Up: The Local Group dwarf
In order to estimate the internal accuracy of our CCD measurements we used
the formal errors from the DAOPHOT package and as additional error estimates we
used the magnitude differences from experiments with artificial stars.
Table 1 lists the derived average standard deviations for successive
intervals of one magnitude in the U, B and V frames.
As can be seen we have large errors (>0.15) at the faint
magnitudes: U > 22.5, B > 23.5 and V > 24.0. The standard errors from
all stars vs. magnitude are displayed in Fig. 3.
The mean standard deviations
Internal errors of the CCD photometry|
The artificial star simulations necessary for the completeness
correction can also be used to determine the measurement errors. A set of
randomly placed on the original frames were re-determined in
the same manner as the program stars. The "input-output" differences in B
and V magnitudes between "added" and "recovered" artificial stars as a
function of the measured output magnitude are shown in Fig. 4. The mean
errors of the "input-output" differences in B and V magnitudes are 0.025
and 0.024 respectively.
The last step in our data reductions was to determine the completeness
functions in the B and V filters. We used the artificial star technique
(Stetson & Harris 1988; Stetson 1991) creating series of artificial frames
by means of the ADDSTAR routine in DAOPHOT II which were re-reduced in the same
manner as the original frames. The completeness correction derived as the ratio
of added to recovered artificial stars was then fitted by a cubic spline. The
completeness function is listed in Table 2. We consider our sample complete up
to 22.5 mag in both B and V filters. In Cols. 4 and 5 as independent
completeness estimation the magnitude distributions for the stars
in U and V filter binned by 0.5 are given.
The turnover interval in U filter is at U=21.5 mag which we will consider
as a completeness limit in this filter. The completeness magnitudes in B and
V agree with the values determined by artificial stars.
The "input-output" differences in B and V magnitudes as a
function of the measured output magnitude|
It is obviously necessary to check the level of field star contamination in the
color magnitude diagram before discussing its detailed structure. Based on the
Bahcall & Soneira (1980) model of the Galaxy,
Ratnatunga & Bahcall (1985)
have predicted the field star densities in the direction of IC1613. Table 3
gives the number of objects expected per bin of magnitude and color in the area
covered by our observations. In summary the total number of expected field
stars in our field is 47, while we can expect approximately two stars brighter
than 19 mag in V and (B-V) < 0.8. Field stars therefore should not seriously
affect the structure of the CMD and no corrections for field contamination
have been applied to the IC1613 data.
Expected numbers of field stars towards IC1613
Up: The Local Group dwarf
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