Southern African Large Telescope

Prime Focus Imaging Spectrograph

Operational Concepts Definition Document

SALT-3170AE0002

Jeffrey W Percival

Modification Record
Version Date Comment
3.0 20-Sep-2001 Included SAAO Detector Subsystem Document
3.1 25-Sep-2001 Complete Merge of SAAO Detector Document
3.11 26-Sep-2001 Consistency and Typo Correction
4.0 01-Mar-2003 New State Descriptions
4.1 10-Mar-2003 Fixed sentence fragments in Overview
4.2 11-Mar-2003 Remove uninteresting mode tables near the end
4.3 20-Mar-2005 Remove etalon zero-point cals from procedures

Introduction

This document replaces version 3.11 of the PFIS OCDD. This document improves upon the PDR version by using a better "parameter space" of instrument configurations and readout modes.

We take care to relate the newly defined instrument states to the ones defined in the old OCDD.

Overview

PFIS is operationally complex. It produces a wide range of science data using various combinations of focal plane masks, polarizing elements, gratings, Fabry-Perot etalons, filters, an articulating camera that allows both imaging and spectroscopy, and a CCD subsystem that supports four readout modes.

To manage this operational complexity, we have designed a state-space representation of the operational modes of PFIS. This representation clearly shows which states are useful, and shows exactly what to do to move between states, and how long it will take.

CCD Readout Modes

The SAAO team's CCD OCDD (whew!) is here.

Focal Plane Masks

PFIS uses focal plane masks and slits in many of its observing modes.

Imaging and Spectroscopy

Normal imaging uses the circular 8 arcminute field of view. An optional long slit is used for spectroscopy.

Imaging Mask

Frame Transfer (High Speed Imaging and Spectroscopy)

For high time resolution observations, the chips are masked off below the frame transfer boundary. An optional long slit supports spectroscopy. In frame transfer mode, the image or spectrum is exposed for some time, and then it is quickly shifted into the masked region. The unmasked upper portion begins collecting new photons while the lower masked portion is read out.

You can shorten the slit and shift fewer lines per transfer to increase the time resolution. For example, a 1 arcminute slit would allow a new spectrum to be transferred in a quarter of the time as a 4 arcminute slit. See the CCD Readout section for more details.

Frame Trasnfer Mask

Polarimetric Imaging and Spectropolarimetry

For polarimetric imaging or spectroscopy, only the central 4 arcminute portion of the focal plane is used. The beamsplitter optic sends the E beam to one side of the frame transfer boundary (say, the upper half of the chip) and the O beam to the other half of the chip. Frame Transfer operations are not possible with polarimetry, because the E and O beams use up space on both sides of the frame transfer boundary.

Polarimetry Mask

Multi-object Spectroscopy

Multi-object spectroscopy requires the use of a carbon-fiber mask in the focal plane. The mask will be milled with a laser cutter and placed in a slitmask magazine designed to hold 20-30 masks (see related mask design study). User designed slitmasks will be cut and stored on site and labeled with a bar code to identify the mask and mask holder (frame) to be used with each mask.

Peakup holes are cut into the mask and correspond to the focal plane positions of reference stars. Peaking up the position of the slits on the sky requires several images taken at slightly different telescope pointings. The CCD control computer will sum and report the counts within pre-programmed sub-arrays on the chips, and the PFIS computer will command the telescope to the position that produced the highest throughput.

Multi-Slit Mask

PFIS Observing States.

PFIS offers both imaging and spectroscopy, and allows combinations of Fabry-Perot and polarimetric optics with 3 styles of CCD readout modes. Defeating the combinatoric explosion requires a careful distinction between instrument configuration, operational procedure, and CCD readout mode.

Managing the instrument configuration is simplified by identifying 6 basic opto-mechanical configurations.

There are 6 unique state transitions (hardware movements):

Here is how the states and transitions are related:

PFIS States

Despite having 6 configuration states, there are only 4 operational procedures (and 3 variants involving CCD charge-shuffle mode). For example, states S1 and S2 share the same operational mode: open the shutter, expose, close the shutter, and read out the CCD. States S1 and S4, however, differ in that state S4 generates multiple exposures, one for each waveplate setting.

PFIS State Transition Table How to use this table: the rows are indexed by the state you are currently in. The columns are indexed by the final (not next) state that you want to end up in. For each (current,desired) state pair, the indexed table cell tells which transition to execute next, and what state that takes you to. Keep iterating until you end up in the desired state (you are done when you end up on the diagonal).
s1 s2 s3 s4 s5 s6
s1
-
 +T1, S2 +T3, S3 +T2, S4 +T1, S2 +T3, S3
s2 -T1, S1
-
 -T1, S1 -T1, S1 -T2, S5 -T1, S1
s3 -T3, S1 -T3, S1
-
 -T3, S1 -T3, S1 +T2, S6
s4 -T2, S1 -T2, S1 -T2, S1
-
 +T1, S5 +T3, S6
s5 -T2, S2 -T2, S2 -T2, S2 -T1, S4
-
 -T1, S4
s6 -T2, S3 -T2, S3 -T2, S3 -T3, S4 -T3, S4
-

Example: Suppose you are in state S5 (Spectropolarimetry), and you want to be in state S3 (Fabry-Perot Imaging) for the next observation.

  1. (S5,S3) indexes the cell (-T2, S2). So execute transition -T2 (Remove waveplates). You are now in state S2 (Spectroscopy).
  2. (S2,S3) indexes the cell (-T1,S1). So execute transition -T1 (Articulate camera back to home). You are now in state S1 (Imaging).
  3. (S1,S3) indexes the cell (+T3,S3). So execute transition T3 (Insert etalons). You are now in state S3.
  4. (S3,S3) indexes the diagonal. You are done.

Note that this state-space representation easily allows one to see the time required to move between two instrument configurations, indicates how the move is to be made, and specifies a central element of the PFIS control system software.

PFIS Operational Procedures

After configuring PFIS into some observational mode, the PFIS control software executes some procedure. The procedure runs on the server side, not the client. The client (MMI) asks for a procedure by name, and supplies the required parameters. The server runs the procedure, then responds to the client.

Procedure P1 - Imaging and Spectroscopy

Procedure P2 - Polarimetric Imaging and Spectroscopy

There are three polarimetric waveplate sequences: linear, circular, and all Stokes (produces both linear and circular measurements). The sequences are:

Linear Circular All Stokes
1/2 wave 1/4 wave
0 -
45 -
22.5 -
67.5 -
11.25 -
56.25 -
33.75 -
78.75 -
1/2 wave 1/4 wave
0 +45
0 -45
22.5 -45
22.5 +45
45 +45
45 -45
67.5 -45
67.5 +45
1/2 wave 1/4 wave
0 0
22.5 33.75
45 67.5
67.5 101.25
90 135
112.5 168.75
135 202.5
157.5 236.25

We can represent this a little differently. The basic quantum of rotation is 90/8 degrees, or 11.25 degrees. The next table gives the waveplate sequences in terms of multiples of the quantum of rotation.

Linear Circular All Stokes
1/2 wave 1/4 wave
0 -
4 -
2 -
6 -
1 -
5 -
3 -
7 -
1/2 wave 1/4 wave
0 +4
0 -4
2 -4
2 +4
4 +4
4 -4
6 -4
6 +4
1/2 wave 1/4 wave
0 0
2 3
4 6
6 9
8 12
10 15
12 18
14 21

Procedure P3 - Fabry-Perot Imaging

Procedure P4 - Polarimetric Fabry-Perot Imaging

Shuffle-mode Procedures

Shuffle-mode procedures are the only ones that interact, in a closed-loop fashion, with some other subsystem. In this mode, photons are collected for some time, and then two things happen: the CCD charge is shuffled up or down on the CCD chip, but not read out, and some piece of hardware is moved to a new position. The move will involve exactly one other item:

Procedure P1-S - Charge Shuffle Spectroscopy

Procedure P2-S - Charge Shuffle Polarimetry

Procedure P3-S - Charge Shuffle Fabry-Perot Imaging

PFIS Observing Mode table

This table presents the observing modes broken down into imaging and spectroscopy, and according to CCD readout mode.

PFIS Observing Modes sorted by PFIS State Number

This table relates the new state-space view of PFIS to the PDR OCDD modes.

Mode# Code Name State CCD Description Team Comm. Comment
1 OIUN Imaging S1 N Stromgren band imaging over 8’ field; narrowband imaging over 8’ field; imaging in preparation for multi-slit spectroscopic work. Yes
2 MIUH High Time Resolution Imaging S1 H Sub-second photometric observations using Stromgren or narrowband filters of transient objects: cataclysmic variables, compact objects with accretion disks, AGN, Gamma ray bursts. No
21 OIUD Drift Scan Imaging S1 D Deep uniform photometry of extended regions, surveys. No
11 SGUN Long Slit Spectroscopy S2 N Conventional single long slit spectroscopy over 7.5’ fields at arbitrary position angles. Used with the SALT ADC, parallactic angle considerations do not restrict the position angle of the observation. Yes
17 MGUN Multi-slit Spectroscopy S2 N Multi-object spectroscopy in fields up to 6’ diameter; redshift surveys; spectral surveys of stars and galaxies. Yes
12 SGUH High Time Resolution Long Slit Spectroscopy S2 H Time-resolved spectroscopy of cataclysmic variables, flares, eclipses, AGN variability, gamma ray burst optical transients. Yes
18 MGUH High Time Resolution Multi-slit Spectroscopy S2 H Multi-object spectroscopy in fields up to 4’ diameter; redshift surveys; spectral surveys of stars and galaxies. No
22 OGUD Drift Scan Spectroscopy S2 D Shallow, wide-area spectroscopic surveys. No
6 OFUN Fabry-Perot Imaging S3 N High resolution imaging spectroscopy of multiple or extended objects; dynamical studies of HII regions, star clusters, and galaxy clusters. Yes
8 SFUN Fabry-Perot Imaging with Coronographic Mask S3 H Spectroscopic imaging of faint extended objects near bright sources. No
3 MILN Linear Polarimetric Imaging S4 N Stromgren or narrow band polarimetric surveys of interstellar polarization; intrinsic stellar polarization. No
5 MICN Circular Polarimetric Imaging S4 N Stromgren or narrow band circular polarimetric surveys of interstellar polarization; intrinsic stellar polarization; low-resolution (R=10) spectral polarimetry. No
4 MILH High Time Resolution Linear Polarimetric Imaging S4 H Stromgren or narrow band polarimetric studies of rapidly varying polarized stars and AGNs. No Not Possible
13 SGLN Long Slit Linear Spectropolarimetry S5 N R=2000 to R=6000 long slit spectropolarimetric studies of cataclysmic variables, young stellar associations, stars with disks, AGNs, and interstellar absorption features. Yes
15 SGCN Long Slit Circular Spectropolarimetry S5 N R=2000 to R=6000 long slit circular spectropolarimetric studies of cataclysmic variables, young stellar associations, stars with disks, AGNs, and interstellar absorption features. No
19 MGLN Multi-slit Linear Spectropolarimetry S5 N Multi-object linear spectropolarimetry in fields up to 4’ diameter; polarimetric surveys. Yes
20 MGCN Multi-slit Circular Spectropolarimetry S5 N Multi-object circular spectropolarimetry in fields up to 4’ diameter; polarimetric surveys. No
14 SGLH High Time Resolution Long Slit Linear Spectriopolarimetry S5 H R=2000 to R=6000 long slit linear spectropolarimetric studies of cataclysmic variables, young stellar associations, stars with disks, AGNs, and interstellar absorption features. No Not Possible
16 SGCH High Time Resolution Long Slit Circular Spectropolarimetry S5 H R=2000 to R=6000 long slit circular spectropolarimetric studies of cataclysmic variables, young stellar associations, stars with disks, AGNs, and interstellar absorption features. No Not Possible
7 OFUV Shuffle Mode Fabry-Perot Imaging S5 S High resolution imaging spectroscopy of multiple or extended faint objects; dynamical studies of HII regions, star clusters, and galaxy clusters; provides good background subtraction. No
9 MFLN Linear Polarimetric Fabry-Perot Imaging S6 N R=500 to R=13000 imaging linear spectropolarimetric studies of extended objects such as HII regions, reflection nebulae, star clusters, young stellar associations, and galactic nuclei. No
10 MFCN Circular Polarimetric Fabry-Perot Imaging S6 N R=500 to R=13000 imaging circular spectropolarimetric studies of extended objects such as HII regions, reflection nebulae, star clusters, young stellar associations, and galactic nuclei. No