What is a passband?

Defines the probability of an incoming photon of a particular wavelength being passed through.

(I concentrate here on modelling just the passband itself, not other measurement characteristics such as rest frame).


The simplest example is that of a red filter on an optical telescope; in principle any red photon (~ 5 x 10^12Hz ?) arriving is let through, and any other photons are blocked.

In practice of course it's not that simple. No filter is perfect - in the above example, some red photons will be absorbed by the filter, and some other ones might get through. Also some photons are more red than others, and the filter will not be even handed; photons of one redness will be more or less likely to pass than a photon of another redness:


Finally a filter might not be completely even across its width.


An instrument's sensitivity also defines a passband. This might be tuned (radio) or part of the physical properties (CCDs).

Instrument, Atmosphere

Other factors influence the probability of a photon being measured - the full chain is given in the Observation Data Model doc (2.8) - depending on where 'incoming' is defined. We can model this set of sequential passbands using a ChainedPassband


How are we going to model it?

I first present the public interface that the Passband object presents to the models that use it, and then how various Passband objects might be defined.

Public Interface

A passband presents the following to the world:

  • getPassrate( Photon ) - returns the probability (0-1) of an incoming photon of the given wavelength/frequency/energy being passed through. [How do we take into account Accuracy?]

  • getMinWavelength() - returns the wavelength (as a Wave ) below which no photons will be passed.

  • getMaxWavelength() - returns the wavelength (as a Wave ) above which no photons will be passed.

  • getUCD() - UCDs have been defined for some passbands.

  • getName() for humans.

  • getCentralWave() - returns approximate central wavelength/frequency/Wave for convenience. Could return Photon instead?


  • SimplePassband - a frequency range between which the passrate is 1, and outside of which the passrate is 0. A library might include a set of these defined for each UCD.
  • GraphPassband - a set of points on a graph that are interpolated to give the passrates.
  • ChainedPassband
  • OpticalFilter - depending on 'completeness', might be a subclass of GraphPassband with added information such as manufacturer. Or it might be a set of Passbands defined by their position on the filter surface. Either way, a set of standard filter instances could be included in libraries.
  • AtmosphericPassband - give it your height in meters, windspeed and distance from the nearest pub, and it will work out an approximate passband. [Is this possible? useful? - MCH]


-- MartinHill - 21 May 2004

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Topic revision: r6 - 2004-05-23 - MartinHill
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