Last updated 12 Jul 2024

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The observatory building consists of a green Sky Domes 8 foot diameter fibreglass dome set on a 2.5 ton concrete base.

Inside is a 16" (0.4m) f/10 Meade LX200 GPS with Meade Ultra High Transmission Coatings (UHTC) mounted on a custom built wedge and pier, installed during June 2005, replacing a 12" Meade LX200GPS that had been operational since May 2002 (see the Equipment 2002-2005 page for details).

The 16" Meade viewed from the south, showing wires from two sets of dew bands leading to the dew band controller mounted using Velcro on the right fork.

Here the telescope, equatorial wedge and pier are viewed from the east. The wedge and pier were obtained with assistance from the British Astronomical Association via the 2005 Ridley Grant.

The dome is relatively small for the 16" telescope and a standard dew shield would not fit on the end of the telescope without hitting the inside of the dome. The shield in use (the black left hand end of the telescope) was made from two dew shields for 8" Meade LX200's supplied by Telescope House, cut to size and fixed together with Velcro.

The blue casing of the U47+ CCD camera is shown attached to the end of the 16" telescope. Leads can be seen powering the telescope fan to help cool the primary mirror, a white USB lead for image downloading from the CCD, a black power lead to the CCD and a black curly lead behind the CCD to allow PC control of the telescopes electronic focusser. Four fans are built into the body of the CCD to help the Peltier thermoelectric cooling unit achieve its maximum 55C cooling below ambient air temperature. 

The inherent accuracy of the drive has been found to be very good, 'training' the drive with the inbuilt periodic error correction (PEC) is not as important as was found with the previous telescope (a Meade 12" LX200), where it was essential to 'train' the drive to allow the telescope to track accurately on the stars. 

Apogee U47+ CCD Camera
The telescope Mirror cell, focal reducer, focussing unit and the U47+ CCD
(move the mouse over the picture to identify components)

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 Apogee 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 Society's 2005 Shoemaker Grant, the CCD was upgraded by Apogee from an AP47p 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.

Maxim 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, see the Check system timing accuracy (using astrometry of GNSS satellites) page for recent determinations of overall system latency.

The pc in the dome is connected to the local area network (LAN) using gigabit ethernet and shares a fibre broadband 300 Mbps internet connection in the house. Broadband came to Great Shefford in June 2004 with an 8 Mbps ADSL connection, replacing a 24x7 56k dial-up connection and in the process increasing the precision of timekeeping, important when following fast moving near earth objects. As an indication of the difference, here is the history log from Dimension4 showing the time corrections made to the observatory pc, including the last two weeks before 8 Mbps ADSL was installed on 23 June 2004. Large corrections of up to ~20 seconds were occasionally being made with the dial-up connection, but the central thick line during that time shows the normal adjustment to the clock was of the order of 1 second. After conversion to ADSL on just a few occasions correction has been made of the order of 2 seconds but the normal adjustment (the very thin line to the right hand side) is of the order of 0.01 seconds.
The dome is not automated or motorised and so requires frequent manual repositioning through the night.
Problems above +68 declination
Prior to the telescope upgrade in June 2005, the short length of the 12" LX200 forks and the size of the AP47p CCD body meant that the telescope could not be pointed north of declination 68, even with the use of a diagonal. However, the introduction of the 16" LX200 with substantially bigger fork mounting has allowed full access to the polar region of the sky.

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