- Limit star magnitude: S/N=10 for V=13 mag in one hour.
- Global trasmissivity: > 10%
- Spectral resolution: R=12000
- Two spectral windows:
- Green: between 495 and 529 nm
- Red: between 634 and 678 nm.
In order to achieve the high global transmissivity (>10%) two telescopes (manufactured by Takahashi) and two VPH are placed in Littrow configuration.
The ARES spectrograph dispersion system is composed by two VPHs developed by Wasatch Photonics, providing the two spectral windows and maintaining the high overall throughput. Both VPHs are mounted on a carriage (prepared to hold up to three VPHs), allowing a fast change from one VPH to the other.
- Sensor type: BV- Back Illuminated CCD, VIS optimized
- Active pixels: 2048 x 512
- Pixel sixe: 13.5 x 13.5 µm
- Image area: 26.7 x 6.9 mm
- Maximum cooling: -100ºC
- Read-out noise: as low as 2.8 electrons
- Dark current: as low as 0.0002 e-/pixel/sec
- Linearity: better than 99%
ARES is equipped with three fibers, providing three spectra per exposure. One fiber is centered on the stellar-like object of interest, another fiber can obtain sky spectra, and the third fiber, placed between the two other fibers, can be used to acquire either calibration lamp flats or wavelength calibration arcs. The observer can choose the calibration spectrum to be inserted in the image when choosing the VPH to be used. Two examples are shown below, with one image showing the two sky fibers illuminated and another image showing a stellar spectrum together with a calibration arc.
Anchor here is required
Apart from the calibration spectrum obtained with each image, the TJO control system is designed to periodically run the acquisition of calibration images, including bias, darks and spectroscopic flat-fields:
- Bias. Bias frames are usually taken in multiples of 5 before the beginning and after the end of the night. Usually, a minimum number of ten bias frames are provided to the observers.
- Dark. Dark current frames are usually taken just before or after the acquisition of bias frames. The exposure time for the dark frames depends on the exposure time of science images. Usually, the exposure time for the dark frames, corresponds to the maximum exposure time for the science frames.
- Spectroscopic flat-fields. The optimum calibration strategy for ARES is still under evaluation. The main goal of spectroscopic flat-fields is to trace the spectra position and to calibrate the intra-pixel response. Four options are being studied: Direct illumination on the CCD, dome flat-fields, fiber-illuminated LED and standard stars. The solution providing the optimum result will be used to provide spectroscopic flat-field images to the observers.
All the calibration images are provided to those observers having science sequences with ARES during the night.