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Plotting realistic star charts


GRIP's main file menu has an option for creating a new image which is a fairly realistic image of a rectangular portion of the sky. It uses publicly available star data, either downloaded to be local files or by using service calls through AstroGrid - more details of both of those possibilities are given below. The charts are plotted using Gaussian profiles for the stars, of varying heights depending on star magnitudes.

This star chart facility is provided for two main reasons:

  1. To produce finder charts that are easier to interpret at the telescope than those provided by the usual planetarium software. The charts are normal 8-bit RGB images so they can be enhanced, inverted and printed just like any photo.
  2. To enable star data (identity, position, magnitude) easily to be attached to objects detected in your photos, as a step on the way to analysing variable stars. To this end the star charts respond to the mouse moving over them and a click shows all known data for the nearest star.

First a dialogue is presented for defining the area to be charted:

The star chart dialogue

The first field here, Name of object, and the button beside it, Get coordinates, only work if you have AstroGrid's Virtual Observatory (VO) Desktop running on your PC. Click here for full details of what is required for GRIP to work with it. If you have that set up, GRIP uses the Sesame service to get the coordinates of the named object, which are then automatically entered into the RA and Dec fields in the dialogue.

Sesame is a very useful service and it seems very flexible in the names it can interpret. Letter case does not matter. Constellation abbreviations and the first 3 letters to spell Greek letters are fine. NGC, M, etc, and popular names of nebulae all seem to work. In the example dialogue I have used HR Del, as I have a page on this site about it.

If you don't use the Sesame service, right ascension (RA) and declination (Dec) text fields can be entered in various ways. For example just filling the RA hour field as 20.5 gives the same result as putting 20 in that field and 30 in the RA minutes field. As it says on the dialogue, any fields left blank will be treated as if zero had been entered in them.

The epoch field refers to the RA and Dec values you have entered. GRIP will convert to the same epoch as the data being searched, to find out which stars to plot.

The field for the half-width of stars adjusts the photo-realism of the resulting star chart; you may wish to vary it but the initial value of 3 is quite suitable. The brightest pixel on each star is adjusted according to the star's magnitude but on a logarithmic scale such that 5 magnitudes reduces it by 10 rather than 100. This may be justifiable on the grounds that the star image is distributed across 2 dimensions. If the reduction were greater the fainter stars would not be seen. The chart image can of course be enhanced within GRIP or by saving it and working on it in another application. It is an RGB image with 8 bits per channel but all 3 channels are identical for every star so they appear grey.

Clicking on the button to "Create chart" will check the validity of the fields you filled in and, if they are correct, proceed to do a cone search (find all stars within a specified angular radius of the given direction in the sky). If you have AstroGrid installed and running, as above, the search will be done through that (at present it uses the Tycho-2 database of 2.5 million brightest stars). Otherwise, or if the AstroGrid process fails for any reason, GRIP will try instead to process local copies of the Hipparcos and Tycho catalogues to load data for the stars that fall in the area defined by the dialogue. For that to work you need to download hip_main.dat and tyc_main.dat from and use GRIP's configuration menu to set paths to where you put those 2 files on your disc (either absolute paths or paths relative to the directory where grip.jar is installed).

All charts are plotted with north at the top (except when the north pole is covered by the chart). GRIP does of course have an option on its geometry menu for rotating any image. Also if you wish to print it the levels menu enables the chart to be inverted (grey stars on a white background).

Here is an example chart and a message that was popped up by clicking on a particular star (highlighted by the red cross and its Hipparcos identifier):

Example star chart drawn by GRIP

The message box contains selectable text, so it can be copied to other applications. In fact the text is also editable, so you could add notes before copying but GRIP does not retain the edited text.

When the chart is first displayed, moving the mouse cursor over it continually marks the nearest star by a cross, labelled with an identifier. If the star only has a Tycho identifier that will be shown (14 characters, containing some spaces) otherwise the Hipparcos identifier will be shown in preference (up to 6 digits). Flamsteed/Bayer names are shown for the brightest stars (above magnitude 4).

There is more about this particular chart here.

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