Question about OAG

[Philip Benson]

I have a QSI camera with the built OAG port.

I know how OAG works. The prism in the OAG port pinches a small amount of the incoming light from the telescope and directs it into the guide camera. I wondered whether that means some of the same stars that are visible in the guider field of view should also be visible in the main camera FOV? 

I tried this last night on the Moon. Aimed telescope at the Moon and then started looped exposures on the guide camera. Light from the Moon was visible but was just out of view.  So I shifted the scope slightly until the Moon came into the FOV of the guide camera and then took a short exposure with the main camera.  The Moon was not visible directly but was obviously just out of the main camera FOV.

If I adjusted the OAG position by pushing further in, would that mean I would be able to marry up stars visible in the main camera and guide camera?

[JamesF]

That's not entirely consistent with the way I understand how they are intended to work.

My understanding is that the prism is supposed to be positioned outside the "cone" of light that hits the main camera sensor, but within the area illuminated by the optics, so effectively it "sees" part of the image that you're not using but does (hopefully) contain stars to guide on.  If the prism is moved in too far then it will shade the main sensor so you don't get an image on that part.

You won't therefore be able to share parts of the field of view between both imaging and guide cameras at the same time.

Unless QSI have done it differently, or my understanding is wrong

James

[Philip Benson]

Makes sense James.   Many thanks.

[BlueAstra]

For interest, this is two separate combined daytime images taken some years ago with my Canon 1000d and a lodestar guide camera, showing the relative field of view size and position. The OAG location is not perfect, ideally it wouldn't interfere with the main camera FOV. The OAG prism shouldn't 'block' any information from the main FOV, just reduce its brightness slightly.

[JamesF]

52 minutes ago, BlueAstra said:

The OAG prism shouldn't 'block' any information from the main FOV, just reduce its brightness slightly.

Yes, I guess it depends how far it sticks out into the light cone.  If we say for example that the prism is at the "top" of the light cone, just in the path of the light travelling from one section of the optics, it still may not be in the path of light travelling from the same target via a different part of the optics, thus the image will be dimmed rather than obscured completely.  The further into the light cone the prism protrudes, the worse this effect will become.

Very useful example image, too :)

James