MARKSTAR: Locate Stars Interactively in an Image

MARKSTAR creates a 'photometry file', which is a record of the positions, coordinates, and magnitudes of stars on an image. (See HELP PHOTOMETRY for a complete list of entries in the photometry file). Typing 'NEW' starts a new list. If you don't type 'NEW', any stars you mark will be appended to the current list (if there is one); in this case the program will show the positions on the TV of the stars that have already been marked - after this marking the program will be ready for you to enter new stars. Thus MARKSTAR with no options will show the positions of the stars in the current photometry file.

There are two ways to operate this program. The first mode lets you interactively mark the positions of the stars. The only keywords you need in this mode are NEW and RADIUS=.

  1. Load an image into the display with the TV command.
  2. Type MARKSTAR or MARKSTAR NEW (with a RADIUS specifier).
  3. Move the cursor near a star.
  4. Hit 'C' or 'J' to mark the star. The 'J' key defines the star's position to be exactly the location of the cursor. The 'C' computes an exact position by finding the centroid of the stellar image. You will probably use the 'C' key most of the time. The 'J' key can be used in very crowded fields.
  5. Repeat steps 3 and 4 until all the desired stars are found. At any time you may type 'H' to get a list of the commands for this program.
  6. Hit 'E' when you are finished.

In interactive mode, the key # will display the number in the photometry list of the star nearest the current location of the cursor, and print the row and column location of that star. This, combined with the STAR= keyword (below) allows you to go back and forth between a list of star positions and a TV image.

The second mode is to have the program mark all the stars that are on an already-present photometry file. This saves you time when you have several exposures of the same field (say in several colors). The syntax for the second mode is MARKSTAR AUTO with the other options. The program takes each position on the current photometry photometry file, then looks at the current TV image for a star. If the star is found, the information is stored on a new photometry list, REPLACING the old list. (Save the old one first!) If the stars on the new image are not exactly in the same positions as on the old image, use DR and DC to specify the change that must be applied to the old coordinates to match the new ones. When the program is finished marking the stars in this automatic mode, it switches to the interactive mode, allowing you to mark more stars. NOBOX prevents display of the newly-marked stars. For images that are not just shifted, but also rotated, you can specify the angle and center of rotation with the ANGLE, AR=, and AC= keywords.

Use RSHIFT and CSHIFT to reject stars for which the AUTO marking gives a new position far from the old one. As MARKSTAR AUTO runs, it prints on the screen the difference in position of stars on the new frame compared to the EXPECTED positions on that frame. This output may be redirected.

Do not confuse MARKSTAR AUTO with AUTOMARK. AUTOMARK automatically finds stars on a frame by locating peaks. MARKSTAR AUTO finds stars on a frame by referring to a photometry file.

The RADIUS specifier gives the size of the region used in computing the centroid of the star. It should be something like the FWHM of the star in pixels.

The STAR= word is used to identify individual stars in a photometry file. It draws a box around the stars whose numbers are given as arguments to STAR=, then exits the program. The word STAR= can appear more than once on the command line.


You can print the contents of a photometry file with the PRINT command. Type 'PRINT PHOT' to see the results on your terminal; type 'PRINT PHOT HARD' to send them to the lineprinter.

It is best to create a new photometry file for each image you reduce. That way there is a one-to-one match between images and files. You do not have to include all the stars in a frame in a photometry file, if you do not so desire, but you are asking for trouble if you have information from several frames in the same file.

The variables R and C are loaded with the position of the last star marked.

MARKSTAR AUTO is most useful to mark a series of images that are very similar - say exposures of the same duration and in the same color. Images of different duration, or images in different colors will reach to different magnitudes. In this case AUTOMARK applied to each frame may be the best.

To find the coordinate offset between different frames, use MARKSTAR on one frame to locate a few bright, uncrowded stars. Then use MARKSTAR AUTO on the second frame. The program will print the difference in location between the star on the second frame and that on the first, then print the average row and column shift. These differences are in the sense:

   SHIFT = (new position) - (old position)
So the shift would be applied to the first image to bring them into alignment.