Set Arcsecond Scale

The Set Arcsecond Scale command attaches a linear arcsecond scale to the x and y axes of the image. This gives you the option to get positional readout, plots, and measurements in units of either arcseconds or pixels. This command can also use the Calculate Image Scale command to calculate a value for the camera and telescope combination. The value is inserted into the Scale field in the dialog, after which you would use [Apply] or [OK] to apply it to the image or image set.

This command applies a World Coordinate System ("WCS") calibration to the image or image set. This calibration replaces any existing WCS. However, this command creates an Undo copy so that you can use the Edit > Undo (Ctrl+Z) command to restore the previous coordinate system. If you want to save the arcsecond calibration, save the image in the normal way.

Using this Command

To apply a linear arcsecond scale to an image or image set,

  1. Enter the image scale value into the Scale field or click [Calculate] to have Mira calculate and insert it for you.

  2. Click [OK] to set the image scale as a WCS calibration for the image or image set.

When you click [Calculate] in step 1, Mira calls the Calculate Image Scale command.

Calculating the Image Scale

If the value to enter for the image scale is not known, you can have Mira calculate it or you can calculate it manually. To calculate the image scale, you must know the pixel size and focal length of the telescope used to create the current image. Use these in the formula below:

     S = 206.265 P / F


           S is the scale in arcseconds per pixel,

           P is the pixel size in microns,

           F is the focal length in mm.

Insert the value of S into the Scale field in the Set Arcsecond Scale dialog.

Related Topics

Set Equatorial Scale, World Coordinate System, Calculate Image Scale