* * * Comparison stars for Guide8 and Guide9 users * * *

Over the last decade or so I have been busy collecting good quality photometric data of stars - with emphasis on the magnitude 10-15 range - that could be incorporated in Guide8 and now Guide9 with the principle aim to have good comparison stars as close to a faint comet as possible.
Below you find a number of files that I feel may be useful for other observers as well. A brief explanation on how to install and (de)activate the data can be found here.


APASS photometry.

The AAVSO is currently running the APASS survey which is going to cover the whole sky down to magnitude 17 in 5 colours.
Now that the survey is nearly complete this is the reference source for comets fainter than magnitude 10.
Unfortunately, the data files are very big and not (yet?) in a form that is easily incorporated in Guide.
Fields up to a 15 degrees radius can be downloaded here and there you can also obtain further information on the APASS program.
I have used this option to prepare monthly files with selected comparison stars between magnitude 10.0 and 15.0 (with some exceptions), that have been measured at least twice, have B-V between 0.00 and 1.00, and have an error =<0.10 magnitude.
Monthly updates will be published around the time of the full Moon for comets expected to be between visual magnitude 10 and about 14.5.
An updated zipfile containing the prn-file with the star selection and a matching tdf-file for the moonless period of July 2018 can be downloaded here.
Date of last update: June 27, 2018
The next update will be available around July 27, 2018.

The ICQ-code for the use of APASS data is AQ.

TASS photometry.

The TASS-4 survey covered the sky north of declination -5 degrees, and all data have been collected in a single file which [~490 Mb!] can be downloaded here.
TASS gives only measurements in V and I. V-I is similar to, or slightly larger than B-V as a first approximation (for stars with B-V ~1.0), so stars with V-I<1.0 should be preferentially choosen for use as comparison stars for comets. It should also be noted that the file has not been edited in any way, so it also contains e.g. variable stars and galaxies. Therefore, data should be used with proper care.
A tdf-file to access the TASS-4 data can be downloaded here. Date of last update: March 6, 2012
Both files should be placed in the Guide directory to work properly.

Recently I have prepared a file with a selection of TASS-stars with V<14.0, V-I<1.2 and 5 or more measurements. This file and a supplement with stars that are possibly slightly variable, but with amplitude less than 0.1 magnitude, can be downloaded here. Date of last update: August 29, 2015

The ICQ-code TA has been assigned to this reference.

Arne Henden's photometry.

Over the years Arne Henden has compiled a large collection of photometric fields north of declination -30o.
Especially for comet observers (but others may find it useful as well) I have made a selection of stars down to magnitude 15.5 that meet a number of criteria: a star must have been measured at least twice, has B-V between 0.00 and 1.00, and has an error <0.10 magnitude.
A zipfile containing the prn-file with the star selection, and a matching tdf-file can be downloaded here. Date of last update: December 26, 2006

In addition I have prepared 3 supplementary files:
* The first supplement contains stars down to magnitude 15.5 with B-V between 0.00 and 1.00, but that have only been measured once. It should be used with some care, because lack of a second (and third ect..) measurement precluded the detection of obvious variable stars.
It can be downloaded here. Date of last update: December 26, 2006
* The second supplement is an extension to the main file with (slightly) red stars having B-V between 1.01 and 1.50.
It can be downloaded here. Date of last update: December 26, 2006
* The third supplement is an extension to the main file with stars fainter than magnitude 15.5.
It can be downloaded here. Date of last update: December 26, 2006
In Guide the stars from the different sets are clearly distinguished by their own colour.
The stars from the main catalog have their magnitude added in green. The 'faint' stars are shown in pale green, the reddish stars in red, and the stars with only a single measurement in orange.

Since 2007 Arne Henden has been director of the AAVSO, and he (and collaborators) have been continuing this work from Sonoita Research Observatory (SRO), measuring many more fields.
Again, especially for comet observers (but others may find it useful as well) I have made a selection of stars down to magnitude 15.0 that meet a number of criteria: a star must have been measured at least twice, has B-V between 0.00 and 1.00, and has an error <0.05 magnitude.
A zipfile containing the prn-file with the star selection, and a matching tdf-file can be downloaded here. Date of last update: March 6, 2012

The ICQ-code for this reference is HN.
The original files, with photometry often down to magnitude 20+, can be downloaded from the AAVSO ftp-site.


Brian Skiff's LONEOS file.

Brian Skiff has compiled a large 'all sky' file with high quality data from the literature.
I have made a special selection for comet observers with stars down to magnitude 16.0 and B-V between 0.00 and 1.00.
A zipfile containing the prn-file with stellar data, and a matching tdf-file can be downloaded here. Date of last update: December 27, 2004

In Guide the stars from the special selection have their magnitude labelled in yellow.
The ICQ-code for this reference is SK.
Brian Skiff's complete LONEOS file can be downloaded here.


Landolt photometry.

Between 1982 and 1993 Landolt published photometry for a large number of stars, mainly along the equator.
A zipfile containing the prn-file with stellar data, and a matching tdf-file can be downloaded here. Date of last update: March 9, 2004

In Guide the stars from this file have their magnitude labelled in pale blue.
The ICQ-code for this reference is L.