Instructions for Observers

These should be read in conjunction with the most up to date user guide that is at the INT.
  • see here and here

    Almanac information

    This is tabulated at e.g. here (but see internal page for links to Almanacs for recent runs). The tables include:
    1. almanac information from skycalc eg [ file ]
    2. quite useful plots that show all the survey field visibilities eg [ gif ]
    Other useful WWW resources:

    Observing scripts

    These are .....

    Pointing centre

    Use the centre of chip 4 as the nominal pointing centre ie Aperture 0. Use the stars that are in the system catalogue that is here They have strange names but they are very bright (V<5) and there is no possibility of making an error about which star is the one you want.

    WFS catalogue files

    These reside in /int/cats. Example ones used for the WFS are:

    Airmass and Zenith distance limit

    No WFS survey data should be taken with an airmass exceeding 1.5 or a ZD of 50degrees. This should only be exceeded for standard stars when extinction is being determined.

    Limiting Seeing criterion

    Ideally we would all like images in 1" seeing. We have selected 5 pixel fwhm as the minimum acceptable seeing that can be used for the survey. This corresponds to 1.65" seeing.

    Nominal Zeropoints (see here for further details) 

    u      22.5
B(KP)  24.5
g'     24.9
r'     24.5
i'     23.8
z'     22.3

    Dark sky sky brightness

    Remember to takeoff the bias levels of ~1600 before comparing with these values 
         600sec   
u     -          
B    600     
g'   700     
r'   1500    
i'   2500
z'   2500

    Exposure times in dark time

    The nominal exposure times in all bands are 600secs. The MGC and Virgo programs use 750secs in B.

    Exposure times in moonlight

  • Formally the exposure time needs to be increased linearly with sky brightness if you want to attain a uniform limiting magnitude.
  • Take action if the sky goes more 25% above the values above by making exposure time 50% longer.
  • When the sky brightness reaches 50% above the above double the nominal exposure.
  • Donot go more than x2 on the nominal exposure times since then the survey speed drops. It is probably better do a redder band or move further from the moon.

    Calibration frames

    Bias frames

    Take 5 bias frames in darkened dome each afternoon or before you start observing in the evening

    Flat fields

    The purpose of flat fields is to remove both small scale pixel to pixel and large scale quantum efficiency variations. There are three types of flat field frames that can be used.
  • Dome flats: Dome flats are taken in a closed dome by illuminating the inside of the dome. Some telescope provide a white screen for this purpose. The ING does not. Dome flats are generally not considered useful since if the camera system is not baffled properly, you can get scatterd light that does not pass through the filters onto the detectors.

  • Sky flats: A Sky flat is made by median combining all the data taken during the night in a single band. In principle this can be used in U through to R with the following important caveat.

    CAVEAT: Since the sky is quite dark in U and B, you may not get enough data during the night to flat field your data. Also the standard WFS mode is to observe a field in all 5 filters as a single observing sequence. This means that you may only have time to observe 6-8 independent fields. One can make the flat from multiple nights but if the weather is band you could end up with data that you cannot flat field. CASU has a library of flat fields but there is no guarantee that they will be good enough.

  • Twilight flats: THIS IS WHAT WE RECOMMEND Take twilight flats in all the filters that you will observe with. This is particulaly important in I/i and z since flats cannot be made from the dark nighttime sky for these bands due to time dependent fringeing. The z flats need to be started soon after sunset since you want to make sure that the flats are dominated by scattered sunlight rather than OH fringes.

    Aim for 20,000 counts per image

    Try to take 5 flats per band. Note you can probably not get more that 10 flats at either dawn or twilight so we accumulate flats over a series of nights

    Below is the actual procedure used at the INT: 

    SYS> guide on pixel 1032 512 - to start the autoguider
    Note the guider must be on so that the smartflat command can measure the sky background in real-time. This command works out the optimal expsoure time on-the-fly using the auto-guider CCD to measure the sky background 
         
Goto a blank field. Some fields are listed below:

    wfcblank1   15 02 00 +29 55 00   J2000.0
wfcblank2   22 58 00 +00 05 00   J2000.0
wfcblank3   02 58 00 -00 06 00   J2000.0
    WARNING: Beware of the standard INT TCS BLANK fields since these are from from Christian et al., 1985, PASP 97, 363. They have been chosen for smaller CCDs. We really need to to check out some 40'x40' fields on POSS and UKST plates at dec=0 and +30 and -30 to define a definitive list for last mosiacs. 
         

 MAKE SURE YOU MOVE THE TELESCOPE AFTER EACH EXPOSURE SO THAT ANY
     BRIGHT STARS GET DITHERED OUT



     DSS resources for field checking 
    
     
     
SYS> smartflat &

A dialog box will pop up and tell the observer the needed exposure
time. Exposures are taken by simply clicking the `take this flat' button.
Remember to OPEN THE SHUTTER! Once all twilight flats are done, type

SYS> guide off
    When taking twilight sky flats the omptimum strategy is to offset in both RA and Dec so that the shift is diagonal. This minimises effects caused by bright star leakage along columns. ie 
         offset arc 30.0 30.0
     offset arc 60.0 60.0
     offset arc 90.0 90.0
     etc
    Note: the offsets are absolute. [However, I suggest you check this]

    Summary of what to do about flat fielding

    Plan to get Twilight flats in at least one filter during each dawn/dusk twilight. A target of 3 per night is a reasonable goal.

    However, get onto you standard stars and focus star as soon as possible each night which means that you will probably concentrate on getting twilight flats during dawn.

    Standard stars

    Make sure you compete your standard star observations well before the end of astronomical twilight. Go to your target fields 15minutes before the end of twilight and focus near this field.

    The time of the start of the first science observation is recorded on the WFS WWW page and can be compared with the time of twilight. [ie big sister/brother is watching you]

    The recommended Landolt fields are these ones

    A minimim of two standards star observations should be carried out during the night. As a minimum observe the filters that you are observing in during the night. However during twilight observe more if time permits.

    A minimum exposure time of 10seconds should be used.

    In U use exposure times of 60seconds are recommended.

    Observing Reports files

    Please fill these in as indicated and note any time lost due to technical problems.

    Observing overheads

    Try to minimise these. Use prepared scripts to avoid human errors None of us is perfect and having to think causes errors.
    Richard G. McMahon <rgm@ast.cam.ac.uk>
    Last modified: Fri Dec 24 19:17:45 2004