[Passband] a useful self-contained model?

[Martin Hill]

Hi Alberto,

Alberto Micol wrote:

> 1.- distinguishing between e.g. a filter transmission curve and the 
> overall bandpass:
> the most useful thing to the astronomer is the multiplication of the 
> transmission
> curves of all the elements in the instrument: optical elements 
> (mirrors/lenses/fibre channels),
> filter, detector quantum efficiency.
> We need to indicate whether it is the overall thing or just one of the 
> components.

yes - I've called this a ChainedPassband, where the overall passband is 
calculated from a chain of consecutive Passbands (which of course themselves 
might be ChainedPassbands - so you can have an instrument chain included in a 
greater chain).

> 2.- Names: There are various names for each bandpass; the obvious one is 
> the name of the filter
> as known within the observatory (e.g. F555W for HST/ACS), but sometimes 
> an alternate name (e.g. "V") is used to indicate that the filter is 
> similar (but almost never identical) to a certain standard filter. I do 
> not like to see a filter named V when it is not really V.

Name sounds good.  I hope that UCDs might cover this also; I think each Passband 
will need to hold a list of UCDs that it might represent.  For standard filters 
(such as V) the transmission curve can be soft-coded and included in a library. 
   Perhaps we should have a way of saying 'nearly-V', but hopefully UCDs will 
cover this.

> 3.- Min/Max: in the RM (if I remember) these two numbers are not 
> mathematically defined
> but are chosen by the data provider in a way to ensure that no search by 
> fravergies
> will miss the given filter;

OK - this sounds important when Passband is part of the metadata.

> 
> 4.- Wavelengths: different observatories use different characteristic 
> wavelengths.
> The one I remember now are: central wavelength, pivot wavelength,
> and someone uses the Schneider, Gunn, and Hoessel (1983, ApJ 264, 337) 
> definition
> which has the property that such "mean" wavelength equals the speed of 
> light over
> the corresponding "mean" frequency, and is half way between a "mean" 
> wavelength, and a "mean" frequency.

OK - We might have a think about whether these are included as part of the core 
Passband model (ie all Passband code writers have to implement them) or whether 
they are put in separate 'helper' classes used by particular disciplines, with 
methods that analyse the passrate shape to calculate them.

> 5.- Filter efficiency: someone characterises the filter by giving the 
> transmission at one of the characteristic wavelengths (see 4. above)

Do you know who?  Is this not just the passrate(central/pivot/etc Fravergy) ?

> 6.- Red-leak; some filters exhibit a red-leak. The min/max above (see 3) 
> might purposely ignore
> the red leak. Probably it would be just good to add a flag to know 
> whether there is such a leak or not, leaving to the full transmission 
> curve the details about the importance of such leak.

Are there blue leaks too? I feel that the passrate shape (and min/max) should 
include complete information about *actual* throughput, not *intended* 
throughput.   Is there a case for having both?

> 7.- Zeropoints; for space based detectors the zeropoint tells how to 
> translate detectors counts
> into magnitudes in some system. On the ground it is a bit more 
> complicated because of varying atmospheric conditions. Nevertheless each 
> measurement should bring along its own zeropoint
> quantity (in the sense of value and error <- it is here that the error 
> is very relevant).

Ooops missed this in my previous email.  I believe this belongs in a different 
model (that would include Passbands), to do with how you calculate Flux/etc from 
the measured values.

Thanks Alberto

Martin

-- 
Martin Hill
www.mchill.net
+44 7901 55 24 66