Pixel Size

The fundamental width of the detector photo-site, assumed to be square. To calculate for a binned image, change the Binning field but do not change the fundamental Pixel Size. For example, suppose your CCD has a 9 micron pixel. To use a 2x2 binned image with effective 18 micron pixels, set Pixel Size=9 and Binning=2. All calculations are made for binned pixels.

Binning

The number of detector pixels per image pixel. This assumes the detector is binned n x n, where n = 1, 2, etc. See the Pixel Size parameter.

Readnoise

The detector readout noise is the intrinsic rms noise in a zero second exposure.

Gain

The number of electrons per ADU (Analog to Digital Unit) or DN (Digital Number) in the image.

Temp

The temperature of the detector.

T doubling

The doubling temperature for dark current approximated as a line in a log-log plot of dark current versus temperature. See the detector's data sheet.

Tref

The reference temperature for the detector Dark Current. See the detector's data sheet.

Dark Current

The rate of thermal electron production at a refernece temperature. See the detector's data sheet. The dark current is calculated using a simple doubling rule starting from a known reference dark current at a reference temperature. This is a very good approximation when the sensor temperature is not too far below the reference temperature, say within 5 or 6 doublings. However, getting even further below the reference temperature, the dark current can become so small that it has negligible effect on the calculated SNR.

Focal Length

The telescope focal length in mm.

Diameter

The telescope primary diameter in mm.

Secondary

The telescope secondary obstruction in mm.

Efficiency

The percentage transmission of the optical system in the selected bandpass.

Name

The name corresponding to the filter set. This selects the photon flux for a star with the spectrum of Vega and magnitude 0 in the selected bandpass above the atmosphere.

Filter Factor

A response or transmission factor for the bandpass filter relative to the standard bandpass. This should usually be 100% unless using a low transmission filter.

QE

The quantum efficiency of the detector in the named bandpass. See the detector's data sheet.

Extinction

The atmospheric extinction coefficient per unit airmass in the named bandpass. This is site dependent. If set to 0.0, then nominal values will be entered into the field when [Calculate] is clicked.

Airmass

The airmass, or optical path length through the atmosphere. The shortest path is through the zenith, which has the minimal airmass value of 1.0. The airmass of an observation is approximated by sec(z), which is the secant of the zenith angle. If set to 0, the observation is considered above the atmosphere.

Magnitude

The magnitude of the target star through the bandpass filter. This magnitude must be adjusted to the bandpass, e.g., use a V magnitude for the V bandpass, B magnitude for the B bandpass, etc.

Exp Time

The exposure time in seconds.

Seeing

The FWHM of the point spread function which is a product of the atmospheric smearing and optical abberations.

Sky Mag

The sky brightness per square arcsecond in the named bandpass. Sample values for a very dark site are around 21.8 to 22 in B and 21.3 to 21.5 in the V bandpass.

Aperture

This is the value of the measuring aperture radius that you would specify in Mira. Some software packages specify the aperture in terms of diameter, not radius. Be sure you enter the appropriate value. Enter a value using binned pixels that you see in the image. For example, setting Aperture = 9 at Binning=1 gives the same arcsecond radius as Aperture = 4.5 at Binning=2.

Aperture Integral

The fraction of the star's light inside the photometry aperture, assuming a circular Gaussian profile and a circular aperture.

Total BG Noise

The total noise in the background underneath the star. This includes readout noise, digitization noise, dark noise, and photon noise from the sky.

Sky / BG

The ratio of sky photon noise to total background noise (which includes sky noise). If this field is 1.0, then the sky photon noise is the only source of noise in the background underneath the star. This result helps you address the question of whether an observation is "sky limited" or "sensor limited".

Reference for equations and theory:

Newberry, M. V. 1991, PASP 103, 122-130.