Blue Dots

Observing strategy, and description of the reduction pipelines.

A typical observing season of the Galactic Bulge starts at the beginning of May every year and lasts four months. Among the 691 alerts available in 2006 (579 from OGLE-III and 112 additional from MOA-II), about 180 are available every night in the middle of the season. Of these, around 20 targets can be monitored by 1m-class telescopes, whereas the Danish 1.54m and the 2m telescopes can follow more events. Therefore, we must apply some criteria to select our 20 targets for every observing night. This is done by one member of the collaboration acting as a coordinator, the so called “homebase”. Depending upon the current magnification, the source brightness, and the time of the last observation, a priority algorithm assigns a worth to each of the events and suggests sampling rates with the goal to maximize the planet detection efficiency. If the magnification of one event becomes very high, it may become the sole designated target during that night. While these suggestions are directly submitted to intelligent agents steering the robotic telescopes of the RoboNet network, the homebase currently tunes them using our gained experience before instructing observers at the PLANET telescopes by means of a web page. We plan to embed our experience into future advanced versions of the priority algorithm and further automate this process.

At the beginning of the night, the observer finds on the PLANET web pages the list of targets with sampling intervals set up by the homebase. He then defines the exposure times for each target and reports them on our private web page, so that the homebase can estimate the observing load at each telescope. Typical sampling intervals are 0.5, 1, and 2 hours, according to the priority of each event. However, in case of high magnification event, when the sensitivity to planet is maximal, the sampling interval can be reduced to a few minutes, to the exclusion of all other candidate objects.

At the end of the exposure, the image is pre-processed (bias, dark removed and flatfielded) and gets a standard name and is passed to an on-line pipeline. Starting in 2006, on all the PLANET telescopes, we shifted from a DoPhot-based on-line pipeline (QUYLLURWASI that was used since 1996) to an image subtraction pipeline based on ISIS (Alard 2000). This robust implementation, named WISIS, has two main tasks: process takes all available images of a given event, chooses the best template and subtracts all images from that template after convolving the reference point spread function (PSF) with the kernel to mimic the current PSF. The update task only processes new images using a previously chosen template.

In the case of OGLE, which has accumulated many images of a given field before a microlensing event is detected there, the template is built from a set of the best images and is not held fixed throughout the season. But in our case, we start observing an event when receiving the OGLE or MOA alert, so we have to build the template “on the fly”. This generates problems when new images appear after a few nights, which are better than the first template. We then have to re-run the process routine on all images of the event. A different image subtraction pipeline is used on the RoboNet telescopes but it follows the same philosophy.

The typical uncertainty of the on-line photometry is 1.2% for an I=17.8 mag Galactic Bulge star at the Danish 1.5m telescope, and allows an on-line detection of a deviating signal. When this appears, excitement grows and an alert to homebase is issued. Homebase then prompts an off-line reduction of the event images which are regularly uploaded to the Paris central archive. The off-line reduction is done with our other image subtraction pipeline, pySIS, which facilitates ’fine-tuning’, so as to get the best possible photometry but is more difficult to automate for real-time use. If the off-line reduction confirms the deviation, an alert is issued to the microlensing community to intensify observations and maximize the chances of a good characterization of the deviation, which is absolutely necessary for future modeling of the event. Moreover, all photometric data are made public immediately as assistance to all teams in order to maximize the planet hunting community’s success.

Friday 6 June 2008 by Jean-Philippe Beaulieu