[Passband] a useful self-contained model?
amsr at jb.man.ac.uk
Mon May 31 09:05:36 PDT 2004
I expect you meant someone other than me to reply but !
Some of the refs from JMD docs I gave you do say a lot of this already....
> The intention is 1) to provide a passband model that can be 'plugged in' to the
> SED model (from SSA) and ObsData when it is ready, and 2) to practice modelling
> a relatively self-contained concept that is familiar to everyone, yet likely to
> be viewed and stored and used in different ways by different disciplines.
Number one need is to define terms. To me, a spectral energy distribution
(SED) means "a series of flux v. wavelength measurements where each single
point is measured across a bandpass which is not necessarily linear" and
has a fairly small (lambda/delta lambda) e.g. <10^3, and the points in
the SED are "often separated by large/irregular gaps"; and the SED is
usually used to measure the average or continuum emission from objects.
In contrast a spectrum has a higher resolution, is made up of a large
number of points in narrow channels which are usually contiguous or
regularly spaced (or is a series thereof), and is usually used to measure
specific spectral lines coresponding to molecular or atomic transitions.
This is not a rigid distinction but I propose that we use SED to cover the
situation I ahve put in """" above, and spectrum to cover the further
subdivision into channels where this can be described as follows ''
'A spectral region described by a bandpass function can be further divided
into channels, where the channel spacing and width are regular in the
appropriate units and the sensitivity of the individual channel is
described by its location in the spectral region bandpass' that is, you
only need one bandpass function. The 'appropriate units' bit is because a
0.5-MHz bandpass divided into 512 equal ~1 KHz channels will only give
approximately linearly-spaced channels in wavelength etc. I think that
the individual channels may sometimes be best described as Gaussian
functions, but all by the same Gaussian (bar the centre ref)?
1 2 3 4 5 6 7 8 eight channels
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(here I use flux to include energy etc. and wavelength/frequency to
include freq., wavel., energy, wavenumber)
> - Who needs errors on passrate()?
REFERENCE FREQ UNCERTAINTY
In some circumstances it is preferable to use a reference wavelength in a
given part of the band (etc.) and a bandpass function. e.g.
reference wavelength 600 nm in channel 256 out of 512
band start nominal 590 nm; band end nominal 610 nm; reference wavelength
600 nm in 0.5 of band
becaues the spectral alignment is done using some reference like an
iodine(?) crystal for optical or a hydrogen maser+GPS for radio. This is
usually sufficiently accurate that it is ignorred but can become an issue
e.g. for looking for extra-solar planets by Doppler wobble.
BAND SHAPE UNCERTAINTY
Depending on how complex a bandpass we can handle, there will certainly be
an uncertainty if we are e.g. using linear bandpasses only.
> - Is 'passrate(Fravergy)', returning a value 0-1 probability, sufficient for
> Radio, X-Ray, etc?
Do you mean
0 1 1 1 1 1 1 0 transmission probability
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This is just about OK for some radio e.g. well-calibrated data from VLA,
MERLIN etc. - which is already in physical units - but not a good
description for raw data and not for X-ray data in counts.
> - Is a simple Min/Max sufficient for the limits of the passband?
This seems to be part of the above question.
You can describe a passband as a max, a min and a binary transmission
probability as above. However you can also describe it as a non-linear
function between the max and min - or as a ref wavelength and a function
about that wavelength, in which case some people might prefer to leave it
to the user to decide if the limits were at the 20% or 50% point etc.
As a bare minimum, I suggest that we allow either max/min or centre+width
(I think that RM1.0 does that).
Initially you might want to keep the interveniing function linear/binary
but you should not make it impossible to include more sophisticated
functions, and errors on the reference standard, in the future.
> - Does anyone tend to use Passbands as just an error on the frequency on a
> measured flux? ie, should Passband also be an Accuracy, or be able to provide an
> Accuracy, for things like plots?
Yes, see the AVO-Aladin SED tool
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Dr. Anita M. S. Richards, AVO Astronomer
MERLIN/VLBI National Facility, University of Manchester,
Jodrell Bank Observatory, Macclesfield, Cheshire SK11 9DL, U.K.
tel +44 (0)1477 572683 (direct); 571321 (switchboard); 571618 (fax).
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