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.
|
Image
|
The name of the image that was measured
|
Obj
|
The label of the object. This is a sequential
number.
|
Name
|
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 Properties page). You can override the
default name using the Edit mode from
the
Aperture Photometry Toolbar. See
Using Edit Mode in Aperture Photometry.
|
Magn
|
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.
|
Std?
|
This column has an * if the object is a
Standard.
|
Error
|
The Empirical estimate of the magnitude
uncertainty (mean error) based on sky noise measurements. See
Photometric Error Definitions.
|
Error(T)
|
The theoretical estimate of the magnitude
uncertainty (mean error). Especially for faint objects, Error(T) is
more stable than the empirically measured Error, and it provides a reference value for
comparison with the noisy Error value.
See
Photometric Error Definitions.
|
X
|
The X coordinate of the object. If the image has a
WCS calibration, this is right ascension, otherwise this is the
column coordinate.
|
Y
|
The Y coordinate of the object. If the image has a
WCS calibration, this is declination, otherwise this is the row
coordinate.
|
Col
|
The column coordinate of the object.
|
Row
|
The row coordinate of the object.
|
Backgr
|
The estimate of local background
|
S/N
|
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.
|
Resid
|
For Standards, the magnitude residual from the
zero point calculation.
|
Net Count
|
The signal above the background value.
|
Filter
|
The filter used for the image, take from the
FILTER keyword in the header.
|
Date
|
The date of observation, from the DATE-OBS
header keyword, in GMT
|
Time
|
The time of observation, from the TIME-OBS
(or DATE-OBS) header keyword, GMT, modified by the exposure
time and the time stamp property . The reported time may correspond
to the beginning, middle, or end of the exposure, See the
discussion of PTIMEREF under Airmass Calculation,
below.
|
JD, HJD, MJD, or MHJD
|
The type of Julian Date is selected using the
Julian Date Type selector in the
Other Properties dialog. See discussion of
PTIMEREF under Airmass Calculation, below. This also
may include a date offset as entered on the
Other Properties page.
The ordinary geocentric Julian Date, JD or MJD, is
calculated from the Date and Time. The heliocentric
Julian Date, HJD of MHJD, also involve the Right Ascension and
Declination of the observation.
The Heliocentric Julian Date calculated by Mira is
extremely consistent results from with a high quality online Julian
Date reference calculator. Based on 14 comparisons between
calculations, the average difference in HJD is about 0.01 seconds,
with a standard deviation of 0.57 seconds (1-sigma). See
Mira's Heliocentric Julian Date
Calculation.
|
Airmass
|
The airmass of the observation. A value of 0.0
indicates that the airmass could not be calculated. See the
discussion below.
|
Exptime
|
The exposure time, in seconds, take from the
EXPTIME header keyword.
|
Weight
|
For standard stars, the statistical weight used in
calculating the photometric zero point.
|
Notes
|
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 Properties 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
Apphot Pane
Photometric Error Definitions
Aperture Photometry
Photometry Keywords
FITS Keywords
Using Edit Mode in Aperture Photometry
Mira Pro x64 User's Guide, Copyright Ⓒ 2023 Mirametrics, Inc. All
Rights Reserved.
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