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Expected Accuracies

Remarks

In Table 14.1 we list a provisional summary of our calibration goals for Cycle 7 and Cycle 7-NICMOS. These goals will be achieved through the analysis of data obtained both from the ground testing, on-orbit during the Servicing Mission Orbital Verification (SMOV) phase, and the Cycle 7 calibration program (see Chapter 15). Observers may, and should, plan on NICMOS achieving these levels of performance. While higher levels of performance may indeed by possible (and even achieved) in Cycle 7, observers should not depend upon NICMOS being calibrated better than defined here. Science programs which require superior calibrations should request and justify additional observing time to reach the necessary calibration accuracy.

Areas of Significant Uncertainty

The present knowledge of photometric calibration does not yet permit a definitive projection of the ultimate accuracies which may be obtainable with NICMOS. We have adopted fairly conservative estimates for the achievable photometric (and, consequently, polarimetric) accuracies. Areas of uncertainty include the quality of the transformations, the long term stability of NICMOS, and (for a small portion of each Camera's field of view) the effects of vignetting.

Perforce our existing calibration data for Camera 3 have been obtained with it out of focus and therefore the estimates for its in-focus performance remain extrapolations.

The astrometric performance of NICMOS is not yet well understood. There appears to be continuing motions occurring within the dewar and the optical bench holding the detectors (which is located inside of the dewar). Additional data may become available as part of the August 1, 1997 STScI NICMOS WWW update on the coronographs performance.

Provisional Cycle 7 Calibration Goals

Table 14.1: Summary of Cycle 7 Calibration Goals

Attribute

Accuracy

Limiting Factor/Notes

Detector dark current and shading

<10 DN

All MULTIACCUM sequences will have good calibrations.
A small subset of the available ACCUM mode exposure times will have a direct calibration.

Flat fields

1% (Cameras 1,2)
2% Camera 3

Color dependence may limit flats in some cases.
The low spatial frequency may only be reliable to £ 3%.

Photometry

5-10% photometric zero point
2% relative over FOV
2% stability

Photometric systems
Intrapixel effects
Filter leaks on red sources

PSF and focus

Maintained within 1 mm

Breathing and dewar motion are approximately equal effects.

Coronographic PSF

.019 arcsec positioning

Hole and detector are not at a common focus

Polarization

~1% relative intensity
3-5% polarization accuracy

Photometric performance
Ghosts/polarizing efficiency

GRISM wavelength calibration

.01 µm

Limited by centroiding of target for wavelength zeropoint determination.

GRISM photometric calibration

20-30% absolute and relative

Expected accuracy over central 80% of spectral range.
Grism C flat field may not achieve this performance.

Astrometry

0.2% plate scale
0.1" to FGS frame

Uncertain due to continued focal plane motions

Table 14.1: Summary of Cycle 7 Calibration Goals
Attribute	Accuracy	Limiting Factor/Notes
Detector dark current and shading	<10 DN	All MULTIACCUM sequences will have good calibrations. A small subset of the available ACCUM mode exposure times will have a direct calibration.
Flat fields	1% (Cameras 1,2) 2% Camera 3	Color dependence may limit flats in some cases. The low spatial frequency may only be reliable to £ 3%.
Photometry	5-10% photometric zero point 2% relative over FOV 2% stability	Photometric systems Intrapixel effects Filter leaks on red sources
PSF and focus	Maintained within 1 mm	Breathing and dewar motion are approximately equal effects.
Coronographic PSF	.019 arcsec positioning	Hole and detector are not at a common focus
Polarization	~1% relative intensity 3-5% polarization accuracy	Photometric performance Ghosts/polarizing efficiency
GRISM wavelength calibration	.01 µm	Limited by centroiding of target for wavelength zeropoint determination.
GRISM photometric calibration	20-30% absolute and relative	Expected accuracy over central 80% of spectral range. Grism C flat field may not achieve this performance.
Astrometry	0.2% plate scale 0.1" to FGS frame	Uncertain due to continued focal plane motions

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