COSMOGRAIL halfnights
(Updated for ECAM !)

You know all this ? Then jump to the summary !

Dear Observer,

Welcome at Euler ! This page is made to help you with the observation of gravitational lenses for the COSMOGRAIL project, which is led by the Laboratory of Astrophysics of the École Polytechnique Fédérale de Lausanne (EPFL). In case of any questions, please fell free to contact me at .

During your run at Euler, you will observe some gravitational lenses typically every third night; these observations take half a night. The dates of these "halfnights" are planned well in advance and I will send you them, as well as the precise target lists, by e-mail. As multiple telescopes are involved in COSMOGRAIL, all observing lenses according to a best possible sampling under the given weather conditions and priorities, we cannot forsee far in advance which lenses will have to be observed in your nights. That's why the target lists for the halfnights are prepared only relatively shortly before the observations.

In case of doubt about the dates of the halfnights, an official schedule can also be found displayed at Euler (look for a dense colorful table somewhere around Greg's seat), and here (access is granted from the computers at Euler). Cosmograil half-nights are marked by a white L on light blue background.

About the project (You could skip this...)

In a few words, the COSMOGRAIL project consists of monitoring strong QSO lenses, to measure their "time delays". We observe distant quasars, gravitationaly lensed by galaxies lying on the line of sight from us to the quasar; as a result of this configuration, we see multiple images (usually 2 or 4) of the same quasar. But the light travel times along the photon paths that form these multiple images are different ! By measuring these differences (aka time delays) between the individual images of the same quasar, we can learn about the absolute geometrical configuration of the gravitational lens. With a model of the mass distribution in the lensing galaxy, these time delays give direct access to the value of Hubble's constant H0 — at relatively low cost, in a way that is fully independent from other techniques, and without a distance ladder or complicated astrophysics.

Two schematic universes; small time delay = large H0, large time delay = small H0. The lens image I show in this sketch is from the HST -- Euler only "sees" the two bright QSO images as blended point sources, but we can nevertheless precisely measure their individual fluxes, using image deconvolution.

To measure these time delays, we repeatedly observe a selection of these gravitational lenses for some years, monitoring the individual brightness variations of the quasar images using our own deconvolution photometry technique ("MCS deconvolution"). This allows us to extract light curves with statistical errors as small as 0.02 magnitudes, while the targets are typically between visual mag 18 to 19 (and highly blended), all this from a 1 meter telescope.

Example of a light curve in which you see the time-delay by eye, between QSO images A and B (astro-ph/0803.4015).

The instrument : ECAM

ECAM (or EulerCAM, also called "CAMERA" by the software) is Euler's imager. It is an E2V 4k x 4k CCD, with a pixel scale of 0.215'', and a field of view of 15' x 15'. ECAM has two different readout modes (specified by the field "ampname" of an OB) : a 4-quadrant readout called "ALL", and a single-quadrant readout called "UL" (for "Upper Left"). The lenses are always observed in the UL mode. Hence I will need bias frames and flats also in UL.

The filter we use for all our observations, "RG" (for "Rouge Genève" :-), is a modified broad Gunn R filter, centered around 660 nm.

ECAM's images are written into /gls/data/raw/[yyyy-mm-dd]. To easily identify them, you could change into this date directory, and type (for instance) :

dfits ECAM.*.fits | fitsort OBJECT FILTER EXPTIME

DS9 and skycat are available to open them (use for instance skycat's object picker (ctrl-p) to get an estimation of the FWHM of the stars), but please be careful to not modify any files in this directories, or litter them with test files.

There is no guiding available with ECAM, and we do not need guiding for our short (360 second) exposures.

If interested, you'll find further information about this instrument in the new ECAM Manual (still in development), here !

The observations

are easy ! Import the OBs from the small ECAMlenses catalog (EDP button "Catalogs: -> Camera -> ECAMlenses/ECAMlenses.rdb"), according to the wishlist of targets that I will send you for each halfnight. Note that we do observe all these lenses, so if you're out of targets you could go for any further lens from this catalog. Once imported, you can still move the lens OBs around your own observations, to plan your night (buttons "swap", "move", "copy", ...) and to optimize airmasses.

Finally, to give you a higher flexibility with your other observations, you can, if you want and in case of relatively stable weather, directly shift by your own up to 2 lenses to an adjacent night. Let me know if you have larger clashes with time critical observations, we can certainly arrange the observing schedule.

Calibration exposures


Many thanks, have fun, clear skies and good luck !