Kermit Operations

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Kermit Operations

Output Filenames

Kermit output files are named kernumber_object_filter_time.fits. number is a four-digit integer that increments for each subsequent exposure. object is the object name (with spaced removed) as entered by the observer. filter is the name of the currently selected filter wheel position. time is the current UTC time as a six-digit integer, HHMMSS.

Exposure Mode Settings

Describe basic settings; nexpose, ncoadds, nreads, nresets,

Subarrays

Describe subarray positioning options here.

Pixel Timing Modes

Kermit has three selectable pixel timing modes: fast readout, slow readout, and slow readout with reset. These modes select the timing of the clock pulses which are sent to the Hawaii 2 detector.

Slow readout with reset mode holds the reset line active during the entire readout process. This mode is purely diagnostic as it allows you to test the read noise only without actually sampling any signal from the detector. It was originally implemented to allow detector noise tests with the array at room temperature. See Murphy et al 1995.

Single Read and CDS

Infrared focal plane arrays are typically read once at the start of an exposure and again at the end of an exposure; the difference between these two values is then recorded. This "Correlated Double Sampling" readout mode provides straightforward removal of chip bias effects, at least to first order. Infrared focal plane arrays are read out nondestructively, and thus may be read multiple times for a single image to reduce the effective read noise. This technique is frequently called Fowler sampling; see Fowler 1991. Kermit has three readout modes:

In "Single read mode", the array is read out at the end of the exposure only. However this mode is slightly misnamed, as the array is actually read NREADS times rather than only once. The point is, it's not CDS, so you'll see the full effect of the chip structure in the image. This mode is mostly for diagnostic purposes.
In "CDS" mode, the array is read out NREADS times at the start of the exposure and NREADS times at the end of the exposure. The difference of the sums of each set of NREADS is then recorded. This is the standard operating mode and will be used for the vast majority of exposures.
In "CDS no subtraction" mode, the array is read out identically to CDS mode, but the subtraction is not performed. The sum of the first NREADS and the sum of the final NREADS are both written to disk as separate image planes in one FITS file. This mode is mostly for diagnostic purposes.

Fast Subarray Mode

Kermit has limited support for a fast readout mode available with small subarrays only. This approach allows multiple images to be recorded in rapid succession with a minimal of overhead, but is limited by memory to a small number of frames depending on subarray size.

In this mode, multiple images are read out rapidly and buffered in the coadder board's memory; after a series of frames has been taken the entire set is written to disk as a data cube in a single FITS file. The reset timing between frames is also modified to use more rapid row-at-a-time resets rather than the normal pixel-at-a-time resets.

Each Kermit coadder board has one megapixel's worth of memory. Normally this is used to store initial and final CDS reads for a 512x1024 subarray. The maximum number of frames available in fast subarray mode is as follows:

Size	Max Frames
64	128
128	32
256	8
512	2

In fast subarray mode, NCOADDS must be 1.

Kermit Motors

Kermit has four motors: a filter wheel, a pupil mask selector, a secondary mirror (M2) selector, and a focus stage. The available positions are as follows:

Filter Wheel

Position	Name	Description
0	Home	Homes the motor. Same physical location as position 1
1	H	H band filter
2	Open	no filter
3	Open	no filter
4	Blank	1/4" opaque aluminum. Used for darks.
5	K	K band filter. Note this is true K not K' or Ks: center wavelength 2.120 microns, bandwidth .340 microns
6	J	J band filter.

Pupil

Position	Name	Description
0	Home	Homes the motor.
1	Blank	opaque.
2	Small	Small pupil; f/32?
3	Large	Large pupil; f/16?

M2 Mirror

Position	Name	Description
0	Home	Homes the motor.
1	GoldMirror	A gold mirror
2	Dichroic	Reflects shortward of 1.8 microns; transmits beyond that.
3	Empty	Empty position.

Focus

The focus slide moves between positions 0 and -8900. 0 is the longest focus (array is farthest from the telescope) and -8900 is shortest focus (array closest to telescope). The physical distance corresponding to this motion is about two centimeters.

This page last modified on 2004-03-24 by Marshall Perrin.