UKIDSS LAS footprint: SGC

Richard McMahon, Bram Venemans, July 2007

Summary

We propose that the UKIDSS LAS footprint is modified in the south galactic cap to overlap with the SEQUE dec=+5 stripe and the SDSS-I dec=+12 stripe. This would increase the UKIDSS LAS footprint with 4hrs>RA>21hrs that overlaps with SDSS from around 200degrees to around 500degrees. This would would faciliate scheduling of the allocated time to the UKIDSS LAS. and increase opportunities for scientific exploitation of the UKIDSS LAS, particularly from ESO.

Currently the UKIDSS LAS footprint UKIDSS LAS footprint is primarily in the north galactic cap(ngc) in the RA range 8 to 16hrs. In the south galactic cap(sgc) region the UKIDSS LAS footprint has around 210deg^2 along the equator (SDSS Stripe 82) with 4hrs>RA>21hrs. Figure 1 below shows the current (DR6, July, 2007) public SDSS imaging coverage in this region. In July, 2007, SDSS released a new SGC stripe as part of the SDSS-II SEGUE survey. This is shown in green. The SEGUE stripes are defined here . This stripe is close to dec=+5. Above it is a partial SDSS stripe with a nominal declination of dec=+12 (Stripe 76). The current status of the UKIDSS LAS converage is available here . Features to note are the ratio of the total area of sky to the area of sky that is full 4-band(JYHK) coverage. Note the area tabulated takes into account of repeat observations i.e. observed area is count once, but it does not take into account regions of sky with no observations that pass the UKIDSS QC.

The results of the recent review of UKIDSS has allocated the LAS 157nights amounting to 47% of all UKIDSS time over the next 5 semesters starting in semester 2007B. In order to reduce the pressure on scheduling this time in the NGC we propose that the LAS footprint in the SGC is increased to include all three SDSS dec>-5 stripes with the constraint that the UKIDSS tiles have b<-30. This would increase the LAS footprint in the SGC from around 200deg^2 to 500deg^2. To put this in context, the current LAS 4-band coverage is 900-1000deg^2 (including observations up to mid-April, 2007).

A increase of the LAS SGC footprint will speed the UKIDSS LAS up in the short term. For instance, the LAS would meet the nominal two year goal faster. It also widens the opportunities for exploitation during the SGC observing season. The top of the the SGC dec=+12 stripe is accessible from ESO and is similar to the northern boundary of the NGC LAS southern dec zone. Thus the area of sky more easily accessible from ESO would increase. We have analyzed the SDSS quality control data for the dec=+5 stripe and it looks OK. See further discussion below and Figure 2-5

Tiling strategy

From Figure 1 one can see that the SDSS sgc stripes are not parallel with the equatorial equator. This will have to taken into account when preparing MSBs and a stepping in declination of the tile centres as a function of RA could be needed. Alternatively, the PA of WFCAM could be rotated to follow the SDSS great circles. The former staircase strategy would probable be less risky from an operational and data flow point of view. A further option is is define a UKIDSS LAS footprint following the normal tiling strategy and observe tiles that lie within the SDSS footprint. This is more future proof. For instance it would make simplify tiling of a future extension of the LAS over the full dec>0 SGC(b<-30) region.

Analysis of SDSS DQC for SGC data

We have analyzed the data quality of the SDSS data using the following tables:
  1. DR6; Field, RunQA
  2. SegueDR6; Field, SegueRunQA
These tables contains the quality control data for each SDSS field. Each field is 2048 pixels wide by 1489 pixels high. The fields overlap along the scan direction by 128 pixels. The pixel size is 0.396. This each field is 13.5'x9.89'. The overlap of 128pixels corresponds to 50.6". A field is 1/27th of a square degree. The camcols, strip and strips also overlap by 1'. SDSS Legacy covers 8417ceg^2 which is covered by 325517 fields i.e. 38.7 fields per deg^2. Figure 2 shows show some DQ plots for the whole of DR6 for the i waveband. Figure 3 shows the same type of data for the SEGUE SGC dec=+5 stripe. Figure 4 shows the data for Stripe82 which is the region that overlaps with the current LAS footprint. Figure 5 shows the data for Stripe75 which is the DR6 dec=+12 stripe. with the current LAS footprint.

Some points to note from the quality control analysis for the SDSS i band data show below:

  1. Seeing: The median seeing for SDSS DR6(excluding SEGUE), Segue p05 and Stripe82 respectively are 1.28", 1.13". and 1.14".
  2. Airmass: The median values are 1.13, 1.14, 1.23.
  3. Photometric zeropoints: The median values are 23.67, 23.64, 23.67.
  4. Sky brightness: The median values are 20.30, 20.22 and 20.09.
  5. Sky noise: Fractional Sigma of Sky Value Distribution, expressed as magnitude. Sky noise = skySig * sky * ln(10)/2.5. We are not sure whether this is per pixel of per square arc second. The sky is defined as mags/arcsec^2 but it ambiguous to us whether the noise is per pixel or per arcsec^2. We will need to check this using some image data. The median values are 0.78, 0.75, 0.71.
Therefore whilst the seeing in Stripe82 is better than average the SequeP05 stripe is close to the median all of the SDSS. The photometric zeropoint distributions are very similar. SEGUE p05 has a similar sky brightness to DR6 with a median of 20.30 and 20.22 respectively. The Stripe82 median i band skybrightness is 0.13 magnitudes brighter. This could be due to the higher median airmass or solar cycle trends. The sky noise for values for SEGUE p05 lies between the DR6 median and the Stripe82 value. We need to check what this means.

TODO

  1. determine the 5sigma point source limiting magnitudes. This needs more work in order to determine whether the skynoise is per pixel or arcsec^2.
  2. look a sky brightness trends with mjd to see if there are solar cycle trends; could do this for z too. Also look at airmass and time after sunset.

Figure 1 SDSS-I/II DR6 coverage in the SGC. Red shows the SDSS-I foot print. Green shows the stripe observed as part of the SDSS-II SEGUE project. [Top of page]

Figure 2 DQC distributions for DR6. [Top of page]

Figure 3 DQC distributions for SEGUE sgc +5 stripe. [Top of page]

Figure 4 DQC distributions for DR6 Stripe82. [Top of page]

Figure 5 DQC distributions for DR6 Stripe76. i.e. the dec>0 SGC stripe with dec around +12. [Top of page]

Richard G. McMahon <rgm@ast.cam.ac.uk>
Last modified: Sun Jul 22 12:41:17 BST 2007