Help needed with OAG

[Dave Smith]

I bought a second hand OAG last year but have only just got around to trying it out on my C9.25 SCT. Initially during the day I pointed the scope at a distant roof top. Made the necessary adjustments to get the image of the tiles in focus with both cameras at the same time. However at night time I can focus the main scope with no problem but the image of a star in the OAG was rather like a large comma shape and there was no way that PHD2 would recognise it as a star and enable guiding. The OAG does not have any fine focusing, just a release screw and manually move the prism. In case it is relevant my main camera is an Atik 460EX and the guide camera is a DMK21AU618. Has anyone any idea what could be the cause? I would have thought that if a star was distorted that much then the distortion would have been clearly visible when looking at roof tiles.

Edit: I've since discovered what is going on - the guide camera is almost in focus but cannot get close enough. It can be solved, I think, by either moving the main camera further out or using a wider OAG. Bright spots on the tiles do look comma shaped when out of focus.

Dave

[BlueAstra]

I think moving the prism/camera together just changes the sampling point in the telescope aperture. You have to move the camera up and down the prism to focus, with the prism stationary. Do the stars in the rectangular corner of the main camera field have a similar distortion? Depending on the relative size of the main camera field and the telescope image circle, you could be sampling beyond the main camera field in a distorted part of the telescope image circle. Or it could be a duff OAG!

[Dave Smith]

3 minutes ago, BlueAstra said:

I think moving the prism/camera together just changes the sampling point in the telescope aperture. You have to move the camera up and down the prism to focus, with the prism stationary. Do the stars in the rectangular corner of the main camera field have a similar distortion? Depending on the relative size of the main camera field and the telescope image circle, you could be sampling beyond the main camera field in a distorted part of the telescope image circle. Or it could be a duff OAG!

Thanks Graham. I have edited my post at the same time as you posted. I think it is now solved. A spacer between the OAG and filter wheel should do the trick.

Dave

[gnomus]

If its solved great.  I believe the issue is that the OAG is necessarily looking at the very edge of the cone of light.  It needs to stay out of the FOV of the sensor (obviously).  That means you are in the part of the light cone where you are getting aberrations,  Many OAGs allow you to adjust how far into the light cone you go.  I found taking flats with the imaging camera helpful.  Just keep dropping the prism into the light cone until you see its shadow appearing in your flats, then back it off a bit.

I think is very common to have misshapen stars in your OAG view, yet PHD still seems able to guide on them.  Clearly in your case they were too misshapen, but they don't need to be perfect.

You might be able to get one of those Baader helical focusers between OAG and guide camera.  That might help.

[ollypenrice]

Do be sure to have the prism heading down towards the long side of the chip, in the middle. This allows you to go as deep as possible into the light cone (its brightest and least distorted part) without shadowning the chip - as Gnomus describes.

Olly