| First step | Bias | Trim Section & Overscan Region | Flats | Flat-Field division | Centering of the images | Combine operation |
Inside Iraf
you can create it typing: files filenames
> output filename
ex --- cl> files jc04* > lisjc
Inside Iraf
you can create it also typing: hselect filenames $I "field=value"
> output filename
ex --- cl> hselect jc09* $I "exptime=0" > listzero
NOTE if something doesn't work pay attention to the spaces
Inside Iraf you can use the task hselect to see a specific field
of the header and to control if the list are correct:
ex --- cl> hselect filenames (or @listname) $I,field
yes
Check if the files inside the list of zero
all have the field 'exptime' =0 if the 'imagetype'
are correct and if you created the correct list for each 'filter'
you have.
Correct what is wrong in the headers and go on.......
In order to change the value of a field you can
use the task hedit like the following:
ex --- cl> hedit filenames (or @listname)
field name new value up+
If the list of Zeros is correct we can start
to work on it. We well use the task Zerocombine. Another possibility
is to use the task
Imcomine.
Let start to editing the parameters typing: epa
zerocombine
cl>epa zerocombine
Image Reduction and Analysis Facility
PACKAGE = ccdred
TASK = zerocombine
input =
List of zero level images to combine
(output =
Zero) Output zero level name
(combine=
average) Type of combine operation
(reject =
minmax) Type of rejection
(ccdtype=
zero) CCD image type to combine
(process=
no) Process images before combining?
(delete =
no) Delete input images after combining?
(clobber=
no) Clobber existing output image?
(scale =
none) Image scaling
(statsec=
) Image section for computing statistics
(nlow =
0) minmax: Number of low pixels to reject
(nhigh =
1) minmax: Number of high pixels to reject
(nkeep =
1) Minimum to keep (pos) or maximum to reject (neg)
(mclip =
yes) Use median in sigma clipping algorithms?
(lsigma =
3.) Lower sigma clipping factor
(hsigma =
3.) Upper sigma clipping factor
(rdnoise=
0.) ccdclip: CCD readout noise (electrons)
(gain =
1.) ccdclip: CCD gain (electrons/DN)
(snoise =
0.) ccdclip: Sensitivity noise (fraction)
(pclip =
-0.5) pclip: Percentile clipping parameter
(blank =
0.) Value if there are no pixels
(mode =
ql)
ESC-? for HELP
To quit the task just type: :q
To understand the meaning of each field just
type cl> help zerocombine
The things you have to set are:
input =
@listZero
(output =
Zero)
Pay attention to set as no the following fields
(process=
no) Process images before combining?
(delete =
no) Delete input images after combining?
(clobber=
no) Clobber existing output image?
so that you will still have the original images at the and of the process.
Remember to set
(ccdtype=
zero) CCD image type to combine
and pay attention that the imagetype field inside
the header is correctly set to zero for those images.
If the field doesn't exist and you are sure about the images type just
leave the parameters as a blank.
When the setting is done you can type: :go from inside epa zerocombine.
Control the final image (Zero.fits) inside your
starting directory running a display package like Saoimage or Ximtools.
First of all you have to display a flat or
one of the others images and determine the overscan and the trim region.
In order to text if you choose the right orientation
of the xy plane you will run ccdproc only with following parameters:
Image Reduction and Analysis Facility
PACKAGE = ccdred
TASK = ccdproc
images =
test List of CCD images to correct
(output =
) List of output CCD images
(ccdtype=
) CCD image type to correct
(max_cac=
0) Maximum image caching memory (in Mbytes)
(noproc =
no) List processing steps only?
(fixpix =
no) Fix bad CCD lines and columns?
(oversca=
yes) Apply overscan strip correction?
(trim =
yes) Trim the image?
(zerocor=
yes) Apply zero level correction?
(darkcor=
no) Apply dark count correction?
(flatcor=
no) Apply flat field correction?
(illumco=
no) Apply illumination correction?
(fringec=
no) Apply fringe correction?
(readcor=
no) Convert zero level image to readout correction?
(scancor=
no) Convert flat field image to scan correction?
(readaxi=
line) Read out axis (column|line)
(fixfile=
) File describing the bad lines and columns
(biassec= [2055:2070,1:2048])
Overscan strip image section
(trimsec= [30:2040,5:2040])
Trim data section
(zero =
Zero) Zero level calibration image
(dark =
) Dark count calibration image
(flat =
) Flat field images
(illum =
) Illumination correction images
(fringe =
) Fringe correction images
(minrepl=
1.) Minimum flat field value
(scantyp=
shortscan) Scan type (shortscan|longscan)
(nscan =
1) Number of short scan lines
(interac=
yes) Fit overscan interactively?
(functio=
legendre) Fitting function
(order =
1) Number of polynomial terms or spline pieces
(sample =
*) Sample points to fit
(naverag=
1) Number of sample points to combine
(niterat=
1) Number of rejection iterations
(low_rej=
3.) Low sigma rejection factor
(high_re=
3.) High sigma rejection factor
(grow =
0.) Rejection growing radius
(mode =
ql)
so that Iraf will read the test image to
compute the overscan
fit on the region ([2055:2070,1:2048])
and
the
trim
section on the region
( [30:2040,5:2040]) in an
interactively way (interac=
yes) Fit overscan interactively?.
The graph you will see depends on the field
(readaxi= line) Read out axis (column|line)
if it is correct set the overscan region in the graph will appear more or less flat and the fit should be a constant, otherwise you should change that field in
(readaxi= column) Read out axis (column|line)
to makes the program able to read the image in the correct way.
If everything is OK you can do the same with
all the images of the same night
images =
jc09* List of CCD images to correct.
You also can set (interac= no) Fit overscan interactively?.
Type: :go
and wait a wile to see the result.
Image Reduction
and Analysis Facility
PACKAGE = ccdred
TASK = flatcombine
input =
@lisflatU List of flat field images to combine
(output =
FlatU) Output flat field root name
(combine=
average) Type of combine operation
(reject =
avsigclip) Type of rejection
(ccdtype=
flat) CCD image type to combine
(process=
yes) Process images before combining?
(subsets=
no) Combine images by subset parameter?
(delete =
no) Delete input images after combining?
(clobber=
no) Clobber existing output image?
(scale =
mode) Image scaling
(statsec=
) Image section for computing statistics
(nlow =
1) minmax: Number of low pixels to reject
(nhigh =
1) minmax: Number of high pixels to reject
(nkeep =
1) Minimum to keep (pos) or maximum to reject (neg)
(mclip =
yes) Use median in sigma clipping algorithms?
(lsigma =
3.) Lower sigma clipping factor
(hsigma =
3.) Upper sigma clipping factor
(rdnoise=
0.) ccdclip: CCD readout noise (electrons)
(gain =
1.) ccdclip: CCD gain (electrons/DN)
(snoise =
0.) ccdclip: Sensitivity noise (fraction)
(pclip =
-0.5) pclip: Percentile clipping parameter
(blank =
1.) Value if there are no pixels
(mode =
ql)
NOTE if you set
(subsets= yes) Combine images by subset parameter?
flatcombine should combine the list of every flat
you have looking at the filter field in the header, so the you can avoid
of making a list
for each filter.
Edit again the ccdproc parameters (epa
ccdproc) and change the fields:
images =
@lisobjU List of CCD images to correct
(flatcor=
yes) Apply flat field correction?
(flat =
FlatU ) Flat field images
In the ex. above we are doing the flat-
field division for the object with the U filter (@lisobjU) ,
so
we have to use the U flat (FlatU).
Choose reference stars in the one you want to
be the reference image:
Run parameters of the task rimcursor
cl>epa rimcursor
PACKAGE = lists
TASK = rimcursor
image =
jd090094.fit image to which coordinates should refer
(wcs =
logical) output world coordinate system name
(wxforma=
) x coordinate output format
(wyforma=
) y coordinate output format
(cursor =
) image cursor
(mode =
ql)
the only think you have to control is:
(wcs = logical) output world coordinate system name
that set the kind of coordinate you want to use.
Display the image you want as a reference
cl> displayjd090094.fit
run rimcursor on your imagemaking it put his result in a file
cl>rimcursor jd090094.fit > output filename
Now on your display program it will appear a cursor (a little ring) that you will use to select the stars on your image, step on the center of each star you want and click on enter.
NOTE: You should select well exposed stars, not galaxies and not cosmic rays.
The next step is to align and shift the images.
Edit the parameters of imalign
PACKAGE = immatch
TASK = imalign
input =
@lisnr298sU Input images
referenc=
jd090094.fit Reference image
coords =
coordnr298U Reference coordinates file
output =
@lisnr298sU Output images
(shifts =
) Initial shifts file
(boxsize=
7) Size of the small centering box
(bigbox =
121) Size of the big centering box
(negativ=
no) Are the features negative ?
(backgro=
INDEF) Reference background level
(lower =
INDEF) Lower threshold for data
(upper =
INDEF) Upper threshold for data
(niterat=
200) Maximum number of iterations
(toleran=
0) Tolerance for convergence
(shiftim=
yes) Shift the images ?
(interp_=
linear) Interpolant
(boundar=
nearest) Boundary type
(constan=
0.) Constant for constant boundary extension
(trimima=
no) Trim the shifted images ?
(verbose=
yes) Print the centers, shifts, and trim section ?
(list =
)
(mode =
ql)
The first two lines are easy to understand
the third is the result of the previous step and the fourth is the
list with the name of the output images.
First of all try without an initial shift file,
leave that field empty.
This tasks without an initial shift file will
try to find the shift among the images by itself. The important fields
you have to set to make it find a good result are:
(boxsize=
7) Size of the small centering box
(bigbox =
121) Size of the big centering box
The first is the dimension of the box for
a fine calculation, usually you can leave the default value, the second
is the dimension of the bigbox it will use to find the same star in all
the images, this should be big enough to contain for each start the same
star in all the images.
Try to set a an expected value and run the task.
It will write on the screen the value of the
shift and the error found for each image referred to the reference image,
the errors should be < 0,1.
The best you can do to control if the alignment
is good is to display the shifted images and blink between them.
Control, with imaximine , if the background level
of the sky is the same, within its the standard deviation, for each images.
If the images look perfectly
align and if you don't need to level them go on with the combination.
cl> epa imcombine
PACKAGE = immatch
TASK = imcombine
input =
@lisnr298sU List of images to combine
output =
nr298U List of output images
(rejmask=
) List of rejection masks (optional)
(plfile =
) List of pixel list files (optional)
(sigma =
) List of sigma images (optional)
(logfile=
com nr298U.log) Log file
(combine=
average) Type of combine operation
(reject =
avsigclip) Type of rejection
(project=
no) Project highest dimension of input images?
(outtype=
real) Output image pixel datatype
(offsets=
none) Input image offsets
(masktyp=
none) Mask type
(maskval=
0.) Mask value
(blank =
0.) Value if there are no pixels
(scale =
none) Image scaling
(zero =
none) Image zero point offset
(weight =
none) Image weights
(statsec=
) Image section for computing statistics
(expname=
) Image header exposure time keyword
(lthresh=
INDEF) Lower threshold
(hthresh=
INDEF) Upper threshold
(nlow =
1) minmax: Number of low pixels to reject
(nhigh =
1) minmax: Number of high pixels to reject
(nkeep =
1) Minimum to keep (pos) or maximum to reject (neg)
(mclip =
yes) Use median in sigma clipping algorithms?
(lsigma =
3.) Lower sigma clipping factor
(hsigma =
3.) Upper sigma clipping factor
(rdnoise=
0.) ccdclip: CCD readout noise (electrons)
(gain =
1.) ccdclip: CCD gain (electrons/DN)
(snoise =
0.) ccdclip: Sensitivity noise (fraction)
(sigscal=
0.1) Tolerance for sigma clipping scaling corrections
(pclip =
-0.5) pclip: Percentile clipping parameter
(grow =
0.) Radius (pixels) for neighbor rejection
(mode =
ql)
The first lines are the usual, if you want you can create a log file with the history of the combine operation :
(logfile= com nr298U.log) Log file
The other two fields you should know are :
(combine=
average) Type of combine operation
(reject =
avsigclip) Type of rejection
the first is the kind of combine operation you
want, that typically is an average, the second is the
type of rejection,
avsigclip seems to work well.
Run the task and control with a display and imexamine
if the result image is good, is the seeing is between those you had in
the original images, and if the standard deviation of the sky is less than
the original.