Zeropoint & Sky Background
ACS Sky Count Rate (in e-/pix/s)
|
WFC |
HRC |
|
Zodiacal Light |
Zodiacal Light |
| FILTER |
Low |
Avg. |
High |
Low |
Avg. |
High |
| F435W |
0.0108 |
0.0180 |
0.0360 |
0.0073 |
0.0122 |
0.0244 |
| F555W |
0.0204 |
0.0340 |
0.0680 |
0.0137 |
0.0229 |
0.0458 |
| F606W |
0.0510 |
0.0851 |
0.1701 |
0.0313 |
0.0521 |
0.1043 |
| F814W |
0.0453 |
0.0755 |
0.1511 |
0.0232 |
0.0386 |
0.0772 |
| F475W |
0.0200 |
0.0333 |
0.0667 |
0.0139 |
0.0232 |
0.0464 |
| F625W |
0.0336 |
0.0560 |
0.1120 |
0.0195 |
0.0324 |
0.0648 |
| F775W |
0.0323 |
0.0538 |
0.1076 |
0.0161 |
0.0269 |
0.0538 |
| F850LP |
0.0223 |
0.0371 |
0.0742 |
0.0126 |
0.0210 |
0.0420 |
| G800L |
0.0755 |
0.1259 |
0.2517 |
0.0414 |
0.0690 |
0.1380 |
|
-
NOTE:
- All calculations are for Earth Shine = SHADOW
- Zodiacal spectrum used in the calculations
is the solar spectrum reddened according to the prescription given in
Leinert et al. 1997 (A&ASS, 127,1) and normalized by the DIRBE (Wright,
astro-ph/0106412) at 1.5 micron and the darkest HDF-N data. We thank Greg
Aldering (Lawrence Berkeley National Lab) for providing the references and
the spectrum to us.
- The convention for the LOW, AVG and HIGH zodiacal
background is as follows: HIGH is the typical value for the ecliptic plane,
AVG is 50% of HIGH, and LOW is 30% of HIGH - representative for the
ecliptic pole (see Leinert et al. 1997 for dependence on the ecliptic
latitude).
- The brightness of the zodiacal light at the
ecliptic pole (the LOW value) corresponds to V = 23.22 mag/arcsec^2,
i.e. about 0.08 mag brighter than the canonical value of
23.3 mag/arcsec^2.
- F850LP (SDSS z) values are probably underestimated,
as the CCD QE and system throughput longward of 1 mic are not well
established.
Zeropoint AB mag
|
|
WFC |
HRC |
| F220W |
NUV broad |
- |
24.29 |
| F250W |
NUV broad |
- |
24.61 |
| F330W |
Johnson U |
- |
24.73 |
| F435W |
Johnson B |
25.41 |
25.24 |
| F555W |
Johnson V |
25.51 |
25.14 |
| F606W |
Broad V |
26.31 |
25.87 |
| F814W |
Johnson I |
25.80 |
25.13 |
| F475W |
SDSS g |
25.82 |
25.52 |
| F625W |
SDSS r |
25.71 |
25.22 |
| F775W |
SDSS i |
25.49 |
24.77 |
| F850LP |
SDSS z |
24.75 |
24.03 |
NOTE:
The quoted zero points refer to an infinite
aperture measurements, after correction for a 5% loss effect not
included in SYNPHOT tables.
How the zero points have been calculated:
We used SYNPHOT and the included ACS tables as in Jan 2000:
a synthetic A0-V spectrum normalized to a Johnson V = 20 VEGAMAG
has been used as input. We determined the AB mag for the syntetic
spectrum in each camera with each filter and the equivalent
counts per second rate. We used a gain factor of 1 e-/DN. The total number
of counts/sec has been multiplied by 0.95 to take into account future
efficiency losses. Conversion to AB mag ( AB = AB_nu + V ) are obtained as:
ABmag = -2.5 log (DN/sec) + zeropoint
detectors 
