RoboDIMM, an overview

The resolution of imaging instruments available to astronomers is rapidly increasing, and performance of large telescopes is highly dependent upon image quality. The larger a telescope's aperture, the greater the degradation of image quality will be, due to atmospheric effects.
RoboDIMM can help understanding the relationship between seeing quality and meteorological conditions. It permits the use of existing meteorogical data in the selection of a new observatory site. That data could also be used to forecast seeing quality on an operational basis, which will improve the efficiency of telescopes and other facilities.

The RoboDIMM is able to autonomously perform astroclimatic measurements throughout the night, where measuring seeing FWHM’s is the key activity.  The seeing is monitored to provide a baseline seeing measurement for quality control of all instruments and telescopes within an astronomical observatory. This will give a real-time assessment of image quality (such as focus optimisation) as well as data for long term remedial work (such as improving the seeing at a site).

The RoboDIMM-system is designed for use in remote and desolate locations. It has its own power supply based on photovoltaic solar panels. RoboDIMM keeps track of the available power and shuts itself down into power saving mode if no sufficient power is present. The RoboDIMM is designed to consume as little power as possible, in order to reduce the total of solar panels needed and to ensure that the system can survive a few days without sun.

RoboDIMM can also use an Automatic Weather Station to take notice of the weather conditions. Insecure situations which might cause damage to the RoboDIMM (e.g. rain) will be translated in an appropriate action, like closing the dome.

Key features

• Fully autonomous device, no operator needed. Very suitable for remote and desolate locations, especially when combined with the solar panels. The dome is opened/closed automatically.
• The RoboDIMM is delivered as a complete package, including dome, weather station (optional), solar panels (optional), telescope OTA, optical wedges, mount, etc.
• Built with very reliable and precise hardware, like the AstroPhysics GTO1200 mount.
• RoboDIMM has an API (application programming interface) for automating common tasks, customizing the user interface and interfacing with third-party software or custom made applications.

Diagrams

All equipment within the dome (computers, telescope mount, CCD cameras, etc) can be connected to a set of deep cycle batteries, which are continuously kept charged by the solar panels (using a dedicated battery charger), supervised by the central computer. The (optional) Automatic Weather Station is also self-supporting, in the sense that it has its own solar panels and batteries.

DIMM Theory

The RoboDIMM, like all classical DIMMs relies on the method of differential image motion between two or four sub-apertures of the telescope to calculate the seeing parameter r₀.

The DIMM method consists on measuring wavefront slope differences over two small pupils some distance apart. It's a differential method, making the DIMM very insensitive to common mode motions like wind shake and tracking errors. Measurements of very good seeing have been seen on several occasions under very windy conditions.
In practice, the incoming star light is split into two beams by positioning a set of optical prisms before the telescope. Hence, two images of the same star are produced, a few millimeters separated from each other. The dual star images obtained exhibits a relative motion in the image plane that represents the local wavefront tilts, which can be expressed in terms of an absolute seeing scale according to the (approximate) formula:

FWHM=0.976*(λ/r₀)

For a detailed explanation of the theories behind an differential image motion monitor, see the paper “The ESO differential image motion monitor”, by M. Sarazin and F.Roddier, Astron. Astrophys. 227, 294-300 (1990). Use the ADS abstract service to find that paper.

RoboDIMM technical details

For more technical details on the algorithms and techniques we use in our RoboDIMM, please click here.