1. The problem statement, all variables and given/known data Hi I have some homework that requires me to write an observing profile for a number of targets. The goal is to obtain B and V band images that can be used to measure integrated photometry as well as surface brightness profiles with a spacial resolution of at least 4kpc. Have been given a number of targets in the form: Ra; Dec; Total V Mag (Vega); Major axis (arcmin); Peak Surface Brightness (mag/arcsec^2); Morphological type; Redshift For one of the targets it is Total V mag =13.4, Major axis=2.3, Peak surface brightness=17.5, type: Sb, z=0.001 Can also assume that the galaxies have a flat surface brightness distribution. 2. Relevant equations 3. The attempt at a solution I am currently trying to work out the exposure times, I have determined a date range that is suitable and the corresponding airmass at different times. I am using an exposure calculator found at https://www.eso.org/observing/etc/bin/gen/form?INS.NAME=FORS+INS.MODE=imaging but it requires a target magnitude. I am not sure how to work out this value from what is given. I was expecting a value around ~24 as a guess. Aiming for a SNR of at least 10. Thanks for any advice or pointers you can give.