APASS: The AAVSO Photometric All-Sky Survey

APASS: The AAVSO Photometric All-Sky Survey

Data release 1 of APASS occurred on 2010 September 10, and contained photometry for approximately four million stars distributed among several hundred discrete fields between -10.15 < Dec < +87.35.  Data release 2 occurred on 2011 February 10, and included an additional four million stars from the southern hemisphere, acquired from our site at CTIO.   Data Release 3 occurred on 2011 August 10, and further extended southern hemisphere coverage; the catalog now contains photometry for 18.9 million objects in about 27,000 square degrees.  Data Release 4 occurred in early January 2012, and Data Release 5 (the current release) occurred on 2012 February 21.  We expect one more data release in mid-2012, and the next in 2013.

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APASS DR5 coverage map (credit: Edward Los)

This current data release is the fifth of several which will eventually cover the entire sky, north and south.  APASS is a public service to the astronomical community, and was funded through generous contributions from the Robert Martin Ayers Sciences Fund [2] and the AAVSO endowment [3].  If you use APASS for your research, please consider supporting the AAVSO [3] with your membership or a contribution.  You should also acknowledge APASS with the following credit line: "This research was made possible through the use of the AAVSO Photometric All-Sky Survey (APASS), funded by the Robert Martin Ayers Sciences Fund."

Access APASS

• Access form
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• Description of APASS and APASS DR1 [5] (in preparation for publication)

Information about APASS Development

Through a grant from the Robert Martin Ayers Sciences Fund, the AAVSO is performing an all-sky photometric survey. This survey is conducted in five filters: Johnson B and V, plus Sloan g′, r′, i′. It is valid from about 10th magnitude to about 17th magnitude. Precise, reliable standardized photometry in this magnitude range is in high demand, both from our observers and from the professional community. Such a catalog will allow many research programs to quickly establish transformation between systems and efficiently achieve conversion of photometry to more fundamental physical properties. It will bridge the gap between Tycho2 [6] and SDSS [7], plus cover the entire sky at the same depth as UCAC [8]. The survey will take approximately two years to complete.

Johnson B and V were chosen to extend the Tycho calibration to fainter magnitudes and to match the many archival published datasets. These passbands can be easily used to create sequences for any VSX [9] / GCVS [10] star. Sloan passbands provide a direct link to the SDSS, SkyMapper, PanSTARRs etc. surveys, and provide a homogeneous catalog of brighter stars.

[11]All photometric nights at the sites will be reserved for the survey. On non-photometric nights, other wide-field projects will be executed. These include an exoplanet transit survey, bright star time series in simultaneous passbands, and long-term monitoring of clusters and specific variable stars. All of these data products will be available on a one-day delay through the AAVSO web site. Some time will also be available for AAVSO member and professional collaborator research.

Astro Systeme Austria [12] (ASA) has given us a substantial discount on two of their N8 20cm astrographs. Software Bisque has loaned us a Paramount ME for the duration of the survey. Both of these astrographs are placed on the Paramount and take simultaneous images of the same field, but in different filters. We use Apogee U16m 4kx4k CCD cameras, plus their 7-position filter wheels, for those astrographs. The system is initially sited at Dark Ridge Observatory [13] (DRO) near Weed, NM, where Tom Smith has agreed to host the system during the northern hemisphere part of the survey. This portion started in October 2009, and continues through summer 2010. At that time, the telescope system will be shipped to Chile to complete the sky.

Dirk Terrell is helping with the necessary computers and software for the telescopes; Doug Welch is helping on the scheduling, data processing and analysis; Michael Sallman is helping with the database design; Stephen Levine (USNO) will be involved in database maintenance and VO access. This is a major project, with 8 million stellar detections per night and terabytes of images every year. All of the data and images will be made available to the general public, and the photometric catalog will be made available as portions of the sky are completed. System characteristics are given in the table below.

All processing is handled with an automated software pipeline at both the observing site and HQ. The image data is archived on external hard drives and sent to HQ on an infrequent basis. The starlists are sent to HQ daily, analyzed and added to the database. Approximately quarterly, we will update an on-line catalog with new photometry for public access. We anticipate covering about 1000 square degrees per night, but will require several nights of observations on any particular field before we consider the calibration to be complete.

The equipment started to arrive at HQ during April 2009. A number of tests were performed here to characterize the CCD systems, install software, and work on the pipeline. The system was then sent to DRO in October for installation in one of Tom's roll-off roof observatories. As of the date of this web page, we have completed approximately 37,000 square degrees of sky.  If anyone wants to help with this project, and has superior computer skills, contact me and I'll put you to work! In particular, we can use web and database expertise. Preliminary results were presented at the 2010 SAS meeting.

APASS update
March 11, 2010

We released a very preliminary subset of APASS data, called DR0, a few weeks ago. This release contained B and V photometry for about 800 square degrees of sky, primarily at the celestial equator and for fields above +70 degrees. That release can be downloaded at ftp://ftp.aavso.org/public/calib/apass_dr0.zip [14]. Only 450K stars are included in this release; the file is about 11MB in size. Doug Welch has generated a 3-D view of where the DR0 fields reside; this is shown below.

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Each field was visited at least twice, and on photometric nights. The AAVSO data-mining section helped in investigating this preliminary release to see where systematic errors were present. We fully expected such errors, as the first phase of processing only performs a linear plate solution and does not correct for possible scattered light.

Patrick Wils was good enough to match this catalog against CMC14, showing that indeed there were systematic astrometric errors in one of the equatorial fields (30172). Stephen Levine has further investigated the astrometry, and finds that the primary residuals follow a radially cubic pattern; that is, we have pincushion distortion in the image. Since these are f/3.6 Newtonian astrographs with a Wynne corrector to both remove coma and to flatten the field, such distortion is common. Shown below are two plots demonstrating both the original error (after the linear plate solution) and the remaining error after applying a simple cubic residual fit. The original errors were in the 1-2arcsec range on average, with obviously larger error away from center; the corrected frames show 0.25arcsec errors over the entire field. Stephen has just about completed the simple program that will go back through the current image database and clean up the astrometry at this level. We fully expect to further reduce the astrometric error in the final global solution, as each star will have about 20 detections (5 colors and 4 epochs), so the error should go down as the square root of the number of detections. With luck, we should have a final catalog with mean astrometric errors in the 100mas range.

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The photometric precision is not too bad. Shown below is a plot generated by Patrick. It is what I commonly call a scatter plot, showing the V-band magnitude on the horizontal axis and the photometric error from repeat observations on the vertical axis. As expected, the system saturates just a bit brighter than 10th magnitude, and performs reasonably well down to about 17th magnitude. This overlaps UCAC3 very nicely, which was the reasoning behind the survey exposure lengths. There is a cloud of stars with Verr greater than 0.4mag; these are a combination of mismatches due to the poor astrometry in DR0, plus some systematic offsets between a given field and those other fields that overlap with it. These problems will be corrected through a combination of solutions, including the improved astrometry steps listed above. Patrick also matched the APASS photometry against the Loneos secondary standards file maintained by Brian Skiff. That match is shown below. It indicates that we are obtaining reasonable photometry over the entire magnitude range. Three stars are obviously discrepant; Patrick has checked these against other photometric sources and indicates that APASS is likely correct and Loneos is discrepant.

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There are scattered light issues with the system; again, not unexpected. Shown below is a test that we did of raster-scanning a single field across the field of view of the chip. The selected field was M67, and the center of the cluster was positioned at 25 different positions on the chip (a 5x5 matrix). The night was not terribly photometric; this is obvious from the plot below, where there are three repeat observations with the cluster centered (radius=0) that do not agree at the 0.1mag level. Still, the plot shows the obvious falloff of the corners, where the Wynne corrector ceases to correct and has severe vignetting. There is also a cluster of 5 points that are in an isolated region of the V-band camera that appear to be significantly different; this is probably scattered light in the system. We have started to work on cleaning up the photometry by flocking various portions of the optical path where reflections might occur, and will do final cleanup with a model of the remaining residuals. We see no reason why we won't be able to meet our goal of 0.01-0.02mag photometry in all 5 passbands over the entire sky by the time the final global solution is performed.

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Our intent is to work on the correction programs over the next few weeks and reprocess the current imagery. Sometime in mid- to late-April, we will make a formal data release (DR1) that will cover several thousand degrees of sky. The weather has been terrible this winter for all-sky photometry in New Mexico, but usually March is the start of the good weather pattern, and we may actually make huge progress before DR1.