Observations in July 2001

Observations were undertaken at the NOT on the nights of 2001 July 25--26 using a CCD65 detector. On the first night of observing the camera showed very poor charge transfer efficiency. On the second night the CCD was heated electrically inside the liquid nitrogen dewar and the charge transfer efficiency appeared to improve. The results presented in this thesis are restricted to data taken on the second night of observing. Table 5.2 lists the targets observed on the second night. Some runs where the target saturated the detector or drifted out of the field have been excluded. All the runs on this night were taken through the HiRac I-band filter with the bandpass shown in Figure 5.5. Some of the short exposure images were affected by one or more white spots which may have resulted from trace amounts of radioactive material in the instrument hardware.

Table 5.2: Observations on 2001 July 6. Some runs where the detector was saturated or the target drifted out of the field have been excluded. For some of the targets several runs were taken with different image sizes. All the observations were taken using the HiRac I filter centred at $810$ $nm$. The bandpass of this filter is plotted in Figure 5.5.

Target	Object type	Number of exposures	Image size (pixels)
CCDM J17339+1747	Binary star	14000	256 x 256 and 160 x 160
HD161796	Planetary nebula progenitor	18000	160 x 160
NGC 6543	Planetary nebula progenitor	2000	512 x 64
aM13 field 1	Globular cluster	3000	624 x 288
aM13 field 2	Globular cluster	6000	624 x 288
M56	Globular cluster	3000	624 x 288
M15 field 1	Globular cluster	2000	624 x 288
M15 field 2	Globular cluster	4000	624 x 288
HD 203991	Binary star	12000	128 x 128
61 Cygni	Binary star	15000	624 x 128 and 624 x 64
8 Lacertae	Binary star	12000	128 x 128

^aCCD electrical heating disconnected.

For all the observations except those of 61 Cygnii, the multiplication register was clocked with a high voltage providing a substantial gain in the signal level.

In order to investigate the size of the isoplanatic patch which prevails at the times of the best exposures, observations of binary stars with a range of separations were undertaken. As the dimensions of the CCD65 detector corresponded to $14\times11$ $as$ on the sky, two of these binaries did not fit within the CCD field of view. In order to observe these targets, an optical arrangement was employed whereby two fields on the sky were superimposed on the detector (as discussed in Figure 5.1). The centres of the two fields which could be superimposed were separated by approximately $21$ $as$, allowing the $22$ $as$ binary 8 Lacertae to be observed in a $128\times128$ pixel sub-array of the CCD. A long $624$ pixel strip across the CCD was used for observations of the $30$ $as$ binary 61 Cygnii. All the observations were taken within two hours of each other and at relatively small zenith angles, so as to minimise the changes in seeing from one observation to the next.

A number of relatively crowded fields in globular clusters were observed in order to determine the limiting magnitude of reference star for exposure selection and image re-centring. Different stars in the field of view were picked as the reference, in order to find the faintest star for which high resolution images could be obtained. The fields chosen had previously been surveyed by the Hubble Space Telescope, and stellar magnitudes were available for some of the stars. These fields were also useful for testing the astrometric performance, as the relative positions of a large number of stars could be compared between different runs.