The STELLA observatory is made up of two telescopes, each with a dedicated instruments: WiFSIP on STELLA-I and SES on STELLA-II.
The STELLA Telescopes
Both STELLA telescopes were manufactured by Halfmann Teleskoptechnik in Neusäß near Augsburg, Germany, which has been part of Halfmann Maschinenbau GmbH, no longer builds telescopes. They feature an Alt/Az mount and an aperture of 1.2m. The first telescope (STELLA-I) is an f/8 Cassegrain system with two Nasmyth foci and a field of view of 30′. This second telescope (STELLA-II) is equipped with a prime-focus corrector unit which feeds the optical fiber at f/8.4 with a patrol field of view of about 2′. The telescopes are equipped with a hydrostatic azimuthal bearing driven by a single stage oil pump, the altitude axis features a ball bearing. The two telescope axes are moved by direct-drive motors from ETEL with optical Heidenhain encoders. One increment on the encoder corresponds to 36″, which is interpolated at a resolution of 11 bits, which translates to a theoretical pointing and tracking accuracy of the drives of 0.18”. This samples well the average site seeing of about 1”.
Wide Field STELLA Imaging Photometer (WIFSIP)
The Wide Field STELLA Imaging Photometer (WIFSIP) is a large-format CCD imager and photometer. Two instruments have been built, one for STELLA and one for RoboTel.
WIFSIP is the large-format CCD imager and photometer for STELLA. It has a usable FOV of 22’x22′ at a scale of 0.32″/Pixel. The detector is a single 4096×4096 back-illuminated thinned CCD (15µm pixels) and a peak QE>90% from the University of Arizona ITL.
Available filters are Stromgren uvbyHβ, Hα-narrow and wide (4 and 18 nm FWHM), the Sloan-u’g’r’i’z’, and the Johnson-Cousins UBVRI set.
STELLA échelle spectrograph (SES)
The SES is a modern white-pupil spectrograph with a moderately high spectral resolution of about 55,000 and a fixed wavelength format of 390 to 870nm. The instrument is located on a passively stabilized optical bench in an insulated box within an actively temperature-controlled room. Until 2014 it was fed by a 24m long octagonal optical fiber with 50µm core diameter. Since then a 65µm optical fiber and a 2-way slicer is used. The spectrograph consists of a 31.6 lines per mm R2 echelle grating from RGL, two equal-sized off-axis parabolic collimator mirrors, one folding mirror and two prisms acting as cross-disperser. The beam with a diameter of 130mm is imaged onto the detector by a f/4 refracting camera (before 2012 a reflecting Schmidt-camera was used). A 4x4k E2V CCD together with a AIP-built Magellan controller is used as the detector system. The readout noise of the CCD is about 3.5 electrons rms, a CryoTiger closed cycle cooler keeps the CCD temperature at -120°C. The exposure time calculator (ETC) can be found on the main user server.
SES – 2nd Generation
Currently an upgrade of the spectrograph is under way. This will consist of a new, more blue-efficient prime-focus corrector and 2 new spectrograph modules for the blue (SES-H&K 380-470nm) and the visual (SES-VIS 470nm-690nm) wavelength ranges. The current SES will be used to cover wavelengths from 690nm to the detector cutoff at approximately 1µm.
More information can be found in the STELLA publications.