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 |