Re: Why does older CoolPix-995 work with scopes, and DSLRs will not?

omcdaniel.abcd@xxxxxxxxxxxxxx writes:

Unfortunately, I keep hearing both (1) DSLRs are not suited for
Digiscoping and (2) if you insist, you must remove the camera's lens
and shoot without it, using a special adapter and a T-Mount. But it's
almost impossible to find an explanation as to why these two
statements are correct.

Can anyone give a good technically sound and logical explanation as to
why the CoolPix series and others like it CAN be coupled up close to
the scope's eyepiece, and the DSLR types cannot without the above
modification and loss of use of its lenses? It is these Image
Stabilized lenses that make a workable arrangement using them so
desirable for this application.

You're talking about two substantially different ways of acquiring an
image from a telescope.

With the CoolPix, you leave the telescope eyepiece in place, and the
telescope provides a magnified image projected to infinity. Then you
place the camera's lens where your eye would normally be, and the camera
focuses on the image and captures it. It works, but there is *a lot* of
glass in that optical path, which reduces contrast and may add
distortion. Also, to work well in this scheme, the camera you're using
needs to be physically mountable on the telescope eyepiece, with a lens
that has an entrance pupil near the front so it can capture most of the
light coming from the eyepiece when set to an appropriate field of view.
Eyepieces with a long eye relief will work with more cameras than one
with a short eye relief.

You *can* use a DSLR with its lens in place with your telescope. But
the DSLR is bigger and heavier and harder to mount than the CoolPix,
and the lens is a longer focal length and so probably has the entrance
pupil deeper inside it, so you may get a lot of vignetting off-centre.

Besides, that just isn't the best way to couple a camera to a good
telescope. The best way is to remove the camera lens and the telescope
eyepiece (thus removing all that glass) and using the telescope
objective directly as the camera lens. You'll get better image quality,
and probably a physically more robust assembly as well. This is called
prime focus photography, because the sensor is at the prime focus of the
telescope objective.

So, you don't *have* to remove the eyepiece to use a DSLR with a
telescope, but it's generally better to do so. With a P&S camera, you
can't remove the camera lens, so you can't do prime focus photography at
all - you're stuck shooting through the eyepiece.

Dave
.

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