Normally a Meade f/6.3
focal reducer is used on the telescope which gives a field of view of 18.4x18.4
arcmin. The actual focal ratio achieved depends on the overall focussing of
the main mirror and can vary from f/6.0 to f/6.3.
The CCD camera is an
Alta U47+ with Marconi (E2V) 47-10 back
illuminated chip. This is a 1024x1024 pixel format chip with 13 micron pixels and the imaging area is 13.3x13.3mm in size.
Quantum efficiencies better than 50% are achieved from 390-870nm and better than 90%
is achieved from 500-660nm. See the
Apogee U47+ specification
and the Marconi CCD47-10 specification, both as PDF
files for details.
With help from the Planetary
Shoemaker Grant, the CCD was upgraded by Apogee from an
to their Alta U47+ model during August and September 2005. The
AP47p is a parallel interface camera and downloads at full resolution (unbinned)
are slow, taking up to 50 seconds. The U47+ uses the same CCD chip but replaces
the electronics to use a USB 2.0 interface and full frame downloads now take
about 2 seconds. One consequence of this upgrade is that because less time is
lost waiting for images to download, about 45% more exposure time each night is
achieved, the equivalent of about half a magnitude. See the monthly observing
statistics here to see the jump in efficiency
(column H) between 2005 Aug and 2005 Sep when the U47+ came into operation.
The CCD camera is generally operated in two binning
modes and with the focal reducer gives:
- 1.1 arcsec per pixel resolution
and image downloads in about 2 seconds unbinned (1x1)
- 2.2 arcsec per pixel resolution,
image downloads in about 1 second binned 2x2.
There is some vignetting at f/6.3, a flat field shows
the effect here
which results in a fully exposed circular area of about 18 arcmin diameter.
The CCD shutter latency
(the time taken between an exposure being requested to start and the actual time
the shutter opens) is important to know when dealing with very fast moving Near
Earth Asteroids. A long delay or worse an unpredictable delay in opening the
shutter would introduce errors into the resulting astrometry.
version 4 introduced a function to measure CCD shutter latency directly and
this has been used at Great Shefford to measure the shutter latency of the
AP47p. The result was an average delay of 0.02 seconds ± 0.02 seconds, allowing
good precision for very fast moving objects. The U47+ has a different shutter,
but with similar characteristics to that in the AP47p. Although its latency has
not been measured yet it is expected to give similar results.