OSIRIS Hieroglyph The OSIRIS User's Manual

HgCdTe Detector Array

This section discusses the characteristics of the HgCdTe array in OSIRIS and in particular the following topics:


The infrared array currently in OSIRIS is a 1024x1024 HgCdTe HAWAII array supplied by CTIO. The array is a hybrid of a silicon multiplexor and an array of infrared sensitive (HgCdTe) detectors. The two pieces are pressed together, with indium bumps on each piece making the electrical connection. The multiplexor is an array of discrete read-out transistors and is unlike an optical CCD. In particular, the array can be read non-destructively and each pixel can be read separately and in any order. Although these differences do not matter for typical observing programs, future upgrades could permit additional observational flexibility. For example, designated sub-arrays could be read-out at high frequency, then shifted and added later to create diffraction limited images.

The band gap energy of this HgCdTe is approximately 0.5 eV, smaller than Si, so this detector is sensitive to radiation with wavelengths as long as 2500 nm. The relatively small band gap energy also causes high dark current at relatively high temperatures and thus in the low count rate regime characteristic of astronomy it must be cooled to liquid nitrogen temperatures (~77 Kelvin).

This HgCdTe array has 18.5 micron pixels. The scale (arcsec/pixel) depends on the telescope and choice of camera lens. The various possibilities are summarized in the section on the Imaging Mode.

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Dark Current and Read Noise

The dark current in the array is low. We currently measure about 0.15 e-/sec, some of which may be due to background radiation in the dewar. If you measure dark current significantly higher than 1 DN/sec, please check that the instrument is still boiling off its liquid nitrogen.

The read noise of the array is 15 e- and the gain is 3.0 electrons per DN. The effective read noise of the detector can be reduced by reading each pixel's signal non-destructively many times; factors of 2-3 improvement are possible. Note, however, that a read noise of 19 e- is comparable to the shot noise for signals of about 100 DN, which happens quickly in most imaging and low-resolution spectroscopic observations. If your observations are read-noise limited, please contact Darren DePoy ( depoy@astronomy.ohio-state.edu) for more information.

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Linearity and Saturation

Here is a plot showing the basic linearity behavior of the array. The plots were generated using large image sections over the upper and lower halves of the array. The array becomes significantly non-linear before the full well capacity of the detector is reached. Further, an accurate non-linearity correction depends on the signal rate as well as the total signal collected. However, non-linearity is 1.0%, for 11,000 DN and 2.0% for 17,000 DN at a signal rate of 800 DN/sec (a typical rate at K looking at the dome white spot with the f/7 camera). Non-linearity coefficients are given on this plot. The array becomes seriously non-linear (>5%) at about 28,000 DN.

The linearity correction is better fit by a 3rd order polynomial than a 2nd order one. The latter has been used previously with CIRIM and is implemented in all previous versions of the IRAF task "irlincor."  This task has been updated (as of 20 March 2000) to allow for 3rd order fits. OSIRIS users can corrrect their data using the updated irlincor task (you must upgrade your IRAF installation for the revised "ctio" package) or with the "linfits" program. This FORTRAN program is available by anonymous ftp. It may be used stand alone, or as part of the basic osiris IRAF data reduction script, osiris.cl (irlincor can also be called from osiris.cl). Both linfits and osiris.cl are included in the CTIO IR reduction package cirred.

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Bad Pixels

The OSIRIS array is generally very cosmetically clean. There are scattered hot and cold pixels on the array, though these only comprise less than 1% of the illuminated area of the array. Dithering the telescope during the course of an observing sequence is an effective way of eliminating the effects of bad pixels.

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After-Image (Residual Charge)

The detector array in OSIRIS exhibits a residual signal from bright sources. The magnitude of the residual image depends on the brightness of and total signal recorded from the source. For typical observations the residual will be 0.5-2% of the originally detected signal. Reading the array several times reduces the magnitude of the residual image to <<1% of the original signal.

We suggest that if a 1% residual image will seriously compromise your results, you dither the telescope and, perhaps, read the array several times (set the exposure time to 0 seconds, do a go 3, and throw those images away) between each science exposure.

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Updated: 1999 December 21