WFCAM meeting Edinburgh 12 & 13 June 2003 ----------------------------------------- (These notes are in addition to what was in the pre-meeting documents) 12 June 10:30 WFCAM Schedule (MMC): Up until last week there would have been a December delivery. There have recently been problems with the optical manufacturer in France (SESO). The problem is with the tertiary mirror flexures (supports?), which are 1mm too short. It is suspected (?) that the problem has been known about some time ago but they haven't been telling ATC. This will mean a 10-12 week delay (& close for August). Delivery will now be March/April 2004 with commissioning to the end of May. Alternative ways to produce these and reduce the delay are being investigated. Following this will be on-sky characterization (not science verification) - sky, flat stability etc. A document is needed to list the tests that should be carried out during this period (AA & MJI). <<<< WFCAM will then be on the telescope for at least a month taking survey data. It will then be off, so that CASU can process and test the pipeline. WFCAM Performance (MMC): For many of the tests described, a Moffat profile had been used for the PSF with a FWHM of 0.5" and beta=2. MJI said that at S/N 5 ~ 20% photometry, you are normally at 50% completeness. There was a bit of concern about this and it was suggested that MJI carry out simulations to test the detection limits. <<<< Read out noise is 13-14 e- It is estimated that there will be 14 cosmic ray hits per minute per detector (Craig Mackay). It was stated that these could be removed by microstepping. (Surely this will interfere with the interleaving. What happened to the idea of 2x5s exposures?). Some of this should be thrown into the simulations - unclear who this applies to. Readout time 0.65s Will try and get timings down to 5s for a store to disc for each frame (ATC). This is currently at 10s. However, no major redesign will be done. <<<< A filter change will take 20s. This has implications for some survey efficiencies. When changing filter, why not read out in order to stop frame saturating? Perhaps the array should be reset continuously if the camera is not doing anything. (This is to do with the fact that there is no shutter). However, it's not obvious that the frame will saturate since it will be so out of focus when the filters are out of the beam. This needs to be tested. <<<< Focus discussion - possibly 1/hour will be needed. The pointing errors quoted in MMC's document are a bit optimistic - the values are for just after a pointing test. The accuracy of N-S/E-W is 0.3 degree for all detectors as a whole. Between frames the mismatch will be < 1 pixel. Bad pixels are stable over time. The number of them is quite variable depending on the criterion set for what is a bad pixel. SJW wanted bad pixels cosmetically dealt with - this was thought to be more of a presentation issue. Hot pixels are variable (equivalent to a variable dark current). Persistence could vary with detector. The reset anomaly can be thought of as a sort of bias. If this is not stable then it will be a big problem. (Many of the above have been carried out on not quite the same setup as will be on WFCAM. Detector tests are needed to verify these effects.) <<<< SJW thought that the crosstalk was high. MJI said that it wad a known problem and in optical it is stable. If this is the case then it is solvable. Use laboratory point source to measure crosstalk matrix (ATC). <<<< (lunch) UKIDSS (SJW): The purpose of the prepared document was to answer questions such that the Query Tool (QT) could be designed such that the 2 year plan could get done in 2 years. A simulator will be essential for this. It looks that the current plan will not work since people are asking for too good seeing. Almost all surveys said that they wanted contiguous data. In order to do this - break up surveys into chunks (differently ranked). Current observing conditions described by seeing, sky brightness and photometricity. These 3 are all pretty much binary ie. useful or not useful. Thus 2x2x2 different conditions. Therefore can work out how many hours of MSBs are needed in any of these 8 conditions. The weighting scheme might be based on air mass and number of hours needed (ie total time remaining for that particular survey) for the MSBs. Discussion: PH - better to use hour angle rather than airmass. AA said to use air mass ratio (as he mentioned at Imperial meeting). How would the weighting change as the surveys progress? MMC - why not just observe close to the zenith? If this was done the UDS wouldn't get finished. Comment: You get worse seeing at the other wavebands. The quoted values are for K. This seemed like a bit of a spanner. Is uniform percentile seeing OK for the WGs? rather than the uniform seeing that some WGs have asked for ie. best 70% (say) of seeing in each passband rather than better than 1" for all passbands. <<<< Moon could be a problem. Perhaps treat it like the start of the night? (high sky brightness). Thin cirrus and moon was discussed a bit. Discussed overhead time of data base access at JAC. AA and PH said that there was a possible problem, so it was intended to only download a chunk of all MSBs at a time. Other people were surprised at there being a problem. The idea of variable exposure times for the shallow surveys was discussed. This was in order to get a uniform survey. It was considered probably better to to back to the same location and top up the observation. The problem with variable exposure times (say if sky increases) is that MSBs might increase over end of night. PH suggested "OR folders": these would contain many defined MSBs with different exposure times. The QT will know what the sky conditions are and select the appropriate one. The H2 survey is going to transfer from GPS to GCS (time will be added to GCS and taken away from GPS). Minor planets - avoid planets, but for the fainter minor planets it is intended to ignore the problem. MSB planning - how will the sky offsets for GPS be done? A possible observing strategy might be: JHK, move to new field, JHK etc. and then pop off to do sky offset every 2 hours OR Group observations by filter - JJJ, HHH, KKK. The sky is varying (by how much?). JRL said that it was OK to do it from one tile (ie JHK is OK). But this would not work in the Galactic Plane, where offset skies are needed. Also remember 20s between filter changes. SJW doesn't like the "wait and see" sort of plan for designing MSBs. Ideal design for pipeline is observe in one filter for the whole night. Discussion: where do the responsibilities lie regarding the design of the MSBs. Most people thought it should be that of the UKIDSS WGs, with some advice given from CASU. SJW thought it should be CASU's responsibility to design the MSBs. He wants as much sorted out before WFCAM gets to Hawaii. There was disagreement within UDS about the request for 3x3 microstepping. Some thought that under really good conditions 3x3 will be needed to sample the PSF and you could add the best images together for some other science. The overlap for macrostepping is 25". If the shift due to microstepping is 5", this implies a true overlap of 15". This is too small to do overlap checking of the photometry. Perhaps it will sometimes be necessary to do a big overlap - this will not be necessary since the Survey Definition Tool (SDT) will effectively do this due to guide star unavailability. Dither strategy - order of microstep, dither, macrostep - you want the stuff that's going to be added together done as close in time as possible in case the PSF changes. "Random" dither direction (30" throw) was asked to be put on the wish list. This would be OK as long as you didn't go too far away. This is due to the distortions within the field making it necessary to do resampling. This limit might be 10-20". Thin cirrus is defined as "0"-20% guide star variation. If it is >20% it is classified as thick cirrus. 2 x 2x2 microstep might not be the plan ie. 2x5s(=10s) exposures to get rid of bad pixels. There was a bit of doubt about all this. There was some suggestions about alternative microstepping plans, but mainly it was confusing. PATT (AA): Skyprobe is still a possibility. 20% of WFCAM to non-UKIDSS (17 proposals). Survey Definition Tool (MF): Has given some thought about how to tile an area if one of the cameras fails. It was generally thought that he shouldn't put more effort into this at the moment. Asked about non-standard field centres to fill in a hole. WSA might not like this. Mentioned subtracting areas from surveys. This was mainly in the context of survey definition, but this could be incorporated into how to avoid bright stars. He then demo'd the programme. 13 June 9:30 Observing Tool (AP): A SKY object needs to be added to the list. This will be added into the headers. <<<< The OT allows you to set up an MSB (template). The SDT sets up multiples of the MSBs. Standard Fields (STH): The goal is 1% accuracy with a requirement of 2%. Colour systematics will be solved as a function of x and y. Various possible complications in the calibration were discussed. MMC warned to not go over half a full well when taking standards since the detectors are probably very non-linear after this. Although the non-linearity is corrected for, it is better to not have standards there. The idea is to have standard fields rather than stars. 100s needed per detector to get colour terms - have many fields of these 1000s needed per detector for doing spatial variation - a few of these Note that the colour terms might change with time cf the Sloan filters where the glue dried out! How does extinction vary? AA said that the data was all available. STH said that he would have a look at this. <<<< PATT observers will be forced to do standards as we want them - there was general agreement on this. STH will set up standard fields <<<< A decision on the frequency of taking the standards was needed. Every 2 hours might not be enough, but every hour would have high overheads since it takes ~5 minutes to do standards, 10%. It was decided on the spot to take standards every hour (even on non-photometric) nights. MSBs at telescope (AA): QT only sets up ~1 hour of queue in advance due to varying weather conditions. WFCAM pipelines (MJI): Summit pipeline - conversion to FITS, write to tape and quality control. JAC have bought Ultrium 1's (Eclipse manufacture the box and HP the drive). Need to find out if Ultrium 2's can read Ultrium 1 tapes (CASU?). <<<< The linearity correction is initially measured in the lab. How will this be monitored? Discussion - this correction will be needed per channel. Perhaps stepped exposures of a standard field or of a dome flat. For the INT, the non-linearity problem originated from the amplifier and ADC. Thus there was a request for a STARE read mode (ATC). <<<< The data will be I*4 from the cameras. The data will be compressed before being put onto tape (Rice - lossless, fast, CFITSIO routine). Data monitoring quantities were described. WFCAM pipelines (JRL): Various calibrations described. Most needed characterization in the lab (reset anomaly, crosstalk, persistence, non-linearity). Why is the dark current not linear with time? Flats - wants twilight flats - can't solve this until on the telescope. Self flats will probably have fringing problems. Dome flats are not really flat. Could use dome flat for pixel-to-pixel variations and then a dense star field to remove residual large-scale gradients. MMC said that there was a variation of the vignetting function with attitude (at few % level). MMC needs to check these numbers. <<<< MMC also showed a report showing the PSF from UFTI. He said that he would provide a copy of the paper to CASU. <<<< Variation of seeing - will this make interleaving pointless? There was some evidence that this wasn't the case (???). Offsets >20" will require resampling due to astrometric distortions. MJI said he would do some more checks on the astrometric distortion since there is currently a factor of 3 ambiguity in the supplied model distortions. <<<< There is no data on intrapixel sensitivity ? 1% or less with microstepping. (not sure about this) (lunch) Survey progress monitoring (SJW): The plan was to have SJW as the Survey Scientist until October 2003 and then the Survey Manager would take over. His job would be to get the 2 year plan out. SJW has this new idea of a Survey Food Taster - a science verifier? He is trying to get 2 years of PPARC funding for this. Has someone in mind (his postdoc, Simon). The idea is that this would be part of a series of tests: Summit pipeline QC Cambridge pipeline QC Food Taster rejection The last would not happen very often - might be a delay of a year before starting these tests. Also, users would contact the Food Taster if they find a problem. Commissioning (AA): There was a start on drawing up a list of things that needed doing during commissioning and on-sky characterization. Fringing - spatial and temporal variation Vignetting - how does this vary (with attitude, etc.) Flat fielding methods Photometric variations - mesostepping Set up primary standards (also try and set up secondary standards - unclear) Check microstepping, interleaving PSF measurements Work out WCS Repeatability of filter positioning (implications for flats) Test crowded field and extended objects Scattered light? Guiding in crowded fields Quantification of overheads Cosmic rays Linearity test Bad pixel stability & masks Who sorts this out - MMC the project scientist. A wiki was suggested for this, but the idea was rejected. 19/6/03 Dafydd Wyn Evans (& corrections from others) Initials glossary: MMC Mark Casali AA Andy Adamson PH Paul Hirst MF Martin Folger SJW Steve Warren AP Alan Pickup MJI Mike Irwin JRL Jim Lewis STH Simon Hodgkin Actions distributed by Mark Casali 20 Jun 2003 Grouped by person responsible. JAC. 10) Andy Adamson. Estimate radial effect of moon on sky. Check sky brightness vs zenith angle relationship. 11) Andy Adamson/Nick Rees. Find out how many survey MSBs can be put in optimised database at once. 14) Andy Adamson. Air mass limit to be added as an MSB scheduling constraint - this needs to be added globally. Also add avoidance of bright solar system objects. Scheduling constraints need to be documented. 16) Andy Adamson. Feedback should be given to the survey groups on the design of their survey observing. 20) Andy Adamson. Ensure that PATT observers use the same calibrations and take the same standards as surveys. 23) Andy Adamson. Document how other parties (e.g. UofH) will gain access to data, and what they will be required to do (e.g. standard fields?) 12) Paul Hirst. Write up example usage of OR folder from OT and explain consequences of its use in survey MSBs ----------------------------------------------------------- Mark Casali. 2) Mark Casali. Generate simulated detector images for CASU to use for detection calculations. 3) Mark Casali/CASU. Simulate cosmic rays in data to find out how detectable they are. 7) Mark Casali. Pass on lab data from arrays to CASU: information on bad/hot pixels, QE measurements etc. 8) Mark Casali. Provide CASU with an IR array crosstalk matrix 15) Mark Casali. MSB creation hints (e.g. filters before tiling) should be created after On sky characterisation. 24) Mark Casali/Derek Ives. Characterise IR array reset anomaly and stability/time dependence of darks. 27) MMC. Lab work will need to determine detector non-linearities, detector crosstalk, detector persistence, stability and time-dependence of darks 28) MMC. communicate vignetting variations with telescope attitude to CASU. 29) MMC. send CASU microstep-simulation UFTI images 31) MMC. Prepare draft plan for On-sky Characterisation period. Circulate for discussion. ------------------------------------------------------------- Andy Vick. 4) Andy Vick. Increase speed of exposures to provide one exposure every five seconds. 5) Andy Vick/ Mark Casali. Find out what the IR array requirements are during filter change: does it need to be continually reset. 6) Andy Vick/Martin Folger. Remove requirement for dealing with inaccurate guide star positions from survey tools. 25) Andy Vick/Mark Casali. Evaluate CCD autoguider problems with bright stars. -------------------------------------------------------------- Martin Folger. 17) Martin Folger. Remove complex missing guide star idea from SDT (survey definition tool). SDT still needs area subtraction and must save its state at various points to allow re-evaluation of changes. 30) Martin Folger. Ensure guide star selection includes limit on brightest allowed star on CCD. --------------------------------------------------------------- Alan Pickup. 18) Alan Pickup. Add requirement for a sky component to OT 22) Alan Pickup. Implement breakpoint (optional as opposed to forced stop points) in translator. 26) Alan Pickup. implement stare mode as a new WFCAM readout mode ----------------------------------------------------------------- Steve Warren. 9) Steve Warren. Consult with UKIDSS groups as to whether absolute or percentile seeing measures are more appropriate 13) Steve Warren/survey groups. The H2 part of the GPS survey should be moved to the GCS (with transfer of time). Prioritisation between the two surveys to be averaged. ---------------------------------------------------------------- CASU. 19) CASU. Evaluate the relationship between extinction and Tau-CSO 21) Simon Hodgkin. Set up standard fields for WFCam centred on UKIRT faint standards. Initial calibration frequency should be conservative. ------------------------------------------------------------------