
adding SDSS object tables:

* autodetect the SDSS files from the header?
* do I need to read multiple files for a single data set?
* how are we going to determine photcodes?
  * do we need to set the photcode on a per-object basis in LoadData or equivalent?

* some re-org might be needed for LoadStars


** fields to keep:

name          sdss 
 X            'objc_colc' (or colc)
 Y            'objc_rowc' (or rowc)
 dX           'objc_colcErr' (or colcErr)
 dY           'objc_rowcErr' (or rowcErr)
 R            ra  + offset Ra (get cos des right)
 D            dec + offset Dec
 dR           (calculate from dX * plate scale?)
 dD           (calculate from dX * plate scale?)
 uR           (from 'propermotion' and 'propermotionangle')
 uD           (from 'propermotion' and 'propermotionangle')
 duR          ?
 duD          ?
 P            X
 dP           X
 M            psfCounts (is already a mag?)
 dM           psfCountsErr (is already a mag?)
 sky          sky (convert to flux?)
 dsky         skyErr (convert to flux?)
 fx           ?
 fy           ?
 df           ?
 Mgal         X
 Map          X
 Mpeak        X
 detID        X
 found        -1
 t            MJD_U + Y*rate
 dt           fixed? NY*rate
 psfChisq     
 crNsigma     
 extNsigma    
 psfQual      
 Mcal         
 airmass      
 az            
 code         
 nFrames      
 flags        
 dophot       
 dummy        

we can use these header fields to get the exp times, and I suppose alt, az, airmass, etc:
NODE    =           XXX.XXXXXX / Ascending node of great circle scanned (deg)
INCL    =           XXX.XXXXXX / Inclincation of great circle scanned (deg)
C_OBS   =            XXXXX.XXX / CCD clock rate (usec/unbinned-row)
TRACKING=            XXXXX.XXX / Tracking rate (arcsec/TAI-sec)
