The Hubble Legacy Archive (HLA) is designed to optimize science from the Hubble Space Telescope by providing online, enhanced Hubble products and advanced browsing capabilities. The HLA is a joint project of the Space Telescope Science Institute (STScI), the Space Telescope European Coordinating Facility (ST-ECF), and the Canadian Astronomy Data Centre (CADC).

The goal of the HLA is to optimize science from the Hubble Space Telescope by:

1. Putting the data online for immediate access
2. Adding a footprint (sky coverage) service to make it easier to browse and download images
3. Providing more extensive "composite images" (e.g., stacked, color, mosaics)
4. Improving absolute astrometry from ~1-2" to ~0.3" (see the astrometry FAQ)

The enhanced data products were generated from the standard HST pipeline products. The ACS, WFPC2, and NICMOS images have been combined using multidrizzle, are aligned north up, and have been astrometrically corrected when possible (for approximately 80% of the cases). All HLA-produced images are in units of electrons/second, which for WFPC2 and NICMOS differs from calibration pipeline products. The photometric zeropoints must be adjusted to reflect the units used.

The HLA at CADC includes only public data. Any publ;ic data which has not been process within the HLA pipeline is available via the same interface than the hla but is tagged accordingly.

ACKNOWLEDGMENT FOR PUBLICATIONS

All refereed publications based on data obtained from the HLA are requested to carry the following footnote:

Based on observations made with the NASA/ESA Hubble Space Telescope, and obtained from the Hubble Legacy Archive, which is a collaboration between the Space Telescope Science Institute (STScI/NASA), the Space Telescope European Coordinating Facility (ST-ECF/ESA) and the Canadian Astronomy Data Centre (CADC/NRC/CSA).

Photometric calibration

ACS

1. ACS images are in ELECTRONS/SECOND. Zero-points have been published in Sirianni et al. 2005, PASP, 117, 1049, Table 10; updated values are included in the ACS Instrument Handbook and can be found at http://www.stsci.edu/hst/acs/analysis/zeropoints. Zero-points can also be computed from the values of PHOTFLAM and PHOTPLAM in the header of the image file using the equation:

ABMAG_ZPT = -2.5 Log (PHOTFLAM) - 21.10 - 5 Log10(PHOTPLAM) + 18.6921

2. If performing aperture photometry, the relevant aperture correction is applied. Published ACS zero points are for an infinite aperture; details of the aperture correction depend on filter and photometry procedure. See the Sirianni et al paper for more details.

3. Convert to the desired magnitude system. ACS zero-points are provided in three systems: ABMAG, STMAG, and VEGAMAG, see the ACS Data Handbook for more details on the systems. Note that the conversion between these systems depends on the filter and, to a lesser extent, on the system throughput curve. However, the conversion DOES NOT depend on the source spectrum. The difference between magnitudes in the three systems is small around 550nm (0.05 magnitude or less for F555W) but it can be quite large (over 1 magnitude) in the UV and near IR.

4. Other adjustments may be necessary, e.g., for CTE losses. These are to date much smaller than for WFPC2, but they may need to be taken into account for high precision photometry. Consult the ACS Instrument and Data Handbooks and the Instrument Science Reports (all found at http://www.stsci.edu/hst/acs/documents) for the most up to date information on the calibration of the camera.

WPFC2

WFPC2 Level 1 (drizzled) and Level 2 (Combined) images produced by the HLA pipeline are also in units of ELECTRONS/SECOND, like ACS images. Note that this convention is different from that adopted by the Archive pipeline, which produces calibrated images (the *c0m.fits files) in total Data Number (DN). One DN is approximately 7 or 14 electrons, depending on the gain level chosen by the original observer. There are thus two differences in the images: HLA images express counts in electrons rather than DN, and they are divided by the exposure time to obtain a count rate rather than total counts.

The calibration information found in the WFPC2 documentation all refers to DN rather than electrons; therefore all zero-points must be adjusted to account for the gain. (Published zero points do assume that instrumental magnitudes are already corrected for the exposure time.) The following adjustments are therefore necessary for HLA WFPC2 images:

1. Convert published zero-points from DN to ELECTRONS using Table 5.1 of the WFPC2 Data Handbook. This is just the Gain, which is listed in Table 4.4 of the Instrument Handbook. This varies by about a percent for the different chip, but an average value of GAIN = 7.04 for the three WF chips should suffice for most purposes. Hence, this correction is 2.5 log10(7.04) = 2.119. So for example, rather than using a zero-point of 22.557 (an average for the three WF chips from Table 5.1), you should use 22.557 + 2.119 = 24.676 magnitudes.

NOTE: Unlike for ACS images, YOU SHOULD NOT USE THE VALUES OF PHOTFLAM AND PHOTPLAM from the header of the WFPC2 HLA images, since neither PHOTFLAM nor PHOTPLAM have yet been adjusted from DN to electrons.

2. Ensure that the instrumental magnitude (-2.5 log10(total pixel value) determined by your photometric package uses an exposure time of 1 second. Some packages may read the value of EXPTIME from the image header and apply a correction of (-2.5 log10(exposure time)) to the total pixel counts from the image; THIS WILL LEAD TO INCORRECT PHOTOMETRY. The value of EXPTIME in the image header is indicative of the total exposure contributing to the image, but it MUST NOT be used in computing the instrumental magnitude as the image units are already expressed in counts per second.

3. If performing aperture photometry, apply the appropriate aperture correction. WFPC2 zero-points are given by adding 0.1 magnitudes to the counts within a 0.5 arcsecond radius; thus if the photometry uses a radius of 0.5 arcseconds, the aperture correction is 0.1 magnitudes by definition. For other apertures, the correction can be determined directly from the data (e.g., by measuring an isolated bright star if available) or estimated using the encircled energy curves from Holtzman et al 1995,PASP 107, 156. If a very small aperture is used (less than 0.1 arcseconds radius), the variation of aperture correction with focus may be significant, and thus a direct measurement from the data is preferable. See the WFPC2 Data Handbook for more information.

4. Convert from VEGAMAG to ABMAG or STMAG if desired. The same considerations apply as for ACS; however, WFPC2 zero-points are only provided in VEGAMAG, not in STMAG or ABMAG. The WFPC2 Data Handbook provides detailed examples on the steps needed to carry out the transformation using SYNPHOT

5. Other adjustments may be necessary, CTE and contamination corrections being the most common. Please consult the WFPC2 documentation at http://www.stsci.edu/hst/wfpc2/documents for up to date information and for more details.

• CTE losses are significant for WFPC2, especially for data taken several years after installation. CTE losses depend on background, flux, and position in the detector; correction formulae for point sources have been developed by Dolphin 2000, PASP, 112, 1397 and following papers; the latest corrections are available at http://purcell.as.arizona.edu/wfpc2_calib. Please note that the CTE corrections depend on the position of the source in ORIGINAL detector pixels, and thus cannot be applied on the basis of the pixel position in the combined HLA images.
• The UV sensitivity of WFPC2 varies substantially with time because of the build-up of contaminants on the CCD windows. These contaminants are largely removed during monthly warming (decontaminations) of the camera; the build-up of contaminants after each procedure is well measured and its impact on photometry can be corrected using the procedures described in Section 5.2 of the WFPC2 Data Handbook.

NICMOS

NICMOS Level 1 (drizzled) and Level 2 (Combined) images produced by the HLA pipeline are also in units of ELECTRONS/SECOND, like ACS images. Note that this convention is different from that adopted by the Archive pipeline, which produces calibrated images (the *cal.fits files) in DN/second. For most images, one DN corresponds to 5.4 electrons (Cameras 1 and 2) or 6.5 electrons (Camera 3); the value of the gain is given by the header keyword ADCGAIN.

The NICMOS photometric calibration is expressed by the header keywords PHOTFLAM and PHOTFNU; photometric zeropoints are obtained from these quantities as described in the NICMOS Data Handbook. The values of PHOTFLAM and PHOTFNU refer to DN/second; counts obtained from HLA-produced images must be divided by the gain in order to apply the zeropoints thus derived.

Please refer to the NICMOS photometry page for more details on various corrections and adjustments that may need to be applied to obtain the best photometry from NICMOS images.