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 9.1 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) to search the Landolt (1992) catalogue of UBV standards. Numerous catalogues of standards are available and Section 7.3 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.
|Anonymous ftp to:||
The file is a compressed tar archive; remember to use ftp in binary mode. Brief details of retrieving, decompressing and extracting the catalogues follow.
anonymous’ to the
Name prompt and give your electronic mail address as the
password. Then type:
(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 ‘
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
photostandards.tar.Zis a compressed tar archive. It must be decompressed before it can be used. Type:
photostandards should be created in your current directory. File
photostandards/0CATALOGUES.LIS gives details of the catalogues available. The Landolt
(1992) catalogue is file:
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
A message similar to the following should appear.
xcatviewa 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:
&’ 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 6 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 7.
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
xcatviewmain display area (see Figure 7). Alternatively, you can list all the column names by clicking on the Listing menu in the bar at the top of the
xcatviewwindow 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
Y you could specify
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
are invalid. The usual convention is to replace the minus sign with an underscore (‘
_’), so the
column names become
U_B. The columns in catalogue
ii183.TXT follow this
magnitude in the range 12 to 15,
colour in the range 0.5 to 1.5,
Right Ascension in the range 15 to 20,
the star was observed more than 7 times.
To generate a selection click on the Selection button in the bar at the top of the
and choose the Create a new selection option. A new window will appear allowing you to specify
the required selection. Enter:
and click on the OK button. This operation selects stars in the magnitude range . 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:
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 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.
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
xcatviewwindow and choose the Save as catalogue option. A new window will appear. Enter the required file name, perhaps:
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
mystandards.TXT containing the selected standard stars should be written in
your current directory.
xcatviewwindow and choose the Save as text file option. Enter the required file name, perhaps:
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.
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.