Background
The Zero Point (ZP) in a given filter is defined as the brigthness of
a source producing a 1 DN/s on the detector. To determine the
ZPs in the DEIMOS broad-band filters, we first observed a
photometric standard star in direct imaging mode. Once the data
were reduced (bias and flatfield corrected), aperture
photometry was performed on the standard star
images using the task PHOT in IRAF, which relies on the
expression:
mag = zmag - 2.5 * log10 (flux) + 2.5 * log10 (itime)
where:
flux = sum - area * sky
and:
- sum is the sum of all DN in the 30 pixel aperture radius.
- area is the area of the aperture.
- msky is the average sky value in an annulus
centered at the star with inner radius of 40 pixels and a
width of 10 pixels.
For each filter, the Zero Point (zmag) was addjusted to
yield the known magnitude of the standard star (mag) in
the exposure time used for the observation (itime).
If the observations are taken at an airmass close to 1, no
airmass correction is required in the precvious expression.
Zero Points
The following table shows the ZPs obtained on Dec 17, 2015 using
the Landlot standard PG0020+132B. The ZPs are magnitues in the
Vega system producing a flux density of a DN/s. The airmass of
the observations was between 1.040 and 1.043.
| Filter |
Zero Point (mag) |
| V |
27.5 |
| R |
27.9 |
| I |
28.0 |
| Z |
27.0 |