UKIDSS DR2 LAS intra-frameset inter-frame waveband positional offsets

Richard McMahon, Melanie Hawthorn, Mar, 2007
[need to run for later releases]

A frameset is a group of associated multi-wavelength frames. Each chip is a separate frame and is 13.7' x 13.7' in extent. LAS framesets have upto 5 frames in 4 different wavebands; Y, J_[1,2], H and K where J_1 and J_2 are two epochs in J.

Normally all frames in a framesets will all have the same centres to less than 15" (see plots below). However due to guide star limiting magnitudes issues frames in different wavebands within a frameset can have different centres where the offsets between wavebands can be greater than 1'. In DR2 a shift greater than 1' occurs between J and H in the same frameset in 494 out of 6684 with J and H band frames (7%). The shift is greater than 5' in 227 cases(3%). See Figure here

The good news is that the number of frames effected should not increase further(needs to be checked) so the fraction of pointings effected in this way should decrease in the future.

In the case of DR1, there is no shift greater than 1' between J and H frames out of 4394 framesets. See Figure here This causes a number of new features in the WSA. Some of these are:

  • A frameset can end up with the YJ frames and HK frames with slightly different pointings within the same framesets. The source merging process can produce two catalogue sources for the same source where one would be a YJ only source and the other is a HK only source. This featured was identified within the EDR and reported in the DR1 paper(section 4.1) and also in the Dr2 paper(section 3.1) However this is not the whole story. In DR2 a new set of features has manifested themselves.

  • framesets with frames with quite different central ra, decs are included. From the plots below it appears that in DR1 (and also the EDR) framesets only contained constituent frames where the offsets between the bands was less than around 15" but this limit has been relaxed for DR2.

  • Therefore sources that lie within the field of view of one or more bands can be outside the field of view in other bands. An example is shown below for a frameset with frames in YJHK. Below we show the pointing offset difference within a frameset. Sources detected in all 4 bands are plotted in black. The yellow points are sources that are detected only in J. The red points are the sources detected only in K and the Green points are the sources detected only Y.
  • It can also happen that a source which is outside the spatial field of view in a frame on one frameset it may lie on a frame that is contained within another frameset. As a result when one queries the WSA for the images that a source lies one you can find that the WSA does not return the frame from the detection frameset but returns a different frame. An example is the source YY that lies at XY. This source need not be detected on this 'foreign' frameset.

    DR2 distributions

    Some features to note are:
  • Generally the offsets are in the RA direcions but in DR2 there is one example of 4 framesets from the same pointing where the HK offsets are 6.7arcmins in the declination direction. The framesets are: 4337917000[31,32,53,54]. See Figure here

    DR2plus plots

    RABase and DecBase have been used for each frame. CentralRA and CentralDec give similar results.

    DR1plus plots




    Richard G. McMahon <rgm@ast.cam.ac.uk>
    Last modified: Thu Jan 18 16:54:07 2007