ATUS

Deutsches SOFIA Institut (DSI)

The Astronomical Telescope of the University of Stuttgart

The ATUS 0.6-meter Ritchey-Chrétien telescope, mounted on a German equatorial Astro-Physics 3600GTO mount with a customized polar fork assembly and counterweight shaft. The fast Andor iXon EMCCD camera ("FPI") can be seen at the RC focus; a QSI 616s camera is mounted sideways on the port of a custom built off-axis guider. A QSI 632ws-8 camera is mounted at the piggy-back 130 mm f/6 refractor ("FFI"). The Wide Field Imager (WFI) is not visible in this picture; it is mounted on a third Losmandy dovetail bar (red) hidden by the telescope structure at about the same location as the small stack of steel plates that balance its weight around the declination axis.
The ATUS 0.6-meter Ritchey-Chrétien telescope, mounted on a German equatorial Astro-Physics 3600GTO mount with a customized polar fork assembly and counterweight shaft. The fast Andor iXon EMCCD camera ("FPI") can be seen at the RC focus; a QSI 616s camera is mounted sideways on the port of a custom built off-axis guider. A QSI 632ws-8 camera is mounted at the piggy-back 130 mm f/6 refractor ("FFI"). The Wide Field Imager (WFI) is not visible in this picture; it is mounted on a third Losmandy dovetail bar (red) hidden by the telescope structure at about the same location as the small stack of steel plates that balance its weight around the declination axis.

The Astronomical Telescope of the University of Stuttgart (ATUS) was established by the university's Institute of Space Systems (IRS) with support of the Stratospheric Observatory for Infrared Astronomy (SOFIA), funded by the DLR Space Administration. The telescope is managed and operated by the German SOFIA Institute (DSI), a department of the IRS. For the university, the telescope serves as a training platform for aerospace engineering students in basic astronomy and in remote control of complex systems. M.Sc. and Ph.D. students use it as a research instrument for their engineering and astronomy projects. For SOFIA, it is used as a test platform to evaluate new hardware and software before the integration on the airborne observatory. In some cases, the telescope is also used to support SOFIA missions by providing preparatory or parallel measurements of a target or to conduct follow-up observations.

ATUS is a 0.6 meter fully reflective telescope in Ritchey-Chrétien configuration made by Officina Stellare, Italy. Its primary and secondary mirrors are made of the ultra-low-expansion glass ceramics CLEARCERAM-Z HS, supplied by Ohara Corporation, Japan. The primary mirror has a conically shaped backside to reduce its weight. The secondary mirror cell is motorized to shift the mirror along the optical axis for focusing. The optical tube assembly is a dual carbon fiber truss structure with titanium alloy joints. A customized German equatorial mount made by Astro-Physics (AP3600GTOPE) carries the telescope and allows slew speeds of up to 3.75 degrees per second. A precision encoder system at the polar axis of the mount provides nominal guiding accuracy better than 0.5 arcsec over periods of 20 minutes.

The first and primary instrument at the Ritchey-Chrétien focus is a back-illuminated EMCCD camera made by Andor, Belfast combined with a 10-position filter wheel containing a Sloan filter set. The same camera and a similar filter set are used in SOFIA's new Focal Plane Imager (FPI+). It offers high quantum efficiency (ηPeak > 90%), high frame rates and virtually gap free imaging thanks to the sensor's frame-transfer architecture. Very low dark currents are achieved by the camera's thermoelectric cooler that cools the sensor as much as 95 K below ambient. A fully custom designed off-axis guider redirects unused light outside the main camera’s optical beam into a guiding camera made by Quantum Scientific Imaging (QSI) with a front-illuminated KAF-1603ME CCD, enabling ultra-deep exposures and steady tracking with sub-pixel accuracy over hours. 

The main telescope is complemented by a piggy-back mounted Wide Field Imager which consists of a ProLine Camera made by Fingerlakes Instrumentation with a back-illuminated e2v CCD47-20 sensor and a commercial 135 mm Canon telephoto lens, focused via a custom Arduino-based interface. This setup is optically similar to SOFIA's Wide Field Imager. A 130 mm f/6 refractor can be used as an imager with an intermediate field size ("Fine Field Imager"), which is particularly useful for satellite tracking and other applications in space situational awareness. It’s camera, also made by QSI, employs a front-illuminated KAF-3200ME CCD. 

ATUS is located at Sierra Remote Observatories (SRO) in the foothills of California's Sierra Nevada, a location with seeing conditions in the 1.0 to 1.5 arcsec range in summer. The telescope's control computer and software are set up for remote access via an internet connection. It is remotely operated from the SOFIA Science Center in Moffett Field, California and from Stuttgart, Germany.

ATUS Optical System

Optical configuration

Ritchey-Chrétien

Clear aperture diameter of primary mirror, D

600 mm

Primary mirror focal ratio

f/3

Clear aperture diameter of secondary mirror

222 mm

Diameter of secondary mirror baffle (central obscuration)

272 mm

Obscuration ratio, ε

45.3 %

Obscured area

20.6 %

Effective focal length, f (nominal)

4870 mm

Effective focal ratio (nominal)

f/8

Diffraction limit, DAiry = 2.44 λ/D

0.46 arcsec

Main Camera ("Focal Plane Imager" / FPI)

Camera model

Andor iXonEM+ DU-888E-C00-BV

Sensor type

e2v CCD201-20, back-illuminated, frame transfer EMCCD

Sensor dimensions

1024 × 1024 pixel

Pixel size

13 × 13 µm2

Field of view on sky

9.4 × 9.4 arcmin2

Plate scale

0.55 arcsec/pixel

Mechanical shutter

No

Filter wheel

SBIG CFW-10; 10 positions for 1¼ inch filter cells

Available filters

- Parfocal Astrodon Sloan 2nd generation filter set (g'2, r'2, i'2, z_s2, Y2) & clear filter (> 395 nm);
- Astrodon Exoplanet Blue Blocking Filter
- Custom VR filter (500  - 700 nm bandpass)
- Opaque filter, used to record bias or dark frames

Achievable frame rates with...

no binning

2x2 binning

4x4 binning

  
 

Full frame readout

8.9 fps

17.4 fps

33.5 fps

  
 

512 x 512 AOI

17.6 fps

34.0 fps

63.9 fps

  
 

32 x 32 AOI

205.3 fps

315.4 fps

427.3 fps

  

Off-Axis Guider (OAG)

Configuration

Custom mechanical design with pick-off prism (25 mm edge length, λ/10) redirecting light outside the main camera’s field of view into a separate camera for precision guiding

Camera model

QSI 616s

Sensor type

ON Semiconductor (formerly Kodak) KAF-1603ME

Sensor dimensions

1536 × 1024 pixel
Pixel size 9 × 9 µm2
Field of view on sky 9.7 × 6.5 arcmin2
Plate scale 0.38 arcsec/pixel
Mechanical shutter Yes
Filter wheel None

Guide Scope ("Fine Field Imager" / FFI)

Optical configuration

Apochromatic refractor

Clear aperture diameter

130 mm

Focal ratio

f/6

Camera model

QSI 632ws-8

Sensor type

ON Semiconductor (formerly Kodak) KAF-3200ME; front illuminated, full frame CCD with microlenses and enhanced response towards blue wavelengths

Sensor dimensions

2184 × 1472 pixel

Pixel size

6.8 × 6.8 µm2

Field of view on sky

65.4 × 44.1 arcmin2

Plate scale

1.80 arcsec/pixel

Mechanical shutter

Yes

Filter wheel

Integrated in camera; 8 positions for 1¼ inch filter cells

Available filters

Parfocal filter set, consisting of
- Astrodon Sloan 1st generation filter set (g', r', i', z')
- Narrowband H-α, O-III, S-II filters (3 nm bandpass)
- Clear filter (> 395 nm)

Readout speed, unbinned

2.78 s (full frame)
2.17 s (128 x 128 AOI)

Wide Field Imager (WFI)

Optical configuration

Commercial telephoto lens

Focal length, f

135 mm

Camera model

Custom FLI ProLine PL4720

Sensor type

e2v CCD47-20; back-illuminated, frame transfer CCD

Sensor dimensions

1024 x 1024 pixel

Pixel size

13 × 13 µm2

Field of view on sky

5.65 × 5.65 deg2

Plate scale

19.9 arcsec/pixel

Mechanical shutter

No

Filter wheel

FLI CFW-2-7; 7 positions for 50 mm filter substrates

Available filters

- Parfocal Johnson/Bessel filter set (UBVRI)
- Opaque filter, used to record bias or dark frames

Frame rates, unbinned

1.5 fps (full frame)
5.6 fps (128 x 128 AOI)
  1. Sickafoose, A.A., Bosh, A.S., Levine, S.E., Zuluaga, C.A., Genade, A., Schindler, K., Lister, T.A., et al., 2019. A stellar occultation by Vanth, a satellite of (90482) Orcus. Icarus, 319, pp.657–668. DOI: 10.1016/j.icarus.2018.10.016.
  2. Bosh, A.S., Sickafoose, A.A., Levine, S.E., Zuluaga, C.A., Genade, A., Schindler, K., Lister, T.A., et al., 2018. The 2017 occultation by Vanth: a revised analysis. AAS/Division for Planetary Sciences Meeting Abstracts, 50, p.311.01.
  3. Sickafoose, A.A., Levine, S.E., Bosh, A.S., Zuluaga, C.A., Person, M.J. & Schindler, K., 2018. Pluto’s atmosphere after New Horizons: results from stellar occultations in 2017 and 2018. AAS/Division for Planetary Sciences Meeting Abstracts, 50, p.502.02.
  4. Sickafoose, A.A., Bosh, A.S., Levine, S.E., Zuluaga, C.A., Genade, A., Schindler, K., Lister, T.A., et al., 2017. A 2017 stellar occultation by Orcus/Vanth. AAS/Division for Planetary Sciences Meeting Abstracts, 49, p.216.02.
  5. Bosh, A.S., Zuluaga, C.A., Levine, S.E., Sickafoose, A.A., Genade, A., Schindler, K., Lister, T.A., et al., 2017. Astrometry of the Orcus/Vanth occultation on UT 7 March 2017. AAS/Division for Planetary Sciences Meeting Abstracts, 49, p.216.01.
  6. Schindler, K., Wolf, J., Bardecker, J., Olsen, A., Müller, T., Kiss, C., Ortiz, J.L., et al., 2017. Results from a triple chord stellar occultation and far-infrared photometry of the trans-Neptunian object (229762) 2007 UK126. A&A, 600, p.A12. DOI: 10.1051/0004-6361/201628620.
  7. Schindler, K., Lang, D., Moore, L., Hümmer, M., Wolf, J. & Krabbe, A., 2016. Computer-aided star pattern recognition with astrometry.net: in-flight support of telescope operations on SOFIA. Proc. SPIE, 9913, pp.991307-991307–14. DOI: 10.1117/12.2231531.
  8. Zintz, K., 2015. Stuttgarter Projekt Atus: Ein ferngesteuertes Teleskop für die Uni (Stuttgart’s project Atus: A remotely controlled telescope for the university). Stuttgarter Zeitung, 13 November 2015.
  9. Timerson, B., Durech, J., Beard, T., McPartlin, C., Morgan, W., Schindler, K., Wolf, J., et al., 2015. Asteroidal Occultation by 82 Alkmene and the Inversion Model Match. Minor Planet Bulletin, 42, pp.129–131.

Pictures of ATUS

The imaging train of ATUS consists of a mechanical focuser, a custom designed off-axis guider with a guiding camera, a 10-position filter wheel and the main scientific camera, an Andor iXon DU-888 with a back-illuminated, frame transfer EMCCD.
The imaging train of ATUS consists of a mechanical focuser, a custom designed off-axis guider with a guiding camera, a 10-position filter wheel and the main scientific camera, an Andor iXon DU-888 with a back-illuminated, frame transfer EMCCD.
The Wide Field Imager (WFI) is mounted sideways. It uses a commercial Canon EF 135mm f/2.8 telephoto lens and an Arduino microcontroller for focusing.
The Wide Field Imager (WFI) is mounted sideways. It uses a commercial Canon EF 135mm f/2.8 telephoto lens and an Arduino microcontroller for focusing.
Close-up view of the secondary mirror assembly and its focus mechanism. The primary mirror cover, consisting of four flaps, is closed in this picture; it protects the primary mirror from dirt and dust when the telescope is not in use. The front truss is covered by a fabric shroud to shield the optics from any potential stray light and dust.
Close-up view of the secondary mirror assembly and its focus mechanism. The primary mirror cover, consisting of four flaps, is closed in this picture; it protects the primary mirror from dirt and dust when the telescope is not in use. The front truss is covered by a fabric shroud to shield the optics from any potential stray light and dust.
In this picture, the primary mirror cover is open and exposes it to incoming light. The fabric shroud has been removed to showcase the truss structure of the telescope; however, the shroud is required for operations as a stray light cover. You can also see the Astro-Physics 3600GTO German equatorial mount, an older configuration of the imaging train, and the piggy-back 130 mm f/6 refractor.
In this picture, the primary mirror cover is open and exposes it to incoming light. The fabric shroud has been removed to showcase the truss structure of the telescope; however, the shroud is required for operations as a stray light cover. You can also see the Astro-Physics 3600GTO German equatorial mount, an older configuration of the imaging train, and the piggy-back 130 mm f/6 refractor.
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