Dave Martindale wrote:
"Rich" writes:
IMO, a good apochromatic telescope while not possessing a fast f-ratio
will beat any camera lens when it comes to long tele work.

The reason these scopes have to be so good optically is that shooting
celestial objects with stars in the field is a severe test of optics.
Any residual off-axis aberrations show up in the star (point source)
images. Most of us have seen the flary, blurred out images at the edge
of a field of lights seen when night shots are taken with camera lenses
used wide open. These scopes can't allow that.

That's part of it. Another factor is that telescope objectives are
normally used as part of a system for visual observing, and the
magnification is changed by changing eyepieces. A typical good
refractor has a 2 inch focuser, and might be used with a very low-power
eyepiece (e.g. 55 mm FL) at about 10X magnification. At that power, it
is expected to produce an image nearly 2 inches in diameter with good
quality.

Or it may be used with a series of successively shorter focal length
eyepieces to give higher magnification. A 4 inch diameter objective is
capable of 200X useful magnification if the objective is diffraction
limited, and refractors are *expected* to be diffraction limited, at
least in the central portion of the image that would be visible at such
high power. This is somewhat comparable to finding a zoom lens that
performs well over a 20X zoom range.

The reason these instruments haven't been made use of by more
photographers is that they don't operate (no IS, focus radically
different) like camera lenses. However, for wildlife photogs who use
blinds and wait for the opportunity to get good shots, they would be
ideal.

I've used a TeleVue Oracle on a film camera. This is a 3 inch diameter
"semi apo" - not as good as the 4" TeleVue apo refractor referred to,
but also a lot cheaper. It was sharp, but it's also quite a pain to use
compared to a camera lens: (These apply to most astro telescopes used
as camera lenses)

1. Focusing is by rack and pinion, which is really too quick to get
precise focus with a camera. (It's fine for telescope use, where
your eye itself provides some degree of final fine focus).

The Oracle is a 16 year old telescope. Most apos now use two-speed
focusers for course and fine focus so that issue is resolved. But they
can't focus fast like a collar focusing lens or an AF lens.


2. There's no auto diaphragm. In fact, there's no diaphragm at all,
so you're always shooting wide open.

That is something they could implement, but most observers want full
aperture.

3. The optical tube is longer and more awkward than a camera lens
of comparable focal length.

Yes, these kinds of scopes don't incorporate (generally) elements
further down the tube to increase focal lengths, but generally rely on
the main lens (objective) to provide the focal length. Additionally,
few are below f5 (Pentax makes a 400mm f4 photographic model
and Takahashi makes a Sky 90 which is a 450mm f5). Both are fairly
compact.