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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. Data Release 4 occurred in early January 2012. Data Release 5 then occurred on 2012 February 21, Data Release 6 occurred on 2012 June 8, and Data Release 7 occurred on March 20, 2013. We expect the next data release in mid-2014. The catalog now contains photometry for 50 million objects in about 97% of the sky.



APASS DR7 coverage map (credit: Edward Los)

This current data release is the seventh 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 and the AAVSO endowment.  If you use APASS for your research, please consider supporting the AAVSO 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."

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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 and SDSS, plus cover the entire sky at the same depth as UCAC. The survey will take approximately two more 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 / GCVS star. Sloan passbands provide a direct link to the SDSS, SkyMapper, PanSTARRs etc. surveys, and provide a homogeneous catalog of brighter stars.  The Johnson B and V magnitudes are on the Vega system; the Sloan g',r',i' magnitudes are on the AB system.

APASS telescopeAll 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 (ASA) has given us a substantial discount on four of their N8 20cm astrographs. Software Bisque has loaned us a Paramount ME for the duration of the survey. Twin astrographs are placed on single Paramounts 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. MaximDL from Diffraction Limited controls the cameras; ACP and ACP Scheduler from DC3 Dreams, Inc. provides the observatory control and queue scheduling. The system is initially sited in the north at Dark Ridge Observatory (DRO) near Weed, NM, (Tom Smith is the observatory owner and operator), and the southern system is sited at CTIO in the Prompt6 clamshell, graciously provided by Dan Reichart (UNC).

Dirk Terrell (SwRI) is helping with the necessary computers and software for the telescopes; Doug Welch (McMaster) is helping on the scheduling, data processing and analysis; Stephen Levine (Lowell) is involved in the astrometric processing.  We have dozens of volunteers, from the contributed efforts of the software/hardware vendors, to deep-sky imaging experts and quality assurance monitors. 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.


Telescope Camera Filter
Wheel
Filters Pixel
Size
Field
of View
Exposure
Time
Table 1 - System Characteristics



Apogee U16m Apogee
AFW50-7S
Astrodon 9x9
micron


APASS 1a ASA N8;
20cm f/3.6
KAF16803
4kx4k
ABG
7 50mm
square
slots
u′, B, g′,
z′, Y
2.57
arcsec
2.9x2.9
degrees
180
seconds
APASS 1b ASA N8;
20cm f/3.6
KAF16803
4kx4k
ABG
7 50mm
square
slots
V, r′, i′,
clear
2.57
arcsec
2.9x2.9
degrees
90
seconds
APASS 1/2    
Location DRO; Weed, NM(1) and CTIO(2)  
Mount Paramount ME  
Camera software Diffraction Limited MaximDL
Scheduler DC3 Dreams ACP
Control computer Virtual Windows XP  
Access/pipeline computer Linux  
Image storage 1.5TB external drive

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 transferred 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 cover about 1000 square degrees per night, but 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 first system was then sent to DRO in October 2009 for installation in one of Tom's roll-off roof observatories, and a second system was added at CTIO in November, 2010. 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 as well as several AAS meetings and other conferences.

Known Problems in DR7


DR7 is not perfect.  We only have sufficient staff resources to keep up with the observing and are only minimally processing the results.  The image quality is high; we know which are the good and which are the poor nights.  We will reprocess all images in about a year to improve both the astrometry and the photometry, especially in crowded fields.

- each new Data Release is a recompilation of all previous photometry.  Therefore, if you obtained a magnitude for an object in DR6, it may well be different in DR7.  This "moving target" won't be stabilized until the last formal release, as the new data should represent improved position and brightness information.

- there are a number of duplicate entries.  These appear to be caused by the merging process, where poor astrometry in one field may cause two seed centroids to form for a single object.  In most cases, just pick the brightest source.

- there are a number of entries with 0.000 errors.  These are usually caused by a duplicate identical entry for a star, which gets over the minimum of two values required for inclusion in DR7, but in this case the values are for the same image.  The magnitudes are ok, just be aware that they may represent a single visit to a field.

- we are beginning the center-to-corner overlap (the secondary survey).  This means that there are now many instances of the corner of one image overlapping with the center of another image.  The corners have inferior astrometry and photometry, and need to be de-valued using a weighting scheme.  Such a scheme is not yet present; the end result is that the photometry has higher error for these regions than will be the case in the end product.  We will also be performing a global solution in the final catalog so that the measures across the sky are homogeneous.  To date, there are no known position systematics.

- we reject individual field observations when other observations of that field confirm an outlier, usually due to passing cirrus.  However, we only have 3-4 observations of any given field, and statistically there may be circumstances where there are two bright measures and one faint measure, and the faint measure is the correct one.  This will be addressed in the global solution, but be aware there may be rare zeropoint offsets on some fields.

- centroiding in crowded fields is very poor; blends will confuse the software.  We will be reprocessing using psf-fitting software, such as PSFextractor, to deblend, improve the astrometry, and also improve the photometry in crowded regions.  Don't trust stars in crowded regions, especially if they are faint.  Currently, we use a 14arcsec diameter aperture for photometry, so blends will cause photometric errors as well as astrometric ones.

- the residual photometric correction scheme can be improved. There are large variations of photometric accuracy over an entire field in the raw data, and the corrections have to be of correspondingly high quality in order to achieve the desired precision.  Right now, this correction is performed by raster-scanning areas of the sky, and is done about twice per year.  We are evaluating a couple of other techniques to improve the photometric residual correction.

- we have not done a linearity correction.  This is a one percent effect over the ADU range of the sensor and will be done for the final catalog.

- there are saturated stars in the catalog.  Anything brighter than V=10 or 10.5 is not usable (except in the equatorial standard fields, which use shorter exposures).  We are starting to include the Bright Extension, which will extend the photometry to about V=7.5.  The overlap between the original survey and the bright extension has not been treated properly yet; the saturated stars from the original survey are averaged together with the unsaturated results from the bright extension.  This will be corrected in upcoming releases.  For now, just use a V=10 limit and ignore the bright extension values.

- the faint completeness limit is V=16.  Sloan i' is the weakest of the band-passes, and is probably a magnitude brighter than this.  We will extend the survey down to its expected V=17 limit by both stacking multiple visits to a field as well as doing psf photometry to use the smallest effective measurement aperture.  This faint extension will take place in a year or two, depending on funding.

- Some early fields had misaligned telescopes, so that there will be (say) a 10arcmin region on the top of images that is only covered by one camera, and a 10arcmin region at the bottom of images that is only covered by the other camera.  This means the overlap between one field and the adjacent field may be poor, and there may be stripes where photometry doesn't exist.  These will be covered by the secondary survey.

- We changed the B filter midway through the survey.  The original Astrodon B has a red leak, which causes red stars to appear brighter at B than is real.  We will go back through the photometry on a later release and correct the B magnitudes of red stars using an analytical approximation to the extra flux, and also give those early measurements lower weight in the final catalog.  In the meantime, inspect g,r,i for any red star, and if B doesn't make sense based on the Sloan measures, then avoid it.  This only occurs for stars with (B-Ic) > 3.5.  The bandpass for the two B filters is essentially identical except for the red leak.

- the northern survey  (+20 to +90) is worse than the south, primarily due to the poorer weather in New Mexico over the past few years.  Many nights had clear sections of a few hours, and then cirrus.  We will be able to utilize the clear sections in the future, but right now the cirrus sections contaminate the good photometry.  There are still some gaps in coverage that should be resolved in DR8.

AAVSO 49 Bay State Rd. Cambridge, MA 02138 aavso@aavso.org 617-354-0484