1.1 HOW-TO Create a CoaddedRegriddedFrame

Several RegriddedFrames created from ReducedScienceFrames (see the ReduceTask HOW-TO or the RegridTask HOW-TO) can be coadded to form a deeper image with a substantial reduction of chip defects and divisions (a CoaddedRegriddedFrame).

NOTE: At this time, the CoaddTask does not allow frames from multiple instruments to be coadded. It should be possible to do this provided all the RegriddedFrames were regridded to the same grid target (R.A., DEC., and pixel scale1), but this operation is currently not supported.

1.1.1 DPU Method

At the moment, the CoaddTask is a purely serial operation which takes place on only one node of the DPU. Some possible reasons to use the DPU for this task are because the machines on the DPU are much faster than the local machine, or because no local machine is available.

For the CoaddTask, the DPU command would look something like:

awe> dpu.run('Coadd', d='2000-01-01', i='WFI', f='#843', o='Science1_?-*')

The CoaddTask options via the DPU are as follows:

  • i (instrument, mandatory): string
  • f (filter, mandatory): string
  • d (date): string of the form CCYY-MM-DD
  • o (object): string or ``regular expression''
  • C (commit): integer=0..1 (optional, default=0)

This example shows how to run the DPU CoaddTask using date, filter, and object information. This allows one to process all data for a given object on a specific day taken in a certain filter, for all CCDs of the mosaic.

The options can be used in any order and can be omitted (except `i'), but the likelihood of locating required data depends on relaying a minimum of information as shown in the examples.

1.1.2 Non-DPU Method

This task can also be performed in on a per chip basis. The CoaddTask is main part of the recipe $AWEPIPE/astro/recipes/Coadd.py. This is most basic front-end for creating CoaddedRegriddedFrames in AWE. An examples of the syntax for the CoaddTask is given below:

awe> coa = CoaddTask(instrument='WFI', date='2000-01-01', chip='ccd50', 
...                  filter='#843', object='Science1_?-*')
awe> coa.execute()

The CoaddTask options are as follows:

  • instrument, mandatory: string
  • filter, mandatory: string
  • date: string of the form CCYY-MM-DD
  • chip: string
  • object: string or ``regular expression''
  • commit: integer=0..1 (optional, default=0)

This example shows how to run the CoaddTask using date, chip, filter, and object information. This allows one to process all data for a given object on a specific day taken in a certain filter, and only one CCD of the mosaic.

The options can be used in any order and can be omitted, but the likelihood of locating required data depends on relaying a minimum of information as shown in the examples.


1.1.3 Coadd algorithm

The algorithm behind the coaddition is described via the coaddition of two RegriddedFrames which have the (object)name reg1 and reg2. The value fout mathend000# of a pixel in the resulting CoaddedRegriddedFrame (which we give object name coad) is computed as follows:

fout = 1#1(wi*FLXSCALEi*fi)/1#1(wi), (1.1)
mathend000#
where the summation is over the RegriddedFrames.
FLXSCALEi mathend000#=value of FLXSCALE attribute of each input RegriddedFrame (i.e.,, reg1.FLXSCALE).
wi = weighti/FLXSCALEi2 mathend000# where weighti mathend000# is the value of the pixel in the associated weightframe (i.e., reg1.weight).
The fi mathend000# is the pixel value in the regridded frame.
The value wout mathend000# of the pixel in the resulting weight frame associated with the coad (i.e., coad.weight) is computed as:

wout = 1#1(wi) (1.2)
mathend000#

1.1.4 Coadd units

The pixel units of the CoaddedRegriddedFrame are fluxes relative to the flux corresponding to magnitude=0. In other words, the magnitude m mathend000# corresponding to a pixel value f0 mathend000# is:

m = - 2.5log10f0. (1.3)
mathend000#

The relation between pixel units in CoaddedRegriddedFrame and the RegriddedFrames from which it was derived is as follows. For a RegriddedFrame object named reg mathend000# we have:

reg.FLXSCALE = = 10.0-0.4 x reg.ZEROPNT (1.4)
mathend000#
A magnitude=0 picels will have counts countsreg(mag = 0) mathend000# in the RegriddedFrame:

countsreg(mag = 0) = = 100.4 x reg.ZEROPNT = 1/reg.FLXSCALE (1.5)
mathend000#
(Note that reg.ZEROPNT is exposure-time specific, therefore counts instead of countrate.) Suppose one makes a CoaddedRegriddedFrame from this single RegriddedFrame reg mathend000#. In this case the relation between the counts countreg mathend000# in the RegriddedFrame and pixel value vcoad mathend000# in the CoaddedRegriddedFrame is made to be:

vcoad = countsreg*reg.FLXSCALE (1.6)
mathend000#
Suppose you know the physical flux density of the magnitude=0 object to be f0 mathend000# Jy. The physical flux corresponding to vcoad mathend000# is then:

fcoad(Jy) = f0*vcoad (1.7)
mathend000#
For example, in the AB magnitude system f0 = = 3631 mathend000# Jy and hence:

fcoad(Jy) = 3631*vcoad (1.8)
mathend000#

If ones make a CoaddedRegriddedFrame out of multiple overlapping RegriddedFrames the resulting flux is the weighted average of the input RegriddedFrames fluxes (see Section [*]).



Footnotes

... scale1
Current default pixel scale is 0.200 arcsec/pixel mathend000#


page generated Tue Apr 4 14:29:08 CEST 2017