FanCam
Near Infrared Linear Polarimetry

IRAF txdump output from 5 exposures in J with each of P1 and P2 for Mar 23, 2006 FanCam observations of polarized standard star HD155528

V band polarization (according to Clemens & Tapia 1990):
p = 4.85 ± 0.05%
 = 93.7 ± 0.3°

sample text file pdat with column labels added:

  xc        yc      sum      flux     mag    merr   image

857.840  790.057  439402.1  422242.9  9.189  0.001  bfc473
925.368  790.216  468634.6  451380.3  9.116  0.001  bfc473
839.130  787.636  418081.8  401303.8  9.244  0.001  bfc474
906.802  787.791  442613.5  426649.3  9.177  0.001  bfc474
809.295  820.834  404801.6  387440.7  9.282  0.001  bfc475
877.077  820.988  429632.5  412037.9  9.215  0.001  bfc475
784.916  821.643  379368.2  358172.2  9.367  0.002  bfc476
852.784  821.826  401264.5  379392.1  9.305  0.001  bfc476
761.873  821.189  387843.1  368072.3  9.338  0.001  bfc477
829.809  821.365  411024.8  390564.5  9.273  0.001  bfc477
763.664  728.187  485164.9  476742.2  9.057  0.001  bfc417
811.372  680.903  481511.3  473458.4  9.064  0.001  bfc417
744.581  721.934  482169.4  474606.5  9.062  0.001  bfc418
792.267  674.647  477410.  471746.9  9.068  0.001  bfc418
727.748  719.520  485568.  476652.3  9.057  0.001  bfc419
775.428  672.269  482965.5  474520.7  9.062  0.001  bfc419
699.044  717.849  481332.9  473132.2  9.065  0.001  bfc420
746.743  670.658  479978.6  472305.5  9.067  0.001  bfc420
685.040  718.106  481202.2  472232.8  9.067  0.001  bfc421
732.646  670.910  477428.9  470224.2  9.072  0.001  bfc421

sample IDL program fred5.pro to reduce pdat:

dat1={pdat0,xc:0.,yc:0.,sm:0.,flx:0.,mag:0.,merr:0.,im:""}

fx0=fltarr(5)
qme0=fltarr(5)
fx90=fltarr(5)
qme90=fltarr(5)
fx45=fltarr(5)
ume45=fltarr(5)
fx135=fltarr(5)
ume135=fltarr(5)
q=fltarr(5)
qme=fltarr(5)
u=fltarr(5)
ume=fltarr(5)

openr,runit,"pdat",/get_lun
for i=0,4 do begin
 readf,runit,dat1
 fx0[i]=dat1.flx
 qme0[i]=dat1.merr
 readf,runit,dat1
 fx90[i]=dat1.flx
 qme90[i]=dat1.merr
endfor
for i=0,4 do begin
 readf,runit,dat1
 fx135[i]=dat1.flx
 ume135[i]=dat1.merr
 readf,runit,dat1
 fx45[i]=dat1.flx
 ume45[i]=dat1.merr
endfor
free_lun,runit

for i=0,4 do begin
q[i]=(fx0[i]-fx90[i])/(fx0[i]+fx90[i])
qme[i]=(qme0[i]+qme90[i])/2.
endfor

for i=0,4 do begin
u[i]=(fx45[i]-fx135[i])/(fx45[i]+fx135[i])
ume[i]=(ume45[i]+ume135[i])/2.
endfor

nq=size(q,/n_elements)
nu=size(u,/n_elements)

q=q(sort(q))
u=u(sort(u))

qm=mean(q)
um=mean(u)
dq=stddev(q)
du=stddev(u)
qem=mean(qme)
uem=mean(ume)
pme=(qem+uem)/2.

print
print,format='("q vals =",4(" ",f8.5,",")," ",f8.5)',q
print,format='("u vals =",4(" ",f8.5,",")," ",f8.5)',u
print
print,format='("q = ",f8.5," +/- ",f7.5)',qm,dq
print,format='("u = ",f8.5," +/- ",f7.5)',um,du
print,format='("qmerr = ",f7.5)',qem
print,format='("umerr = ",f7.5)',uem
print

;instrumental corrections

qi=0.
ui=0.
pacorr=0.

;correct q and u

qm=qm-qi
um=um-ui

;find the polarization

p=sqrt(qm^2.+um^2.)
dp=(dq+du)/2.

;find the position angle

if ((qm eq 0.) and (um ge 0.)) then th=!dpi/4 else $
if ((qm eq 0.) and (um lt 0.)) then th=3*!dpi/4 else $
th=0.5*atan(um/qm)
if (qm lt 0.) then th=th+!dpi/2
if (th lt 0.) then th=th+!dpi
th=th*!radeg

if (p eq 0.) then dth=180.d else dth=!radeg/2*dp/p
if (dth gt 180.) then dth=180.d

;correct position angle

th=th+pacorr
if (th ge 180.) then th=th-180
if (th lt 0.) then th=th+180

;reconstruct q and u after position angle correction

qm=p*cos(2*th/!radeg)
um=p*sin(2*th/!radeg)

print,format='("q(%) = ",f6.2," +/- ",f5.2,"  (",i1,")")',100.*qm,100.*dq,nq
print,format='("u(%) = ",f6.2," +/- ",f5.2,"  (",i1,")")',100.*um,100.*du,nu
print,format='("p(%) = ",f5.2," +/- ",f5.2)',100.*p,100.*dp
print,format='("pmerr(%) = ",f5.2)',100.*pme
print,format='("th(deg) = ",f5.1," +/- ",f5.1)',th,dth
print

end

q vals = -0.03335, -0.03077, -0.03061, -0.02965, -0.02877
u vals = -0.00346, -0.00302, -0.00224, -0.00213, -0.00087

q = -0.03063 +/- 0.00172
u = -0.00234 +/- 0.00099
qmerr = 0.00110
umerr = 0.00100

q(%) =  -3.06 +/-  0.17  (5)
u(%) =  -0.23 +/-  0.10  (5)
p(%) =  3.07 +/-  0.14
pmerr(%) =  0.11
th(deg) =  92.2 +/-   1.3