[Aavso-photometry] U mag work with CCDs (previously - UBV Photon Counting)

[Arne Henden]

Richard Miles wrote:
> Arne Henden wrote:
> I've been testing some of the newer CCDs, such as the SXV-H9, ST-7XME 
> and Meade DSI, and find they have U response, probably adequate for 
> brighter targets.
> 
> Arne,
> That sounds real interesting as I have tended to rule out U-band stuff 
> with CCDs.
> I've a few questions :-
> 
> (a) How do you know what you are measuring is all U photon response and 
> not significant red leak.
> 
Bessell prescription U filters usually use S8612 blocking glass, which
cuts down red leak enormously.  The best way to see whether you have
significant red leak is to see if there is any systematic trend with
color when observing standards.

> (b) Would your filter combination work properly on a Mira type which are 
> like searchlights in the near IR?
> 
U is hard with Miras.  Not only do they emit mostly in the red, and so
can pass through an IR-blocked filter at some level (no blocking is perfect),
but that also means they have very low flux in the blue.  I consider the
Bessell filter to be adequate for normal-colored stars, but not for
the really red variables.  There, you really need CuSO4 to obtain
adequate IR blocking while preserving the U bandpass shape.

> (c) Previously I have noticed that your ftp file sequence data (which I 
> should say are a great resource for we amateurs, as well as for GRB 
> professionals, etc.) only give "9.999" as the error estimate for the U-B 
> color index.  Is this some reflection of a really serious uncertainty in 
> the measured U magnitude?
> 
I don't think this is true when (U-B) is available in those files.
See, for example,
ftp://ftp.nofs.navy.mil/pub/outgoing/aah/grb/grb021004.dat
The error column for any particular filter/color is set by the number
of measurements combined to form the filter/color magnitude.  In most
cases, a 9.999 error either means no magnitude is available, or that
only one measure was used to create that magnitude and so it is
highly uncertain.  Unfortunately, my file format only has a single column
to indicate number of measures, while I might have observed on some
nights with all filters and others with a subset.  I don't often
do U-band calibration, as it literally doubles the total time required to
do all-sky calibration when you count the added time for all standards.
For the bright objects for which amateurs would attempt U-band photometry,
I bet there exists adequate UBV calibrations in the literature.
Arne