or diagrams Ken?

[Merlin66]

The attached sketch shows the simplicity....
AstroEngineering in the UK used to sell a 1.25" to 2" excentric adaptor, where the 1.25" eyepiece aperture was off-set to the centre line....this looks like an ideal match...

[Astro_Bot]

Good idea, AFAICS.

With the appropriate optical treatment (barlow/reducer/corrector/extension/focussing ring/whatever), couldn't you use the entire FOV for a guide star? It looks effectively to be a reflector-like arrangement with large CO.

[Merlin66]

Hmmm
Don't think so..
The clearance aperture for the imaging beam is very large relative to the available field of view IMHO it would be a guide field of view with a bright perimeter, the width restricted by the size of the chip and the focuser drawtube (2"???)
If you use a sliding in/out flip mirror/OAG you only see the reflected FOV from the mirror surface......

[Peter.M]

I could be wrong here but wont your guide camera need to be larger than your imaging camera for this to give any advantage? The center of the image is going to be black and only the edges visible, If the "black" center of the image circle is the size of your imaging camera then the guide chip would need to be larger than that too see all of the guide stars. you could move the guide cam around to find the best star of the frame but this is already possible with conventional OAGs. Also this would presumably take up a large amount of back focus.

[Merlin66]

Could be....
I suppose the easiest way to check would be to place an aperture mask (a 25mm diameter "black spot" paper) in the centre of a diagonal and see what the final image and FOV looks like.....