combining lots of flats:

- make one flat per night, per central wavelength

- Make list of A and B Flats

 
   - hselect > flat_a.lis 
     images = *.fits[0]
     fields = $I
     expr = obstype ?= 'FLAT' && grwlen == 810
    
   - hselect > flat_b.lis
     images = *.fits[0]
     fields = $I
     expr = obstype ?= 'FLAT' && grwlen == 815
    

- seems like max of flats is about 20.
if have more than that, can make two flats.

- emacs to get rid of [0]

- Note: There is some debate as to whether it is better to bias subtract and/or overscan subtract. Gemini-N biases have structure so it's important to bias subtract Gemini N data. Gemini S doesn't have this structure. There are differing opinions about overscan subtracting the flat in addition to bias subtracting. Some at the Gemini Help desk have strongly recommended overscan subtracting ONLY for Gemini S data, and doing both bias and overscan subtracting for Gemini N, whereas others recommend against using overscan subtracting at all because of light contamination, which is
speculated to affect brighter targets more than fainter objects. As of now, there is no concensus on this subject, but we've chosen to do both.

- For N&S reductions, make sure you fit the overscan regions interactively to fit the middle region. (Use s, s, f to change the region)

- When prompted: fitline = 200-300 for standard reductions, 1000-1300 for N&S
order for each chip: 1,1,6-9 for GS; 1,3,6-9 for GN

GSFLAT parameters:

inflats =          @flat_a.lis  Input flatfields
specflat=               flat_a  Output normalized flat (MEF)
fl_answe=                  yes  Continue interactive fitting?
(fl_slit=                   no) Correct output for Illumination/Slit-Function
(slitfun=                     ) Slit Function (MEF output of gsslitfunc)
(fl_keep=                   no) Keep imcombined flat?
(combfla=                     ) Filename for imcombined flat
(fl_over=                  yes) Subtract overscan level
(fl_trim=                  yes) Trim off overscan region
(fl_bias=                  yes) Subtract bias image
(fl_dark=                   no) Subtract (scaled) dark image
(fl_fixp=                  yes) Interpolate across chip gaps
(fl_vard=                   no) Create variance and data quality frames
(bias   = gS20070913S0213_bias.fits) Bias image
(dark   =                     ) Dark image

(fl_inte=                  yes) Fit response interactively?
(fl_dete=                  yes) Fit response detector by detector rather than sli
(functio=              spline3) Fitting function for response
(order  =                    1) Order of fitting function, minimum value=1
(low_rej=                   3.) Low rejection in sigma of response fit
(high_re=                   3.) High rejection in sigma of response fit

(ovs_fli=		   yes) Interactive overscan fitting?