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. 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, narrowband filters, and a Y band filter 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 |
| Y | 17.9 | 11.9 | 14.2 | 16.2 | - | - | - | 2-5 |
| 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.)