Question about telescope design.

Just wondering... If much of the input ends up digitised anyway, what
is the point of dished telescopes?
Wouldn't it be enormously cheaper and simpler just to use a strip bent
into a parabolic shape (rather like a slice through a parabolic dish
and sensor) and let the movement of the earth do the scanning in a
raster fashion. That way the expensive sensor only needs to be one
pixel wide by as long as you like.

Has this already been done or have I come up with a new astronomy tool?

On Sat, 26 Jan 2008 07:06:32 -0800 (PST), wrote:

Just wondering... If much of the input ends up digitised anyway, what
is the point of dished telescopes?
Wouldn't it be enormously cheaper and simpler just to use a strip bent
into a parabolic shape (rather like a slice through a parabolic dish
and sensor) and let the movement of the earth do the scanning in a
raster fashion. That way the expensive sensor only needs to be one
pixel wide by as long as you like.

Has this already been done or have I come up with a new astronomy tool?

The technique is called "drift scan", and has been done by both
professional and amateur imagers. Some radio telescopes, such as
Arecibo, also work this way since their dishes can't be steered [much].
It is an interesting thing to try, and useful in certain circumstances,
but is generally limited to imaging fairly bright targets, since a pixel
is only able to collect light from one spot for a short period of time
before that spot rotates away. Most astronomical imaging requires that
each pixel remain aligned with the same area of sky for many minutes or
hours to get a high S/N result.

However, I've never seen it done with a strip mirror like you describe.
That's probably because making such a mirror (at least in the optical
wavelengths) would be very difficult. Grinding a large circular blank
into a spherical shape, which is then tuned to a paraboloid, is actually
easier than trying to make an accurate parabolic strip.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

wrote in message
...
Just wondering... If much of the input ends up digitised anyway, what
is the point of dished telescopes?
Wouldn't it be enormously cheaper and simpler just to use a strip bent
into a parabolic shape (rather like a slice through a parabolic dish
and sensor) and let the movement of the earth do the scanning in a
raster fashion. That way the expensive sensor only needs to be one
pixel wide by as long as you like.

Has this already been done or have I come up with a new astronomy tool?

The parabolic shape of the mirror in a reflecting telescope
is intended to gather light and concentrate it so that the image
will be bright enough to see or detect with instruments. It also
provides directional selectivity, as essentially only light
arriving from a target that is on-axis will reach the focal
imaging area. A parabolic strip CCD imager wouldn't provide
concentration or magnification or directionality for a target as
a mirror does without doing some fancy mathematical footwork
with phased array detection.

On 26 Jan, 17:57, Chris L Peterson wrote:
On Sat, 26 Jan 2008 07:06:32 -0800 (PST), wrote:
Just wondering... If much of the input ends up digitised anyway, what
is the point of dished telescopes?
Wouldn't it be enormously cheaper and simpler just to use a strip bent
into a parabolic shape (rather like a slice through a parabolic dish
and sensor) and let the movement of the earth do the scanning in a
raster fashion. That way the expensive sensor only needs to be one
pixel wide by as long as you like.

Has this already been done or have I come up with a new astronomy tool?

The technique is called "drift scan", and has been done by both
professional and amateur imagers. Some radio telescopes, such as
Arecibo, also work this way since their dishes can't be steered [much].
It is an interesting thing to try, and useful in certain circumstances,
but is generally limited to imaging fairly bright targets, since a pixel
is only able to collect light from one spot for a short period of time
before that spot rotates away. Most astronomical imaging requires that
each pixel remain aligned with the same area of sky for many minutes or
hours to get a high S/N result.

I see...
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".
Something like the scanning part from a flatbed scanner (but maybe
more sensitive) is what I imagine as the sensor.


However, I've never seen it done with a strip mirror like you describe.
That's probably because making such a mirror (at least in the optical
wavelengths) would be very difficult. Grinding a large circular blank
into a spherical shape, which is then tuned to a paraboloid, is actually
easier than trying to make an accurate parabolic strip.

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. It would be relatively easy to do on a frame, and to tune
with evenly spaced adjustment bolts. In fact the parabola could be
adjusted to focus onto a straight sensor like from a scanner.
It would actually be easier to adjust if it were quite big, say 2-3
metres.
If it's really big, a flat strip of thin glass could be bent to such a
parabola.



_________________________________________________

Chris L Peterson
Cloudbait Observatoryhttp://www.cloudbait.com

On 26 Jan, 18:25, "Greg Neill" wrote:
wrote in message

...

Just wondering... If much of the input ends up digitised anyway, what
is the point of dished telescopes?
Wouldn't it be enormously cheaper and simpler just to use a strip bent
into a parabolic shape (rather like a slice through a parabolic dish
and sensor) and let the movement of the earth do the scanning in a
raster fashion. That way the expensive sensor only needs to be one
pixel wide by as long as you like.

Has this already been done or have I come up with a new astronomy tool?

The parabolic shape of the mirror in a reflecting telescope
is intended to gather light and concentrate it so that the image
will be bright enough to see or detect with instruments. It also
provides directional selectivity, as essentially only light
arriving from a target that is on-axis will reach the focal
imaging area. A parabolic strip CCD imager wouldn't provide
concentration or magnification or directionality for a target as
a mirror does without doing some fancy mathematical footwork
with phased array detection.

I think the directional selectivity could be done by a small lense
strip near the sensor, if that's even necessary.

On 26 Jan, 17:51, Sam Wormley wrote:
wrote:
Just wondering... If much of the input ends up digitised anyway, what
is the point of dished telescopes?
Wouldn't it be enormously cheaper and simpler just to use a strip bent
into a parabolic shape (rather like a slice through a parabolic dish
and sensor) and let the movement of the earth do the scanning in a
raster fashion. That way the expensive sensor only needs to be one
pixel wide by as long as you like.

Has this already been done or have I come up with a new astronomy tool?

You made the assumption that astronomical phenomena static. How
would your approach work for.

o pulsars
o novae
o supernovae
o quasars
o viable stars
o comets
o rotating planets and moons
o binary systems
o gamma ray bursters

OK, using tracking you could repeatedly scan the same bit of sky.The
frame rate would be dictated by how fast your tracker is. You are also
not restricted to using one strip, or one sensor per strip.

Maybe it's possible to have the parabolic strip stationary and move
the sensor slightly for the raster.
Another method would involve a smaller movable strip lens near the
scanner.

It's an analogy to using a flatbed scanner instead of a webcam to scan
documents.

markzoom wrote:
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).

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.

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.

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.

--
Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.html

wrote in message
...
On 26 Jan, 17:57, Chris L Peterson wrote:


However, I've never seen it done with a strip mirror like you describe.
That's probably because making such a mirror (at least in the optical
wavelengths) would be very difficult. Grinding a large circular blank
into a spherical shape, which is then tuned to a paraboloid, is actually
easier than trying to make an accurate parabolic strip.

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. It would be relatively easy to do on a frame, and to tune
with evenly spaced adjustment bolts. In fact the parabola could be
adjusted to focus onto a straight sensor like from a scanner.
It would actually be easier to adjust if it were quite big, say 2-3
metres.
If it's really big, a flat strip of thin glass could be bent to such a
parabola.

The 2D actually makes it more difficult. A 3D lens or mirror is
a shape that is self supporting and rigid for grinding and polishing.
A 2D strip would flex.

How do you bend a strip to a parabolic curve that's uniformly
within 1/4 wave or less of the perfect shape?

On 26 Jan, 21:11, "Greg Neill" wrote:
wrote in message

...



On 26 Jan, 17:57, Chris L Peterson wrote:

However, I've never seen it done with a strip mirror like you describe.
That's probably because making such a mirror (at least in the optical
wavelengths) would be very difficult. Grinding a large circular blank
into a spherical shape, which is then tuned to a paraboloid, is actually
easier than trying to make an accurate parabolic strip.

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. It would be relatively easy to do on a frame, and to tune
with evenly spaced adjustment bolts. In fact the parabola could be
adjusted to focus onto a straight sensor like from a scanner.
It would actually be easier to adjust if it were quite big, say 2-3
metres.
If it's really big, a flat strip of thin glass could be bent to such a
parabola.

The 2D actually makes it more difficult. A 3D lens or mirror is
a shape that is self supporting and rigid for grinding and polishing.
A 2D strip would flex.

Obviously it's on a rigid frame.


How do you bend a strip to a parabolic curve that's uniformly
within 1/4 wave or less of the perfect shape?

By having regular spaced adjustments on the frame. (How big is a 1/4
wave?)

markzoom wrote:
Obviously it's on a rigid frame.

How will you make sure the frame is properly shaped? It would have to
have the same level of precision, if the strip can't be relied on to
keep its own shape.

By having regular spaced adjustments on the frame. (How big is a 1/4
wave?)

About 5 millionths of an inch, at visible wavelengths. (The center of
the visible spectrum is sort of greenish light, at 550 nm, or about 20
millionths of an inch.)

--
Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.html