NIRI Detector Characteristics and Read Modes

PROPERTIES OF ARRAY AND ITS READ MODES

The properties of the Aladdin detector array for the NIRI science channel are given in the table below. The array has nicely uniform response and very low dark current. Various size centered subarrays may be read out instead of the full 1024x1024 array. The bias voltage may be adjusted to modestly increase the well depth for thermal IR (L and M band) observations.

Three read modes have been defined to optimize use of the array. For high background environments (e.g., in the thermal IR), the array is read once at the beginning and once at the end of the exposure and the difference is recorded. In medium background situations (e.g., f/6 broad band JHK imaging) the same basic mode is used, but the beginning and end reads are digitally averaged 16 times. In low-background observations (e.g., f/32 observations, 1-2.5um narrow band imaging and 1-2.5um faint object spectroscopy), the array is read 16 times at the beginning and the end of the exposure, with the above digital averaging also taking place during each read. The read noises associated with these three modes are shown below.

*IMPORTANT NOTE regarding GCAL flats and flatfield accuracy:

Flats taken with the calibration unit make use of shutter open and closed images to explicitly correct for dark current and thermal emission from the camera optics. The calibration unit produces a beam that matches the telescope pupil very nicely except that it contains no central obscuration. The light path between GCAL and NIRI obviously excludes the primary and secondary mirrors as well. The GCAL illumination is therefore subtly different from that of NIRI on the sky.

GCAL flats are reproducible from night to night to about 0.3%, ie, the sensitivity of a given pixel varies by 0.3% over many nights as measured by GCAL. Obviously, for a star that subtends many pixels, the photometric accuracy will be approximately 0.3% divided by the square root of the number of pixels.

We installed a pupil mask in NIRI with a central obscuration to attempt to better match the GCAL and telescope pupils. The diffraction from the support vanes on the mask is quite severe, and we have not used this mask in practice for science observations.

Alternatively, sky flats can be constructed from data that sees exactly the same pupil. Sky flats, however, cannot make use of the equivalent "shutter closed" images to subtract dark current and thermal emission, which may be significant at K-band. Dark current subtraction is essential for making sky flats, and our experience shows that dark current variations over periods of a few hours limit the accuracy of sky flats. Dark current images really must be taken at the same time as the data to be used to construct sky flats. The difference between a given pixel in a sky flats vs. a GCAL flat can be 2% to 3%. The large-scale illumination pattern differences are completely swamped by the dark current variation, which has a spatial pattern of its own.

Last update 2006 September 1; Andrew Stephens, Tom Geballe & Joe Jensen