In article ,
Martin Brown writes:
Are there any EHT candidate radio galaxies near enough to image with the
spin axis perpendicular to our line of sight? Cygnus A is too far away.

M87 and Sgr A* were chosen because they are by far the best
candidates. I don't know what the next best would be.

Would the EHT be capable of taking a look at a starburst galaxy like M82
and making sense of the various odd compact objects lurking in there?

The southern hemisphere telescopes -- ALMA being by far the most
important -- can't look at M82. Even if they could, I doubt there
would be any sources with the enormous brightness temperature
required to give a signal.

Or even closer to home could EHT do M1 the crab nebula and look into a
much smaller accretion disk very much closer to home. I guess temporal
variations in the emission might stymie any such attempt.

That and again perhaps brightness temperature.

I presume SgrA* has caused problems because its emissions were varying
during the observations.

That's my guess too, but the EHT collaboration hasn't said anything
so far as I know. We know from the GRAVITY results
https://ui.adsabs.harvard.edu/abs/20.....10G/abstract
that emission in the Sgr A* accretion disk varies on 10-minute time
scales.

It is an impressive achievement to image the accretion disk/black hole
shadow.

Indeed. They are in effect synchronizing telescopes a whole earth
apart to a fraction of a millimeter of light travel time. The
hydrogen maser clocks used for that are only one of the amazing
technical achievements that were needed to make EHT work.

[[Mod. note -- Getting either higher resolution, or a wider field of
view (probably at lower resolution) would be somewhat difficult.

I wasn't sure what limits the field of view, so I ask an EHT
expert. The response was:
For VLBI the FoV is often set by bandwidth and integration
time. This is because widely separated structure on the sky causes
high frequency corrugations on the fourier plane, and these will be
averaged over if the spanned bandwidth or averaging time is too
long. But these limits are typically fairly large - much larger
than the FoV we used. We did search for larger scale structure and
didn't find any.

That last doesn't surprise me. As mentioned above, the brightness
temperature has to be huge for VLBI to see anything.

UV coverage doesn't seem to be a limitation: at least little more for
imaging far from the phase center than near it. Bandwidth smearing
and time-average smearing are factors in conventional
interferometry. They can be overcome with a combination of more
complex equipment and higher data rates, but EHT is already pushing
data rate hard. I think the real limit here is that there just
aren't high T_b sources far from the center.

Higher resolution requires either shorter wavelengths -- challenging
but perhaps 0.8 or 0.9 mm might be possible -- or larger baselines.
That could in principle be done from space, but it wouldn't be quick
or cheap.

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