Features & Optics

The 1.2m Cassegrain telescope of the Astronomical Station Kryoneri, made by Grubb Parsons Co., was installed in 1975. Its original optical system consisted of a parabolic primary mirror of 1.2m diameter and f/3 focal ratio, and a hyperbolic secondary mirror (31 cm). The mirrors were made of Zerodur material (a type of glass with zero expansion). Its mounting is based on a modified torque-tube equatorial system. During its first 40 years of operation the telescope’s effective focal ratio was f/13, with a field of view about 40’ and an image scale of 12.5″/mm.

In May 2016, the telescope was upgraded by DFM Engineering Inc. for the needs of the NELIOTA project. This upgrade included the conversion of the original optical configuration to a prime focus telescope, the replacement of some mechanical parts of the telescope, the installation of a new control system, dome automation, and the installation of a Prime Focus Instrument (PFI) able to provide a large field-of-view as required for NELIOTA. The new optical configuration yields a focal ratio of f/2.8 (Xilouris et al. 2018).

Fast-Frame Imager

The Fast-frame imager consists of two sCMOS Andor Zyla 5.5 cameras, installed at the prime focus of the telescope, offering the ability to record a target simultaneously in two different wavelength bands. A dichroic with a cut-off at 730 nm splits the light beam, sending each split beam to a different camera. The cameras are equipped with R and I-band Johnson-Cousins filters.

Kryoneri Telescope Camera
Parameter Value
Model Andor Zyla 5.5
Type of sensor Front Illuminated Scientific CMOS
Shutter options i) Global
ii) Rolling
Gain settings options i) High well capacity (12bit)
ii) Low noise (12 bit)
iii) Low noise & High well capacity (16 bit)
Read out rate options i) 560 MHz
ii) 200 MHz
Sensor size (mm) 16.6×14.0
Pixel size (μm) 6.48
Total pixels 2560×2160
Field of view (arcmin) 17.0 x 14.4 (R-band)
16.8 x 14.2 (I-band)
Pixel scale (arcsec/pixel) 0.4
Maximum frame rate 40 fps
Maximum exposure time 30 sec
Linearity < 60,000 ADUs (16-bit); <4,000 ADUs (12-bit)
Cooling Thermoelectrical (constant at 0 degrees)
Connection USB 3.0
Rolling Shutter Global Shutter
System RO rate Gain settings CMOS Sensitivity
(e- per A/D count)
Median Read| Noise
(e- RMS)
CMOS Sensitivity
(e- per A/D count)
Median Read Noise
(e- RMS)
560 MHz High well capacity (12bit) 7.53 6.62 7.59 7.06
Low noise (12 bit) 0.30 1.39 0.42 2.58
Low noise & High well capacity (16 bit) 0.47 1.41 0.47 2.62
200 MHz High well capacity (12bit) 7.40 6.93 7.73 6.59
Low noise (12 bit) 0.30 1.19 0.42 2.45
Low noise & High well capacity (16 bit) 0.45 1.15 0.46 2.51
Camera Input light Filters

The performance of the Prime Focus Instrument (PFI) optical system (dichroic, filters, cameras) is calculated taking into account the Quantum Efficiency (QE; blue solid line) of the cameras, the transmittance of the R and I filters (red and green solid lines, respectively) and the dichroic (black dashed and dot dashed lines). The dichroic is centered at 730 nm with a > 90% throughput. The figure is from Xilouris et al. (2018).

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