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:

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