Photometric Measurement Definitions

The quantities measured by the Mira Aperture Photometry package are defined in the table below. Also see Magnitude Calculations and Photometric Error Definitions for further discussion about the values described here.

Photometric Measurement Definitions


The sequence number of the measurement.


The name of the image that was measured


The label of the object. This is a sequential number.


A name for the object. The default name is created by combining a prefix with a sequential number (see Default Name Prefix in the Other Preferences page). You can override the default name using the Edit mode from the Aperture Photometry Toolbar. See Using Edit Mode in Aperture Photometry.


The measured or computed magnitude.

  • If the photometric zero point does not exist, this is a raw magnitude with a value less than 0.

  • If the zero point exists, this is a positive number.


This column has an * if the object is a Standard.


The Empirical estimate of the magnitude uncertainty (mean error) based on sky noise measurements.


The theoretical estimate of the magnitude uncertainty (mean error).


The X coordinate of the object. If the image has a WCS calibration, this is right ascension, otherwise this is the column coordinate.


The Y coordinate of the object. If the image has a WCS calibration, this is declination, otherwise this is the row coordinate.


The column coordinate of the object.


The row coordinate of the object.


The estimate of local background


The Signal to Noise Ratio of the detection, including the background.

Mag Std

The magnitude of the standard star. This is the value entered by you or imported from a catalog file.


For Standards, the magnitude residual from the zero point calculation.

Net Count

The signal above the background value.


The filter used for the image, take from the FILTER keyword in the header.


The date of observation, from the DATE-OBS header keyword, in GMT


The time of observation, from the TIME-OBS (or DATE-OBS) header keyword, GMT, modified by the exposure time and the time-stamp preference . The reported time may correspond to the beginning, middle, or end of the exposure, See the discussion of PTIMEREF under Airmass Calculation, below.


The Julian Date calculated from the reported Time. The type of Julian Date is selected using the Julian Date Type selector in the Other Preferences dialog. See discussion of PTIMEREF under Airmass Calculation, below. This also may include a date offset as entered on the Other Preferences page.


The airmass of the observation. A value of 0.0 indicates that the airmass could not be calculated. See the discussion below.


The exposure time, in seconds, take from the EXPTIME header keyword.


For standard, the statistical weight used in calculating the zero point.


Notes you add into the Standard Data or Target Data dialog in Edit mode using the Aperture Photometry toolbar. See Using Edit Mode in Aperture Photometry.

Airmass Calculation

The airmass is calculated using the first 6 keywords entered into the Photometry Keywords dialog and the result is saved in the image header using the AIRMASS keyword. The keyword PTIMEREF is also saved to the image header to document whether the AIRMASS value is effective at the beginning, middle, or end of the exposure (see the Exposure Time Reference choice on the Other Preferences page).

If the airmass cannot be calculated (for example, because the required keywords are not in the image header), then the airmass is assigned the value 0.0. If all 6 required keywords are indeed in the image header, check the image header(s) to verify that the longitude has the proper sign; Mira expects to use a positive value for longitude west of Greenwich, England. You can correct this using the Flip Sign option in the Photometric Keywords dialog.

Related Topics

Magnitude Calculations, Photometric Measurements Report, Photometric Error Definitions, Aperture Photometry, Photometry Keywords, FITS Keywords, Using Edit Mode in Aperture Photometry