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The optimum focus is a function of wavelength since LDSS-2 has a dioptric optical system in which the lenses introduce longitudinal chromatic abberation that cannot be perfectly corrected. Also the camera focal surface is not quite planar and there is some chromatic distortion which increases with off-axis angle, so there will be some slight degradation of focus as you go off axis. It is therefore necessary to determine the best focus for each filter that you wish to use. This will give you the best focus for direct imaging in the appropriate band and will also indicate the focus value to choose in spectroscopic mode. For example, if you want your spectra to have the best (spatial) focus at 6000Å, you would be wise to choose the focus appropriate for the V filter which has a central wavelength around 5600Å. For this reason, it would be wise to determine the optimum focus for the B, V and R broad band filters even if you do not plan to use all of these for direct imaging.
If LDSS is focussed for best imaging at V, then the image spread at B due to longitudinal colour terms is 125m, and at R it is 30m. The spatial resolution of the instrument when used in spectroscopic mode over a wide wavelength range is severely wavelength dependent.
The camera focus is adjusted using the command LFOCUS on the instrument control system. A Hartmann test with a given filter selected, should produce Hartmann shifts close to zero, but these numbers will never all be zero because of the residual curvature of the camera focal surface. The observer has to choose whether to optimize the focus for the on-axis position or for the whole field. An example of satisfactory Hartmann shifts is given in Table 9(b). Each shift has an associated uncertainty of about .
The relationship between Hartmann shift and the units used by LFOCUS is shown in Table 11. Thus a Hartmann shift of +1.5 can be corrected by reducing the focus by about 160 units.
Examples of focus values obtained for the broad band filters are shown in Table 12. The offsets can be loaded into the instrument control system using the LFILTER_SETUP command so that they will be automatically applied when the appropriate filter is selected using the command LFILTER filter.
From all these data, it can be seen that if Hartmann shifts are more positive than +1.2 units or more negative than -3.5 units when LFOCUS is set at 380 for the V filter, then there is a risk that the optimum focus will be out of range of the camera focus drive for some passbands.