2.3.1 Telescopes and instruments
The Wendelstein 0.8 m telescope has a focal length f of 9.9 m, which results in an aperture ratio f/D =12.4. Since the beginning of the
project in September 1997 we used a TEK CCD with 1024×1024 pixels of 24µm corresponding to 0.49 arcsec on the sky. With this CCD chip we were able to cover 8.3×8.3arcmin2 of the bulge
of M31. To increase the time sampling of our ob- servations we started to use the Calar Alto 1.23 m telescope (f =9.8m, f/D=8.0) in 1999. The
observations were partly carried out in service mode. Six different CCD chips were used. Three of these CCDs cover a field of 17.2×17.2arcmin2
and were used to survey the whole bulge for lens- ing events. A detailed overview of the properties of each CCD camera used for WeCAPP is given in Table2.1.
Most of the sources for possible lensing events in the bulge of M31 are luminous red stars, i.e. giants and supergiants. Consequently, the fil- ters used in our project should be sensitive espe- cially to these kind of stars. We therefore chose
R and I filters for our survey. At Wendelstein we used theR2 (λ '650nm,∆λ '150nm) and Johnson I (λ ' 850nm, ∆λ ' 150nm) wave- bands. The Calar Alto observations were carried
2Modified section.
out with the equivalent filters, R2 (λ '640nm, ∆λ'150nm) and JohnsonI(λ'850nm,∆λ'
150nm). Since June 2000 we are using the newly installed filters Johnson R (λ '640nm, ∆λ'160nm) and JohnsonI(λ'850nm,∆λ'
150nm) at Calar Alto.
Despite of the combination of different tele- scopes, CCDs, and slightly different filter systems we observed no systematic effects in the light curves depending on these parameters. We could therefore combine the data points of Wendelstein and Calar Alto to one common and homogeneous data set.
2.3.2 Observing strategy
During the first two campaigns the survey was re- stricted to Wendelstein and therefore to a 8.30×
8.30 field of view (FOV). Following the sugges-
tion of Tomaney & Crotts (1996) and Han &
Gould(1996a) we selected a field (F1 in the fol-
lowing) along the minor axis of M31, which con- tains the area with the highest expected rate for pixellensing events. The main fraction of the field is covered by the bulge of M31 with the nucleus of M31 located at one corner of the field (see Fig.2.1). From 1999 on, F1 was observed simul- taneously from Wendelstein and Calar Alto. In 2000 we extended the Calar Alto observations to a field of 17.20×17.20, which was centered on the
Figure 2.1: V-,R-, andI-band composite image of the observed fields F1 to F4, taken at Calar Alto Observatory during the campaign 2000/2001. The black lines mark the positions of fields F1 to F4.
with the maximal lensing field F1. At Wendel- stein we continued with the observations of F1, accompanied by observations of F3, the opposite field along the NW minor axis. Due to the si- multaneous observations we reached a very good time sampling during the observability of M31. Since summer 2002 we are mosaicing fields F1 to F4 with the Wendelstein telescope. An image of fields F1 to F4 taken at Calar Alto Observatory is shown in Fig.2.1.
As gravitational lensing is achromatic, the amplification of the source is the same in dif- ferent wavebands. However, as shown in sev-
eral publications (e.g.,Valls-Gabaud, 1994;Witt,
1995; Han et al., 2000), blending and differen- tial amplification of an extended source can lead to a chromatic, but still symmetric, lensing light curve. Under certain circumstances chromatic light curves permit to constrain the physical prop- erties of the source-lens system (e.g., Gould &
Welch, 1996; Han & Park, 2001), however, the
light curves of intrinsic variable stars will gener- ally change color in a different way. To be able to check the light curves for chromaticity, we there- fore had to observe the M31 field in two wave- bands.
Figure 2.2: Illustration of PSF vs. time coverage during 1997-2000. Shaded regions mark the peri- ods of time when M31 was not observable.
We split our observations in cycles, one of them comprising 5 images in the R-band and 3 images in the I- band, taking about 45 min in- cluding readout time. The cycles were repeated as often as possible during one night, usually at least twice. As we had to avoid saturation of stars in the observed field we made exposure times depen- dent of the actual seeing, whereas exposure times in theI bands where generally longer.
Stacking the images with an average exposure time of 150 sec inRand 200 sec inI results in a magnitude limit between (20.8 – 22.1) mag inR
and (19.1 – 20.4) mag inI for a point source on the background of M31 and a signal-to-noise ratio
S N
=10 in over 95 % of the frame. The back-
ground of M31 typically has a surface brightness between (18.7 – 21.2) mag/arcsec2inRand (16.8
– 19.3) mag/arcsec2inI.
2.3.3 The data
We began our observations at Wendelstein with a test period in September 1997, observing on 35 nights until March 1998. The second observa- tional period lasted from 1998 October 22nd un- til 1999 March 24th . During the first Calar Alto campaign we received two hours of service ob- servations on 87 nights (1999 June 27th - 2000 March 3rd). From November 1st until November 14th we were able to observe during the whole night. In parallel we got data from 65 nights at Wendelstein. In this way we achieved during 1999/2000 an overall time coverage of 132 nights (52.6 %).
During the 1997/1998 test campaign condi- tions at the Wendelstein telescope were improved
significantly. A newly installed air condition- ing system reduced dome seeing to a low level. Further improvements like fans just above the main mirror finally lead to a leap in the image quality obtained with the telescope. Figure 2.3
which presents the PSF statistics of Wendelstein images from the 1997/1998 and 1998/1999 cam- paigns respectively illustrates this fact. Compar- ing these values with the statistics of the Calar Alto data shows furthermore, that the Wendel- stein data have a marginally better PSF distribu- tion than Calar Alto (see Fig.2.3).
Figure 2.2 shows the time sampling we reached with WeCAPP during 1997-2000. Be- cause of time loss during the upgrades of the tele- scope, time coverage of the 1997/1998 campaign is only fragmentary. The same applies to the sub- sequent campaign due to a camera shutdown and a further time consuming project. The time cov- erage of the first joint campaign of Wendelstein and Calar Alto is very good due to the often op- posite weather situation in Spain and Germany. In the following joint campaigns we enhanced the time sampling further, culminating in a coverage of 85% during the first two months of 2002.
In Figs. 2.4, 2.5, and 2.12 we give the basic properties of the complete data set being accu- mulated between autumn 1997 and spring 2005. Figure 2.4 shows the cumulative distribution of epochs with data taken at the two sites. One epoch corresponds to nightly stacks. Due to the earlier start of the observations and the continued high priority observations of field F1 at Wendelstein, F1 shows the best time coverage of all fields un- til spring of 2002. Since the summer of 2002, with only Wendelstein contributing to the obser- vations, the time sampling is comparable for all four fields. In Fig. 2.5 we present the distribu- tion of the full-width-half-maximum (FWHM) of the star PSFs in the co-added images. TheI-band data shows a slightly better quality (i.e. smaller PSF values) than the R-band data. The median values for the R- and I-band data are shown in Table2.2. Figure 2.12 finally shows the cumu- lative distribution of the weighting factors of the
Figure 2.3: Histograms of the full-width-half-maximum (FWHM) of the point spread function (PSF) of the frames taken at Wendelstein Observatory during the 1997/1998 campaign (first panel), the 1998/1999 campaign (second panel), the 1999/2000 campaign (third panel), and at Calar Alto Obser- vatory during the 1999/2000 campaign (fourth panel). Frames in the R-band are marked by a solid line, frames in theI-band by a dashed line. The lower limit of the PSF is restricted by a pixel size of 0.5 arcsec. Note, that the images used to construct this figure do not correspond to nightly stacks.
field F1 F2 F3 F4
R 1.38 1.39 1.34 1.39
I 1.33 1.30 1.26 1.35
Table 2.2: Median values of the stellar PSFs, given in arcsec, for theR- andI- band data in the fields F1 to F4.
co-added images for fields F1 to F4. The weight- ing factors (see below) can be understood as qual- ity flags and give the inverse of the noise present inside the area of the PSF in the photometrically aligned images. The weightings were normalized to unity in each field separately, giving maximal weight to the best quality frame in each field.