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ESO-LV magnitudes and RC3 is withdrawn by the RC3 authors; actually, contrary to what is sayed there, ESO-LV magnitudes were copied straight into RC3, without any correction (Paturel Priv.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.