Comment by watersb | Hacker News Reader

Comment by watersb | Hacker News Reader

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Another tricky thing about pulsar observations is that the integration times are very short.

Obviously. You want the thousands to millions of beats per second.

With short exposure time, not many photons to grab. You need a bigger aperture.

Aericibo used to be a nice telescope for pulsars. Sigh...

China has a huge dish, sort of a next-gen Aericibo, the 500-meter FAST https://en.m.wikipedia.org/wiki/Five-hundred-meter_Aperture_...

At a conference in 2000 or 2001, they had a brief presentation where they noted that they have multiple such geological features that could be used to create such telescopes.

Interferometric arrays have so far been easier to build, I think because they have proven to be capable of more types of observation than single fixed dish, and therefore a better investment.

The NRAO can do long baseline observations that include large single dishes, although Areicibo was never able to do so; they used the Green Bank Telescope. Which is a monster.

https://en.m.wikipedia.org/wiki/Green_Bank_Telescope

https://greenbankobservatory.org/science/telescopes/gbt/

A dedicated VLBI array of FAST-class dishes would be mighty nice, though, and could spend many hours to gravity wave observations.

Arecibo and FAST are giant immovables dishes (essentially a parabolic valley covered by some electrically conducting material). The receivers suspended above these dishes are somewhat moveable, but that is about it. That means they can only observe a small region of the sky exactly overhead (this region changes together with the rotation of the earth). VLBI requires multiple telescopes significantly separated from each other in physical space (think thousands of km) to track the same object on the sky during an observation. Thus VLBI is hard/impossible for giant immovable dishes like FAST :)

> VLBI requires multiple telescopes significantly separated from each other in physical space (think thousands of km) to track the same object on the sky during an observation. Thus VLBI is hard/impossible for giant immovable dishes like FAST. :)

Good point... But the VLBI people I know are "here, hold my beer..." engineers. And astronomers will use Vegemite for thermal paste, if it lowers their Tsys in a tight spot.

If FAST-class dishes are cheap enough, you could build an assortment of them, pointing in different (fixed) directions. The primary azimuth would still train close to zenith, but the hard part is that suspended receiver assembly, the moving bits, a relatively heavy platform (that eventually fell into Arecibo's dish, after decades of minimal budget and maintenance:-( ... Dang, the Canadians have proposed large dirigibles, tethered like weather balloons, for the primary focus assemblies.

I miss that crowd. My first drive was the NRAO VLBA correlator, 23 (!) years ago. I'm out of the loop now.

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