next up previous contents
Next: 11.3 Extinction and Night Up: 11.2 Aperture Photometry Previous: 11.2.1 The Focused Images

11.2.2 The Defocused Images

phot was run on 4 well isolated stars (F81, F153, F170, & I27) in the defocused images, with apertures $5,6,\ldots,42$ pixels, and with salgorithm=gauss, annulus=42.0 pixels, and dannulus=14.0 pixels. Even though the defocused stars did not have a peak to define their centers, they were centered well using a somewhat large box, centerpars.cbox=15 pixels, which still was small enough to exclude the neighboring stars.

A large effort went into trying to get mkapfile to fit the profiles of the defocused stars, and compute the aperture correction. This did not succeed.

Instead, aperture corrections at 12 and 15 pixels were manually read from the growth curves that mkapfile produced. These growth curves, one for each image, were based on the above 4 stars. An example of such a curve is shown in Figure [*]. The resulting aperture corrections are shown in Table [*], and plotted in Figure [*], the two left panels. In the top right panel, the difference between the two is shown.

  \begin{figure}% latex2html id marker 23945\makebox[\textwidth]{
\epsfxsize=\h... is also $-0.0173$ .
The estimated uncertainty on this is 0.002.

From the plots it can readily be seen, that the aperture correction is not the same for the 4 filters, and that it is not the same from night to night. Further, the difference between the aperture corrections at the two apertures is not the same for the 4 filters and from night to night.

Table: The manually measured aperture corrections, by night
night filter image apcor12 apcor15 delta
1 GR d1263 -0.032 -0.018 0.015
1 JV d1262 -0.027 -0.014 0.012
1 JB d1264 -0.034 -0.019 0.014
1 JU d1266 -0.048 -0.023 0.025
2 GR d1388 -0.025 -0.017 0.009
2 JV d1389 -0.020 -0.012 0.007
2 JB d1390 -0.032 -0.020 0.012
2 JU d1391 -0.044 -0.026 0.018
3 GR d1534 -0.017 -0.012 0.006
3 JV d1535 -0.015 -0.010 0.005
3 JB d1536 -0.021 -0.016 0.006
3 JU d1537 -0.043 -0.030 0.013
4 GR d1689 -0.021 -0.014 0.007
4 JV d1690 -0.017 -0.013 0.004
4 JB d1691 -0.024 -0.015 0.009
4 JU d1692 -0.040 -0.027 0.013
6 GR d1948 -0.022 -0.017 0.005
6 JV d1949 -0.017 -0.012 0.005
6 JB d1950 -0.023 -0.015 0.008
6 JU d1951 -0.038 -0.024 0.013
7 GR d2092 -0.021 -0.015 0.006
7 JV d2093 -0.016 -0.010 0.005
7 JB d2094 -0.021 -0.014 0.008
7 JU d2095 -0.038 -0.027 0.011
Notes: apcor12 and apcor15 are the aperture corrections for the defocused images at an aperture of 12 pixels and 15 pixels, respectively, in magnitudes. delta is minus the difference between the two, i.e. delta = -(apcor12-apcor15). The aperture corrections were manually measured from growth curves based on the 4 stars F81, F153, F170, and I27. Uncertainties were estimated from the scatter of the used individual points. Typical values are: GR: 0.002 mag, JV: 0.001 mag, JB: 0.002 mag, JU: 0.006 mag. When doing the eye fit, the JU values from night 4 was noted as uncertain, and a more realistic uncertainty for these values is probably 0.010-0.015 mag.

  \begin{figure}% latex2html id marker 23979\epsfxsize=\textwidth
...e of 59 stars,
cf.\ Figure~\ref{dmag_GR_JV} and \ref{dmag_JB_JU}).

The M67 field of this study had 23 stars in common with the standard stars of Jørgensen (1994). Besides of these standard stars, 36 stars were selected as ``program'' stars on the following three criteria: they should be somewhat bright (we required a peak > 500 ADU in the GR image d1262), they should not be too close to other stars, and they should have a unique name. The reason for including these program stars was: 1) the derived magnitudes for these stars could be useful in another context (they could perhaps serve as tertiary standard stars), and 2) the more stars, the better statistics for the test of the aperture correction, see below.

phot was run on these 59 stars, using two apertures: 12 and 15 pixels. cbox was increased to 20 pixels. The reason for using two apertures was to test the aperture corrections determined above at these two apertures. Since per definition

$\displaystyle m({\rm total})$ = $\displaystyle m(12{\rm px}) + {\rm apcor}(12{\rm px})$ (11.1)
  = $\displaystyle m(15{\rm px}) + {\rm apcor}(15{\rm px}),$ (11.2)

it follows that

 \begin{displaymath}m(12{\rm px}) - m(15{\rm px}) =
- \left[ {\rm apcor}(12{\rm px}) - {\rm apcor}(15{\rm px}) \right].
\end{displaymath} (11.3)

This only holds in the absence of contributions from neighboring stars and cosmic ray events. $m(12{\rm px}) - m(15{\rm px})$ for the above 59 stars for each image (with one image per night per filter) is shown in Figure [*] for Gunn r and Johnson V, and Figure [*] for Johnson B and U. The median value of $m(12{\rm px}) - m(15{\rm px})$ for each of the 24 images is shown in Figure [*], bottom right panel. In the top right panel of the same figure is shown $- \left[ {\rm apcor}(12{\rm px}) - {\rm apcor}(15{\rm px}) \right]$.

Table: Mapping between star names and ID numbers
ID Name Type ID Name Type ID Name Type
1 M67-F81 standard 21 M67-F141 standard 41 M67- I56 program
2 M67-F83 program 22 M67-F143 program 42 M67- I60 program
3 M67-F93/ I12 standard 23 M67-F145 program 43 M67- II20 program
4 M67-F94 program 24 M67-F147 program 44 M67- II22 standard
5 M67-F95 standard 25 M67-F149 program 45 M67- II30 program
6 M67-F105 standard 26 M67-F153 standard 46 M67- III1 program
7 M67-F106/ I11 standard 27 M67-F164 program 47 M67- III2 program
8 M67-F108 standard 28 M67-F170 program 48 M67- III7 program
9 M67-F111 standard 29 M67- I1 program 49 M67- III12 program
10 M67-F115 program 30 M67- I5 program 50 M67- IV2 program
11 M67-F117 standard 31 M67- I9 program 51 M67- IV3 program
12 M67-F119 program 32 M67- I14 program 52 M67- IV4 program
13 M67-F124 standard 33 M67- I15 standard 53 M67- IV6 program
14 M67-F127 standard 34 M67- I17 program 54 M67- IV12 program
15 M67-F128/ I198 standard 35 M67- I20 standard 55 M67- IV13 program
16 M67-F129/ I199 standard 36 M67- I24 program 56 M67- IV21 program
17 M67-F130 standard 37 M67- I25 standard 57 M67- IV34 program
18 M67-F132 program 38 M67- I27 standard 58 M67- IV35 program
19 M67-F134 standard 39 M67- I31 program 59 M67- IV59 program
20 M67-F135 standard 40 M67- I49 standard      
Notes: ID is the identification serial number which phot uses internally. It is also the number used in Figure [*] and [*].

  \begin{figure}% latex2html id marker 24060\epsfxsize=\textwidth
\epsfbox{Ap_st... and star name
can be found in Table~\protect\ref{def_names_ID}.

  \begin{figure}% latex2html id marker 24067\epsfxsize=\textwidth
\epsfbox{Ap_st... and star name
can be found in Table~\protect\ref{def_names_ID}.

As can be seen, the form of the ``curves'' is qualitatively the same, but there is a small offset. This offset is probably due to contributions from neighboring stars for the sample of 59 stars. This contribution is much less in the sample of 4 stars, since they were selected from being isolated. This can be checked by inspecting these 4 stars in Figure [*] and [*]. They are almost always in the lower range of the 59 points. The mapping between names and serial ID number for these 4 stars is F81=ID1, F153=ID26, F170=ID28, and I27=ID38. F81 with and ID of 1 is particularly easy to locate. The conclusion is, that the manually determined aperture corrections pass the test of equation ([*]).

The M67 field is quite crowded. For example, F128/ I198 and F129/ I199 are 25 pixels (13'') apart, F129/ I199 and F130 are 23 pixels (12'') apart, and F106/ I11 and its fainter neighbor I10 are only 16 pixels (8'') apart. This also shows up in the magnitude difference plots in Figure [*] and [*]. The mapping between names and serial ID number for these 4 stars is F128/ I198=ID15, F129/ I199=ID16, F130=ID17, and F106/ I11=ID7. Especially F106/ I11 is very contaminated. The above speaks in favor of a small aperture

Also, the images are quite defocused. Inspection of screen dumps of the images used in Jørgensen (1994) shows, that that these images were less defocused than the images of this study, despite that both used a defocusing of +200 encoder steps. This is probably because the filters are now placed in the DFOSC, instead in the instrument adapter as they previously were.

Jørgensen (1994) used two apertures for her defocused images, 7.52'' and 9.40''.

It was decided to use an aperture of 12 pixels (6.09''), and to use the corresponding aperture corrections listed in Table [*]. As previously mentioned, the background region was 42-56 pixels, i.e. 21.31-28.41''.

Finally, we can compare the aperture corrections for the focused and the defocused images. This in done in Figure [*]. The variation with night is quite similar for the focused and defocused images.

  \begin{figure}% latex2html id marker 24093\epsfxsize=\textwidth
... JV: 0.001 mag, JB: 0.002 mag, JU: 0.006 mag (night 4: 0.010 mag).

next up previous contents
Next: 11.3 Extinction and Night Up: 11.2 Aperture Photometry Previous: 11.2.1 The Focused Images

Properties of E and S0 Galaxies in the Clusters HydraI and Coma
Master's Thesis, University of Copenhagen, July 1997

Bo Milvang-Jensen (