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Photometry Calibrated File

The description of a FITS file with fully calibrated data.

The file is result of run of munipack phcal action.

The specification has been created for internal use in Munipack. Any backward compatibility is not guarantied.


Raw frames contains an array of counts, the array of digitised amount of electrical charges due to dropped photons. The detected signal is proportional of quality of detector, area of telescope and fluency of many additional things appears.

To by able to compare, the raw data, we needs combine observed quantities with calibrated ones. The dirty job is provided by phcal action. And results of calibration are stored in this kind of FITS file.

Crucial properties:

Note that the calibration requires knowledge of both filter and photometric system.

Primary Array

The primary array has modified header with items:

Keywords compatible to HST added by Photometry Calibration in primary image
PHOTSYSPhotometric system
BUNITSPhysical units of array values, always 'photons'
PHOTFLAMflux for 1 photon/s/cm2erg/s/cm2
PHOTZPTmagnitude zero-point for fluxeserg/s/cm2
PHOTPLAMeffective wavelengthÅ

All pixels of result image are converted to a required quantity.

The key conversion is from observed counts c to photons n (see BUNITS) for every pixel in the image. For one-filter approximation, the relation is used:

n = r c

where r = 1/η is photon to counts ratio. η is coded in FITS header by CTPH keyword.

The conversion is more complicated in case of multi-filter observation. Every pixel in a standard photometric system is computed as the linear combination of pixels in an instrumental photometric on the same position. This is main reason for creation of this kind of photometric calibration file.

The conversion from counts to photons will probably not visible on the first sight due to algorithms used for scaling of high-range images, ones suppress simple linear scaling in intensity.

Photometry Table

This PHOTOMETRY extension contains a photometry which would be used for further processing.

Header Keywords

EXTNAMEPHOTOMETRY as the identifier of this table
APERaperture radiusdeg
ANNULUS1inner sky annulus radiusdeg
ANNULUS2outer sky annulus radiusdeg
CTPHcounts per photons
CTPHERRStatistical Error of CTPH

The parameters ANULLUSes an APER are directly copied from instrumental table and with known astrometry calibration converted to degrees.

The photometry calibration is summarised in the comments of this table.

COMMENT === Photometric Calibration by Munipack ===
COMMENT Reference photometric sequence: UCAC5 Catalogue (Zacharias+, 2017)
COMMENT Number of objects used = 10
COMMENT Counts rate per photon rate =   1.1420     +-  7.9E-03
COMMENT Catalogue RA,DEC [deg]   Photons [ph/s/m2] Rate [cts/s/m2]   rel.err.
COMMENT   58.45428950   0.04279090    145.975E+03     59.052E+03       -0.00040
COMMENT   58.26729090   0.04719370     54.135E+03     21.358E+03        0.01220
COMMENT   58.36659530   0.10020920     25.089E+03     10.164E+03       -0.01539
COMMENT   58.32388300   0.07288640     20.058E+03      7.913E+03       -0.00444
COMMENT   58.32013680  -0.04973250      9.143E+03      3.632E+03       -0.03498
COMMENT   58.29327650  -0.11260250     13.611E+03      5.616E+03       -0.03224
COMMENT   58.38659270   0.07895420      5.104E+03      1.940E+03        0.01366
COMMENT   58.36115270  -0.04099480      4.532E+03      1.791E+03        0.00234
COMMENT   58.38704450   0.09433840      2.199E+03    810.295E+00        0.02770
COMMENT   58.28163680   0.01965840      3.141E+03      1.217E+03        0.01977
COMMENT Description:
COMMENT === End of Photometric Calibration by Munipack ===

The table is designed likely of the astrometry residual page. First and second columns identifies calibration star, the third is photon flux in given filter computed from catalogue magnitude of star, the fourth is counts rate and last the ratio of [(3) - CTPH *(4)]/(3) as an analogy of residuals.


Photometry table of calibrated frame
XHorizontal coordinatepix
YVertical coordinatepix
SKYMean sky levelphotons per square arcsec
SKYERRStatistical error of SKYphotons per square arcsec
PHOTONStar photon countphotons
PHOTONERRStatistical error of PHOTONphotons

The structure is perfectly same as structure of PHOTOMETRY table in Processing file except that COUNTs are replaced by PHOTONs.

Available Photometric Quantities

Table of available photometry quantities for non-calibrated data
COUNT[α]Counts ccount
RATE[β]Count ratecount/s/m2
MAG[γ]Instrumental magnitude m

[α] Counts means number of detected electrons by captured photons. Ones are derived from raw data dn (data number (DN)) in relative units (ADU) with help of gain g (photo-electrons per ADU): c = g*dn.

[β] Counts rates are counts per area A of a detector per a time period T. A is derived from AREA, T from EXPTIME header keyword as c/(A T).

[γ] Instrumental magnitudes are derived from rates. Theirs shift against to right magnitudes is given by optical system attenuation. Magnitudes are derived as m = 25 - 2.5 log10 c/(A T). Note, that an instrumental shift has been chooses as 1 cts/s/m2 for magnitude 25 (see -2.5 log10 10-10).

Table of available photometry quantities for fully calibrated data
PHOTON[a]Photon counts nphoton
Photon flux Φph/s/m2
PHOTNU[b]Photon rate per frequency nνph/m2/Hz
PHOTLAM[c]Photon rate per wavelength nλph/m2/nm
FLUX[d]Energy flux in a band fW/m2
FNU[e]Energy density flux per frequency fνW/m2/Hz
FLAM[f]Energy density flux per wavelength fλW/m2/nm
MAG[g]Magnitude m
ABMAG[h]Magnitude per 1 Hz of frequency mAB
STMAG[ch]Magnitude per 1 nm of wavelength mST

[a] Photon count n and photon flux Φ are the core of calibration. The fitting routines estimates ratio of detected c and expected n photons and derive quantity (efficiency by mean) η = c / n (0 ≤ η ≤ 1). The typical values are between 0.1 — 0.5. The η is included in header keywords as CTPH and its dispersion as CTPHERR. Photon count is derived from original data as n = c / η. The reference photon count in a band B is derived from a known (catalogue) star magnitude mB
Φ = (fνB ΔνB / h νB) 10-0.4 m = (fλB ΔλB) (h c / λB) 10-0.4 m,
where fνB and ΔνB are spectral density flux and passband FWHM (in this order) defined by photometry system. Count of photons is n = Φ A T. Note that product fνB ΔνB has meaning of energy flux and h νB is the mean energy of photon in the given band.
The photon flux is also frequently is used quantity defined as ϕ = n / A T.

[b] Photon rate per unit frequency defined as nν = n / Δν.

[c] Photon rate per 1nm of wavelenght defined as nλ = n Δλ c / λeff2.

[d] Energy flux in given band (filter) defined as F = ϕ h νeff = ϕ h Δλ c / λeff2.

[e] Spectral energy flux density per unit frequency defined as fν = ϕν h νeff = ϕ h νeff / Δν.

[f] Spectral energy flux density per 1nm of wavelength defined as fλ = ϕλ Δλ c / λ2eff = ϕ h c / λ2eff / 10-9.

[g] Magnitudes in the given filter are computed as m = -2.5 log10 F / (f Δν) = -2.5 log10 F / (f Δλ). Undefined values are marked 99.999.

[h] Magnitudes in the given filter are computed as m = -2.5 log10 fν / fAB, where fAB = 3.631 .10-23 [W/m2/Hz] is spectral flux density for star of magnitude zero in V (545nm) band for AB system. Undefined values are marked 99.999.

[ch] Magnitudes in the given filter are computed as m = -2.5 log10 fλ / fST, where fST = 3.6335 .10-10 [W/m2/nm] is spectral flux density for star of magnitude zero in V (545nm) band for ST system. Undefined values are marked 99.999.

Photometry table for fully calibrated data
RARight Ascensiondeg
SKYSky intensity[-]
SKYERRSky intensity error[-]
QA selected quantity[+]
QERRThe quantity standard error[+]

Note that for fully calibrated data, keywords TUNITn are presented in the header.

[-] Units of sky intensity are the same as quantity Q, but, in addition, they are related to the cone 1 arcsec2.

[+] There is many of possible related quantities, which can be directly derived from calibrated photons.

Magnitudes Are Considered As Obsolete

Please, have in mind. The photometry calibration is designed for a photon counting detector, eg. a device that can detect an incoming single photon. CCD, CMOS and many modern detectors are that photon counting devices. Ones are extremely sensitive with linear response. The calibration naturally take the advantage.

The magnitude scale is considered as obsolete in this framework and provided just for backward compatibility. This approach for the photometry has many advantages for modern astronomy for following reasons:

See Also

Aperture Photometry, Photometry Calibration