Frequently Asked Questions

What is this website for?

This website is a tool for finding and learning about resolved stellar photometry for nearby galaxies as viewed by the Hubble Space Telescope. The files contain tables of photometry for the ANGST sample, as defined in Dalcanton et al. 2009 (PDF). Also included are associated reference images, raw photometry tables, and parameter files.

What data can I get from this website?

We have provided links to high-quality stellar photometry, all stellar photometry, and raw stellar photometry organized by galaxy and target name.

Where do the completeness diagrams come from?

We have performed 100,000 artificial star tests on each target in order to determine the completeness as a function of color and magnitude. The results of the tests are shown in the completeness diagrams as a grayscale plot of completeness in the color-magnitude plane.

How was the photometry performed?

ACS measurements were made using the DOLPHOT (Dolphin 2000) package, and WFPC2 measurements were made using the HSTphot pipeline described in Holtzman et al 2006, updated with the new August 2008 CTE corrections derived by Dolphin. ACS photometry uses CTE corrections as determined before the switch to side 2 electronics, but the zero points in the binary fits tables have been corrected to the most recent values published on the STScI website as of 10-Sep-2008. New ACS CTE corrections will be incorporated into upcoming data releases.

Is all of the data complete and reliable?

There were a few targets whose processing had issues that may not be obvious to the user. The M82 fields are very crowded and shallow, leading to low-precision photometry whose errors are significantly underestimated by the quoted errors from DOLPHOT. Photometry near bright diffraction spikes is unreliable; data in these regions are provided on a shared risk basis. Archival F814W data for M81 tilings from GO-10250 is not included in the current release, since the significant misalignments with the F435W and F606W data prevents automated pipeline reductions; these data will be incorporated in subsequent releases. Not every ACS/WFPC2 image of target galaxies is included in this release; pointings were excluded when images were not of sufficient depth or wavelength coverage to be useful for stellar photometry. Please contact the ANGST team if you find images in the archive that you think should be included in subsequent releases.

How were the targets named?

The names were taken from the image headers and are of the format PROPOSID_TARGNAME.

What are the "reference images"?

These were the images that were used by the photometry routine to align all of the individual exposures of a given target in order to measure the total photon flux from each star. Their names are:

*ref.fits: Reference image used for the alignment of the data and determination of the X, Y, RA, and DEC (J2000) coordinates of each star. WFPC2 data has a separate reference image for each chip: *ref[1-4].fits.

*ref.jpg: JPEG image of reference image used for the alignment of the ACS data and determination of the X, Y, RA, and DEC (J2000) coordinates of each star.

*refmos.jpg: JPEG image of "batwing" mosaic of the 4 chips of the reference image used for the alignment of the WFPC2 data.

What information is included in the photometry files?

The naming scheme and column names for the photometry files are summarized below, as well as in the headers of the fits files themselves. File names also include the filters of the photometry. For fields with three filters, the *.st.fits and *.gst.fits files are generated for each pair of filters; complete 3 filter photometry can be found in the much larger *.phot files. The headers for the binary fits tables include extensive information on the data used for photometry (name of reference image, filters, camera, etc).

*.st.fits: Star files: these files contain the photometry of all objects classified as stars (object type <= 2) with S/N > 4 and data flag < 8. These files will contain more objects than the *.gst.fits files, but will have less clean CMD features. Columns are X, Y, RA, DEC, MAG1_ACS, MAG1_STD, MAG1_ERR, MAG2_ACS, MAG2_STD, MAG2_ERR. Filters are in the file name as well as the header.

*.gst.fits: Good Star files: These files contain the stars that pass the ANGST quality cuts for sharpness and crowding (sharp_filter1 + sharp_filter2)^2 < 0.075 && crowd_filter1 + crowd_filter2 < 0.1), in addition to the S/N and flag criteria. Columns are X, Y, RA, DEC, MAG1_ACS, MAG1_STD, MAG1_ERR, MAG2_ACS, MAG2_STD, MAG2_ERR. Filters are in the file name as well as the header. For WFPC2 data, the crowding cut was 0.7. All of this information is also in the headers of the binary fits tables. Caution should be used when using these files in regions of high crowding, such as stellar clusters.

*.phot: Raw output from DOLPHOT (or in case of WFPC2 data, raw output from HSTphot). For DOLPHOT output, the columns are defined as follows, as can be seen in the DOLPHOT manual:

#### DOLPHOT/ACS OUTPUT ##############################
######################################################
1 extension
2 chip
3 x position
4 y position
#### GLOBAL SOLUTION #################################
5 chi
6 signal-to-noise
7 sharpness
8 roundness
9 major axis
10 crowding
11 object type (1,2 == stars)
#### PHOTOMETRY ######################################
12 counts
13 background
14 magnitude (instrumental)
15 magnitude (standard)
16 magnitude uncertainty
17 chi
18 signal-to-noise
19 sharpness
20 roundness
21 crowding
22 FWHM
23 ellipticity
24 PSF a
25 PSF b
26 PSF c
27 error flag (<8 usable)
######################################################
#### COMBINED PHOTOMETRY IS REPEATED N TIMES FOR ####
#### N FILTERS (IN ORDER OF WAVELENGTH), FOLLOWED ####
#### BY PHOTOMETRY FOR EACH INDIVIDUAL IMAGE ####
######################################################

For raw HSTphot output, the columns are defined as follows, as can be
seen in the HSTphot manual:

#### HSTPHOT OUTPUT ##############################
######################################################
1 chip
2 x position
3 y position
#### GLOBAL SOLUTION #################################
4 chi
5 signal-to-noise
6 sharpness
7 roundness
8 major axis
9 object type (1,2 == stars)
#### PHOTOMETRY ######################################
10 counts
11 background
12 magnitude (VEGAMAG)
13 magnitude (standard)
14 magnitude uncertainty
15 chi
16 signal-to-noise
17 sharpness
18 roundness
19 crowding
######################################################
#### COMBINED PHOTOMETRY IS REPEATED N TIMES FOR ####
#### N FILTERS (IN ORDER OF WAVELENGTH FOR FILTERS ###
#### FOR WHICH MORE THAN ONE IMAGE WAS ENTERED INTO ##
#### HSTPHOT), FOLLOWED BY PHOTOMETRY FOR EACH ######
#### INDIVIDUAL IMAGE ###########################
######################################################

What are the "parameter files"?

These files contain the parameters used to run DOLPHOT for all of our ACS targets so that the user can know exactly how DOLPHOT was run in order for us to make our measurements. Since HSTphot does not make use of a parameter file, the WFPC2 targets do not have such a file available.

What resources are available for reading binary FITS files?