PFIS Home > Documents > Code 3170: Operations > OCDD

Southern African Large Telescope

Prime Focus Imaging Spectrograph

CCD Readout Modes

SAAO Detector Subsystem Team
Sam Baloyi
Dave Carter
Geoff Evans
Willie Koorts
James O'Connor
Darragh O'Donoghue
Modification Record
Version Date Comments
1.0 05-Mar-2003 Transferred to html by jwp

Table of contents

Overview

This section describes the readout modes for the SAAO Detector Subsystem of PFIS.

Introduction

The PFIS OCDD defines 4 readout modes for the detector subsystem. This document describes these readout modes and includes information about the tradeoff between readout speed and readout noise. The readout speeds and noise values discussed in the Detector Subsystem Functional Performance Requirements Document (FPRD) are assumed. Note that these noise values and rates are TBC1 (readout times given below are listed to 1 millisec accuracy so that the calculations that produced the figures can be verified, not because it is believed that the figures are accurate to 1 millisec!).

Normal Readouts

This is the standard mode for operating CCD cameras involving shuttered operation. It is assumed that 1x1 prebinning will never be used and that the minimum prebinning will be 1x2 prebinning (high resolution spectroscopy) or 2x2 prebinning (all the rest).

Detector subsystem operations will include:

Faster readout is possible (by about a factor 3) with 5 e-/pix readout noise.

High Time Resolution Readouts

It is assumed that 1x1 prebinning will never be used and that the minimum prebinning will be 1x2 prebinning (high resolution spectroscopy) or 2x2 prebinning (all the rest). Frame transfer operation is required to enable a continuous sequence of exposures, frame transfers and readouts (while the next exposure is accumulating). The shutter will be open throughout the sequence.

Detector subsystem operations will include:

with repetitions of frame transfer and readout for as long as desired. The minimum exposure time should be a little longer than the above readout times (say 0.2 sec: TBC2) to ensure that the next readout does not begin before the data are read out, displayed (if necessary) and stored. Minimum total cycle times of about 11.5, 6.0, 3.5 and 2.0 sec (assuming a 0.2 sec safety margin) would be realized for the parameters shown. Note that the image is smeared during the 0.102 sec needed for frame transfer.

The above readout times assume that the image is formed in the image section half of the chip, which extends 4 arcmin from the frame transfer boundary in the cross dispersion direction and away from the readout register. If a slit plane mask is used to limit (window) the imaging area to less than 4 arcmin, significantly higher time resolution can be gained with frame transfer operation. In such an arrangement, a frame transfer does not take place at the end of an exposure; instead, only the imaged area is transferred across the frame transfer boundary to save on row transfer time (50 msec/row). The following table shows transfer times, vertical transfers during readout and readout times as a function of imaged area for 2x2 prebinning and 5 e-/pix readout noise (2.9 msec/pix readout rate).

The first row is a repetition of the information given previously. The minimum exposure times should be significantly longer than these readout times in order to avoid too much degradation by image smearing during the row transfers.

Overheads such as associated with data storage within the host PC may play a significant role in the fastest applications so that the above performance needs verification (TBC3).

Image Height (arcseconds) Image Height (Pixels) Transfer Time (s) Store Transfer Time (s) Readout Time (s)
4.000 2048 0.102 0.102 1.595
2.000 1024 0.051 0.051 0.797
0.500 256 0.013 0.013 0.199
0.125 64 0.003 0.003 0.050

Charge Shuffle Mode

This is not strictly readout; instead, it involves shuffling the charge vertically in either direction. Each vertical transfer takes 50 msec/row. The Detector subsystem will be supplied with the direction and number of row operations required and will execute the sequence (starting after some time still to be specified?).

Drift Scanning

This will require the rows of the detector to be clocked at a tunable rate (from 30 to 118 rows/sec), and the readout register to be readout in a time much shorter than this vertical clock rate.

Detector subsystem operations will include:

with repetitions of vertical transfers and readout register readouts for as long as desired.

List of TBCs

TBC1 - Readout rates and associated readout noise

TBC2 - Exposure time overhead to ensure readout is complete before the next frame transfer operation

TBC3 - Performance in subsecond exposures needs to be verified