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Given enough exposure time the sensitivity of an astronomical instrument is ultimately limited by systematic errors, and the dominant source of systematic errors for most optical/infrared instruments is imperfect sky subtraction. In turn the limiting factor for sky subtraction accuracy is frequently the accuracy of flat field calibration, making these calibrations critical to the overall performance of the instrument.
The Anglo-Australian Telescope’s fibre-fed spectrographs, and in particular the multi-object integral field spectrograph SAMI, are reaching sky subtraction systematic error limits and this has motivated an upgrade to the calibration systems. SAMI and its successor HECTOR are calling for sky subtraction accuracies of at least 0.25%, with a goal of 0.06%, an improvement of 4-17 times.
Flat field calibrations can use dark sky, twilight sky or an illuminated screen (‘dome flats’). For multi-object spectrographs such as SAMI recalibration is required for each set of targets. This makes twilight flats impractical as it is impossible to guarantee the availability of clear twilight sky for every configuration. The dark night sky is the ideal calibrator, but the long integration times required result in onerous overheads. What is needed is a dome flat field system accurate enough to replace dark sky flats. To achieve this we have replaced both the existing screen and its illumination system.
The effective throughput of optical fibres feeding an instrument vary slightly as their paths change, so high accuracy demands that calibration be done with the telescope in the same position as the science observations. We have applied two new screens to the dome windscreen, either side of the aperture, so that it is possible to move a screen in front of the telescope while in any position.
There are two distinct purposes for flat fielding: photometric calibration and sky subtraction. For an ideal telescope these are equivalent but the existence of stray light creates subtle differences, and this has implications for design of the screen. When the primary purpose is sky subtraction the highest possible accuracy will be achieved with a screen that illuminates the telescope from all the directions that the night sky does. Consequently our screens match the size, shape and relative position of the windscreen aperture. The screens are implemented as Avian D diffuse reflectance coating applied to the dome windscreen itself. Avian D is highly Lambertian, has high reflectance and is durable enough for the observatory environment.
The screens must be illuminated uniformly, in terms of spatial variations of both total intensity and spectral energy distribution (SED). We use an array of lamps around the end of the telescope tube. By using LEDs we are able to customise the SED and obtain a signal to noise ratio that is more consistent across wavelengths than is possible with traditional quartz tungsten halogen flat field lamps.
We present the design of the new flat field calibration system, explain the main design decisions and discuss results from commissioning. These include comparisons between dome and dark sky flats, and measurements of the sky subtraction accuracy.
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