Techniques of
Observational Astronomy
AST3722C
Basic Astronomical Photometry
Reference: Romanishin Ch. 3 (which I follow closely in much of this)
The purpose of astronomical photometry is to determine the brightness of an
object or a region of an extended object. In fact, all astronomical photometry
is a form of spectrophotometry; the determination of the flux from the object
in some region of the electromagnetic spectrum. In this context, flux is the
energy received per second per unit area per unit frequency (or wavelength)
interval:
in frequency
units, 
in
wavelength units. A "spectrum" is the variation in flux as a function
of frequency (or wavelength) over some range of frequency (or wavelength). For
the remainder of this discussion we will assume that we are looking at spectra
as a function of wavelength since that is most common usage for visible light.
True spectrophotometry passes the incoming radiation through a dispersing element (diffraction grating or prism). Filter photometry measures the flux in fairly broad wavelength regions using (typically) glass filters. This can be considered low resolution spectrophotometry
The process sequence is
- radiation leaves object
- passage through interstellar (and/or interplanetary) medium
- passage through Earth's atmosphere (unless observed with spaceborne instrument)
- passage through optics of telescope
- passage through dispersing element or filter
- photons incident on detector
- response from detector recorded
The observed spectral energy distribution can be altered at each of these stages. Usually an observer attempts to process the data in order to estimate the distribution at the top of the atmosphere. This processing is non-trivial and usually is done via a differential measurement rather than as an absolute measurement.
Differential vs. Absolute Photometry
Absolute Photometry
To do absolute spectrophotometry we would need to know
- the absolute sensitivity of the detector in output units per flux unit
- the absolute transmission of the dispersing element or filter
- the absolute transmission or reflection of every element in the telescope
- and (unless observed with spaceborne instrument) the absolute transmission of the Earth's atmosphere
In practice it is nearly impossible (and certainly not routine) to do such absolute calibration of a system. Changes in conditions due to dust accumulation, aging or deterioration of surface, etc. would have to be known at all times. Even if the telescope system was well calibrated the Earth's atmosphere is not. What to do?
Differential photometry
If we can observe a star of known characteristics nearly adjacent to our program object in position and time, we can assume that the both objects are effected in the same fashion by the system and hence that the characteristics of the program object can be determined by their ratio or difference relative to the "comparison" star. In practice, nearly all filter photometry is differential although this is possible only because a few astronomers have developed a system of suitable comparison stars through absolute photometry.
Practical Approach to Differential Photometry
- make observations of the program object and at least one comparison star.
- any observation of an object will include also "sky" in the vicinity of the object; make the necessary measurements to allow subtraction of the sky from the object measurements
- calculate the flux from the program object in units of the flux from the comparison star
- use the known flux (or magnitude) of the comparison star to get the flux (or magnitude) of the program object
Standard Magnitude and Color Systems
In the visual region of the spectrum, astronomers use a logarithmic system of flux measurement called magnitudes.
The magnitude system has its roots in a system of describing star
brightnesses that goes back to the astronomer Hipparchus 2200 years ago. Today
the system is specified algorithmically by 
.
Since optical astronomers still frequently refer to stars and
observations of stars in units of magnitude, it is useful to have a few
magnitude terms committed to memory.
Note that the magnitude system is an inverse system in that brighter
objects have algebraically smaller values and that very bright objects have
negative magnitudes.
|
Brightest naked eye stars |
Faintest naked eye stars |
Full moon |
Venus |
Limit at .46m scope |
HST limit |
|
-1.5 |
About 6.5 |
-12.7 |
Up to –4.5 |
About 19th |
About 30th |
Frequently we think in terms of magnitude differences which are equivalent to flux or brightness ratios.
|
Mag diff. |
1 |
2.5 |
5 |
0.75 |
.01 |
|
Flux ratio |
2.54 |
10 |
100 |
2 |
.01 |
There are a number of magnitude systems related to the wavelength regions
involved. In most cases colors, defined as a difference in magnitudes, are also
used in the description of a stars characteristics. The standard system in the visible region of the spectrum is
the Johnson-Cousins UBVRI system.
Stellar characteristics are frequently plotted in terms of a color-magnitude
diagram (V vs. B-V) or a color-color diagram (U-B vs. B-V). The wavelength coverage of a standard
UBVRI filter set are plotted at
http://www.astro.ufl.edu/~oliver/ast3722/lectures/BasicPhotom/filtersets.htm
. These are the filters installed
at the 0.46m and 0.76m telescope at Rosemary Hill Observatory.
References
Lists of standard stars http://sofa.astro.utoledo.edu/SOFA/photometry.html
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This page was last edited 11/4/2004 11:46 AM