Re: ASTRO SAA Re: Question about telescope design.

On Sat, 26 Jan 2008 17:16:50 -0800 (PST), markzoom@xxxxxxxxxxxxx wrote:

In the end, you have to ask yourself what advantage a 1-D imaging system
obtains.

A fraction of the weight and hence a larger size.

Not necessarily. You still need an accurate tracking system, and you
need to transport an active support, since the strip isn't naturally
stable in the way a disk-like mirror is.

You could get the resolutions of large mirror telescopes at a
fraction of the cost.

No, you couldn't. Mirrors become seeing limited by the time they reach
about 0.5 meters in diameter. The reason that larger telescopes have
been created is mainly to increase light gathering ability, not to
increase resolution. Only recently have adaptive optics techniques
allowed us to really take advantage of the resolution that large
apertures can provide. However, your strip mirror would not collect
enough light to make adaptive optics possible.

Better digital resolution. If you think of a megapixel but all in a
line, that could potentially be the vertical resolution alone. The
horizontal resolution depends on how well the tracking is done: By
tilting the telescope or by moving the sensor on it's own a small
distance.

Now you have added another difficult engineering problem- a 1M x 1 CCD!
Not only will this prove difficult to make, but I doubt the economics
could ever be made to work. Astroimagers ride along a bit on the coat
tails of the many other applications for conventional 2D image arrays.
You're talking about a very specialized sensor, here- one that isn't
going to be very useful outside a very narrow field. So the small number
of users will need to carry not only the low volume manufacturing costs,
but also the very large development costs.

It is vastly less sensitive than a 2-D system.

No it's not, It's as sensitive as any rectangular digital camera
sensor chip would be, it all just happens to be in a strip.

There are two things to consider here. First, the number of pixels. With
commercially available sensors, the number of pixels on 2D devices is
much greater than on 1D devices. And the total exposure time required to
produce an image is proportional to the number of pixels. Second, you
need to consider the area of the mirror. Total exposure time is also
proportional to mirror area. To collect the same amount of light as an
ordinary 1 meter diameter mirror, your strip mirror (assuming a 9um
pixel sensor) would need to be 87 kilometers long. I'd like to see how
that could be made inexpensively! At best, you might make it a few
meters long, and it would be collecting thousands of times less light.

Consider a
typical astroimage that is 1K x 1K.

That sounds woefully low. Modern consumer flatbed scanner sensors are
4800 dpi wide. I don't know how that translates to pixels but it
sounds substantially more than just 1k for one column:

1K square is not "woefully low" at all. Some especially large cameras
might be several thousand pixels on a side, but in general there isn't
any value in larger sensors for most telescopes, since you will not map
pixels to much less than an arcsecond of sky (because of seeing limits),
and because you can normally only cover so many arcseconds and still be
in the field that your optics are well behaved.

It will take 1000 times longer to
construct that using a 1K x 1 linear detector.

I disagree. For a start 1k is pretty low for a linear detector.
Secondly, I see no reason why a drift scan can't be done with a linear
detector, (yes it would take longer, but not 1000 times)

It will take longer by a factor of the ratio of the pixel counts,
further increased by the ratio of the mirror areas. In all, that's
likely to be a lot more than 1000 times longer. The reason people use
drift scan imaging is because they don't have to have a tracking
telescope- a huge advantage, even if it comes at the price of being
unable to record dim objects. But with a linear detector, you still need
a tracking drive, so you lose that advantage.

You could get the resolutions of large mirror telescopes at a
fraction of the cost.

As previously noted, large telescopes with passive optics do not produce
better resolution than ordinary, inexpensive amateur telescopes.

If you follow some links on the above pages and do some googles for
digital scan/scanning cameras you'll find some are capable of pictures
measured in hundreds of megapixels.

You can't fool Mother Nature. S/N in an image is determined primarily by
the number of photons you can collect. With a scanner, you have a lot of
control over that, since you control the light source. Have you ever
seen a high end production scanner? They use dangerously bright light
sources, and have protective measures designed to prevent that light
from reaching eyes. With astroimages, you simply don't get much light,
and there's not much you can do about that except to make your objective
area larger. Sensors are already approaching perfect devices, which can
record nearly 100% of the photons that strike them. High S/N astroimages
are acquired by using very long exposure times. With your linear
detector, and a typical exposure time of one hour, you'll need to spend
one hour on each strip of sky, say about 1 arcsecond wide. That means
something on the order of a year or longer to produce an image that is
just 1 degree wide (regardless of how long the array is). And that's
ignoring the low area of the mirror. The same image with conventional
amateur equipment could be made in a few nights.

_________________________________________________

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

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