Re: Question about telescope design.

In message <nGImj.2520$9j6.351@attbi_s22>, Sam Wormley <swormley1@xxxxxxxxx> writes
markzoom@xxxxxxxxxxxxx wrote:
Just wondering... If much of the input ends up digitised anyway, what
is the point of dished telescopes?

You get the full resolution in 2 dimensions in a single step acquisition of data. NxN numbers in time T

Using a linear sensor (or logical equivalent) on a 2d imaging system is done - some of the zenith only observing mercury mirror scopes do this. But it is a fully 2d imaging system sampled on a linear detector.
N numbers in time T

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.

It doesn't quite work like that. If you only have a parabolic trough reflector you can form a 1-D image of the sky brightness. Roughly equivalent in a perfect idealised world to measuring all the light that would come through a geometrically perfect narrow slit placed across the sky.

Your 1-D high resolution graph has almost no resolution in the other dimension. You can ameliorate this deficiency by rotating the 1-D imaging system with respect to the sky. A trick that is still used for shadow masks at ultra high energy where no focussing optics are possible.

Cylindrical optics are not all that common. The only thing I can see in the home that would allow you to see how this basic scheme would fail is a Galilean thermometer which consists of a cylinder full of water.
(Any other clear cylindrical prism would do)

Taking a very good 1-D imaging system and then rotating the object under study (or letting it rotate with the sky) is the fundamental basis of most indirect imaging techniques. But you cannot get around the fact that you are trying to make an NxN image by taking N measurements at a time.

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

It is as old as the hills in radio astronomy. The 4C deep radio galaxy catalogue that effectively destroyed the steady state universe theory used a telescope configuration of parabolic trough reflectors (and interferometry of connected antennae to sort out the horrible ambiguities).

In fact the limitations of computing power and the side lobe structure of real antennae at the time led to an over estimate of the number of faint sources. It led to one of the most bitter cosmological disputes of all time. Ryle vs Hoyle. The 4C aerial is in a few online photos. eg

http://www.furrfu.org/gallery/displayimage.php?album=24&pos=4

Once Earth rotation aperture synthesis was perfected parabolic troughs were abandoned by radio astronomers as dishes were so much easier to fabricate and with a bit of engineering cunning could be made so that they stayed parabolic no matter what angle of elevation they were pointed at (the focal length changed slightly instead).

And the imaging was indirect by measuring the properties of the signals combined and correlated from pairs of the dish antennae.

The latter is so successful for the highest resolution observations that it is now being applied in the near IR and optical. Taking interferometric radio astronomy into the optical window (admittedly with massive technical difficulties).

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

o pulsars

The system that detected pulsars was a pretty crude phased array with enormous area. A parabolic trough would work OK for that.

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

This is a rather an unkind criticism as the vast majority of astrophysical objects *are* constant over the timescales needed for meaningful observations. Compact bright calibrators used for radio astronomy baseline calibration being one of the few notable exceptions (the dodgy ones are now well known and avoided but that was not always the case). Reliable flux calibrators which were partially resolved at the longest baselines also cause problems.

At visible and near IR wavelengths linear trough designs are basically only of use for thermal solar heating. Philips holds various patents on cylindrical and parabolic solar water heaters with a pipe along the focus.

Regards,
--
Martin Brown

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