General instructions for defocussed photometry of planetary transits

This project aims to obtain high-precision light curves of transit events in extrasolar planetary systems. The approach is to defocus the telescope so each point spread function covers hundreds to thousands of pixels. We then continually image the field with relatively long exposure times (60-120 s) from some hours before the transit starts to some hours after it has ended. Features of our defocussing approach:

This page contains general instructions for performing defocussed photometry. Any specific instructions given on the main webpage override the notes below.

Observing procedure

  1. Filter. We want to detect plenty of light from these relatively red stars, but we also want to avoid the fringing which affects CCDs at red wavelengths. The filter to use is normally indicated on the main webpage - if it is not then default to Cousins I or Sloan i.
     
  2. Acquisition. At or before the indicated start time, point to the target with the telescope focussed. Coordinates and finder charts will be available on the main webpage. Take an image of the field of the target, and check that you have the correct field. Please save this image as it is an important part of the data analysis.
     
  3. CCD positioning. Check that the target and comparison stars are on optically good bits of the CCD (no dodgy pixels, bad columns, dead spots, dust grains etc).
     
  4. Binning. For the defocussed photometry project we do not normally bin the CCD.
     
  5. Windowing. Now the target and comparison stars are on good bits of the CCD, change the CCD window parameters so the images cover only the highlighted area on the finder chart. This is done to decrease readout time. If in doubt, make the window a bit too large instead of too small. Record the CCD window pixel values for inclusion in a night report, as they may be needed during the data reduction.
     
  6. Defocussing. It is a good idea to experiment with this in advance, either earlier in the night or the night before, as it could take up to 30 minutes to be sure you have got the optimum defocus. Defocus the telescope to some reasonable value and take an image with the correct exposure time. We want the light from the target (and hopefully the brightest comparison stars) to form an annulus with maximum counts per pixel about 25000 to 35000. If the maximum counts are outside this range then adjust the defocus and try again. Take a note of the focus value and include this in your night report.
     
  7. Observe. Once the telescope is defocussed properly, start the sequence of images of the target field.
     
  8. Autoguiding. It is good to autoguide, as keeping the light from the stars on the same pixels makes the final light curve better. If autoguiding is impossible (e.g. if the autoguider is also defocussed) then you will need to monitor the incoming data and manually adjust the pointing if the stars of interest get close to the edge of the CCD or a set of bad pixels.
     
  9. Relax and monitor. Check the images every so often to ensure all is well, and keep a note of the weather conditions.  
  10. Timing check. At some point during the transit (or alternatively at the start or end of the night) it is important to check whether the timings imprinted on the fits file headers are correct. Choose one exposure and note down the time the shutter opens using either a www resource (for example timeanddate) or your watch. Include this information in the nightlog.
     
  11. Finishing. Continue the observing sequence until the given finishing time. Refocus the telescope and take one more image of the field for reference.
     
  12. Flat fields. It is good to take plenty of flat fields for this project. These can be taken in the twilight before or after the transit observing sequence, or even on adjacent nights. Remember to use the correct filter and to not window the CCD down. Counts for the flat fields should be in the range 25000 to 35000 ADUs.
     
  13. Report. It is very useful to issue a short night report. Please include the following: Many thanks in advance!