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Hubble Space Telescope
Absolute Photometry

First on-orbit absolute photometric calibrations for NICMOS were obtained during SMOV. The results of these limited calibrations were used to establish in July 1997 preliminary corrections to the predicted photometry based on ground-based vacuum measurements.

The first photometric update based on cycle 7 calibration measurements was delivered in March 1998 and included new photometric values for all NIC1 and NIC2 filters. A second photometric update has been delivered in May 1998 including new photometric values for all NIC3 filters. A third update is expected towards the end of NICMOS cycle 7 observations.

July 1997 Update

A preliminary on-orbit absolute calibration of all NIC1, NIC2 and NIC3 filters went out in July 1997 soon after the end of the Servicing Mission Observatory Verification (SMOV) phase. This preliminary update was based on on-orbit measurements of a couple of standard stars in 4/5 filters per camera and covering the entire NICMOS wavelength range. The advertised accuracy of the final absolute calibration obtained in this way was 10% - 15% for most filters (Colina & Rieke 1997; HST Calibration Workshop).

March 1998 Update

This update is based on a complete NIC1 and NIC2 on-orbit absolute photometry calibration program executed in August 1997. Images of G191-B2B and P330E were taken in August 1997 (proposal ID 7691) through all available NIC1 and NIC2 narrow-, medium- and broad-band filters (32 in total). Independent measurements of each of the stars were taken at three separated positions within the detectors.

Countrates (i.e. counts per second) were measured in the final calibrated images using a 0.5 arcsec radius aperture in both cameras. The total countrates in a nominal infinite aperture have been computed applying an aperture correction equivalent to multiplying the 0.5 arcsec aperture countrates by 1.15, irrespectively of filter.

The observed total countrates obtained in this way were compared with the predicted total countrates obtained using synthetic photometry (SYNPHOT) on the spectra of G191_B2B and P330E for all NIC1 and NIC2 filters.

The average of the observed/predicted countrates for each of the two stars was used to compute corrections and therefore to match the predicted and observed countrates. These corrections were applied in practice by renormalizing the filter throughputs without changing their shape and without changing the predicted DQE of the detectors. Once the corrections were in place, SYNPHOT was used to compute the new photometric keywords for all NIC1 and NIC2 filters.

May 1998 Update

Images of G191-B2B and P330E were also taken with all available NIC3 filters during the first NIC3 campaign in January 1998 (proposal ID 7816). Independent measurements of each of the stars were taken at three separated positions within the detectors.

Countrates (i.e. counts per second) were measured in the final calibrated images using a 1.0 arcsec radius aperture in both cameras. The total countrates in a nominal infinite aperture have been computed applying an aperture correction equivalent to multiplying the 1.0 arcsec aperture countrates by 1.075, irrespectively of filter.

The observed total countrates obtained in this way were compared with the predicted total countrates computed using synthetic photometry (SYNPHOT) on the spectra of G191_B2B and P330E for all NIC3 filters.

The average of the observed/predicted countrates for each of the two stars was used to compute corrections and therefore to match the predicted and observed countrates. These corrections were applied in practice by renormalizing the filter throughputs without changing their shape and without changing the predicted DQE of the detectors. Once the corrections were in place, SYNPHOT was used to compute the new photometric keywords for all NIC3 filters.

Absolute Photometry Results:

To quantify the accuracy in the absolute photometry, the observed/predicted countrate ratios for NIC1 , NIC2 and NIC3 filters have been computed with SYNPHOT after photometric corrections for each camera were applied. The results for the two primary standards G191-B2B and P330E and the two additional standards GD153 and P177D indicate that the absolute photometry is accurate to 2% (1 sigma uncertainty) and is always better than 5%, for stars in the J-K range covered by the standards, i.e. from about -0.25 to +0.35. Measurements with deviations at the 5% level or larger are identified with measurements where the star was positioned close to the edge of the detector (i.e vignetting can affect the measurements) or stars for which the template spectra have additional uncertainties at the few percent level.

A note of caution is necessary here when converting countrates into flux and/or magnitudes. The absolute photometry, and therefore the photometric keywords delivered by the NICMOS calibration pipeline, are valid when the images are calibrated with the same darks and flat-fields reference files used in the calibration of the standard stars. In addition, the total countrates have to be computed using a 0.5 arcsec (NIC1 & NIC2) or 1.0 arcsec (NIC3) radius aperture and multiplying the result by the ``infinite'' aperture correction (1.15 for NIC1 & NIC2 and 1.075 for NIC3) used in the measurements of the standard stars.

Below are listed the Reference Files used in the calibration of the P330E and G191-B2B 7691 data:

Images calibrated with different darks, flat-fields or countrates obtained using apertures different than the ones indicated above need appropriate corrections before multiplying the countrates by the photometric keywords.