Being above the atmosphere, NICMOS is not forced to adopt filter bandpasses like instruments used at ground-based observatories. Instead it has filters which were designed to meet the anticipated scientific demands. Thus in practise NICMOS does not have filters matched to any of the ground-based photometric bands. Obtaining photometric calibrations for NICMOS data is discussed in Chapter 5 of the NICMOS Data Handbook.
The NICMOS team has determined that NICMOS suffers from a count rate dependent non-linearity which influences accurate photometry, especially at the shorter wavelengths. If your science requires better than 15% accuracy in photometry, apply the non-linearity correction rnlincor routine described on the non-linearity page to your reduced data.
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The NICMOS group has derived new calibration zeropoints for the NICMOS cameras, both pre- and post-NCS installation. The values presented here were determined using all calibration data taken before June 2004 and supersede all previous tables and values in the handbook. Values on this web page will always be the most up-to-date known. Due to the change in the nominal operating temperature of the detectors from ~62K before the NCS-installation to the current 77.1K, the photometric keyword values have changed. The QE has increased by 20-90% (depending on wavelength) and different sets of calibration constants have to be used for pre- and post-NCS data.
In the effort to calibrate NICMOS after NCS installation, all pre-NCS Cycle 7 calibration data were also analyzed again with the latest data reduction techniques. This has resulted in new calibration values for these data as well. The PHOTTAB photometric table that is in the calibration database and that is being used to populated image headers during on-the-fly-reduction is still an old 62K, pre-NCS table. These photometric keywords have changed for Cycle 7 data by typically 5-10% due to improved aperture corrections and these keywords are NOT APPROPRIATE for data taken at 77.1K in Cycles 11 and beyond. The new PHOTTAB tables with calibration constants were installed in the pipeline on July 29, 2004. Any observations retrieved from the archive before July 29, 2004 will have the old, incorrect photometric header items. Retrieving this data again after July 29, 2004 will fix the problem, as the on-the-fly-reduction will automatically populate the headers with the correct calibration constants. Alternatively, headers can be updated by hand using the tables listed below on this webpage.
The new NICMOS Photometric Keywords were derived from principle calibration stars P330E (a Solar analog star) and G191B2B (a white dwarf). The calibration values are the error weighted averages of the values derived for the two stars individually. We have set the error in the calibration keywords such that they reflect the uncertainty in the individual star measurement errors and the discrepancy between the two stars. We have not included the uncertainty in the absolute calibration of the reference star spectra in our error estimates. These absolute calibration uncertainties are believed to be less than 5%. This reference spectrum uncertainty will also introduce a much smaller error in colors derived from NICMOS observations in different passbands, which are thought to be less than 3%. This effect is most significant for the narrow-band filters, when the exact shape of absorption bands is not exactly modeled in our reference spectra and falls in the narrow passband. In our new calibration tables on the web we also list the wavelength dependent aperture corrections we have used to go from our fixed aperture calibrations (11.5 pixels for NIC1, 6.5 pixels for NIC2, and 5.5 pixels for NIC3) to infinite aperture calibration listed. We have derived these aperture corrections from TinyTim simulations.
We have identified variations in the photometry calibration data at the less than 1% level that are still not fully understood. The individual cameras show sensitivity variations at this level, which means that all passbands in the same camera are affected in the same way and therefore color measurements should still be very accurate over time. We also see some discrepancy between the two primary standards in NIC3 that are not fully explained.
No new photometric calibration of the polarimetry filters of NIC1 and NIC2 and the grisms in NIC3 has been performed. The calibration values for these optical elements in the calibration tables reflect the initial calibration of NICMOS performed in Cycle 7 and have only been corrected for the observed detector sensitivity gain for the post-NCS era. THESE VALUES WERE DERIVED WITH A DIFFERENT METHOLOGY AND SYSTEMATIC OFFSETS WITH THE ZEROPOINTS OF THE OTHER FILTERS OF ORDER 5-10% CAN BE EXPECTED.
Any updates to the calibration standards will be announced through this website, as well as through STAN announcements.
Photmetric Keywords and Vegamag Zeropoints for 77.1K Detectors (Appropriate for all data taken after January 1, 2000)
Photmetric Keywords and Vegamag Zeropoints for 62K Detectors (Appropriate for all data taken prior to January 1, 2000)
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This page summarizes all the information concerning NICMOS photometry including absolute calibrations of the cameras, its photometric stability and the absolute calibration of the NIC3 grisms.
Aside from Chapter 5 of the Data Handbook, other important photometry resources also provide valuable high level sources of photometry information: Differential Photometry Across the Detectors, Intrapixel Sensitivity, Photometric History of NICMOS, Absolute Spectrophotometric Standards, and Systematic Uncertainties and Special Cases. Another important photometry reference is NICMOS to Ground Based Systems Transformations. For more specific details on photometry follow the links to the documents listed below.
NICMOS Photometry Documentation
Advisories:
Important updates, discoveries and developments
that could potentially affect NICMOS observations,
calibration, or data analysis.
Flux Conversion Correction
An error in one of the NICMOS flux conversion factors was identified and must be updated.
Cycle 11 Update to Tiny Tim
Instructions for updating Tiny Tim with Cycle 11 aberration parameters.
FAQs:
Frequently Asked Questions.
Q: Photometry A: ...
Performance Summaries:
Status reports reflecting the current
understanding of instrument characteristics,
performance and calibration.
Red Leaks
No indications of measurable red
leaks have been found in the blue
filters.
Absolute SpectroPhotometric Standards
The white dwarf G191-B2B and the solar analog P330E provide NICMOS absolute calibration in the white dwarf and solar analog scales, respectively.
Photometric History of NICMOS
The photometric stability of all three NICMOS cameras over time is discussed.
Differential Photometry Across the Detectors
Differential photometry for cameras 1 and 2 is better than 2%, in the filters for which measurements are available.
Absolute Photometry
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.
Systematic Uncertainties & Special Cases
Various systematic uncertainties
and special cases which could
affect the final accuracy of
NICMOS photometry are discussed.
Intrapixel Sensitivity
The sensitivity of NICMOS detectors
is lower near the edges of the
pixels than in their centers. In
practical terms this effect means
that for a source whose flux changes
rapidly on a size comparable with
or smaller than the pixel size,
the measured countrate, and
therefore flux, will depend on
where the center of the source
lies with respect to the center of
the pixel.
Focus History from 1997-1999
A brief discussion and table describing the history of the NICMOS focus through January, 1999.
NICMOS Focus Monitoring Programs
Due to varying mechanical stresses in the dewar, the focus of the NICMOS
detectors was constantly changing on-orbit. A series of programs were
designed to monitor the focus for each of the three cameras.
Handbooks:
The NICMOS Instrument Handbook is the primary
guide regarding the characteristics and use of
the instrument. The HST Data Handbook is the
primary guide for calibration, reduction and analysis
of NICMOS data.
Instrument Science Reports:
ISRs are technical reports written by members
of the NICMOS Group about various aspects of the
instrument and data. They usually contain in-depth
information about specific topics.
NICMOS ISR 2006-002: NICMOS Count Rate Dependent Non-Linearity in G096 and G141
Since the discovery of the NICMOS count rate dependent non-linearity
documented in NICMOS ISR 2005-002, additional tests have been conducted to
further understand and quantify the effect. Long integrations of up to 25 minutes
exhibited the same level of non-linearity as the original 1-3 minute integrations.
Observations of the star P041C on high background from an internal flat field
lamp provides another measure of the wavelength dependence of the nonlinearity
to compliment the measures based on pure hydrogen WD models.
NICMOS ISR 2006-001: NICMOS Count-rate Dependent Non-linearity Tests using Flatfield Lamps
We investigate the recently discovered NICMOS count-rate dependent non-linearity
(Bohlin et al. 2005) using the flatfield lamps to artificially increase the count rate. A star
cluster field was imaged in a lamp off-on-off sequence in all cameras in a selected set of
filters, followed by a series of darks to investigate persistence and to clean the images
from any remaining charge for the next orbit. Subtracting the lamp-off images from the
lamp-on images clearly shows residual ADUs at the star positions, indicating that a
higher background (and thus total) count rate increases the number of ADUs registered
from an object. We model the non-linearity with a power law (count-rate å flux¿) and fit
this model to the data. Both NIC1 and NIC2 (NIC3 was not tested in this program) show
non-linearity, becoming stronger at shorter wavelengths, but with larger amplitude than
predicted by the Bohlin et al. NIC3 measurements. The non-linearity in NIC1 and NIC2
amounts to 0.06-0.10 mag offset per factor ten change in incident flux for the shortest
wavelength (F090M and F110W), about 0.03 mag/dex at F160W, and less at longer
wavelengths. Archival data from Cycle 7 are also analyzed, showing that the nonlinearity
has not changed in NIC2 F110W, and suggesting that this effect is independent
of detector temperature.
NICMOS ISR 2003-010: Removal of Cosmic Ray persistence From Science Data using the Post-SAA Darks
Latent or persisten images of cosmic ray hits following HST transits of the South Atlantic
Anomaly (SAA) can signicantly increase the noise in subsequent NICMOS science images. By
taking a pair of DARK exposures immediately following the exit of the SAA, a map of the persistent
signal can be made and then used to subtract this signal from the impacted images in that
orbit, thereby recovering much of the original S/N. We here describe an algorithm to do this
NICMOS ISR 2003-009: NICMOS IntraPixel Sensitivity
A study of the NICMOS (Camera 3) intrapixel sensitivity is carried out after installation of
cryo-cooler in Cycle 11. The consistency of the procedure adopted here is tested by applying
it to the data in Cycle 7, as analyzed by Storrs et al (1999), and reproducing their
results. The intrapixel sensitivity in Cycle 11 is then measured and compared to that in
Cycle 7 (before the installation of the NICMOS cryo-cooler). We find 27% decrease in the
intrapixel sensitivity from Cycle 7 to Cycle 11 for both F110W (J-band) and F160W (Hband)
filters. The possibility of this effect being due to an increase in the stellar PSF over
this period is considered and discarded, as we only find 5% change in the PSF between the
two cycles.
.
NICMOS ISR 2003-001: High Signal-to-Noise, Differential NICMOS Spectrophotometry
We report analysis for NICMOS CAL/9642, High S/N Capability Characterization.The
purpose of this three orbit test was to establish NICMOS time domain stability in a
domain not previously tested for NICMOS. Observations of the bright (H = 6.13) G0 V
star HD 209458 were obtained using 1.8 second MULTIACCUMs with G141 on NIC3.
Exposure-to-exposure stability summed over the full spectrum provides S/N of 2000, i.e.
we demonstrated 0.5 mmag photometric precision treating the observations simply as
broad-band, time-series photometry. Orbit-to-orbit means in a 0.2 micron band taken in
ratio to neighboring continuum, without applying any corrections for correlated variations
in contemporaneous auxiliary parameters, differed by only 3x10-4. A much better
result was obtained with application of such corrections, but the result cannot be considered
secure with comparison of only two orbits data. There is an excellent chance that in
applications with sufficient flux to generate the required Poisson limit, that well planned
NICMOS observations can provide differential spectrophotometric stability better than 1
part in 10,000 if not even significantly better.
NICMOS ISR 2002-001: Bad Pixel Masks
A study of the frequency and location of the NICMOS bad pixels in each of the three cameras
had not been undertaken since the instrument was commissioned on-board HST in
1997. The original MASKFILEs located in the calibration database (CDBS) were compiled
with data taken during Systems Level Thermal Vacuum (SLTV) ground testing in
August 1996. This analysis examined dark frames taken during the initial lifetime of NICMOS,
March 1997 - January 1999, and details the creation of new bad pixel masks for
each camera. Updated flat field reference files were also created as a result of this analysis.
NICMOS ISR-95-006: Effects of the NICMOS Array Flat-field Response on Observations.
In this report we use the results obtained from our detailed flat-field response tests to analyze numerically the likely effect of this behaviour on NICMOS photometry and imaging.
Papers and Proceedings:
Selected NICMOS related published papers and workshop
proceedings.
Characterization and Subtraction of Well-Exposed HST/NICMOS Camera 2 Point-Spread Functions for a Survey of Very Low Mass Companions to Nearby Stars
D. Golimowski,D. Schroeder,T. Henry
On-orbit Properties of the NICMOS Detectors on HST
L. Bergeron,A. Schultz,J. MacKenty,A. Storrs,W. Freudling,D. Axon,H. Bushouse,D. Calzetti,L. Colina,D. Daou,D. Gilmore,S. Holfeltz,J. Najita,K. Noll,C. Ritchie,W. Sparks,A. Suchkov
Absolute Flux Distributions of Solar Analogs from the UV to the Near-IR
Tutorials:
Cookbook style instructions, prescribed procedures, and
helpful tips.
NICMOS to Ground Based Systems Transformations
A total of six stars (2 white dwarfs, 2 solar analogs and 2 redden stars) have been observed through the NICMOS JHK system (i.e. F110W, F160W and F222M filters) to help establish transformations between NICMOS and ground-based near-infrared systems.
Photometric Calibration CookBook
A brief summary of the steps needed
to perform absolute photometric
measurements in NIC1, NIC2, and NIC3 images. The steps described here are no longer valid for most of the current calibration values, which have wavelength dependent aperture corrections. However, polarimetry and grism calibrations should still be performed this way, as they have not been recalibrated in the method with aperture corrections.
Examples:
Examples of how to use various NICMOS-related tools, calibration and analysis techniques.
Space Telescope Analysis Newsletters:
STANs contain useful information regarding
calibration and data reduction.
NICMOS STScI Analysis Newsletter 15
NICMOS standard stars
NICMOS End-of-Life monitoring
NICMOS Prime Science Observations Update
NICMOS at the January 1999 Texas AAS Meeting
NICMOS STScI Analysis Newsletter 10
Announcement of the NICMOS Photometry Update web page
Update of NIC1 & NIC2 photometry
New Special Requirements for Proposal Processing