A Description of Cluster Format Files

The first character (excepting spaces) on the top line should be a number, indicating how any colours the catalogue contains (e.g. 2 for a catalogue of V vs B-V). The next line should be the names which describe those colours, and the third line is left for comments. The catalogue then has one line per star with the following columns.
1) Field number. Sometimes integer, sometimes real of the form 3.02 where the number after the decimal is the CCD number.
2) Star identification number.
3 to 8) Star RA and declination. A negative declination is always marked by a minus sign before the degrees, even if the latter is zero.
9 & 10) Star X and Y position in the CCD image.
11) A magnitude.
12) An uncertainty. For reasons explained in Naylor et al (2002), this is actually the bright side error bar, since in magnitude space uncertainties are technically two sided. The same paper explains how to derive the faint side error bar (should you need it).
13) A flag.
Columns 11 to 13 are then repeated for each colour.

The flags are two character strings. Each character is the flag for one of the two filters which went to making up the colour. For simple magnitudes, one flag is for the filter for that magnitude, the other flag is for the other filter which was used for the colour term. The order of the flags is the same as the order of the colour, thus for B-V the B flag comes first and the V flag second. The first "colour" is always a magnitude constructed using a colour term based on the second colour (e.g V constructed using B-V). So the flags for the first "colour" are always a copy of those from the second. (There used to be a rule that the flags were ordered such that the bluest filter used came first. The catalogues constructed using this rule also conform the the above rule.) For example, a V magnitude whose colour term was derived from B-V, might have a flag of NB. This would mean that the instrumental B measurement had been flagged N and instrumental V as B. The flags are divided into strong and weak. A strong flag will always be written, overwriting any other flags, whilst a weak flag will only be written if the measurement is unflagged so far. Thus the order the flags are applied is important. Most flags are strong, a weak flag is only used if there might be reason to trust the resulting photometry, or if there is an underlying reason for it. The ill-determined background criterion is very cautious, so if inspection of the image suggests the the background is not strongly structured, such data can be useful. Similarly patching bad pixels can work very well, at which point images on flagged pixels may be worthwhile. The list of flags, in the order they are applied, is as follows.

Flag	Flag Name	Algorithm	Type	Flag Number
O	O.K.			0
S	Saturated pixel.	Pixel flagging.	Strong	4
L	Counts above pixel Linearity limit.	Pixel flagging.	Strong
F	Bad pixel	Pixel flagging.	Strong	8
R	Uncalibrated Region	Pixel flagging.	Strong
W	CroWding test	Star searching.	Strong	2
N	Non-stellar	Star shape estimator.	Strong	1
B	Background fit failed	Sky determination.	Strong	3
I	Ill-determined background	Sky determination.	Weak	5
P	Position fit failed	Flux measurement.	Weak
E	Too close to Edge	Flux measurement.	Strong	2
M	Negative (Minus) counts	Flux measurement.	Weak	9
A	Absent input data	Data input to calibration.	Strong	7
H	Poor profile correction	Profile correction.	Strong
V	Variable	Combining catalogues.	Strong	6
C	Large relative transparency correction (Cloudy)	Combining catalogues.	Strong
G	Poor seeing	Combining catalogues.	Strong
T	AperTure photometry	Combining catalogues.	Strong

Older versions of the software used numerical flags, whose values are given in the last column.

If a star is flagged as having negative counts (M), the magnitude is replaced with the uncertainty in flux converted into magnitudes. Thus magnitudes for objects flagged M should be treated as bright (i.e. lower) limits. It also means that colours which are quoted for these objects are limits. For example a flag of MO in the B-V colour, means that the B magnitude is a lower limit, so B-V is a lower limit. The uncertainty becomes meaningless, unless you want the description for real aficionados...

Error bars for points flagged as negative

On encountering a negative flux, the uncertainty is normally set to the modulus of the flux, converted into magnitudes. Older versions of the code then multiply this by minus one, but newer versions will not (which has happened is obvious, as these stars are faint and should have positive magnitudes). The reason we suggest you normally ignore this number, is that thereafter it will be treated as the uncertainty. So, for example, as soon as a colour is made, it becomes meaningless.

The two-sidedness of error bars

The bright end of the error bar for objects with positive fluxes must match smoothly onto the upper limit for objects with negative fluxes. For this to be true it means that as the flux for an object tends to zero, the result of subtracting the magnitude error from the magnitude (i.e. calculating the bright end of the error bar) must tend towards the value obtained by converting the flux error into a magnitude. This forces our definition of the magnitude error bar (E) to be the bright end of the error bar, subtracted from the magnitude;

E = -2.5log10(f) - (-2.5log10(f+e)) = 2.5log10((f+e)/f).

Of course, in reality the error bar in magnitude space is not single sided. This definition we have chosen is, of course, the error bar in the bright direction. Rather than quote two-sided errors, we simply note that the faint side error bar can be calculated from the bright-side error bar in the following way. The difference between the bright-side error bar and faint side error bar is given by,

2.5log10(f/(f-e))-2.5log10((f+e)/f)=-2.5log10(1-(10^(E/2.5) -1)^2).