The previous commit asserted that the 21 Jul Centaurus A data came back
with a saturated nucleus, and booked a short-exposure core set on T32 to
fix it. The assertion was wrong.
It came from taking the maximum pixel inside a 300 x 300 px box centred
on the frame. That measures the brightest thing near the middle of the
image, which is not the same as measuring the galaxy. The 65313 ADU
belonged to a foreground star 128 px (69 arcsec) from the nucleus, well
inside the box. Median filtering the inner 800 x 800 px to remove stars
leaves the galaxy peaking at 1944 ADU, roughly a thirtieth of the clip
level, with not one star-free pixel above 30000 ADU anywhere in that box.
The brightest pixel in the whole frame is at the right-hand edge, 2370 px
from centre.
The core looked blown in the processed image because of the stretch, not
the sensor: the white point sat at the 99.995th percentile, a level set
by field stars, so the galaxy's entire range from a 7 ADU sky to a 1944
ADU peak was compressed into the top few percent of the tone curve.
Re-rendering the same frames with a second curve scaled to the galaxy
recovered the core completely, with no new data.
The T32 reservation has been cancelled without running, so it cost
nothing. The other three reservations were verified intact afterwards
and quota is back to 6:05 of 8:00. plans/hdrcore-t32.txt stays in the
repo and on the scope: 47 Tuc and the SMC may genuinely saturate, a
dense cluster core being the fastest thing there is to clip, and the
ladder is ready if they do.
The corrected habit is recorded in CAMPAIGN.md: measure whether a core is
saturated before buying time to fix it, filtering stars out of the
measurement first. The check takes a minute and would have saved this
whole detour.
TODO.md also records a second result that affects future planning: the
faint outer halo of that stack is limited by the sky-plane subtraction,
which absorbed about 17.9 ADU/px of real halo light, rather than by
exposure time. More integration alone would not have gone deeper.
Laurence
7dc6119359