Guide to FOCAS imaging exposure times

Background-limited operation

With a read noise of 8-10 e- rms, FOCAS achieves background-limited performance (BLIP) when the background level reaches ~300 e-, as indicated by the following table.

Filter Background Min. BLIP
time (s)
(mag/arcsec2) e-/s/pixel
U 21.3 0.6 500
B 21.9 4.0 80
V 21.1 7.6 40
R 20.5 12.0 25
I 19.5 22.9 13
Note: these background levels are for dark time. BLIP times will be significantly reduced at other lunar phases.

Saturation magnitudes

The following table lists the point source saturation magnitudes in good (0.3'') seeing for a number of different exposure times.

Filter Saturation magnitude Maximum
exposure (s)
1 s 10 s 60 s 300 s 900 s
U 11.9 14.4 16.4 18.1 19.3 52 000
B 14.6 17.1 19.1 20.9 22.1 7 500
V 14.5 17.0 19.0 20.8 22.2 4 000
R 14.4 16.9 18.9 20.8 22.2 2 500
I 14.1 16.6 18.6 20.6 22.8 1 300



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