# HOW-TO Photometric reference catalog and standard extinction curve¶

The Photometric Reference Catalog is a catalog which contains magnitudes in common bands, coordinates and names for standard stars. The Photometric Reference Catalog and the standard extinction curve are two calibration files that are provided by the system. Both calibration files are directly accessible for the user from the awe-prompt.

## The Photometric Reference Catalog¶

### Its present contents¶

The Photometric Reference Catalog present in the Astro-WISE system is the result of an ongoing project. This section describes the contents of the most recent version of this Photometric Reference Catalog wich has the filename cal569E_v11.cat. The catalog file can be found in the catalog project. This catalog contains stars from four sources: the Landolt catalog, the Stetson catalog, the Sloan Digital Sky Survey (SDSS) Data Release 5 (DR5) catalog and the Preliminary Catalog for the OmegaCAM Secondary Standards Programme which is based on our own observations. For all four sources stellar magnitudes are given in both the Johnson-Cousins $$UBVRI$$ photometric system and in the Sloan $$ugriz$$ photometric system (i.e., unprimed).

• Landolt stars: the Photometric Reference Catalog contains all 544 stars from the Landolt catalog. The Johnson-Cousins $$UBVRI$$ magnitudes are taken from Landolt 1992. The Sloan $$ugriz$$ magnitudes are computed from the Johnson-Cousins $$UBVRI$$ magnitudes using the transformation equations of Jester et al. 2005 which apply only to stars with $$R-I<1.15$$. The $$ugriz$$ magnitudes are not computed for stars which have $$R-I>1.15$$. Improved coordinates are used for the 526 Landolt stars available at this ESO webpage.
• Stetson stars: the Photometric Reference Catalog contains all 39109 Stetson stars (see Table 1). The Johnson-Cousins $$BVRI$$ magnitudes are taken from the on-line catalog of Stetson ($$U$$-band magnitudes are not available). The Sloan $$griz$$ magnitudes are computed from the Johnson-Cousins $$BVRI$$ magnitudes using the transformation equations of Jester et al. 2005 which apply only to stars with $$R-I<1.15$$. The $$ugriz$$ magnitudes are not computed for stars which have $$R-I>1.15$$.
• SDSS DR5 stars: the Photometric Reference Catalog contains only SDSS DR5 stars which are located inside 1 square degree fields centered on 22 SA fields (see Table 1). The Sloan $$ugriz$$ magnitudes are the psfMags for objects classified as stars available in the SDSS DR5 database. SDSS DR5 quality control flags were taken into account to exclude stars with saturated pixels, being blended with neighboring stars or being too close to the edge of a frame for a given band. This means that stars were excluded if any of the following SDSS DR5 quality flags are raised for any band: COSMIC_RAY, SUBTRACTED, SATURATED, BLENDED, BRIGHT and EDGE. The SDSS DR5 data was retrieved using the python code sdss2refcat.py The Johnson-Cousins $$UBVRI$$ magnitudes are computed from the Sloan $$ugriz$$ magnitudes using the transformation equations of Jester et al. 2005 which apply only to stars with $$R-I<1.15$$. The $$UBVRI$$ magnitudes are not computed for stars which turn out to have $$R-I>1.15$$.
• Preliminary Catalog stars: we observed 7 of the 22 SA fields listed in Table 1 with the WideField-Camera (WFC) on the 2.5m Isaac Newton Telescope at La Palma using Sloan $$ugriz$$ filters. The derived catalog is called the Preliminary Catalog. The SDSS DR5 stars were used as calibrators. The Johnson-Cousins $$UBVRI$$ magnitudes are computed from the Sloan $$ugriz$$ magnitudes using the transformation equations of Jester et al. 2005 which apply only to stars with $$R-I<1.15$$. The $$UBVRI$$ magnitudes are not computed for stars which turn out to have $$R-I>1.15$$. Further details of the data reduction, photometric calibration and other properties of the Preliminary Catalog are given in Verdoes Kleijn et al. 2006. The PC data have much larger measurement errors than the DR5 data. Additional plots with :math:u-g vs $$g-r$$ for PC only <http://www.astro-wise.org/Public/UminGvsGminRdensity.eps>__ and :math:u-g vs $$g-r$$ for PC plus DR5 <http://www.astro-wise.org/Public/UminGvsGminRdensityWithDR5.eps>__ show the same data as in Fig 4 of Verdoes Kleijn et al. 2006 but as density plots to better show the consistency in stellar locus between the DR5 and PC data. Similar plots for :math:g-r vs $$r-i$$ for PC only <http://www.astro-wise.org/Public/GminRvsRminIdensity.eps>__ and :math:g-r vs $$r-i$$ for PC plus DR5 <http://www.astro-wise.org/Public/GminRvsRminIdensityWithDR5.eps>__ and for :math:r-i vs $$i-z$$ for PC only <http://www.astro-wise.org/Public/RminIvsIminZdensity.eps>__ and :math:r-i vs $$i-z$$ for PC plus DR5 <http://www.astro-wise.org/Public/RminIvsIminZdensityWithDR5.eps>__ are also available. The Preliminary Catalog will be used as starting point to derive the OmegaCAM Secondary Standards with OmegaCAM itself in the first year of operations.

Table 1: The stars present in the 22 SA fields contained in the Photometric Reference Catalog. The fields cover an area of 1.1x1.1 degree centered on the tabulated coordinates. The number of Landolt standard stars, Stetson standard stars, SDSS DR5 stars and stars from the Preliminary Catalog (PC) for the OmegaCAM Secondary Standard Programme are listed.

Field $$\alpha$$ (J2000) (deg) $$\delta$$ (J2000) (deg) Landolt Stetson SDSS DR5 PC
SA51 112.663 $$+$$29.828 0 0 214 0
SA57 197.171 $$+$$29.384 0 0 952 0
SA68 4.146 $$+$$15.844 0 0 1302 0
SA92 13.946 $$+$$0.949 41 213 1094 6475
SA93 28.783 $$+$$0.824 4 0 1128 0
SA94 44.033 $$+$$0.571 7 0 1099 0
SA95 58.500 $$+$$0.000 45 426 1093 0
SA98 103.021 $$-$$0.328 46 1116 0 23840
SA100 133.529 $$+$$0.546 6 1 3343 0
SA101 149.112 $$-$$0.386 35 117 1776 5591
SA102 163.779 $$+$$0.866 5 66 1517 0
SA103 178.779 $$+$$0.556 2 0 1507 0
SA104 190.4875 $$-$$0.5292 34 76 1576 5701
SA105 204.533 $$+$$0.676 4 0 2172 0
SA106 220.533 $$+$$0.427 2 15 2864 0
SA107 234.8250 $$-$$0.2631 28 728 3889 12006
SA108 248.033 $$+$$0.369 6 3 6148 0
SA110 280.6000 $$+$$0.34583 39 589 0 38562
SA112 310.529 $$+$$0.524 7 73 12087 0
SA113 325.3750 $$+$$0.49944 42 483 4046 13947
SA114 340.529 $$+$$0.689 9 5 1957 0
SA115 355.779 $$+$$0.888 10 0 1170 0

The catalog is in FITS table format and its columns are described in Table 2. Whenever a magnitude or its associated error has value 0.0 it means that no value has been determined.

Table 2: Description of the 29 columns in the Photometric Reference Catalog cal569E_v*.cat.

column name description
SeqNr sequence number
origin origin of stellar magnitude:
Landolt: Landolt catalog, Stetson: Stetson catalog,
SDSS5: SDSS DR5,
AW2S: Preliminary Catalog for OmegaCAM Secondary Standards
Name Name of star
Ra/Ra_err Right Ascension / its error (deg)
Dec/Dec_err Declination / its error (deg)
Epoch epoch of coordinates: all J2000
Flag flag, (not used currently)
JohnsonU/JohnsonU_err Johnson U / its error (mag)
JohnsonB/JohnsonB_err Johnson B / its error (mag)
JohnsonV/JohnsonV_err Johnson V / its error (mag)
CousinsR/CousinsR_err Cousins R / its error (mag)
CousinsI/CousinsI_err Cousins I / its error (mag)
SloanU/SloanU_err Sloan u / its error (mag)
SloanG/SloanG_err Sloan g / its error (mag)
SloanR/SloanR_err Sloan r / its error (mag)
SloanI/SloanI_err Sloan i / its error (mag)
SloanZ/SloanZ_err Sloan z / its error (mag)

### Retrieving the Photometric Reference Catalog¶

The Photometric Reference Catalog can be retrieved from the database to the local directory as follows:

awe> from astro.main.PhotRefCatalog import PhotRefCatalog
awe> refcat = PhotRefCatalog.get()
awe> refcat.retrieve()


which will automatically give the most recent Photometric Reference Catalog in the system. Note the retrieve operation in the last line; this is important. The contents of the catalog thus retrieved to the local directory can then be queried using the methods described in §[query_methods].

### Query methods¶

The Photometric Reference Catalog has six methods for querying/accessing its content. The first four of these are simple methods without parameters. These are the following:

1. refcat.get_number_of_sources(), which returns the number of sources in he catalog.
2. refcat.get_list_of_bands(), which returns a list of the photometric bands supported by the catalog.
3. refcat.get_source_attributes(), which returns information of the data content of a source. The information is stored in a dictionary with the attribute names of the source as keys and their types as values. To just get the attribute names, do: refcat.get_source_attributes().keys().
4. refcat.make_skycat(), which dumps the catalog for overplotting in skycat.

The following query methods have a more elaborate interface. One thing that should be mentioned is that every single star in the catalog has an index for cross-referencing purposes. The methods below all return dictionaries which use these indices as keys.

In order to retrieve the magnitudes for all the stars in the standard star catalog for one particular photometric band (mag_id), type:

awe> refcat.get_dict_of_magnitudes(mag_id)


which will return a dictionary with the indices of the sources as keys, and as values 2-tuples containing the magnitude and its uncertainty. The mag_id is a string that should match one of the entries in the list generated by the get_list_of_bands method. If one wants to retrieve the magnitudes for only a subset of stars, it is possible to provide the method call with an additional list of indices:

awe> refcat.get_dict_of_magnitudes(mag_id, index_list = [1, 2, 13500])


which will only give the magnitudes for the stars 1, 2 and 13500 in the list.

Besides this dedicated method, the Photometric Reference Catalog also has an allround query method that can be used to retrieve any information that is needed. This method and its signature are:

awe> refcat.get_source_data(column_list, index_list = index_list)


which will return a dictionary with the indices of the sources as keys, and as values lists of the requested data items in the same order as specified in the input column_list. The input column_list is the list of data items to be retrieved, and the optional index_list is used to get data from a subset of stars only. The entries in column_list should match the keys of the dictionary that is generated by the get_source_attributes method. These entries are strings.

### Examples of use¶

Retrieve the V magnitudes of all the stars in the catalog:

awe> mag_dict = refcat.get_dict_of_magnitudes('JohnsonV')


Retrieve the g:math:’ magnitudes of stars 10 to 500:

awe> inds = range(10, 500)
awe> mag_dict = refcat.get_dict_of_magnitudes('SloanG', index_list = inds)


Retrieve the Ra-Dec-Epoch information from all the stars:

awe> info_dict = refcat.get_source_data(['ra', 'dec', 'epoch'])


Retrieve the name (star_id) of the stars 1 and 2:

awe> info_dict = refcat.get_source_data(['star_id'], index_list = [1, 2])


Retrieve the names of the catalogs from which the sources originate (origin):

awe> info_dict = refcat.get_source_data(['origin'])


### Using a subset of the catalog for photometric calibration¶

As was mentioned in Its present contents, the Photometric Reference Catalog contains stars from various contributary catalogs. To allow the user (and the system) to see/use/select the stars of only one or a select few of the contributaries, the standard star catalog is outfitted with a filter on the origin column. In the current version of the Photometric Reference Catalog this attribute can have one of the following values: Landolt, SDSS5 and Stetson for Landolt, SDSS DR5 and Stetson catalogs, respectively, and AW2S for the Preliminary Catalog.

To select/see/use only stars from the Photometric Reference Catalog that originate from the ‘Landolt’ catalog use:

awe> refcat.origin_filter.activate('Landolt')


To select/see/use only stars from both the ‘Landolt’ and ‘Stetson’ sub-catalogs use:

awe> refcat.origin_filter.activate('Landolt', 'Stetson')


To select/see/use only stars from the ‘Stetson’, ‘SDSS5’ and ‘AW2S’ sub-catalogs use:

awe> refcat.origin_filter.activate('Stetson', 'SDSS5', 'AW2S')


Note, that the order in which the origin identifiers appear is not important.

To de-activate the filter so that the full view on the catalog is restored:

awe> refcat.origin_filter.deactivate()


To check whether the filter is actually switched on:

awe> refcat.origin_filter.is_active()


which will return either True or False. By default, the filter is disabled.

It is obvious that the setting of the filter affects the return values of the query methods described in §[query_methods].

### Standard and non-standard photometric bands for OmegaCAM¶

The OmegaCAM system distinguishes two types of bands: key bands, and user bands. This distinction has its origin in the OmegaCAM calibration plan, and has implications for the inner workings of the photometric pipeline. The key bands are the Sloan $$ugri$$ bands. These bands are fully supported by the OmegaCAM system, and form the core of the contents of the Photometric Reference Catalog. Any other band than these four Sloan bands is a user band.

To process data that have been observed in a user band, a transformation table is needed. This table contains information about which key bands to use in the transformation, and a set of transformation coefficients. Such a table should be present in the database for every filter that belongs to a user band. The contents of such a table could be so simple as providing a substitute key band for a given user band (e.g. for a Gunn $$r$$ filter, the Sloan $$r$$ magnitudes will be used), but could also contain a full-blown transformation with given color and a color-term. The transformation table will be discussed in a separate chapter.

The Johnson-Cousins bands and the Sloan $$z$$ band are strictly speaking also user bands. However, given the wide-spread use of these systems and for convenience, the necessary transformations have already been performed and the results have been stored together with the Sloan $$ugri$$ key bands. The transformation equations used are discussed in Its present contents. It is, of course, always possible to override these transformed magnitudes stored in the catalog by providing the system with an appropriate transformation table.

### Using your own catalog with standard stars¶

The Photometric Reference Catalog described above is the default Photometric Reference Catalog automatically used by AWE. This standard catalog has the reserved name cal569E_v*.cat, with the version number at the asterisk. This naming convention should never be used for your own catalog! In the event that you need to use standard stars which are not provided by the default standard catalog you can create your own catalog and ingest it in AWE.

A Photometric Reference Catalog requires a specific format. One can create this format from an input ascii format using the refcat_generator.py code available in catalog/tools
Typing at the operating system command line
linux> awe refcat_generator.py


will generate the help documentation.

Once created, you can ingest it into the system doing the following:

awe> from astro.main.PhotRefCatalog import PhotRefCatalog
awe> refcat = PhotRefCatalog(pathname = 'myowncatalog.cat')
awe> refcat.store()
awe> refcat.commit()


Note the extra store command that will put the file on the fileserver.

## The standard extinction curve¶

To be written.

### Retrieving the standard extinction curve¶

To retrieve the standard extinction curve from the database, do the following:

awe> from astro.main.PhotExtinctionCurve import PhotExtinctionCurve
awe> extcurve = PhotExtinctionCurve.get()


after which it can be used immediately. For example :

awe> extcurve.get_extinction(4861.0)
0.13652400000000001


with 4861.0 the wavelength in Å, and the answer in mag/am. This method is the only query method defined on the extinction curve.

### Ingesting the standard extinction curve¶

Before the photometric pipeline can be used, the system should be initialized by putting a standard extinction curve into the system. Note: this should only have to be done once by the maintainer of the system. After initialization, every user of the system has access to the standard extincintion curve. To put a standard extinction curve into the system, do the following:

awe> from astro.main.PhotExtinctionCurve import PhotExtinctionCurve
awe> extcurve = PhotExtinctionCurve(pathname = 'cal564E.dat')
awe> extcurve.commit()


The standard extinction curve file can be found in a separate catalog project. The file cal564E.dat represents the La Palma extinction curve in units of mag/am.

Landolt, A.U. 1992, AJ, 104(1), 340

Jester, S., et al. 2005, AJ, 130, 873

Verdoes Kleijn et al. 2006