Offset Keywords

Most of the keywords involved in offsetting are illustrated in See Telescope Offset Conversions (ignoring on-sky scaling issues) . The keywords that actually initiate an offset (the *OFF keywords do not take immediate effect) are not shown. These are:

• REL2CURR ("relative to current"): set to 1 or TRUE to move relative to the current position (so, for example, an offset of (0,0) will do nothing; this is the most common type of offset)
• REL2BASE ("relative to base"): set to 1 or TRUE to move relative to the base position (so, for example, an offset of (0,0) will return to the base position; this type of offset is less common)
• MARK ("mark base"): set to 1 or TRUE to mark the current position as the base position
• GOTOBASE ("go to base"): set to 1 or TRUE to return to the base position (not available in the current Keck I keyword library; on Keck I set RAOFF=0, DECOFF=0, REL2BASE=TRUE)

Further keywords can be read to determine the current telescope position:

• RABASE and DECBASE are the telescope base position
• RA and DEC are the telescope current position
• RAOFF and DECOFF (when read) are the "current - base" offset

Refer to See KSD/46, "Keck I DCS Keyword Manual" and See KSD/106, "Keck II DCS Keyword Manual" for full details.