Re: ASTRO SAA Re: Question about telescope design.

On 26 Jan, 20:58, br...@xxxxxxx (Brian Tung) wrote:
markzoomwrote:
Surely it's just a matter of good tracking then. If a telescope can be
tracked accurately for a crisp still picture, the same can be done
with a "reflective strip scanning telescope".

In your original proposal, the strip stayed still, while the sky moved
past. There is no tracking to do. If, on the other hand, you don't
keep the strip still, but instead use it to track, then it will only
take images of a small section of the sky, but be worse than a big
"dished" telescope (a reflector of some sort, I'm assuming you mean).

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.


I don't think it would be, since it's only a 2D item, no compound
curves involved. It's just a matter of bending a nice reflective strip
into shape.

No. All stars must be focused by the mirror into a point before your
scanning detector can record it properly. If you only have one strip,
each star is focused into a line as long as the strip is wide, rather
than a point. This will not result in good images, obviously. In order
to correct for this, the strip would have to be curved inward both
lengthwise as well as widthwise.

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

Take the analogy of the webcam versus a flatbed scanner.


I suppose you could run your scanning detector parallel to the strip
(and therefore perpendicular to the linear star images, but then the
effective aperture is a mirror only as wide as the scanning detector.
Very inefficient.

A design could be made using *two* strips of the sort you mention, one
to focus in one axis, and one to focus in the other. Such a design has
been proposed in the past (even here on this newsgroup, a few years
ago!). However, it will either be very inefficient light-wise, or it
will exhibit substantial aberrations. (The strips, in order to collect
enough light to be worthwhile, have to have very low geometric f/ratios.
This results in bad images, especially off-axis--away from the center.)

If it's really big, a flat strip of thin glass could be bent to such a
parabola.

Unlikely that it would naturally deform to a parabola. It would have to
be very carefully controlled.

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".


As Chris points out, it is just not that hard to grind a sphere, and
then parabolize it--especially in the apertures and accuracies needed in
the visible wavelengths. Radio telescopes, which require much larger
apertures, are a different story, I suspect.

It's quite expensive to make a large mirror. This could be metres
across at a fraction of the cost.
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. The sensor might as
well be 1D/single-dimensional (or a pixel's width by whatever height,
anyway). It's only because human eyes can't store the previous raster
lines that we desire 2D all at once. Well the computer can store the
other lines for you now, to make a picture.


--
Brian Tung <br...@xxxxxxx>
The Astronomy Corner athttp://astro.isi.edu/
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