Re: ASTRO SAA Re: Question about telescope design.

markzoom wrote:
There is no reason why it couldn't be quite large. Say 3m across,
mounted on the edge of a suitable flat ***. It would be a fraction
of the cost of a 3m diameter mirror. The reason I posted this on
sci.astro.amateur is because such a strip reflector might be buildable
by a competent amateur, whereas a 3m mirror wouldn't.

An accurate 3 m strip mirror would be harder to make than a round mirror
of equivalent aperture. I think you underestimate the accuracy of the
figure that is required. The strip would have to be parabolic along one
axis, and very flat across the other. It's pretty hard to make it flat
on one axis and curved on the other than to make it curved radially.

Note that it is harder to build an accurate flat mirror than it is to
build an accurate curved one, because you don't care if the focal length
of the mirror is off by an inch or two, but a flat can have only one
accurate shape.

...or a smaller lens in front of the sensor could do that? It rather
depends on how narrow the strip sensor is.

No. The optics are wrong, as the intermediate image is linear. An
ordinary lens cannot focus that to a point. You need a second strip,
which can be smaller: It only has to be as long as the first strip is
wide. It still has to be as accurate in figure, though.

Take the analogy of the webcam versus a flatbed scanner.

The analogy works optically only because you can shine a bright on the
item to be scanned, enhancing the signal-to-noise ratio significantly.
You can't do that with the night sky, so the narrow width of the optical
element hurts you much more (and a larger width worsens your optical
aberrations).

Well no, imagine a flat *** of something with a parabola cut on one
edge. The strip rests in that. It doesn't have to do anything
"naturally".

How do you propose to cut the parabola to an accuracy of millionths of
an inch? It can't just rest in there. Thermal expansion and rigidity
issues prevent simple resting from working properly. These aren't
problems with radio, because the corresponding accuracy of figure is on
the order of inches, or at least significant fractions of an inch. A
round optical mirror doesn't have this problem, either, because the
shape is ground into a more or less rigid disc.

It's quite expensive to make a large mirror. This could be metres
across at a fraction of the cost.

Yet it would cost more than an ordinary but smaller mirror that still
collects more light than the longer strip. And the smaller mirror can
be built by an amateur, which the strip can't.

It seems illogical to me to try and reflect a 2D image onto a 2D
camera sensor array and then convert it through 1D back to 2D
electronically to end up on your puter screen.

Is that what you're trying to avoid? This seems strange, since the
conversion, as illogical as it might seem to you, is pretty cheap and
still fairly quick.

You've asked whether there's a problem in principle, because you don't
care about problems of engineering. In my opinion, since the problem
you're trying to avoid is essentially an engineering one, not an optical
one (there's nothing wrong with the Newtonian design in principle), it
is illogical to ignore problems of engineering. However, if you are
determined to go ahead with this project, I suspect you will find the
issues out for yourself.

--
Brian Tung <brian@xxxxxxx>
The Astronomy Corner at http://astro.isi.edu/
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