A guide to Suprime-Cam exposure times
Saturation is almost inevitable for broad-band observations with Suprime-Cam. The large pixel size of the instrument and light gathering power of the primary mirror produce high count rates per pixel, and a number of bright stars will always be present in the large field of view, even at high Galactic latitudes.
The following table shows saturation magnitudes (defined as >150 000 e-) for different exposure times in good seeing (0.5''), when approximately 10% of the light from an unresolved source falls in the peak pixel. Also listed in the last column is the time it takes for the sky background to reach the CCD satuation level. A moderately dark sky (approximately 3 days from New Moon) has been assumed. Magnitudes for the Johnson-Cousins filters are on the Vega system, and those for the SDSS filteres and narrowband filters are on the AB magnitude system.
| Filter | Sky brightness (mag/arcsec2) |
Saturation magnitude | Typical exposure time (min) (2) |
|||||
|---|---|---|---|---|---|---|---|---|
| 1.2 s(1) | 10 s | 60 s | 300 s | 600 s | 900 s | |||
| B | 21.9 | 13.6 | 15.9 | 17.9 | 19.6 | 20.4 | 20.9 | 15-20 |
| V | 21.1 | 13.8 | 16.1 | 18.1 | 19.9 | 20.8 | 21.3 | 8-12 |
| Rc | 20.5 | 13.8 | 16.1 | 18.1 | 20.0 | 20.9 | - | 6-10 |
| Ic | 19.5 | 12.8 | 15.1 | 17.0 | 18.9 | - | - | 4-8 |
| g' | 21.4 | 13.8 | 16.1 | 18.0 | 19.8 | 20.7 | - | 10-15 |
| r' | 20.6 | 14.0 | 16.3 | 18.3 | 20.2 | - | - | 6-10 |
| i' | 19.7 | 14.1 | 16.4 | 18.5 | - | - | - | 4-8 |
| z' | 18.3 | 13.6 | 16.0 | 18.1 | - | - | - | 2-6 |
| NA656 | 20.6 | - | - | - | - | - | - | 20-30 |
| NB711 | 21.1 | - | - | - | - | - | - | 20-30 |
| NB816 | 20.5 | - | - | - | - | - | - | 20-30 |
| NB921 | 20.4 | - | - | - | - | - | - | 20-30 |
(2)Commonly-used exposure times. The sky background levels in each band are approximately half the saturation level with these exposure times. Since the sky background in bands redder than Rc-band varies with time, the observer must adjust the exposure time to avoid the sky background saturating. (The variation is as large as a factor of 4 in z'-band, smaller in Rc-band.)