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Chapter 32
FOS Error
Sources

In This Chapter...

Target Location in Aperture / 32-2
Mechanism Stability / 32-6
Location of Image on Diode Array / 32-7
OTA Effects / 32-15
Calibration Accuracies / 32-16
Photometric Accuracies / 32-17
Flatfield Calibration / 32-38
Wavelength Calibration / 32-49
Other Data Problems / 32-52
PSF/LSF / 32-60
Astrometry and Aperture Location / 32-62
Timing Accuracies / 32-62
Polarimetry / 32-64

Overall FOS calibration accuracy and the quality of your observations are governed by a variety of factors both internal and external to the instrument, which include:

The location of the target within the science aperture as determined by the target acquisition strategy that was employed and by the quality of the FGS acquisitions and guiding.
The location of the target on the photocathode, which, in addition to having been influenced by acquisition-related consequences, is affected by FOS mechanism positional repeatability.
The location of the mapping of the photocathode output image (the spectrum) onto the detector diode array.
Orbital position-related phenomena such as GIM, background levels, and telescope breathing.
Intrinsic properties of the FOS such as grating-scattered light and LSF and PSF characteristics imposed by the FOS optics, time-dependent photocathode changes, and sporadically dead or noisy diodes.
Telescope focus (pre-COSTAR).

The photon statistics of the detected signal. Table 32.1 summarizes the accuracies you can expect from your data when you have recalibrated it with the latest calfos software and the best reference files (as recommended by StarView-see Chapter 1 in Volume 1), assuming your target acquisition was successful and appropriate for your science aperture, and, for photometry, assuming the 1.0 or 4.3 aperture was used. Note that a typical science observation may have systemic errors as much as two or three times those listed in Table 32.1, if the acquisition miscentered the target or the observations were taken during a period when the Y-bases were in error. In some cases improvements can be made further by using the calibration information and post observation refinement tasks described in this Data Handbook.

Below we first address important factors affecting various calibration accuracies and the data quality (e.g., target centering, mechanism repeatabilities, and image location). Then we discuss the limitations associated with each type of FOS calibration and their associated impact on FOS data quality, providing information to help you estimate the accuracy in your particular science observations.

FOS Calibration Accuracies (1 )

Attribute

Limiting Accuracy

Comments

Absolute photometry

3-4%

Requires accurate centering and Y-bases;
Substantial color effects if pointing
(or equivalent Y-base) worse than 0.1".
Extended source correction required.

Relative photometry within a given spectrum

+/- 2%

Full range of uncertainty may be seen as a systematic effect on scale-lengths of 200-300 Å.
Requires accurate centering and Y-bases.
Substantial color effects if pointing
(or equivalent Y-base) worse than 0.1".

Relative photometry over large wavelength range

+/- 2%

Full range of absolute flux system color uncertainty over wavelength range 1200-5500 Å. Effect is roughly proportional to wavelength baseline.

Flatfield correction

Pre-COSTAR:2% rms achievable
Post-COSTAR:1% rms achievable
3-5% temporal limits in some regions

Requires accurate centering.

Relative image photometry

Limited by photon statistics

"White light" images only.

Absolute wavelength

Without WAVECAL: +/- 1.0 diode
With WAVECAL: +/- 0.11 diode (28 km/sec at high dispersion)

Systematic offsets currently set limit without WAVECAL

Relative wavelengths

+/- 0.1 diode (25 km/sec at high dispersion)

FOS/RD G570H shows systematic 0.2 diode offsets from H to H.

Timing

From headers: absolute: ~0.25 sec;
Relative: ~0.125 seconds

From engineering telemetry: absolute:
~0.125 sec; relative: ~0.02 sec.

Polarization

Pre-COSTAR: ~0.5%; post-COSTAR: ~0.1%

Typically photon statistics limited.

**FOS Calibration Accuracies (1 )**
Attribute	Limiting Accuracy	Comments
Absolute photometry	3-4%	Requires accurate centering and Y-bases; Substantial color effects if pointing (or equivalent Y-base) worse than 0.1". Extended source correction required.
Relative photometry within a given spectrum	+/- 2%	Full range of uncertainty may be seen as a systematic effect on scale-lengths of 200-300 Å. Requires accurate centering and Y-bases. Substantial color effects if pointing (or equivalent Y-base) worse than 0.1".
Relative photometry over large wavelength range	+/- 2%	Full range of absolute flux system color uncertainty over wavelength range 1200-5500 Å. Effect is roughly proportional to wavelength baseline.
Flatfield correction	Pre-COSTAR:2% rms achievable Post-COSTAR:1% rms achievable 3-5% temporal limits in some regions	Requires accurate centering.
Relative image photometry	Limited by photon statistics	"White light" images only.
Absolute wavelength	Without WAVECAL: +/- 1.0 diode With WAVECAL: +/- 0.11 diode (28 km/sec at high dispersion)	Systematic offsets currently set limit without WAVECAL
Relative wavelengths	+/- 0.1 diode (25 km/sec at high dispersion)	FOS/RD G570H shows systematic 0.2 diode offsets from H to H.
Timing	From headers: absolute: ~0.25 sec; Relative: ~0.125 seconds	From engineering telemetry: absolute: ~0.125 sec; relative: ~0.02 sec.
Polarization	Pre-COSTAR: ~0.5%; post-COSTAR: ~0.1%	Typically photon statistics limited.

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stevens@stsci.edu

Chapter 32 FOS Error Sources

In This Chapter...

Attribute

Limiting Accuracy

Comments

Chapter 32
FOS Error
Sources