Large trace shifts

[julienguy]

Code to compute large trace shifts based on detection and match of spots in CCD image from an arc lamp exposure.
Example (based on data from the SM10 collimator mirror exchange):

cp $DESI_SPECTRO_REDUX/daily/calibnight/20220612/psfnight-r1-20220612.fits .
desi_preproc -i $DESI_SPECTRO_DATA/20220612/00139415/desi-00139415.fits.fz  --cam r1 preproc-r1-00139415.fits
desi_preproc -i $DESI_SPECTRO_DATA/20220613/00139516/desi-00139516.fits.fz  --cam r1 preproc-r1-00139516.fits

desi_compute_large_trace_shifts --ref-image preproc-r1-00139415.fits --image preproc-r1-00139516.fits  --ref-psf psfnight-r1-20220612.fits  -o psf-r1-first.fits --plot

# INFO:large_trace_shifts.py:87:main: Refined delta_x = 2.93 delta_y = -2.75 rms dist = 0.08

desi_compute_trace_shifts --image preproc-r1-00139516.fits --psf psf-r1-first.fits --arc-lamps --outpsf psf-r1-refined.fits

# INFO:trace_shifts.py:201:fit_trace_shifts: Subtract median(dy)=-0.029203938233156583
# INFO:trace_shifts.py:242:fit_trace_shifts: polynomial fit of measured offsets with degx=(2,2) degy=(2,2)
# INFO:trace_shifts.py:247:fit_trace_shifts: dx dy error floor = 0.000 0.000 pixels
# INFO:trace_shifts.py:249:fit_trace_shifts: check fit uncertainties are ok on edge of CCD
# INFO:trace_shifts.py:264:fit_trace_shifts: max edge shift error = 0.007 pixels
# INFO:trace_shifts.py:317:fit_trace_shifts: central shifts dx = -0.025 +- 0.001 dy = -0.005 +- 0.001 

scatter plot of measured spots CCD image coordinates for the reference and current image

blue crosses: reference spots ; red dots: current spots; orange dots: current spots with offsets subtracted