NICMOS Photometry

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.

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.

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.
NICMOS Group 06 Feb 2003 (html)

Cycle 11 Update to Tiny Tim
Instructions for updating Tiny Tim with Cycle 11 aberration parameters.
NICMOS Group 22 Jan 2003 (html)

FAQs:
Frequently Asked Questions.

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.
NICMOS Group 06 Nov 2001 (html)

Systematic Uncertainties & Special Cases
Various systematic uncertainties and special cases which could affect the final accuracy of NICMOS photometry are discussed.
NICMOS Group 01 Nov 2001 (html)

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.
NICMOS Group 01 Nov 2001 (html)

Photometric History of NICMOS
The photometric stability of all three NICMOS cameras over time is discussed.
NICMOS Group 01 Nov 2001 (html)

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.
NICMOS Group 01 Nov 2001 (html)

Differential Photometry Across the Detectors
Differential photometry for cameras 1 and 2 is better than 2%, in the filters for which measurements are available.
NICMOS Group 01 Nov 2001 (html)

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.
NICMOS Group 01 Nov 2001 (html)

Focus History from 1997-1999
A brief discussion and table describing the history of the NICMOS focus through January, 1999.
NICMOS Group 04 Jan 1999 (html)

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.
NICMOS Group 04 Jan 1999 (html)

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.
R. Bohlin 15 Feb 2006 (pdf)

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 fluxƒ¿) 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.
R. de Jong 15 Feb 2006 (pdf)

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
E. Bergeron and M. Dickenson 06 Oct 2003 (pdf)

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. .
C. Xu and B. Mobasher 16 Sep 2003 (pdf)

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.
R.L. Gilliland and S. Arribas 07 Jan 2003 (pdf)

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.
M. Sosey 15 Jan 2002 (pdf)

Papers and Proceedings:
Selected NICMOS related published papers and workshop proceedings.

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

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.
NICMOS Group 01 Nov 2001 (html)

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.
NICMOS Group 01 Nov 2001 (html)

Examples:
Examples of how to use various NICMOS-related tools, calibration and analysis techniques.

Examples
Section from NICMOS Instrument Handbook version 4.1 for Cycle 11
NICMOS Group 01 Nov 2001 (html)

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 Group 01 Nov 1998 (html)

NICMOS STScI Analysis Newsletter 10
Announcement of the NICMOS Photometry Update web page
Update of NIC1 & NIC2 photometry
New Special Requirements for Proposal Processing
NICMOS Group 01 Apr 1998 (html)