Sigma QUick Infrared Camera User Manual

Observer’s Guide for QUIRC

K.-W. Hodapp J.L. Hora M.R. Metzger

University of Hawaii

October 30, 1997

1SystemOverview

The University of Hawaii (UH) QUick Infrared Camera (QUIRC) utilizes a 10241024 pixel HgCdTe Astronomical Wide Area Infrared Imaging(HAWAII) array produced by Rockwell Science

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Center. This array is sensitive to radiation from 1 to 2.5 scale, givingthe pixel scales listed in Table 1 for the various telescopes and conﬁgurations.

Table 1. QUIRC pixel scales

Telescope Optics arcsec/pixel FOV (arcsec)

UH 88-inch f/10 0.1886 193x193

f/31 0.06084 62x62

CFHT f/8 0.150 154x154

0.61-m f/15 0.43 440x440

QUIST 0.25m f/10 1.5 1550x1550

The QUIRC system is comprised of four functional components: (1) the detector, optics, and dewar; (2)The detectorreadoutelectronics;(3)A DSP controller;and (4) the instrument controlSparcstation and ﬁber optic communications interface. The ﬁrst three components are physically integrated and mounted on the telescope, while the fourth is typically located in the observing room and/or the computer room.

m. The reimaging optics provide a 1:1

The QUIRC electronics are controlled from a Sparcstation by issuing commands and receiving data via ﬁber optic cables. The control program on the Sparcstation is called “qcdcom”. The qcdcom program is based on the ccdcom program by M. Metzger and was modiﬁed for use with QUIRC. The qcdcom program controls taking exposures and writing data in FITS format to disk, operates t he moving parts of the instrument such as the shutter, ﬁlter wheel, and pupil mask, communicates with the t elescope and guider to obtain information and perform mosaics, and provides a script capability for automatically performing simple observing tasks. qcdcom is a command line interface only and does not directly provide image display, but can be used with any popular display program that can read FITS ﬁles (e.g. saoimage, Vista, IDL). A link has also been provided to the viewﬁts program to automaticallydisplay images (see below).

2 Near-Infrared Observing Techniques

Imaging in the near-infrared (1–2.5m) generally requires more effort than at optical wavelengths, because the background is so much higher. There are two general data reduction techniques in common use—both of these require frequent observation of sky ﬁelds.

The ﬁrst data reduction philosophy is one in which the sky ﬁelds are used for subtraction,and the sky subtractedimage is divided by normalizeddome ﬂats to remove the variationsin quantum efﬁciency. The advantage of this technique is that the dome lights have similar color temperature to the typical sources being studied.

The second data reduction technique is one in which the sky exposures are also used as ﬂats, so the image is sky subtracted, then divided by a normalized sky ﬂat. This technique often will work better

QUIRC User Guide

in cases where the background has been changing rapidly. It may also give better results if the dome ﬂat was not evenlyilluminated(it is difﬁcultto achieve even illuminationat the 0.6m telescope). The dark should be subtracted from the sky before division. The disadvantages of this technique are that the sky ﬂat shows the response of the detector to the OH airglow + thermal emission. In particular, fringing may be present in certain conﬁgurations (e.g. 2.2m f/10 1:1), and fringing is something which should be subtracted, not divided.

Thenumberofbadpixels usuallydictatesaspecialtechnique forobservinginwhichseveralexposures are made of the ﬁeld being studied, with each exposure shifted slightly from the others (dithering). When the images are combined,the bad pixels in one image can be “ﬁlled in" with good pixels from a shifted image. This technique also improves ﬂat-ﬁelding relative to a single long exposure at the same position.

It is recommended that at the start and end of each night dome ﬂats and darks be taken. The darks should ideally be exposures of the same length as the object exposures. Even if the darks are not directly used in the reduction, they will serve to show which pixels have high dark counts so that these pixels can be included in a bad pixel mask. Dome ﬂats are generally taken as a lights on/lights off pair. Using this strategy results in a differenceimage (ON – OFF) which represents the detector’s ﬂat-ﬁeld response to a source with color temperature of a few 1000 K, which is roughly the same temperatureas some of the sources being studied.

The shutter is a leaf type shutter, meaning than the center part of the aperture is open slightly longer than the outside. Recent tests showed signiﬁcant center - to-edge illumination differences for integrationtimeslessthan1 second. Therefore,short exposures should be avoided, particularlywhen exposing dome ﬂats—it is far better to dim the lights with the domelight dimmer switch and use an exposure of a few seconds than to use the dome lights at full intensity and an exposure which is less than a second (this can introduce spurious radially varying structure into the ﬂat-ﬁeld). There is an uncertainty in the timing of the shutter of the order of 10 millisec. Therefore, short standard star exposuresshouldalso be avoided—onthe2.2m,theEliasstandardsm ay need to be slightlydefocused to allow reasonable exposure times in the broad ﬁlters.

At the 2.2m telescope there is a slight rotation in the nominal cassegrain rotator position (270). The rotation was measured in February 1996 to be 0.883 degrees CCW (e.g., N is rotated 0.883 deg E of vertical when displayed in the normal way). One could attempt to adjust slightly for this by changing the rotator position, or adjust for it later during data reduction. If the precise rotation and scale is importantto the observations, one must measure t his carefully during the run since the exact rotation value is likely to change slightly between runs when the instrument is taken off the telescope and remounted.

2.1 Detector Linearity, Saturation, Read Noise, Dark Current

Hard saturation of thedetector occurs at 50,000 ADU’s. The total gain of the system results in a scale factor of 1.85 electrons/ADU. Recent tests (2/96) showed the device to be linear to better than 1% for values up to about 44,000 ADUs. However, the gain and illumination is variable across the array so care must be taken so that parts of the array are not saturating when the averageADU value is getting close to the non-linear region. The average value should be kept below 40,000 ADUs to ensure that

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one is not saturating areas of the array. The average detector dark current is

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the read noise is

15 electrons rms.

0.8 electrons/sec, and

October 30, 1997

2.2 Dewar Temperature

The dewarnowhas a temperaturesensor and heater. For normaloperation,the temperature controller should be used to set the detector temperature to 80.0K. If the controller is not used, drifts can occur as the telescope is moved, resulting in dark current instabilities in the array. The new array is not thought to be more sensitive to these effects than the previous detector, but some of the anomalies previouslyseen by observers with the original QUIRC detector may have been due to this effect.

2.3 Camera Sensitivity

The following sensitivity estimates are based on observations of 2 standard stars at the UH 2.2m

telescope on 2/6/96. The Point SRC (source) and Mag/arcsec

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integrationtime, 5

. The point source detection values below assume a PSF of 0.5 arcsec FWHM.

values are for 1 minute of on-source

Table 2. QUIRC Sensitivity

UH 2.2m Telescope

Filter Point SRC(Mag) Mag/arcsec2Jy/ADU (1 sec)

J 18.6 17.9 1.052e-6 H 17.8 17.1 8.295e-7 K 16.2 15.5 9.622e-7

H+K 18.6 17.9

2.3.1 QUIST sensitivity

The following numbers are based on observations of standard stars using QUIST at the UH 0.6m

telescope on 2/9/97. The Point SRC (source) and Mag/asec

are for 1 minute of integration time, 5

sigma. The point source detection values below assume a PSF of 2.5 pixels FWHM.

Table 2. QUIRC Sensitivity on QUIST

Filter Point SRC(Mag) Mag/arcsec2Jy/ADU (1 sec)

K’ 14.5 16.4 7.644e-5 Hs 14.7 16.6 2.291e-4 Hl 14.3 16.2 2.391e-4

2.12 12.0 15.0 4.563e-6

2.26 12.7 15.5 1.412e-5

3 Setup and Observing with QUIRC

The nominal orientation for QUIRC at the 88-inch telescope is for the black electronics box on the dewar (the utility box) to point North (toward the control room). Then the resulting FITS data ﬁles written by the program have N up and E to the left (i.e. the (0,0) pixel is in the SE corner).

QUIRC User Guide

3.1 Workstation setup

The program is run from a workstation in the control room, currently io (or halley on the 88”). There is one conﬁgurationﬁlethatqcdcom reads upon startuptodeterminethetelescope,secondary, and otherinformation. There are four presetconﬁguration ﬁles currently used, located in the directory

/aux/inst/qconf:

tel_config_06_quirc tel_config_22_f10_quirc tel_config_22_f31_quirc tel_config_cfh_quirc

These ﬁles are for the 0.6m, the 88” at proper ﬁle, a symboliclink should be made in that directory called tel appropriate conﬁguration. For example, to set up for the 88” at

% rm /aux/inst/qconf/tel_config % ln -s tel_config_22_f31_quirc /aux/inst/qconf/tel_config

10, the 88” atf=31, and CFHT atf=8. To install the

config that points to the

10,

This step only needs to be done once when t he telescope or secondary changes, and should already be set up properly for the current conﬁguration. Be careful not to delete the conﬁguration ﬁles themselves.

One xterm should be devoted to the camera program. The qcdcom program does not provide an integrated image display capability. A separate program should be used to display the data. One option is to use the viewﬁts program (vf). This is a display program developed by Tony Denault at the IRTF that displays images and has statistical and graphical analysis features. A link has been provided from qcdcom to this program that displays images automatically. The program can be run by typing the following in any window:

%vf&

To display images automatically, one must set the data directory to be the same as where qcdcom i s storing the FITS ﬁles. Thisis done by clicking on the “File" button and entering the proper path. One must also issue the command set vfout in qcdcom to enable the link so that data are displayed automatically. See the section on vf below for m ore information.

The saoimage program can also be used to display the QUIRC images. The observer should open a saoimage window on the camera workstation. The command line usually must be edited to read in the ﬁts ﬁle. This can be done by pressing the “n" key (for the “new" command)while in the saoimage window. For example,

-imtool -fits /scr/aug11/q940811.004

would read in ﬁle number 4. When in chop mode, qcdcom writes each chop position to a separate ﬁle, and t he chop difference to a ﬁle ending in “chop" instead of the ﬁle number.

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