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The end of the line for the Arecibo radio telescope

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coppercone2:

--- Quote from: nctnico on March 07, 2021, 01:05:19 am ---
--- Quote from: Kleinstein on December 06, 2020, 08:18:20 am ---Compared to space missions rebuilding the dish would be relatively cheap. The tricky part could be more  like getting the funding for the ongoing costs, so we don't get the next collapse by 2050. A chance would be if NASA decides to forgets about maned space missions - lots of money to set free.

--- End quote ---
Late reaction but from some indirect interactions with the radio atronomy community I understand that they are no longer into building really big dishes but are going to link up many smaller dishes. The advantage is that this also allows to be able to pinpoint from where signals are coming from.

--- End quote ---

I think you will have a lower bandwidth though. But the minimum they measured is 300MHz which is only 1 meter, so this is fine. But if I recall, they had low frequency transmitters on there. 5MHz. The VLA can do 90MHz.

The 300 meter dish is 22 dB at 5MHz.

If you have a bunch of small dishes how does the math work out with that? Not sure if its identical.

David Hess:

--- Quote from: nctnico on March 07, 2021, 01:05:19 am ---Late reaction but from some indirect interactions with the radio atronomy community I understand that they are no longer into building really big dishes but are going to link up many smaller dishes. The advantage is that this also allows to be able to pinpoint from where signals are coming from.
--- End quote ---

The huge amounts of cheap digital processing available now allow aperture synthesis unlike back when Arecibo was built.  Integration makes individual transceivers inexpensive.

fourfathom:
A single large dish will have a lower noise floor than an array of smaller dishes.  This is because a small dish has lower gain before the preamp.  In an array, each dish/preamp sends the amplified signal to a delay/combiner stage where the Synthetic Aperture beam steering is done.  Assuming similar amplifier noise figures, the higher-gain antenna will have better sensitivity.  But a very large array can create a smaller beam angle than a practical big dish, reducing the cosmic background noise and other off-angle pickup.  The large arrays have significant operational performance advantages, just not for very weak signal work.

This reminds me of a discussion I had regarding radar reflectors (used on boats), and how they compare to optical reflectors (mirrors).  An X-band radar runs around 10 GHz.  My 12" diameter octahedral radar corner reflector compares (in wavelengths) to an optical light mirror about 2.5 miles x 2.5 miles.  There's probably some analogy here to the angular resolution of radio astronomy antennas vs optical telescopes.

[edit: I got these backwards.  My 12" radar reflector is wavelength-equivalent to an optical mirror 0.000188" in dia.  Or, taking my 2x3" emergency signaling mirror and scaling up to radar wavelengths would give a radar reflector 2 miles x 3 miles. ]

CatalinaWOW:

--- Quote from: fourfathom on March 07, 2021, 03:59:31 pm ---A single large dish will have a lower noise floor than an array of smaller dishes.  This is because a small dish has lower gain before the preamp.  In an array, each dish/preamp sends the amplified signal to a delay/combiner stage where the Synthetic Aperture beam steering is done.  Assuming similar amplifier noise figures, the higher-gain antenna will have better sensitivity.  But a very large array can create a smaller beam angle than a practical big dish, reducing the cosmic background noise and other off-angle pickup.  The large arrays have significant operational performance advantages, just not for very weak signal work.

This reminds me of a discussion I had regarding radar reflectors (used on boats), and how they compare to optical reflectors (mirrors).  An X-band radar runs around 10 GHz.  My 12" diameter octahedral radar corner reflector compares (in wavelengths) to an optical light mirror about 2.5 miles x 2.5 miles.  There's probably some analogy here to the angular resolution of radio astronomy antennas vs optical telescopes.

--- End quote ---

I think you got your optical/radar comparison backwards.  A 12 inch optical reflector is roughly the same number of waves across as a 2.5 mile radar reflector.  Or said another way the two reflectors would have comparable beam patterns.

fourfathom:

--- Quote from: CatalinaWOW on March 07, 2021, 04:34:05 pm ---I think you got your optical/radar comparison backwards.  A 12 inch optical reflector is roughly the same number of waves across as a 2.5 mile radar reflector.  Or said another way the two reflectors would have comparable beam patterns.
--- End quote ---

Yes, thanks for catching that.  I was grabbing quotes from a previous conversation and got things scrambled.  Speaking of scrambled, I realized my error while I was eating breakfast, and rushed back here to discover that you had already found it.  (I hoped I could sneak in and fix it before anybody noticed!)

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