The current prime focus camera uses an irising Uniblitz shutter. This shutter has a combined open-close time of about 30 milli-seconds. This means that at exposure times less than 5 seconds the shutter will not expose the CCD uniformly.
The following figure shows the resulting `over-exposure' pattern, when you take a 0.5 second exposure with this shutter. It was made by flattening a 0.5 second dome flat exposure with a 10 second dome flat exposure. This flattened image was then normalised by the number of counts expected in the short exposure based on 1/20th of the counts seen in the long exposure. As you can see the `exposure map' shows the centre of the short exposure to be over-exposed by about 7%. Moreover, there is a pattern to this over-exposure which is related to the way the shutter "irises" open and closed.
The moral of the story is don't take short exposures with this shutter! In particular, don't take short flat-field exposures, and don't take short standard star exposures. The shortest exposure you can get away with is 5 seconds. This will produce a central over-exposure of <0.6%, which is tolerable for most programs. Of course, if you want high precision photometry, then you should suitably increase this minimum exposure limit.
Introduction The Telescope & Optics The Detectors
The Imaging Cameras An Imaging Cookbook The Data you Take Away
Exposure times OFFSET_RUN files CCD Windows Data Catalogs
On-line Reduction Filters Flat-fields Blank Fields Orientation Shutters
This Page maintained by : Chris Tinney (firstname.lastname@example.org)
This Page last updated: 6 Sep 1997, by Chris Tinney