Green-yellow 'Oil mark' visible with telescope on Nikon

The hexagonal pattern is clearly a manufactured pattern in the focusing screen and not a defect, oil spot, or other issue. Please, Tony, again, what kind of camera is it? There is a reason I am asking. Later AF and most digital SLRs have an active focusing screen that is very complex - depending on model it includes a layer with the light up indicators for active focusing, switchable on/off grid lines, etc. On many of these cameras, the focusing screen doesn't actually snap into sharp focus until you power the camera on. The patten could be related to light interference between the layers of the screen.

I agree with others that the pattern is likely visible because the telescope is illuminating the focusing screen and fresnel differently than the lenses the screen was designed for. (Many interchangeable focusing screens are rated for a range of focal lengths, and odd effects can happen outside that range.)

If you want to test this or want to eliminate the pattern, try the telescope on an older manual focus camera with a simple focusing screen.

I get this 'illusion' on a Nikon F65 and a Nikon D50

Closer versus distance views - marginal difference - if any in the illusion.
The major difference is when the light level changes.
For instance looking into bright sunshine the illusion is much more visible than when the light level is low and/or over cast skies

I just checked with a Nikon F50. It shows very pronounced that iridescent effect when looking through the viewfinder without lens at a pointlight source, as I hinted at in post #18.

I guess the Minolta 7000 was the first to show this effect though lesser pronounced than some later SLRs.

So far I have not perceived this effect at any non-AF SLR.

There may be a beam splitter in the optical path of the viewing system that is interacting with the very long focal length of the telescope.