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Dear All,

I would like to ask a few things about IMCOMBINE, COMBINE, FLATCOMBINE tasks.

- I noticed that IMCOMBINE performs scaling differently than COMBINE and FLATCOMBINE. Why is it so?

- Why is it not possible to set the 'weight' parameter in FLATCOMBINE via epar? Is there some indirect way to set and apply weighting parameter for FLATCOMBINE?


Below I have shown the output for all three tasks for easier communication.

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output for IMCOMBINE

Jun 26 9:42: IMCOMBINE
combine = average, scale = mean, zero = none, weight = none
blank = 0.
Images Mean Scale
linearity_expTime-42_enum-56_biasCor.fit 48774. 1.000
linearity_expTime-42_enum-57_biasCor.fit 48806. 0.999
linearity_expTime-42_enum-89_biasCor.fit 48722. 1.001
linearity_expTime-42_enum-90_biasCor.fit 48711. 1.001
linearity_expTime-45_enum-59_biasCor.fit 52003. 0.938

Output image = testImcombine, ncombine = 5


output for FLATCOMBINE.

Jun 26 9:44: IMCOMBINE
combine = average, scale = mean, zero = none, weight = none
blank = 1.
Images Exp Mean Scale
linearity_expTime-42_enum-56_biasCor.fit 42.0 48777. 1.013
linearity_expTime-42_enum-57_biasCor.fit 42.0 48806. 1.012
linearity_expTime-42_enum-89_biasCor.fit 42.0 48724. 1.014
linearity_expTime-42_enum-90_biasCor.fit 42.0 48712. 1.014
linearity_expTime-45_enum-59_biasCor.fit 44.8 52002. 0.950

Output image = testFlatcombine, ncombine = 5


output for COMBINE.

Jun 26 9:47: IMCOMBINE
combine = average, scale = mean, zero = none, weight = none
blank = 0.
Images Exp Mean Scale
linearity_expTime-42_enum-56_biasCor.fit 42.0 48777. 1.013
linearity_expTime-42_enum-57_biasCor.fit 42.0 48806. 1.012
linearity_expTime-42_enum-89_biasCor.fit 42.0 48724. 1.014
linearity_expTime-42_enum-90_biasCor.fit 42.0 48712. 1.014
linearity_expTime-45_enum-59_biasCor.fit 44.8 52002. 0.950

Output image = testCombine, ncombine = 5

Hello,

The most fundamental and general task is IMCOMBINE. The COMBINE task is a variant that has some features built in to recognize keywords based on the translation mechanism; i.e. to recognize biases and dome flats. The task FLATCOMBINE and other similar tasks are scripts around COMBINE that are specialized by fixing and hiding certain parameters. This is done to simplify for typical users. In all cases the core library for the combining part is the same. If you feel comfortable you can pretty much use COMBINE or even IMCOMBINE. Weighting is not a common thing in creating master calibrations. The use of the mean for multiplicative scaling is the common whay to stack calibrations without having to measure and setup more general scaling. In summary, the tasks in the CCD reduction package(s) are meant to simplify and do the correct thing as defined my developer (me). If you want to do things differently then that is possible with COMBINE or IMCOMBINE keeping in mind that certain things will have to be done explicitly; like selecting exposure types.

Frank Valdes