TAUVEX Calibration Techniques

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Contents 1 Overview 2 Photometric Calibration 2.1 Overview 2.2 Procedure 2.3 Requirements 2.4 Potential Problems 3 Edited up to here 3.1 Co-Alignment 3.2 Coincidence Loss 3.3 Distortion Analysis 3.4 Calibration from Field Observations 3.4.1 Overview 3.4.2 Steps in Calibration 3.5 Flat Field 3.6 Red Leak

[edit] Overview

The first three months of the mission will be devoted to the characterization and calibration of the 3 TAUVEX telescopes in all operational modes. This document details the procedure to extract the calibration parameters. Although these may be part of the automated pipeline, if feasible, the calibration parameters will not be incorporated into the pipeline without manual and informed intervention.

[edit] Photometric Calibration

[edit] Overview

The three TAUVEX telescopes will scan over the sky and will observe sources continuously. There are three types of calibration sources:

1. Primary calibrators: Sources with known stable UV fluxes, typically HST primary calibrators.
2. Secondary calibrators: Sources that are known to be stable. These will act as relative calibration sources and as continuous monitors of the instrument stability. There may be three types of secondary calibrators:
   1. Sources that have been previously observed by TAUVEX.
   2. Sources with predicted fluxes from other UV observations (IUE, GALEX, TD1).
   3. Sources with B and V magnitudes which will be extrapolated.
3. Non-calibrators: These cannot be used as calibration sources either because they are variable or because they are too bright or too dim for the instrument. Sources may drop from the secondary calibrator list to here as the mission progresses.

[edit] Procedure

1. Simulate field star brightnesses. If there are primary calibrators in the field, those can be used; otherwise use model fluxes.
2. Run a point source extractor on the field and extract point sources.
3. Match observed sources to predicted using the position.
4. Reject confused sources in either the model or the observed data.
5. Fit a straight line to the observed vs model fluxes to find the scale factor.
   1. Primary calibrators should be given the highest weight. If there is a primary calibrator in the field then the scale factor is just the ratio of observed to predicted.
6. Identify sources that are far from their predicted brightnesses. This may be due to an incorrect calibration or to errors in the predicted brightness. If the latter, flag the source and do not use in further calibration.
7. Report single number which represents the conversion from observed units to physical units.

[edit] Requirements

1. Model brightnesses for all stars in field. Sources which should not be used should be flagged.
2. Flat field should be known.

[edit] Potential Problems

[edit] Edited up to here

[edit] Co-Alignment

-- All three telescopes point at the same source

-- Determine offset relative to the centre of the central telescope detector

[edit] Coincidence Loss

--Obtain raw data in counts/s from pipeline

--Find expected counts rate

--Correct count rate for dead time using the frametime and deadtime fraction

[edit] Distortion Analysis

-- Perform initial distortion correction using ground based map

-- Run source detection program. IRAF tasks daofind can be used to detect as many sources as possible.

-- Accurate positions can be found using imexamine which uses source list from daofind as input

-- Scale and rotate grid pattern to minimize the difference between predicted and actual source positions for a sources

-- Correlate source positions to sky co-ordinates. Using star near centre as reference co-ordinate of each detected source is converted to an undistorted pixel position assuming 3"/pixel.

-- For each pixel 4 nearest data points are identified and a weighted interpolation algorithm is used to calculate a displacement vector

[edit] Calibration from Field Observations

[edit] Overview

As TAUVEX scans over the sky, it will observe a large number of point sources which we may use for the purpose of checking and refining the calibration of the instruments. Unfortunately, many of the sources will not have accurate fluxes, particularly in the UV, and so we cannot blindly extract a calibration from these ratios.

[edit] Steps in Calibration

[edit] Flat Field

-- Get sky background image

-- Remove all small scale variations due to stars by filtering

-- Combine all images

-- Plot histograms showing sky background count rate as a function of x and y pixel positions and shade plots showing any large scale variations

-- If variations are more than a few percent create a map

[edit] Red Leak

--Obtain raw counts/s for each source

--Compare with expected

--Difference gives red leak

--Repeat for other filters