Exercises 2 for RÖ2 (12 Feb)



  1. Work out the field of view and plate scale for the WHT Cassegrain focus (Primary mirror D=4200mm, f/11 focus, with a 2000x2000 chip having square 15 micron pixels). Also work out the size of the diffraction-limited spot and the image size equivalent to a source with diameter 1 arcsec

  2. CCD image defects. Follow the link to find 3 images affected by some imaging problems. The page with the images also contain a link to a web site wihich contain numerous exampels of problems that can occur during CCD imaging and how they might be identified. So before trying to solve the problem yoy need to spend quite some time going through those examples. (The aim of this exercise is to make you look through a whole bunch of image defects that can occur in CCD-imaging.....)

  3. Consider an observation where the flux of a star is measured through a diaphragm and the only noise is due to photon statistics.
    1. How will the signal-to-noise ratio depend on source intensity and on observing time when Flux(star) >> Flux(sky)?
    2. The same for Flux(star) << Flux(sky)?
    3. How does the signal-to-noise ratio depend on the diameter of the diaphragm in the two cases?
    4. What affects the choice of diaphragm size?
    5. At what stellar magnitude is the Flux(star) = Flux(sky) limit reached for a 5 and 10 arcsecond diaphragm resp. when the sky brightness is 20 magnitudes per square arcsecond.? (note: magnitudes are NOT additive!)

  4. During one night you have made the following observations of a standard star (V magnitude = 13.20). Integration time in each case is 100 seconds.

    Zenith distance
    (degrees)
    		 Counts (Source + sky)
    		 Counts Sky
    0
    		 824512
    		 100902
    5
    		 824800
    		 100434
    20
    		 820523
    		 100030
    30
    		 824320
    		 112111
    45
    		 804126
    		 113223
    60
    		 752515
    		 113121

    1. Using these data correct the countrate of the star for the atmospheric extinction.
    2. Estimate the V magnitude for a start with a measured countrate of 1022 counts per second at a zenith distance = 33 degrees.
    3. For the star in b), calculate the monochromatic flux at the center of the wavelength band as well as the total flux in the band.