(Updated for ECAM !)
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.
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.
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 !
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.
- Each "lens" consists of 5 x 360 s dithering + overheads, this sums up to about 35 minutes per target.
- Number one observational constraint for our project : the seeing !. If the DIMM seeing (as shown on the ESO MeteoMonitor) is worse than 1.5 arcsec, do not observe lenses, but move them to the next night, or even the night thereafter (yes, this is an official agreement). In case the MeteoMonitor is not available, you could measure the FWHM of some stars using skycat on the ECAM images (pixel scale 0.215"). If the DIMM seeing is significantly above 1.5" (or the FWHM of the stars on the actual images is above 2.0", i.e. FWHM > 9 pixels), the images will be essentially useless.
- There is no need for a perfectly photometric sky. Light cirrus clouds are ok. But if it's too cloudy, the same rule as for bad seeing applies : shift the lenses to the next night.
- Try to observe the lenses at best possible airmass, at least below 1.5, but not at zenith, as Euler sits on an alt-az mount. So keep elev < 80.0 deg (anyway this applies only to HE2149-274 and Q1355-2257, other lenses don't come close to the zenith).
- You can start observing in nautical night (the planning if forseen to do so, same applies for the other Euler programs). If you end your night with a lens that is just rising, it is perfectly ok if the last exposure gets shifted out of the nautical night.
- The wishlist of lenses that I'll send you takes into account the position of the Moon; you can observe any of these lenses at the planned date. In case you want to judge by you own, a rule of thumb : the sky background on the ECAM images should stay below 3000 ADU.
- Bias frames : EDP Insert -> CAM_BIAS, then switch the ampname to UL and duplicate the exposure to get 5 images. I need only one such set of 5 UL biases per night. You can take these bias frames for instance after the Coralie calibs, or after the evening flats, waiting for the night to fall.
- Flat fields are very important to us, as we perform differential photometry over sometimes large distances across the image. With ECAM, skyflats have to be taken daily, in all filters and readout modes used during the night. For the lenses, the only filter we use is RG, readout mode UL, and 3+ of such flats in the evening and 3+ in the morning would be optimal, at least for the nights containing lenses. You are always welcome to take flats on any other evenings or mornings of your run : we tend to build masterflats over periods of the order of one week, so any RG flat of your run is useful, the more the better !
The "skyflat OB" corresponding to the current month is sent into the EDP from : Catalogs: -> Camera -> ECAMutils/flat.rdb. For each twilight, insert the corresponding OB. For each ampname, some desired filters/ampname combinations have now to be specified in the "flatlist" field, separated by spaces or commas. Each combination (like "RG/UL") written into this flatlist corresponds to 3 exposures with this filter and ampname; the exposure times will be adapted automagically, always aiming at a flux of 20000 ADU. But, you have to respect a given ordering of the filters that you write into the flatlist (e.g., in the evening, we take flats starting with the darkest and ending with the most transparent filter). A list giving this order is displayed on the pinboard behind the printer.
If you lack of inspiration, you could try this :
- Evening flats : UG/ALL,BG/ALL,VG/ALL,ZG/ALL,RG/ALL,RG/UL,IC/ALL (note that UG starts already just before sunset !)
- (ALL biases, UL biases, night, and again ALL biases just after the night)
- Morning flats : RG/UL,RG/UL + whatever filters were most used in the night
- Focus sequences are essential to get sharp images, even if Euler's autofocus will try to compensate for temperature changes during the night. Always leave the autofocus on, even while taking the manual focus sequences ! The latter will serve as zero point for the autofocus.
Focus sequences are inserted into the EDP using the menu Catalogs: -> Utils -> focus/focus_for_camera.rdb. Choose any star whose RA is relatively close to the sideral time (visualized at the very bottom of the EDP window, label "ST"), and thus high up in the sky.
Start the night with two successive focus sequences (of the same star). This will not cost observing time, as you can launch these observations 7 minutes before the nautical night. There is nothing else to configure for these focus exposures, but the software will almost certainly ask you to pick the focus star by hand. Be ready to do so when observing a focus sequence :
- Minimize all the big windows of the lower right screen (catalogs, ECAM RTD) exept for those with the fancy rainbow colours : the manual star selection will be done in the latter.
- At the beginning of the focus sequence, a dialog might appear in the lower left screen, inviting you to select the focus star, by clicking on it in the "Image Xaff" window on the lower right screen.
- If you cannot see stars in that window, click on "Cuts" and then "Redisplay", or tweak the cuts by yourself. Then just click (left mouse button) on the brightest star of the field. It shouldn't have a companion, and usually it should be close to the center of the image.
- One after the other, 5 images of the same star will now appear in the Image Xaff window, with different focus positions. Image #3 should be clearly sharper then images #1 and #5 (both in terms of FWHM and visual aspect), at least in the second focus seqence you've inserted at the beginning of the night. The software will determine the optimal focus by a parabolic fit on these 5 measurements. Ideally the best FWHM (given in arcseconds) would be close to what the DIMM gives; however this will not be the case : currently Euler is not seeing-limited.
- 5 UL biases per night
- Aim at 3 RG UL flats in the evening, and 3 in the morning (minimum)
- Start the night with a duplicated focus sequence, launch them say 7 minutes before the nautical night.
- Focus sequence : expect manual star selection !
- Observational constrains : DIMM seeing < 1.5'' (or FWHM < 9 pixels), airmass < 1.5
- In case of bad weather, shift the lenses to the next nights
- Insert extra (non-duplicated) focus exposures before each large bloc of lenses