Equipment (2002 - 2005 July)

Equipment in use 2002 - 2005

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

Important: See details of the equipment upgrade made during June 2005 here.

Inside is a 12" (0.3m) f/10 Meade LX200 GPS with Meade Ultra High Transmission Coatings (UHTC) mounted on a BCF wedge and pier.

For good automatic sidereal rate tracking it is essential to 'train' the drive using the Meade Periodic Error Correction (or PEC) facility.

Normally an f/6.3 Meade focal reducer is used on the telescope which gives a field of view about 25x25 arcmin.

The CCD camera is an Apogee AP47p with Marconi 47-10 back illuminated chip. This is a 1024x1024 pixel format chip with 13 micron pixel size and the imaging area is 13.3x13.3mm in size. The AP47p is a parallel interface camera and downloads at full resolution (unbinned) are rather slow.

The CCD camera is generally operated in two binning modes and with the focal reducer gives:

1.5 arcsec per pixel resolution and image downloads in about 43 seconds unbinned (1x1)
3.0 arcsec per pixel resolution, image downloads in about 13 seconds 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 25 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 pc in the dome uses a wireless LAN connection to share an ADSL 2Mbps internet connection from the house. Broadband came to Great Shefford in June 2004, 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 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, therefore unfortunately little opportunity to operate remotely from indoors. Warm clothing is required in winter!

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.