This page documents the columns of the source catalog data tables exposed by the daschlab analysis toolkit in RefcatSources tables. Different data sources (e.g. the “Cannon” data access portal) may use different names or expose somewhat different sets of data.

Columns by Category

Some columns are listed in multiple sub-sections here, if appropriate.

Detailed Column Information (Alphabetical)

class

• Synopsis: GSC2.3.2 classification code, if available
• Units/type: Integer

Most of the entries in DASCH’s “APASS” reference catalog are actually sourced from the Guide Star Catalog, version 2.3.2. (These can be identified because their ref_number fields begin with the digit "1".) For these sources, the class field gives the GSC 2.3.2 “classification code” for the source, with the important caveat that all codes besides 0 (“star”) and 3 (“non-star”) were filtered out. (The other codes are “galaxy”, “blend”, “unclassified”, and “defect”.)

For the ATLAS reference catalog, this field is always zero.

If you care about this kind of source classification, you should almost surely match your sources to a more modern database and use its values instead of these values.

color

• Synopsis: The catalogued source color
• Units/type: mag

The astronomical color of the source, according to the source catalog. For the APASS reference catalog, this is a B - V color. For ATLAS, it is g - r. For the APASS reference catalog, the mag_flag column may encode notes about how this value was obtained.

ddec

• Synopsis: The separation in dec between this source and the query center
• Units/type: arcsec

This column is provided primarily so that it is easy to sort the resulting table by distance from the query center.

dra

• Synopsis: The separation in RA between this source and the query center
• Units/type: arcsec

This includes a factor of cos(δ). This column is provided primarily so that it is easy to sort the resulting table by distance from the query center.

gsc_bin_index

• Synopsis: A spatial binning number for this source
• Units/type: Nonnegative integer

Under the hood, many DASCH data systems bin sources spatially according to a system referred to as “GSC binning,” although it is not the system used by the Guide Star Catalog itself. The details are unimportant, but due to the architecture of the DASCH data files, the low-level lightcurve query API (POST /dasch/dr7/lightcurve) needs to be provided with this number.

For the curious: the sky is divided into declination bands, and then each declination band is divided into whatever number of bins in RA results in the bins being most nearly square. In this usage, the declination bands are 1/64 of a degree tall, resulting in a total of 168,966,386 bins. Bins are numbered in increasing dec and RA, so that bin zero is at the south pole. More modern binning schemes like TOAST are almost surely preferable if you want to bin anything yourself.

local_id

• Synopsis: A unique number identifying this source within this daschlab session
• Units/type: Nonnegative integer
• Note: This column is added by daschlab and is not present when using the underlying web APIs directly

Every source in a daschlab catalog table is assigned a unique "local id", which is its index within the table. Local IDs should only be used internally in an analysis; they are not meaningful except with reference to the particular session in which they were generated.

mag_flag

• Synopsis: Flag indicating the origin of the magnitude and color data, for APASS refcat
• Units/type: Nonnegative integer

For the APASS reference catalog, this number provides information about the origin of the magnitude data. The meanings of the values, and the total number of rows in the catalog with them, are as follows:

Most of the entries in DASCH’s “APASS” reference catalog are actually sourced from the Guide Star Catalog, version 2.3.2. (These can be identified because their ref_number fields begin with the digit "1".) Only records sourced from the GSC have non-zero values for these fields. Meanwhile, only records sourced from APASS are used in the DASCH photometric calibration, which means that we can state robustly that the photometric system used by that calibration is Johnson B: the (many) GSC records using Jpg or Fpg magnitudes are never involved in the photometric calibration.

Another exception is that a relatively small number of sources are named with Tycho-2 identifiers (ref_number beginning with the digit "5"). These all have a mag_flag equal to 6.

“Skymap” refers to an early version of the SKY2000 catalog.

For the ATLAS reference catalog, this field is always zero.

num_matches

• Synopsis: The number of detections of this source in the DASCH lightcurve database
• Units/type: Nonnegative integer

If you obtain the DASCH lightcurve of this source, it will contain this many detections. For all of the reasons that DASCH lightcurves need to be reduced, this number should be treated as being at best indicative of the true number of DASCH detections of the source. In particular, the Source Splitting known issue can cause detections of one source to be reassigned to another. If the number of detections seems surprising given the source magnitude, examine nearby sources in the refcat to see whether detections were potentially mis-assigned to another object.

pos

• Synopsis: The canonical source position, sometimes with proper-motion metadata
• Units/type: Astropy SkyCoord

This column gives the catalogued source position. Astropy coordinate objects allow positions to be annotated with space motion terms (see Accounting for Space Motion in the Astropy docs), when available these fields are populated here. In particular, pm_ra_cosdec, pm_dec, and obstime are filled in.

The positions and proper motions in the ATLAS refcat come from Gaia DR2, while those in APASS come from older sources and so will be of lower quality. If you care about (e.g.) the actual proper motion of some object, you should obtain it by consulting a modern catalog such as the latest Gaia data release. The data in this field remain significant, however, because the capture the parameters that were used by the DASCH pipeline during its astrometric and photometric processing.

ref_number

• Synopsis: A unique identifying number for this source
• Units/type: Nonnegative integer

Within each reference catalog, every source is assigned a unique “reference number”. This number is not meaningful outside of DASCH and can generally be ignored. However, it is required by the low-level lightcurve query API (POST /dasch/dr7/lightcurve).

This number can be interchanged with ref_text fairly directly. The reference numbers are not sequential and do not have good hashing properties.

ref_text

• Synopsis: A unique textual name for this source
• Units/type: string

This field provides a unique name for each DASCH reference catalog source.

These names may be derived from identifiers used in either ATLAS-refcat2 (for the ATLAS refcat) or from one of GSC 2.3.2, APASS DR8, or Tycho-2 (for the APASS refcat). However, they do not match these precisely and in general will not resolve with SIMBAD. When reporting catalog source names in publications, it is recommended to provide both the DASCH ref_text (to aid anyone aiming to examine the source in DASCH data systems) as well as the canonical identifier used by one of these external catalogs (to link the DASCH object with the nomenclature used outside of DASCH).

refcat

• Synopsis: The reference catalog to which this source belongs
• Units/type: String

This column indicates the reference catalog from which the source record was obtained. Its value will be either "apass" or "atlas". In typical usage, every row will have the same value for this column, but in principle one might find oneself merging source lists from the two refcats, in which case this column could come in handy.

stdmag

• Synopsis: The catalogued source magnitude
• Units/type: Magnitude

The cataloged magnitude of the source. When using the APASS reference catalog, this is in the Johnson B system; when using ATLAS, it is SDSS g. The v_flag field provides a binary assessment as to whether this value is “uncertain”, in an ad-hoc way. For the APASS reference catalog, the mag_flag column may encode notes about how this value was obtained.

u_pm_dec

• Synopsis: The catalogued uncertainty in the declination proper motion
• Units/type: mas/yr

If the source catalog contains an estimate of the proper motion uncertainty, the declination component is stored in this column. The actual proper motion value is attached to the pos column, within the Astropy SkyCoord object.

u_pm_ra_cosdec

• Synopsis: The catalogued uncertainty in the RA proper motion
• Units/type: mas/yr

If the source catalog contains an estimate of the proper motion uncertainty, the RA component is stored in this column. The actual proper motion value is attached to the pos column, within the Astropy SkyCoord object.

v_flag

• Synopsis: Flag indicating uncertain source magnitude
• Units/type: Integer

This field is either 0 or 1. If 1, it indicates that the DASCH pipeline considers the source to have an uncertain magnitude, based on several ad-hoc cutoffs. For downstream analysis, it would be better to go back to the original catalogs and retrieve actual magnitude uncertainty values.