Previous: Field of view
Up: Specification
Previous Page: Field of view
Next Page: The nasmyth A&G Unit

Image quality

Spot diagrams for the theoretical design of the Nasymth image derotator are reproduced in La Palma Technical Note 9.

The diagrams show some transverse chromatic aberration, giving a slightly larger image scale at longer wavelengths. If the images from 3300 to 10000Å are superimposed, these will make a line of about 0.70.2 arcsec in a radial direction. This has two effects:

• The scale along the slit will vary very slightly with wavelength. The variation in scale over the entire wavelength range is of order 1 per cent. This may be relevant for some longslit spectroscopy.

• Offset guide stars may appear to be elongated when viewed in white light.