The MEIA2 instrument is the optical imager of TJO. It consists in two components: the CCD camera and the filter wheel. In addition, a system to acquire dome flats is available. Please, read the full information provided here when preparing observations for the TJO.
The CCD camera is an iKon L (model DW936N-BV), with a back-illuminated 2k×2k chip manufactured by Andor:
- Model: CCD42-40
- Sensor Manufacturer: E2V
- Sensor Type: Back Illuminated
- Coating: BV
- Number of pixels: 2048x2048
- Pixel Size: 13.5x13.5 μm (0.36x0.36 arcsec @ TJO)
- Field of view at TJO: 12.3x12.3 arcmin
- Sensor Diagonal: 39.1mm
- Peak Quantum Efficiency: 96%
- Typical Working Temperature: -50º C
- Temperature Stability: 1º C
- Typical Gain: 1.05 e-/ADU
- Typical Dark Current: <1 e-/pixel/sec. @ -50º C
- Typical Readout Noise: 9 e- RMS @ 3 MHz
- Non linearity: <1%
- Read-out time: 10 seconds
The filter wheel is physically coupled to the telescope at the back of the primary mirror support. It can hold up to 12 3-inch filters that are placed at the optical axis of the telescope by rotating the filter wheel. Currently, 5 Johnson-Cousins photometric filters (manufactured by Custom Scientific) are installed: U, B, V, Rc, and Ic.
The limiting magnitude of MEIA2 for an exposure time of 300 seconds (the maximum recommended) to reach S/N=100 for each one of the five Johnson-Cousins filters is (preliminary results):
The maximum S/N that can be reached with MEIA2 in a single exposure strongly depends on seeing and sky brightness. However, values with a S/N=750 should ensure an optimum S/N within linerarity regime of the CCD, except in cases with very good seeing.
The time required to change from one filter to another is 30 seconds.
Although currently only sky flats are being obtained, dome flat-field images are expected to be routinely obtained with a homogeneity better than 1% RMS using the following instrumental set-up:
- A white screen mounted on the inside of the dome.
- A full-spectrum light source that illuminates the screen.
- A control system to turn on the light when required by the observatory control system.
The TJO control system is designed to periodically run the acquisition of calibration images, including bias, darks and sky flat-fields:
- Bias. Bias frames are usually taken in multiples of 5 after the acquisition of sky flats or between two science sequences along 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.
- Sky flat-fields. Sky flats are routinely taken by pointing to several blank fields during twilight. When sky conditions do not allow the acquisition of sky flat exposures, images taken previously can also be provided to the observers. Flat-field images are routinely obtained with an illumination that provides a homogeneity better than 1% RMS.
Calibration exposures are expected to be used for an on-the-fly evaluation of the quality of the science images taken with MEIA2. All the calibration images are provided to the observers having science sequences during the night.