RVS Calibration Workshop, Paris RVS RVS Calibration RVS Calibration & First Look Workshop, Paris Mark Cropper

RVS Calibration Workshop, Paris

RVS

RVS Calibration

RVS Calibration & First Look Workshop, Paris

Mark Cropper


RVS

Contents

1. Which parameters will be calibrated?2. How will they be calibrated?3. Type/volume of calibration data?4. Auxiliary data required for the calibrations?


RVS

1. Which Parameters? Guidelines

• The aim of calibration is to transform measurements from the internal instrument system to a universal system (cgs/SI).

• Simply: all those parameters that are necessary to do this transformation should be included.

• Obvious ones easily come to mind – examples:– wavelength reference point– throughput as a function of wavelength– … etc.

• However, a complete list is more difficult to compile and depends on a number of other parameters, such as temperature, and others we may not realise yet some flexibility needs to be retained.


RVS

1. Which Parameters? Guidelines (ctd)

• Also, calibration has to be practical and driven by scientific needs (no point in measuring something for the sake of it) implications for the policy on sub-system level testing, eg.– information per optical surface– information per CCD etc.

• In-orbit calibration uses as a basis the ground calibration/performance verification, together with – calibration file structures– infrastructure (conceptual, harware/software systems

etc.) – simulation and analysis software

which it is sensible to re-use to some extent if this is helpful (but issues of information and knowledge transfer).


RVS

1. Which Parameters? Guidelines (ctd)

• Calibration has to take note of the particular circumstances for Gaia:– no dedicated observations will be made– all data will be taken in TDI mode– fixed instrumental configuration (very helpful)– the dataset will be very large automation is required

• Calibration data must be sufficient to:– provide scientific data products– allow data to be combined on board prior to telemetry– provide long-term monitoring of instrument health

• …All of these considerations need to be combined in making decisions on which parameters to include.


RVS

1. Which Parameters? Suggestions

• Calibration items required – all of these are f(tlong-

term)– photometric throughput as f(CCD#,y,)– AC line spread function as f(CCD#,y,,scan_law)– AL line spread function as f(CCD#,y,)– distortion map as f(CCD#,y,)– wavelength scale and zero reference as f(y,, tshort-term)

– CCD bias as f(CCD#)– CCD readout and dark noise as f(CCD#,y)– CCD TDI flat field as f(CCD#,y,)– CCD blemishes as f(CCD#,y)– signal linearity as f(CCD#)– saturation level as f(CCD#)– scattered light/ghosts as f(CCD#,y)

we come back to these later


RVS

Which Parameters? Suggestions (ctd)

• Many of these need to be known for each individual CCD (CCD#): this is particularly true for the on-board processing software.? – is this also true for the ground processing?

• Data at individual CCD level is available only via diagnostic mode

• Could adopt the policy for ground processing that the science calibrations operate only on the co-added data.

• In this case have two calibration streams:1. diagnostic mode data calibration for on-board

software2. science data science calibration (mostly via

SGIS)• Else - use individual diagnostic mode calibrations

to construct end-to-end calibration? end-to-end detailed calibration ?


RVS

1. Which Parameters? Suggestions (ctd)

• On consideration, probably best to separate requirements and use two-stream approach:– ensures best compatibility between measurements for

where they are required– may be difficult to combine the detailed information

via the diagnostic stream to use in the science calibration

• NOTE however, – ground calibration will be responsible for both streams

(on-board and science processing)– long term trends and monitoring will need to be done on

both streams– cross checks will need to be made to ensure

compatibility between the two calibration streams


RVS


• Splitting the two streams, then, on-board processing requires the following parameters:– photometric throughput as f(CCD#,y,)– AC line spread function as f(CCD#,y,,scan_law)– AL line spread function as f(CCD#,y,)– distortion map as f(CCD#,y,)– wavelength scale and zero reference as f(y,, tshort-

term)– CCD bias as f(CCD#)– CCD readout and dark noise as f(CCD#,y)– CCD TDI flat field as f(CCD#,y,)– CCD blemishes as f(CCD#,y)– signal linearity as f(CCD#)– saturation level as f(CCD#)– scattered light/ghosts as f(CCD#,y)


RVS


• Science processing requires the following parameters:– photometric throughput as f(CCD#,y,)– AC line spread function as f(CCD#,y,,scan_law)– AL line spread function as f(CCD#,y,)– distortion map as f(CCD#,y,)– wavelength scale and zero reference as f(y,, tshort-term)

– CCD bias as f(CCD#)– CCD readout and dark noise as f(CCD#,y)– CCD TDI flat field as f(CCD#,y,)– CCD blemishes as f(CCD#,y)– signal linearity as f(CCD#)– saturation level as f(CCD#)– scattered light/ghosts as f(CCD#,y)

note CCD#


RVS

2. How will they be calibrated? overview

• In-orbit calibration needs to proceed in as automated manner as possible, especially as mission proceeds

• Calibration objects will be observed as part of the routine observations

• Ground processing needs a database (or database partition) identifying the calibration objects and what they are used for

• Standard ground processing pipelines will process calibration objects, after which they will be extracted and piped to the calibration pipeline for further processing

• Calibration pipeline will derive instrument calibration parameters

• Calibration files will then be updated automatically

• Instrument monitoring and alerts will be produced automatically


RVS

2. How will they be calibrated? science data stream

• Calibration objects will be:

1. objects identified a priori by humans as suitable calibrators

2. objects identified by the Gaia-RVS processing itself (SGIS)

• A mixture of both is required, using the SGIS approach for the major (self) calibration, and the external calibrators used to provide zero points/references to the SGIS calibrations

• In any case, suitable calibrators will need to be identified before launch, and this may require specific observations from other telescopes

• In the SGIS approach, suitable robust techniques will also need to be identified – this will take place when the data centres are established post the ESA-AO.


RVS

2. How will they be calibrated?

• Standards will be required for– spectrophotometric throughput and out-of-band rejection– linearity of photometric response– radial velocity zero points– rotational velocity determinations

– astrophysical parameter determinations (Teff, log(g) etc.)

• Suitable stable stars selected by SGIS will be sufficient to monitor– LSF in AL and AC directions– distortion map– dispersion law– radial velocity stability– spectrophotometric throughput stability


RVS


• SGIS works by monitoring the output spectra from the single-transit pipelines

– bright stars, but not too bright to induce non-linearity/saturation

– range of appropriate types (F, G, K) - may be useful to have input from photometry to select

• SGIS then selects a subset of those which produce a consistent wavelength, throughput, position etc. parameters

• Note these are not constant parameters, as there will be drifts at some level, particularly on the wavelength scale.

• SGIS will need to allow for and track parameter changes in order for consistent sample to be selected some learning/iterative process?


RVS



RVS

2. How will they be calibrated? science data stream (ctd)


RVS

2. How will they be calibrated? science data stream (ctd)


RVS

2. How will they be calibrated? diagnostic mode data stream

• Diagnostic mode data has to be compared with co-added data from all 10 CCDs to – update the on-board software parameters/look-up tables– determine the additional calibration to operate on the co-

added data telemetered from the RVS • Sequence:

– determine the calibration parameters– update the calibration files automatically

(calibration database version control and management)– visualise the calibration parameters and provide tracking as a

function of time– provide automated alerts for deviations

• All of this requires specific software tools (both manual and automatic) to be developed, and for the process to be overseen in some way by humans


RVS

2. How will they be calibrated? diagnostic mode data stream


RVS

3. Type/volume of calibration data

• Calibration data for the science stream will not add any particular telemetry overheads - these are normal observations

• Calibration data for on-board processing will incur overhead telemetry with the diagnostic mode data stream– data rate can be adjusted according to timescale that

variations occur in the calibration– limited by overall telemetry allocation– priority when data is to be lost before telemetry?

• Additional volume will be required to host the calibration database and the calibration pipelines– SGIS– external standards– calibration for on-board processing– tools

– status monitoring etc. TOTAL volume TBD


RVS

4. Auxiliary data

• Auxiliary data is required for the processing of the calibration (and other) pipelines. This includes– instrument/spacecraft parameters (temperatures, solar

angles, time, attitude, operating voltages) – these provide the correction for long-term and short-term calibration drifts

– MBP/BBP photometry– astrometry from Astro– catalogue information (especially early in mission –

astrometric positions etc.)– external standards (radial velocity, spectrophotometric

etc.)

• Auxilliary data should include time history of the processing applied on the (calibration source) data up to that point.

• List is non-exhaustive – further thought required

Documents

RVS Calibration Workshop, Paris RVS RVS Calibration RVS Calibration & First Look Workshop, Paris Mark Cropper