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Masking the field of view!

18/04/2006

An astronomer sits down and decides that tomorrow he wants to eliminate all the light he is not interested in from the focal plane so that only the light he wants to work on remains. He designs a mask with a few well-placed slits bored through it, and leaves the specifications so that, using a machine specially set up for the purpose, an engineer will have the mask ready for the following night…The OSIRIS masks are not for fancy dress but rather for selecting light. To equip it to perform the tasks that will be demanded of it, this instrument has a quite exceptional Mask Unit.

The Mask Unit is the part of the instrument responsible for picking up the mask chosen by the astronomer and fixing it into the focal plane.

Manufactured at the Instituto de Astrofísica de Canarias (IAC), the Mask Unit ran into design problems early in its development: OSIRIS’ field of view is spherical and very large. The masks needed to be adapted to this field of view and had to be slightly curved. From a structural perspective this was a small challenge, but it was not the only one.

The Mask Unit is made up of two principal components: the store and the loader.

The store has enough room for 13 masks which can be of five different types: long slit; for fast photometry; a charge shuffling mask; a frame transfer mask (these four are made of a single unit) and finally a mask for multi-object spectroscopy (multi-slit, which consists of a layer of laminate on which the slits are cut and a frame onto which the laminate is mounted). This last type of mask will be specially designed for each project when requested by an astronomer, using programmes developed specifically for the purpose. Each mask will have a barcode so that a reader can locate and install it.

When the mask has been selected, the loader takes it from the store and positions it in the focal plane to within a margin of error of just a few microns. The loader has a brake, a decoder that displays the position of the mask, a brushless motor (to avoid any potential for interference with the electronics in the detector’s reader), a reductor (which changes the transmission relation like the gearbox in a vehicle) and an axle, which moves the mask.

Loaders are traditionally designed with more motors than this, but in this case, they have been replaced with springs for greater reliability. According to Lorenzo Peraza, the engineer at the Instituto de Astrofísica de Canarias (IAC) who designed this part of the instrument, "one of the engineering challenges posed by this type of mechanism is finding a compromise in the between giving it as long a life as possible and making maintenance easy."

The whole process of changing a mask will take 30 seconds, much less than in other instruments, and inserting a mask will take just 12 seconds. This is less time than it takes for information from the instrument's CCD detector to be read so that there will be no additional waiting time, allowing best use to be made of observation time.

What better way to understand all this than watching the video?

Natalia R. Zelman

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