Provisional strategy of the observing plan for XID INT/WFC 2002 DEC run

                                            W. Yuan 

General: the emphasis is on completing fields properly and
         doing a full (i r g ?) set of filters for new fields.
         
Folllowing are a proposed general guildline of selecting fields
for observations in deceasing priotities.  

A)  Normal case (good observing condition, eg., weather, seeing )
If the seeing is OK for observing (say, <2.0), 

1.  Fields need data in just one extra band to complete [i,r,g]-band sets
    Priority: higher priority giving to fields with no data at all 
      than bad-seeing ones

2.  Fields with bad photometry in i, r, g bands
    Priority: in the order of bad zero-point error (worst highest priority)
    At first we aim at repeating ZPTERR>0.1 fields and may go down to 
    small values if have time (upto 0.05, perhaps).

3.  Fields with bad seeing in [r i g] bands
    Priority: [seeing >2.0] - high;  [seeing >1.6] - low

4.  Fields need data in just one extra band to complete all 5-band sets
    Priority:  same priority policy as in 1.

5.  Fields with bad photometry in bands U,z
    Priority: in the order of bad zero-point error (worst highest priority)
    At first we aim at repeating ZPTERR>0.1 fields and may go down to 
    small values if have time (upto 0.05, perhaps).  

6.  New fields
    Priority: as given at selection

(1) Note that the lists I put on the web are not exclusive.
    Once observed, a field will be removed from other lists and
    the actual total number of fields is thus less the sum of the 
    fields in all the list.  
(2) Note for the first several nights:
     Considering the effect on the Moon in the later part of the night ,
     We will mainly do i-band and/or z-band, depending on the sky brightness
     Detailed switching criterion to be decided.
     The least priority fields pools are fields without i-band data
     and fields without z-band data
(3) For repeating fields with poor seeing, only do this when the seeing is
    much better than the ones in existing data at observation, of course.

B)  In case of bad seeing  (>2.0)
 
1.  New XMM fields 
     (low priority ones)

2.  Existing fields without data in one particular band
    Priority: i r g z U

3.  What else?