A recent WFS run using the SDSS r' filter showed no measurable sky fringes, highlighting the pitfalls caused by the extended the red tail of the conventional "Harris" R-band filter.
The filter specifications for passbands used in the WFS are itemised below for convenience.
Focus offsets (mm)
U 1mm UG2 + 5mm CuSO4 + 1mm UBK7 0.95
B 1mm BG12 + 2mm BG39 + 1mm GG385 0.00
V 2mm GG495 + 2mm BG39 0.00
R 2mm OG570 + 2mm KG3 0.00
I 3mm RG9 + 1mm RG715 0.00
Z 4mm RG850 0.05
KPB 3mm GG385 + Hoya GM/C500 + cut off at 4900A 0.60
g' 2mm GG400 + 3mm BG38 + cut off at 5500A 0.50
r' 4mm OG550 + 1mm BK7 + cut off at 7000A 0.15
i' 4mm RG695 + 1mm BK7 + cut off at 8500A 0.30
The latter three were chosen to match the SDSS filters. For a visual comparison between the current standard INT WFC filters and the SDSS set click here . For more details on filter reponses see ING filter database .
The overall QE of the recommended U ,g',r',i',Z filter set and EEV CCD is available as a postscript file here .
Using these filters results in the following fringing levels - for reference rms dark sky photon noise is approximately 1% of sky for a 600s exposure.
After optimally removing the zeroth order fringes the residual fringe pattern is generally < 1/10th of the original level. In particular the results so far indicate that:
Zeroth order fringe removal has been done for all R, I, i' and Z-band data taken between the October 1998 runs and August 1999, inclusively.
Examples of I-band data before and after zeroth order defringing are available via anonymous ftp, as unsigned 16 bit FITS files.
Normal flatfielded I-band data run no. r129887 gzipped fits file CCD#1
Normal flatfielded I-band data run no. r129888 gzipped fits file CCD#2
Normal flatfielded I-band data run no. r129889 gzipped fits file CCD#3
Normal flatfielded I-band data run no. r129890 gzipped fits file CCD#4
Zeroth order defringed I-band data run no. r129887 gzipped fits file CCD#1
Zeroth order defringed I-band data run no. r129888 gzipped fits file CCD#2
Zeroth order defringed I-band data run no. r129889 gzipped fits file CCD#3
Zeroth order defringed I-band data run no. r129890 gzipped fits file CCD#4
Examples of Z-band data before and after zeroth order defringing are also available via anonymous ftp, in a similar format.
Normal flatfielded Z-band data run no. r142464 gzipped fits file CCD#1
Normal flatfielded Z-band data run no. r142465 gzipped fits file CCD#2
Normal flatfielded Z-band data run no. r142466 gzipped fits file CCD#3
Normal flatfielded Z-band data run no. r142467 gzipped fits file CCD#4
Zeroth order defringed Z-band data run no. r142464 gzipped fits file CCD#1
Zeroth order defringed Z-band data run no. r142465 gzipped fits file CCD#2
Zeroth order defringed Z-band data run no. r142466 gzipped fits file CCD#3
Zeroth order defringed Z-band data run no. r142467 gzipped fits file CCD#4
The master zeroth order fringe frames (or sky super flats if you prefer that term) were constructed using imcombine within IRAF to produce a nightly stacked image for each passband. Each of the nights was them imcombined to produce the "average" or zeroth order fringe frame for each separate observing run. These have been stored as 16 bit, gzipped binary files of roughly 10 Mbyte size and have (.gif) preview images, size of order 100 Kbyte, attached to the list below.
A series of tests on R-band data taken since October 1998 indicates that the fringe pattern is sufficiently stable that the above October fringe frames can be used to successfully remove fringes on subsequent runs. The caveat is that adequate sky flats must have been obtained during each run to remove the temporal foibles (dust etc...).
For further examples of fringe frames contact mike@ast.cam.ac.uk
