NOTE on photometric
calibration of ESO-LV optical disk images
Edwin Valentijn Kapteyn Institute
Groningen 19 April 1995
Users have reported that when they derive surface photometry from the ESO-LV images (the calibrated
images distributed by ESO)
that they obtain magnitudes for the ESO-LV targets that are on average
0.21m fainter in the B and 0.10 m fainter in the R than the
corresponding values of similar parameters in the ESO-LV parameter catalogue - e.g.
Peletier etal A&A Suppl. 108, 621 (1994).
Here, it is explained that this is due to an airmass = 1 calibration of
the images, while the ESO-LV catalogue has been reduced
to airmass = 0.
I investigated this issue and came to the following conclusions:
i) the ESO-LV parameter catalogue
contains the proper magnitudes and surface brightness values (for zero
airmass). The ESO-LV magnitude
scale uses an atmospheric airmass unity extinction in B of 0.20 and in
R of 0.10. The ESO-LV magnitude scale corresponds closely, within
a few hundreds of a magnitude, to the RC3 scale and the note given in
the printed text of RC3 (page 42) about a systematic difference between
ESO-LV magnitudes and RC3 is withdrawn by the RC3 authors; actually,
contrary to what is sayed there, ESO-LV magnitudes were copied straight
into RC3, without any correction (Paturel Priv. communication,
A&A paper 1994).
ii) The sky brightness both in the
ESO-LV parameter catalogue and in the header of the ESO-LV images are made with
precisely the same calibration scheme as used for the galaxian
photometry. As a result, the listed sky brightness values
correspond to zero airmass sky and are in fact brighter than the real
sky brightness (weird, but this is due to the fact that the sky value
is essentially used for setting the photometric
scale). In order to obtain the true sky surface brightness one
has to add 0.2m in B and 0.1m in R to the values listed in the ESO-LV
parameter
catalogue and to the headers in the images.
iii) The ESO-LV images have
been calibrated using both the first step and the second step in the
ESO-LV calibration procedure (first step
using one high quality photo-electric standard per plate and the
calibration spots, second step using additional standards). The
amplitude of the second correction is small (the mean correction is
less than 0.02m, the sd of the correction is 0.13 in B and 0.22 in R).
The preceeding conclusions do not result into the difference in
results from images and the parameter catalogue, but merely stress
the accuracy of both.
For reasons I could not trace anymore, the image data of ESO-LV have been
calibrated to the airmass=1 situation.
This means that, given the ESO-LV adopted atmospheric extinctions,
galaxian photometry directly on the images
should be corrected according:
B = B_on_image - 0.20
R = R_on_image - 0.10
The sky brightness as derived from the images correspond to the true
sky brightness, which is, as explained above, fainter than the sky
brightness given in the header and in the catalogue.
By applying the above correction to photometry obtained from the images
one should obtain zero airmass magnitudes, constistent with the
ESO-LV catalogue and RC3.