Selecting Standard Stars

This recipe shows you how to select standard stars for inclusion in your programme of photometric observations. Obviously you would use the recipe as part of the preparations for your observing run. Section  outlined the procedure for selecting standard stars. Briefly, you usually want to choose between fifteen and twenty standard stars with a similar, or slightly larger, range of air masses, magnitudes and colours than your programme objects.

Traditionally suitable standards are identified by manual inspection of the paper copies of catalogues and this technique must still be used if a computer-readable version of the catalogue is not available. However, this recipe uses the catalogue and table manipulation package CURSA (see SUN/190[16]) to search the Landolt (1992)[53] catalogue of UBV standards. Numerous catalogues of standards are available and Section  gives some of the details.

Irrespective of whether you are using the paper or computer readable version of a catalogue it is always advisable to read the paper or other notes which accompany the it. This documentation will typically contain important information about the limitations, applicability and use of the catalogue which you should be aware of in order to use it effectively.

1. The first step is to obtain a copy of the catalogue of standards. A collection of catalogues of photometric standards which are in a format accessible to CURSA is available via anonymous ftp. The details are as follows.

   Anonymous ftp to:	ftp.roe.ac.uk
   Directory:	/pub/acd/catalogues
   File:	photostandards.tar.Z

   The file is a compressed tar archive; remember to use ftp in binary mode. Brief details of retrieving, decompressing and extracting the catalogues follow.

   1. To start ftp type:

      %  ftp  ftp.roe.ac.uk
      

      Reply `anonymous' to the Name prompt and give your electronic mail address as the password. Then type:

      ftp>  cd /pub/acd/catalogues
      ftp>  binary
      ftp>  get photostandards.tar.Z
      

      (Note that messages from the ftp commands have been omitted and `ftp>' is the ftp prompt rather than something that you type in.) Once the file has been retrieved type `quit' to leave ftp. The file photostandards.tar.Z should have been created in your current directory. Note the use of the binary command to set ftp to the appropriate mode for retrieving non-text files. If you encounter problems with ftp then seek assistance from your site manager in the first instance. There is now a veritable plethora of books about using computer communications networks. However, one which gives a good description of the ftp utility is The Whole Internet User's Guide and Catalog by Krol[48].

   2. File photostandards.tar.Z is a compressed tar archive. It must be decompressed before it can be used. Type:

      %  uncompress  photostandards.tar.Z
      

   3. To extract all the files in the archive type:

      %  tar  xvf  photostandards.tar
      

      Subdirectory photostandards should be created in your current directory. File photostandards/0CATALOGUES.LIS gives details of the catalogues available. The Landolt (1992) catalogue is file:

      photostandards/ii183/ii183.TXT

      The name of the subdirectory refers to the numbering of the catalogue by the CDS. The catalogue is in the CURSA Small Text List (STL) format for which the file type is .TXT (or .txt).

   4. Finally, move a copy of the catalogue to a convenient directory and make this directory your current directory.

2. Start CURSA. Simply type:

   %  cursa
   

   A message similar to the following should appear.

      CURSA commands are now available -- (Version 6.3)
   

3. The CURSA applications include xcatview a GUI-based catalogue browser which can be used to select standard stars that match your criteria. To start it, ensure that your terminal is configured to receive X-output, then type:

   %  xcatview  &
   

   (The `&' merely makes xcatview run as a detached process so you can, if you so desire, continue to issue Unix commands from the command line.) A window similar to Figure  should appear. Use the Select Catalogue window to open catalogue ii183.TXT. The Open Catalogue window is similar to the file-selectors often found in GUI-based applications and if you have used similar ones you should not have any difficulty using it. However, in case of difficulty, click on the Help button for assistance. The catalogue should open and the appearance of xcatview should be similar to Figure .

   You can, if you wish, use the various functions in xcatview to browse the catalogue. Note that all the windows in xcatview contain a Help button which can be clicked for assistance.

   Figure: Starting up the CURSA catalogue browser xcatview

   

   Figure: xcatview displaying a catalogue

   

4. In CURSA (and similar systems) each column in the catalogue has a name which is unique within the catalogue and you use this name to refer to the column. The names of the columns are shown at the top of the xcatview main display area (see Figure ). Alternatively, you can list all the column names by clicking on the Listing menu in the bar at the top of the xcatview window and choosing the Show summary of columns option.

   In CURSA you can calculate new columns `on the fly' by specifying algebraic expressions involving existing columns. Thus, if you had columns called X and Y you could specify ${\tt X + Y}$. The actual details are not germane here. However, an important consequence which you should be aware of is that column names themselves cannot contain arithmetic operators (because such names would be ambiguous). Thus, the obvious names for colours such as $B - V$ and $U - B$ are invalid. The usual convention is to replace the minus sign with an underscore (`_'), so the column names become B_V and U_B. The columns in catalogue ii183.TXT follow this convention.

5. The next step is to select the stars which match the required criteria. Suppose that standard stars were required which met the following conditions:

   $V$ magnitude in the range 12 to 15,
   $B - V$ colour in the range 0.5 to 1.5,
   Right Ascension in the range 15$^{\rm h}$ to 20$^{\rm h}$,
   the star was observed more than 7 times.

   The range of Right Ascension would, of course, be constrained by the place and date of your observing run as well as the coordinates of your programme objects. Note that most of the stars in the chosen catalogue are relatively close to the celestial equator, so there is little point in selecting on Declination. The final criterion (that the star was observed more than 7 times) follows a suggestion in Landolt's discussion of the catalogue[53] that stars with multiple observations make better standards.

   To generate a selection click on the Selection button in the bar at the top of the xcatview window and choose the Create a new selection option. A new window will appear allowing you to specify the required selection. Enter:

   V > 12.0 AND V < 15.0

   and click on the OK button. This operation selects stars in the magnitude range $12 < V < 15$. Repeat the procedure to further refine the selection by limiting the range of colours, Right Ascension and number of observations. The selections to enter are:

   B_V > 0.5 AND B_V < 1.5
RA > 15:00:00 AND RA < 20:00:00
OBS >= 7

   Note that in order to indicate that the Right Ascension is being specified as sexagesimal hours the value is entered unsigned and with a colon (`:') to separate the minutes and seconds. CURSA interprets an unsigned sexagesimal value in this format as hours. A signed sexagesimal value is similarly interpreted as degrees. Thus positive angles in sexagesimal degrees must be preceded by a plus sign. See SUN/190[16] for further details.

   Alternatively, if you prefer, you can generate the required selection in one go by entering all the criteria in a single selection, with the individual elements separated by `AND'. However, it is probably easier to make typing mistakes this way. Whichever way the selections are specified you should finally select 24 standard stars.

6. You will probably not need to keep all the columns in the catalogue. For example, if you were just planning to observe in the $U$, $B$ and $V$ bands you might only want to keep the corresponding columns plus the star name and coordinates. The names of the required columns are:

   NAME
   RA
   DEC
   V
   B_V
   U_B

   Click on the Listing button in the bar at the top of the xcatview window and choose the Choose the columns to be listed option. A new window appears which allows you to choose the required columns. Use it to select the above set of columns. Click on the Help button in case of difficulties and on OK when you have selected the required columns. Subsequently, only the chosen columns will be listed on the screen, written to output catalogues etc.

7. The next step is to save the selection as a new catalogue, so that you can refer to it again (you will need the catalogue magnitudes when you come to calibrate your own photometry; the recipe in Section  is an example of this process). Click on the File button in the bar at the top of the xcatview window and choose the Save as catalogue option. A new window will appear. Enter the required file name, perhaps:

   mystandards.TXT

   Note that CURSA uses the file type to recognise the format in which the catalogue is to be written. The most appropriate format for these small lists is the Small Text List (STL) format, for which the corresponding file type is `.TXT' or `.txt'. Also set the Columns button to current list (otherwise all the columns in the catalogue will be written). Then click on the OK button. A catalogue called mystandards.TXT containing the selected standard stars should be written in your current directory.

8. It is probably useful to also save the selected stars as a text file. Click on the File button in the bar at the top of the xcatview window and choose the Save as text file option. Enter the required file name, perhaps:

   mystandards.lis

   set the other options as required and click on the OK button. File mystandards.lis will be written in your current directory. It is suitable for printing out, editing etc.

9. You now have a preliminary list of standard stars. The final step is to check the visibility of each star at the location and date of your observing run. A number of utilities are available to assist with this process. The document SG/10: Preparing to Observe[63] summarises what is available. One alternative is OBSERVE (see SUN/146[61]). To run it simply type:

   %  observe
   

   and enter the required details. A series of plots and graphs are generated. You can use this output to arrive at a final list of fifteen to twenty standards. You would, of course, probably also use OBSERVE to check the visibility of your programme objects.