For a fixed satellite pointing the field of view of EIS is determined by two things: the size of the CCD and the range of movement of the mirror fine motion mechanism.
CCD size governs the field of view in Solar-Y. The CCD is 1024 pixels high, and one pixel corresponds to 1 arcsec. Up until April 2008 only the central 512 pixel band of the CCD was used for science observations and thus the field of view in Solar-Y was 512 arcsec. Towards the end of April 2008, a software update was performed which enables the full 1024 pixel height of the CCD to be used for science observations with the restriction that no more than a 512 pixel region can be observed in a single exposure. E.g., the EIS Chief Observer can choose to observe a 512 pixel region at the bottom of the CCD and then adjust the pointing to observe a 512 pixel region at the top of the CCD, but the complete 1024 pixels can not be observed in a single exposure.
The field of view in Solar-X is governed by the range of motion of the mirror's fine motion mechanism. When using the raster design software (eis_mk_raster) you are allowed to specify up to 589 steps of 1 arcsec when creating a raster. The field of view in Solar-X then depends on the size of the slit being used. E.g., with the 40" slit you have to take account of the field of view of the actual slit, giving an additional 40" field of view in this case. The fields of view in Solar-X allowed by the software are thus:
1" slit: 590"
2" slit: 592"
40" slit: 630"
266" slit: 856"
A word of caution, though. Some large rasters performed with the 40" slit have demonstrated that the fine motion mechanism seems to 'stick' at the ends of the range thus EIS is not actually accessing the extreme ends of the field of view. This effect is being investigated with the likely result being that the allowed range of the mechanism will be reduced by around 50".
Peter Young
It's probably worth clarifying that the although the "sticking" of the fine mirror rastering was discovered using the 40 arcsec slot, the mechanism is common to all observations, and thus affects all slit or slot choices.
--David R Williams, 02-May-2008