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SPIRE Flux Calibration: Implementation SPIRE Flux Calibration: Implementation

SPIRE Flux Calibration: Implementation - PowerPoint Presentation

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SPIRE Flux Calibration: Implementation - PPT Presentation

George J Bendo and the SPIREICC Basic Equations Derivative of flux calibration curves from empirical analysis Flux calibration curve integral of above equation V 0 is a zeropoint voltage that is selected to match the background in dark sky ID: 604017

calibration flux map neptune flux calibration neptune map scan densities uncertainty bolometers pcal data curves measured dotted voltages peak

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Slide1

SPIRE Flux Calibration: Implementation

George J. Bendo

and the SPIRE-ICCSlide2

Basic Equations

Derivative of flux calibration curves (from empirical analysis):

Flux calibration curve (integral of above equation):

V

0

is a zero-point voltage that is selected to match the background in dark sky.

K1, K

2

, and K

3

are unique parameters for each bolometer that are derived using the techniques discussed here.

An additional K

4

term is used to scale the RSRF-weighted flux density to produce monochromatic flux densities (assuming υF

υ

is constant).Slide3

Calibration Strategy

PCal

flash observations taken against backgrounds with different surface

brightnesses

give the relation between V and 1/ΔV. These data can then be used with the derivative of the flux calibration curve to give unscaled versions of K1 and K

2 and a scaled value of K3.Fine scan observations in which each bolometer scans over Neptune are used to scale the K1

and K2 parameters.Slide4

PCal Flash Observations

Instrument was pointed at a series of locations near

Sgr

A* during

PCal flash observations to get 1/ΔV vs V measurements. Each bolometer falls within a region that should be ~75% of the peak surface brightness.

Additional PCal flash measurements against dark sky were used to constrain the part of the curve near V0.Slide5

Results: Unscaled Derivatives of Calibration Curves

Dotted lines:

Voltages

for V

0

and V0-VPeak (Neptune)Slide6

Results: Unscaled Derivatives of Calibration Curves

Dotted lines:

Voltages

for V

0

and V0-VPeak (Neptune)Slide7

Results: Unscaled Derivatives of Calibration Curves

Dotted lines:

Voltages

for V

0

and V0-VPeak (Neptune)Slide8

Neptune Fine Scan Observations

In these observations, each bolometer in SPIRE passed across Neptune in a very fine pattern. This gave measurements of the peak and background voltages that could be used to scale the calibration curves.

To derive the peak and background voltages, we fit the timeline data (not the map data) with two dimensional Gaussian functions. An example is shown below.Slide9

Results: Scaled Calibration Curves

Solid line:

V3-0

flux calibration

Dotted line: V2-3 flux calibrationSlide10

Results: Scaled Calibration Curves

Solid line:

V3-0

flux calibration

Dotted line: V2-3 flux calibrationSlide11

Results: Scaled Calibration Curves

Solid line:

V3-0

flux calibration

Dotted line: V2-3 flux calibrationSlide12

Problematic Bolometers

Some of the dead, noisy, and slow bolometers could not be calibrated because of difficulties with performing analysis on the sample. The following number of bolometers were not calibrated:

PSW 7

PMW 1

PLW 1 16 bolometers in PSW and 7 bolometers in PMW saturated on Neptune. To calibrated these bolometers, we used fine scan observations of 3C 273. We calibrated the signal from 3C 273 for the “good” bolometers, and then used the signal from 3C 273 to scale the bolometers that saturated on Neptune.Slide13

Sources of Uncertainty(Individual Bolometers)

Uncertainty from fits to

PCal

flash data.

Uncertainty in scaling terms.Uncertainty in model flux densities of calibration source.Slide14

Uncertainty from PCal Fits

Because of degeneracy problems with the nonlinear equation fit to the

PCal

flash data, we cannot derive simple uncertainties in the K parameters from the fit.

Instead, we used a Monte Carlo approach to create plots that show the uncertainty in the calibration curves as a function of voltage.

The typical fractional uncertainties from the PCal flash fits derived this way are ~0.0002. The maximum uncertainties do not supercede 0.005. Slide15

Uncertainty from PCal Fits

Dotted lines:

Voltages

for V

0

and V0-VPeak (Neptune)Slide16

Uncertainty from PCal Fits

Dotted lines:

Voltages

for V

0 and V0-VPeak (Neptune)Slide17

Uncertainty from PCal Fits

Dotted lines:

Voltages

for V

0 and V0-VPeak (Neptune)Slide18

Uncertainty from Scaling Terms

The table below lists the uncertainties for the scaling of the calibration curves for individual bolometers based on the uncertainty in the peak voltage measurements of Neptune or 3C 273.

The maximum fractional uncertainties in the PSW and PMW arrays are for bolometers that saturated on Neptune. Most other bolometers have uncertainties much closer to the mean.

Array

Mean

Fractional Uncertainty

Maximum Fractional

Uncertainty

PSW

0.0098

0.11

PMW

0.010

0.12

PLW

0.0016

0.040Slide19

Tests of Flux Calibration

The new Flux Calibration Product was used to process standard large and small scan map data for two sources:

Neptune (primary calibrator)

Gamma

Dra (secondary calibrator)The results can be used to gauge the random uncertainty in the flux calibration as well as variations in the flux calibration over time.

Measurements of the flux densities were performed on the timeline data. Mapping the data increases the dispersion in the flux density measurements, as binning the data into map pixels will effectively blur the data. The source extraction tools currently included in HIPE (DAOPHOT and Sussextractor

) both systematically undermeasure flux densities, and Sussextractor also functions very poorly for >100

mJy

sources.Slide20

Neptune Measured/Model Flux Densities

Triangles: Small scan map

Squares: Large scan mapSlide21

Neptune Measured/Model Flux Densities

Triangles: Small scan map

Squares: Large scan mapSlide22

Neptune Measured/Model Flux Densities

Triangles: Small scan map

Squares: Large scan mapSlide23

Neptune Measured/Model Flux Densities

Array

Measured/Model

Flux Density Ratios

All

Large

Map

Small Map

PSW

0.995 +/- 0.007

0.996

+/- 0.009

0.994 +/- 0.003

PMW

0.993

+/-

0.010

0.997

+/-

0.013

0.992

+/- 0.004

PLW

1.001

+/-

0.003

1.000

+/-

0.003

1.002

+/- 0.003Slide24

Gamma Dra Measured Flux Densities

Triangles: Small scan map

Squares: Large scan mapSlide25

Gamma Dra Measured Flux Densities

Triangles: Small scan map

Squares: Large scan mapSlide26

Gamma Dra Measured Flux Densities

Triangles: Small scan map

Squares: Large scan mapSlide27

Gamma Dra Measured Flux Densities

Array

Model Flux Density (

Jy

)

Measured Flux Densities (

mJy)

All

Large

Map

Small Map

PSW

251

258 +/- 3

260 +/- 2

258 +/- 4

PMW

127

139 +/- 4

140 +/- 5

138 +/- 4

PLW

61

74 +/- 5

75 +/- 5

73 +/- 5

These are monochromatic flux densities without color corrections (and without a

u

in color).Slide28

Conclusions

Although a few individual bolometers have calibration uncertainties of >5%, each array as a whole can measure flux densities with the following accuracies:

PSW 1.5%

PMW 1.7%

PLW 0.5%The accuracy of the flux calibration for >100 mJy sources will primarily be limited by the Neptune models.