SupIRCam

SupIRCam was decommissioned at the end of July 2010 following a failure of the cooling system. It will be replaced by IO:I, scheduled for late 2014.

Introduction

The SupIRCam near-IR camera was funded by PPARC and was built jointly by JMU (responsible for software and integration) and IR Labs (responsible for hardware and electronics). Initial on-sky commissioning was completed in April 2004. Robotic commissioning was completed in early 2005.

Status History

2006
The camera was removed from the telescope in August 2006 and returned to the UK for improvements to the cooling and electronics systems. It was refitted onto the telescope in February 2007. Overall these changes resulted in a reduction in read-noise, removal of the variable bias level and better behaviour of hot pixels preventing them affecting neighbouring cells.

2007
On 27th June 2007 operations with SupIRCam were suspended due to failure of the mirror deployment mechanism. Operations recommenced in September 2007 following a servicing mission.

2009
In August 2009 the cooling system failed and the instrument was taken off-sky while an assesment was made of future options. In November 2009 the cooling system was repaired, and SupIRCam was returned back on-sky.

2010
On 11th July 2010 the cooling system failed again and the decision was made not to attempt another repair but instead focus effort on the construction of our new infrared-optical camera IO. The instrument was therefore effectively decomissioned at that point, and removed from the telescope in September 2010.

Specifications & Performance

Detector	256x256 pixel PICNIC HgCdTe array
Pixel scale	0.413 arcsec/pixel (no binning available)
Field of view	1.7 arcmin (no windowing available)
Gain	6.8 electrons/ADU
Dark current	15 electrons/second
Read Noise	26 electrons
Saturation Limit	20,000 counts
Linearity (below 20,000 counts)	Better than 1% in reduced data
Exposure Times available	1,2,5,10,20 seconds
Approximate Zeropoint (1 count)	21.5 at J and H
Approximate Zeropoint (1 electron)	23.5 at J and H

Filter Set

There were no cooled optics in the instrument, so only the J and H band filters were available for use. Transmission curve tables can be downloaded by clicking on the filter names below. Column 1 in the table is wavelength in Angstroms, column 2 is transmission in percent.

Filter Name	Wavelength Range (Angstroms)
Barr J	11200-13800
Barr H	15200-18200
Barr K'	19600-23100 (not available)

Standards

Standards were spaced approximately every few hours of RA and were observed in all bands every three hours by the robotic control system. Click on a name to bring up a 2MASS J band image of the field. North is at top, east to the left.

Name	J2000		J	H
	RA	Dec
FS109	03h 13m 24.16s	+18° 49' 38.4"	11.434	10.989
FS14	07h 24m 14.40s	-00° 33' 04.1"	14.101	14.163
FS21	11h 37m 05.15s	+29° 47' 58.4"	12.975	13.042
FS134	14h 28m 43.37s	+33° 10' 41.5"	11.884	11.235
FS146	18h 54m 04.01s	+37° 07' 18.6"	10.704	10.213
FS30	22h 41m 44.72s	+01° 12' 36.5"	11.939	11.972

Magnitude data is obtained from here: http://www.jach.hawaii.edu/UKIRT/astronomy/calib/phot_cal/faint_stds.html

Pipeline

All SupIRcam data since April 2007 was run through a data reduction pipeline before being distributed to users. This pipeline performed linearity correction, dark current subtractions, flat-fields and sky subtraction. It is important to note that the sky image was generated by building a median stack of all the dither frames (suitably smoothed) within a single observation, typically either 5 or 9 frames. If a large extended source was in the field, it was possible for it to be included in the sky image and therefore some small fraction of its flux to be subtracted from each image. The sky image used was included in a FITS image extension called SKY within each distributed image for user inspection, allowing users to remove the sky subtraction stage from the reduced image if needed.

As an example, to use ds9 to inspect the sky frame which was automatically generated and used in the reduction of frame s_e_20081010_1_1_1.fits, one would have used the command
ds9 -zscale s_e_20081010_1_1_1.fits "s_e_20081010_1_1_1.fits[sky]"

By contrast, the flat field used in the reduction was generated from a very large sample (many thousands) of contemporary observations and so was not affected by that problem (the flat field stability being much greater than the sky stability).

Data which predated the availabilty of this pipeline (i.e., prior to April 2007) are still only available as raw data. They were not loaded into the main data archive but may still be downloaded from Unloaded Data.

SupIRCam overheads

One would use the exposure time calculator to get the total exposure time needed for a source. One then had to select from a fixed range of individual exposure times and dither patterns to match the total exposure time, such that:

(individual exposure time) x (dither pattern) ≥ total exposure time

Individual Exposure Times

Only set exposure times were available, of 1, 2, 5, 10 and 20 seconds. Because the dither overhead was currently large (20 seconds per dither position), it was better to have fewer, longer exposures within the constraints of saturation and obtaining sufficient dither positions to allow good sky subtraction and flat field creation. For most targets individual exposure times of 20 seconds were recommended.

Dither Patterns

There were four dither patterns, selected by option number. There was no overhead for filter changes.

Dither Pattern	Option number	Details
	1	Gave a single pointing. Note that this made accurate sky subtraction difficult unless a separate offset target was specified. Even then it was difficult to construct a flat field from the data alone.
	2	Gave a pair of pointings offset by 7 arcseconds. This was sufficient to do a simple sky subtraction for point sources, but was not ideal for deriving the best flat fields from the data alone unless a separate offset target field was also obtained.
	5	Gave a "cross"-shaped mosaic of five pointings with offsets of 7 arcseconds. It was the best choice for general purpose use, allowing good sky subtraction and flat field generation.
	9	Gave a "square" mosaic of nine pointings with offsets of 7 arcseconds. It allowed the generation of the best quality flat fields, and was recommended for crowded or complex fields.

Additional SupIRCam overheads

Additional overheads added were:

• Acquisition time (time taken to slew the telescope on target): 60 seconds
• Dark Time overhead: equal to twice the individual exposure time.

Worked Example

Obtain J band photometry of a 14.8 magnitude object in Bright/Average conditions at SNR=100:

• The exposure time calculator gives 178 seconds (remember to set 1x1 binning in the calculator - SupIRCam is never used binned).
• Selecting individual exposures of 20 seconds combined with dither pattern option number 9 gives a total exposure time of 180 seconds.
• The total time required would therefore be:
  • 60 seconds acquisition
  • +9 x 20 seconds (exposures)
  • + 2 x 20 seconds (darks)
  • + 9 x 20 seconds (dither overhead)
  • = 460 seconds.