One Nasmyth port of the 2m Fraunhofer telescope at the Wendelstein Observatory has been equipped with a wide-field three lens field corrector. It preserves the excellent image quality (< 0.8 median seeing) of the site (Hopp et al. 2008) over a field of view of 0.7 degrees diameter and keeps distortion to a minimum. A camera was designed to image astronomical objects to very faint light levels in the presence of bright source. Science applications include the structure of massive elliptical galaxies and the monitoring of gravitational lensing events towards the Andromeda galaxy M31, see below.
The available field is imaged by a mosaic CCD camera (WWFI, the Wendelstein Wide Field Imager) built in-house (project manager C. Goessl). It places two filter wheels into the telescope beam followed by a large Bonn shutter (Reif et al. 2007). The third lens of the corrector is placed next and serves as the entrance window for the evacuated CCD camera system, which is cooled to about 160 K with a closed loop cooler system. Before the filter wheels, two plane mirror pick-off two guide windows outside the science field of view and feed them into two 2k*2k e2v FLI CCD cameras which allow to correct for telescope tracking errors along all motion axis (azimuth, elevation, image rotation) of an altaz mount.
The system is based around customized components like the CCD camera from Spectral Instruments, Tucson, the Bonn Shutter, off the shelf slide mechanisms, the Cryotiger cooling system, etc. and is described in length by Kosyra et al. (2014). A brief summary of the technical details is presented in the table below.
The instrument builders of our group designed the opto-mechanical system, realized the mounts including the filter wheels, build the control electronics and wrote the control software, and purchased, integrated and tested the customized components. They did also the final integration at the telescope in July 2013 including verification at the sky. The data reduction pipeline was developed together with the scientists in our group using data of the WWFI for their science topics.
Wide-Field Imager at the Wendelstein 2-m Telescope (WWFI)
|Field of view||26.6 × 29.0 arc min (gaps: 22″, 98″)|
|Number of pixels||4 × 4096 × 4096|
|Pixel scale||0.199 arc sec/pixel integrated in a SI 900 camera system by Spectral Instruments, Tucson|
|Read-out time||8.5 sec at 500 kHz, 40 sec at 100 kHz|
|Read-out noise||7.8 e− at 500 kHz, 2.2 e− at 100 kHz|
|Gain||5.8 e−/ADU at 500 kHz, 0.7 e−/ADU at 100 kHz|
|Dynamic range||16 bit|
|Full well capacity||> 250 ke−/pix|
|Filter positions||12+2 in 2 wheels|
|Filters||Sloan u′, g′, r′, i′, z′|
|Zero points (SDSS filters)||u′: 24.4, g′: 25.4, r′: 25.3, i′: 24.7, z′: 23.8|
|Auto guiding||2 off-axis 2k × 2k CCDs|