2 The telescope system

The telescope is a Grubb Parsons refractor with an objective of 178 mm diameter and focal length 2.66 m. After initially being located at Brorfelde, Denmark, the telescope was moved in 1984 to La Palma in the Canary Islands to take advantage of the better observing conditions there.

Originally, the detector used was a scanning-slit photoelectric micrometer, but was replaced in 1998 by a CCD camera operating in a drift scan mode. This was a major change in the method of observing, since relative astrometry with respect to a dense grid of standards within the same data frames would be used rather than absolute astrometry and offsetting the telescope with respect to the standards.

The two significant advantages of a CCD system are that fainter stars can be observed and that many stars can be observed simultaneously. This has increased the number of stars that can be observed in a night by a factor of more than 100. However, there is a disadvantage with the new system in that close to the celestial pole the images become distorted. This is discussed further in Sect. 4.4. Although this restricts normal observing to declinations South of $+60\hbox{$^\circ$ }$, it is more than compensated by the amount of high-quality data that it produces.

More recently (April 1999) the CCD system was upgraded to a larger detector (Kodak ${\rm 2~k\times2~k}$ with 9 $\mu$m pixels) and a filter equivalent to the Sloan Digital Sky Survey r' passband was fitted.

The increase in the CCD size has a number of advantages which include: completing the survey faster due to a larger field of view; providing a deeper survey due to longer exposures; and improvements in the calibration due to increased frame sizes. Also, with the fitting of the r' filter, the project is now also able to provide photometry on a commonly-used photometric system.

The new CCD pixel size corresponds to 0.7'' and considering that the median seeing at the telescope is just under 3'' the images are well sampled. It should be noted that the site seeing is much better than this.

The CCD can be cooled to -65  $\hbox{$^\circ$ }$C by a Peltier cooler. Currently, the chip is cooled to -30  $\hbox{$^\circ$ }$C since this reduces the effect of a charge transfer efficiency (CTE) problem with the chip (see Appendix 8). The higher operating temperature does not affect our magnitude limit.

These improvements with the CCD and the new filter have increased the number of stars observed by about 4 times as many stars per night than with the old CCD system. The current magnitude limit is $r'_{\rm CMT}=17$ and between 100 000 and 200 000 stars a night are observed. On a typical night, more than 50 square degrees are covered.

More details about the telescope can be found in Helmer & Morrison (1985) and about the recent upgrades in Evans (2001b). A summary is given in Table 1.

 

 

Table 1: A summary of the telescope and camera parameters for the current configuration.

Telescope:	Located on La Palma, Canary Islands
	178 mm objective
	2.66 m focal length
Camera:	CCD chip - Kodak (KAF-4202 Grade:C1)
	$2060\times2048$ pixels
	Pixel size 9 $\mu$m (0.7'')
	CUO built
	Operating temperature -30 $~\hbox{$^\circ$ }$C
System:	Automatic and remotely controlled
	Drift scans
	(generates $\sim$3 Gb of data per night)
	Data automatically parameterized
	(reduced to 6-7 Mb)
	Daily reductions take about 30 min.
	100 000-200 000 stars observed per night
	Calibrated with respect to Tycho 2
	Magnitude limit (Sloan) $r'_{\rm CMT}=17$