[Cmunipack] New version released (1.1.1)

cmunipack at amper.ped.muni.cz cmunipack at amper.ped.muni.cz
Sat May 27 11:24:37 CEST 2006

> Hi David,
> thank you for your new version. I tested the SNR-feature and wondered 
> why the output is in mag and negative (Barnard's star: -10 mag). Isn't 
> it a datum without a dimension?

Yes, it is. But the magnitude is also a unit without dimension. It means 
logarithm of luminosity of a measured star with respect to some agreed 
value. So I thought what representation is suitable for displaying SNR. 
For it is also the ratio - signal of a star and a sky noise (both in 
ADU). If I had use simple ratio, it would result in very great numbers 
(-10 magnitudes correspond to 100,000 in linear scale). In technics, 
decibels (dB) are used for this value, but in astronomy, there is one 
unit, which suits better - it is logarithm-based and well known - 
magnitudes. So when Muniwin displays SNR = -5 mag, it means, that the 
star is 100 times brighter than the sky signal.

> When doing photometry on Barnard's star the program gives me an 
> instrumental value of 13.66 mag which is really too low.
> My photometry settings:
>            readout noise 15 e
>            gain: 24 e/ADU         (full well capacity: 100 000 e)
>            low good datum: 5 sigma
>            high good datum: 4096 ADU
>            FWHM: 3 pix
>            threshold: 3 sigma
> When I measure the same frames with Astrometrica I receive a value of 
> about 9.1 mag for this star. Astrometrica uses the USNO-B1 star catalogue.

I think it is no good idea to rely on instrumental magnitudes and 
compare them with star atlas, for example Tycho-2. To do so, you had to 
count in many conditions, which are not always constant and predictable 
- extinction for example. The Muniwin program is designed to produce 
relative magnitudes - to use one star as comparison star, which is 
believed to be constant and with well known brightness, and to derive 
brightness of some another star in neighbourhood. Their distance should 
be small enough to disregard the difference in extinction. I've done 
listing of instrumental magnitudes for users, which need raw data to 
perform their own further corrections and processing, it wasn't intended 
as final product.

Best regards,


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