Reducing ALFOSC dual amplifer data using iraf/mscred-package

NOTE: This is for ALFOSC data taken before 26.06.2011

Basic reductiuons of the ALFOSC dual amplifier data. All the reduction steps are made using tasks from iraf/mscred. Please note that I have renamed the output processed image for calrity. Usually it is not necessary. Using iraf the main difference in reducing the dual amplifier data in comparison to a single amplifier are that (1) the data reduction package is iraf/mscred (mosaic reduction package) instead of noao/imred/ccdred and (2) after the basic reductions the amplifers are mergerd.

The data

The images were taken on 22.2 2006 unless stated

Science frames:

Data: nine i-band (filter 12) 240 seconds integrations
pb220140-149

Calibration data:

-ten bias frames (19.2.2006)
-five dome flats (19.2.2006)
to create the fringe frame:
-four 900 second i-band (19.2.2006)
-eleven 240 second i-band (dithered images)

Data reduction

Below is the listing of the mscred/ccdproc parameter file:
lpar ccdproc

Raw image:

"Raw" frame with the cuts of z1=1110.6 z2=1227.5, which is about +-5% of the median sky.

Image pixel statistics of the "raw frame":
im1 (left) [920:1020,5:2048] (white line)
im2 (right) [4:104,5:2048] (red line)
Note that the last column of im1 is 1024

trimmed & overscan subtracted images

plot of the raw_ima 920-1020, 1029-1129
raw_ima_colplot.eps

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mscred>ccdpr raw_image out=oversc_sub trim+ overscan+
Below is the the overscan subtracted frame. The cuts are z1=865 z2=1015 which is about +-8% of the median sky.

Image pixel statistics of the overscan & trimmed image:
im1 (left) [920:1020,5:2048] (white line)
im2 (right) [4:104,5:2048] (red line)

Creating the master bias frame:

Master bias (from ten bias frames):
mscred> zeroc @bi.lst out=biaspb19 nl=2 nh=2
The "master" bias frame with the cuts z1=-10 z2=+10

Image pixel statistics of the "master bias" frame:
im1 (left) [920:1020,5:2048] (white line)
im2 (right) [4:104,5:2048] (red line)

Master bias correction:

mscred>ccdpr oversc_sub out=oversc_sub_zero zerocor+ zero=biaspb19
Master bias subtracted image with the the cuts of z1=865 z2=1015, which is about +-8% of the median sky.

plot of the oversc_sub_zeroc image 920-1020, 1029-1129

Image pixel statistics after master bias subtraction.:
im1 (left) [920:1020,5:2048] (white line)
im2 (right) [4:104,5:2048] (red line)

Dome flat correction:

mscred>ccdpr oversc_sub_zero out=oversc_sub_zero_flat flatc+ flat=iflatpb19
Image after dome flat correction with the cuts: z1=904 z2=960, which is about +-3% of the median sky.

oversc_sub_zeroc_flat.ps

Image pixel statistics after dome flat correction:
im1 (left) [920:1020,5:2048] (white line)
im2 (right) [4:104,5:2048] (red line)

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i-band fringe correction

i)select long(ish) i-band integrations "i2.lst"

ii) ccreate objmask

mscred>objmask @i2.lst @i2.mask omty=numbers skys=@i2.sky
The contents of files:
i2.lst: pb190230 pb190231 etc.
i2.mask: pb190230om pb190231om etc.
i2.sky: pb190230sky pb190231sky etc.

iii) combine masked images-> fringe image

mscred>combine @i2.lst ifringev1 masktype=!objmask scale=mode

iv) subtract backgroung from the fringe image
mscred> mscmed ifringev1 ifringev1med 428 428
mscred>mscarith ifringev1 - ifringev1med pb22fringe_i
mscred>rmfringe oversc_sub_zeroc_flat out=oversc_sub_zeroc_flat_fringe pb22fringe_i429 objmask back=sky

Fringe corrected image. The cuts are z1=899.9 z2=955.5, which is about +-3% of the median sky.

oversc_sub_zeroc_flat_fringe.ps

Image pixel statistics after fringe correction:
im1 (left) [920:1020,5:2048] (white line)
im2 (right) [4:104,5:2048] (red line)

Sky flat correction

v)create superskyflat mscred> combine @i2f.lst Skyflatpb22i masktype=!objmask scale=mode

vi) superskyflat correction:
mscred>ccdpr oversc_sub_zeroc_flat_fringe oversc_sub_zeroc_flat_fringe_sky sflatc+ sflat=Skyflatpb22i

The final image with the cuts of z1=916.05 and z2=943.9, which is about +-1.5% of the median sky.

Image pixel statistics after sky-flat correction:
im1 (left) [920:1020,5:2048] (white line)
im2 (right) [4:104,5:2048] (red line)

Final image with one extension

merge amplifiers:
mscred>ccdpr oversc_sub_zeroc_flat_fringe_sky out=final_image merge+

Last modified: April 19 2023