Macro lens down the diagonal?

[25585]

Instead of using an eye piece with a camera body attatched, I was considering a macro (close focusing) lens. 

Using spot metering & with a high enough ASA dialled in, might it be possible to take snap shots of brighter objects from a diagonal's image using auto focus & exposure? Moon obviously...

[ronin]

33 minutes ago, 25585 said:

Instead of using an eye piece with a camera body attatched, I was considering a macro (close focusing) lens. 

Not entirely sure what you are thinking of. As far as I was aware Afocal is scope with eyeiece and camera with lens.

The scope and eyepiece are set as "normal" to deliver collimated light as the output. This means the "image" exiting the eyepiece is at infinity. So the the DSLR and its lens "simply" focuses this infinite object as normal on to the sensor. Go draw out the ray trace of the system.

If you used the camera body and just the eyepiece you simply get collimated light out of the eyepiece and no actual image. The camera lens or your eye forms the image.

So adding using macro lens is just the same as normal Afocal imaging. Just a bit of a different lens on the camera.

Hold a piece of card up to the moon and look at the card - no image just light falling all over it. That light is the collimated light from the moon. It holds the data to create an image but in itself is not an image. You need an eye or a lens to create the image.

[vlaiv]

If I'm thinking what you are thinking - it will not work

I think original idea was to use macro lens to focus on image on focal plain - that image being small and macro lens having ability to take small image and project it to sensor.

There is however way to do it, but you will loose a lot by doing it. You can take a piece of tracing paper, and put it right at focal plane - Moon image will form on it - then you can use macro lens to record it in full detail. Unfortunately it will not be of high quality. When image forms on focal plane rays of incoming light are sort of collimated - not parallel but falling to focal plane in cones - or parallel rays being focused to a point. In order for macro lens to work you need different kind of rays - ones that emanate from point in spherical fashion - this is what tracing paper gives you - each point on tracing paper being hit by focused rays acts as spherical source.

There is however better way to project image at focal plane to sensor - just place sensor at imaging plane - this is what is normally done in astrophotography and we call it prime focus imaging