Service Mode Tips for WFI on the
MPG/ESO 2.2-m Telescope

Through the experience gained during our first period of service mode observations at the WFI, we have found many ways in which OBs could be made in order to maximize their chances for execution. The aim of this page is to convey that information to our users with the hope that future service observing campaigns can be even more efficient. We have formatted this page as a series of points which complement what is already in the service mode instructions page, that is, the information contained here is not complete.

Tips and Tricks:

• Photometric constraints:
  One of the scarce resources in any observatory are photometric nights. Thus, if you need photometric calibration data, try to implement one or more of the following strategies:
  • Do not mix the PHOT constraint with a very stringent FLI (fractional lunar illumination) constraint. Ideally you should supply us with photometric calibration constraints with no seeing or FLI constraints (see below).
  • If doing a large survey type of program you can overlap your images so that you can tie the zero points of a large series of images together. For OBs of this type you can specify CLR or THN for the weather constrains to maximize chances of execution.
  • If you are doing series of deep dithered exposures, separate your OBs into a very shallow photometric calibration OB, and a deeper science OB. For example, if you have 6 pointings towards Fornax, each pointing consisting of a dither of 3x20 minutes exposures, in 3 filters each (say UVI), for a total of 18 hours shutter time. If you just give us a set of OBs named Fornax_Pi_Fj, where i numbers the field, and j numbers the filter, we will need to use upwards of 20 hours of photometric time, or two to three photometric nights. On the other hand, if you just specify CLR for the above OBs, and in addition you supply us with a set of OBs named PHOT_Fornax_Pi_Fj, with one or two minute exposures each, no guiding, no FLI and no seeing constraints, we will then take only 35 to 40 minutes to provide you with data that will allow you to calibrate your observations. If all observers do this we could use the photometric nights near full moon to carry out calibration campaigns.

• Good seeing constraints:
  • Before requesting an OB with excellent seeing, please check the environmental conditions database. Please notice that most of the nights have highly variable seeing. Thus, if you request OBs with seeing better than 1.0 arcsec make sure that you give us short integration time OBs. For example, if you are doing a lensing study towards the south galactic pole, and have 4 pointings in the R filter, DO NOT supply us with 4 OBs, SGP_P1_R, SGP_P2_R, SGP_P3_R, and SGP_P4_R, each with an integration time of 1 hour in 4 dithers of 15 minutes. In this case it is better to supply OBs of 15 minutes integration each. Thus, you would divide the above OBs in 4, one for each dither, for a total of 16 OBs of the type SGP_P1_Di_R.
  • Do not mix good seeing with the PHOT constraints. If you need excellent seeing and photometry, request also a photometric set of OBs with very short integration time, with no seeing and no fractional lunar illumination constraints (see example above). Many programs that requested FLI=0.0, PHOT, and seeing of 0.8 arcsec or better, did suffer because such constraints made the observation extremely unlikely.
  • Beware that seeing constraint and airmass constraints can be in conflict. WFI does not have an atmospheric dispersion corrector. Thus, for broad band imaging in filters bluer than V the images are elongated for high airmasses. In the case of U even at an airmass as low as 1.2 the effect can be noticeable. Please read the WFI Handbook. In the case in which the image is elongated due to diferential refraction we will assign to that image the smallest of the dimensions of the stellar image and classify accordingly.

• FLI and moon distance constraint:
  Again, if you need deep observations make sure that you do not choose also the PHOT constraint. Use a second set of shallow OBs to calibrate your dark time data (as above). Mixing dark time and excellent seeing will reduce your chances of getting your OBs executed, but this is one of the reasons to have service mode observations in the first place. So, follow the previous guidelines of short OBs with CLR wheather constraint.

Send comments to <ls-220@eso.org> Last update: Mar 15, 2006 -- Jan 07, 2002