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Averaged Flat-Field Frame

An average flat-field frame of a set of flat-field frames.

Synopsis

munipack flat [.. parameters ..] file(s)

Description

The flat-field frame is a map of the photometric response of an instrument (both detector and optical configuration together). Flat-fields are used to remove defects like dust, vignetting and other imperfections on a single pixel base. Flat-fields can be acquired on sky during twilight, as night sky images and the dome flats. This routine creates the flat-field frame by averadging of a serie of such images.

The output flat-field Fij is computed as the (robust) average of every pixel of pre-scaled input images (I'k)ij:

Fij = 1/(1 - 〈I'kij),

where i,j is an index of a pixel and k is an index of an image. Input images (Ik)ij are pre-scaled as

I'ij = (Iij - 〈Iij〉) / 〈Iij,

where the average of whole image is 〈Iij . The rescaling is used to unifying possible different light levels (in twilight flat-fields).

The averaging of flat-fields is computed by the robust algorithm. Amount of frames is not limited. Just a single frame can be provided, which normalizes the input. Robust method works very well for thirteen and more frames.

The input flat-fields can be pre-corrected for both bias and dark:

Iij → Iij - x Dij - Bij.

The meaning of x and the algorithm itself are the same as in photometric corrections.

The flat-field can be scaled onto an output level. The default value is 1.0 which is suitable for any situation when output is stored as float numbers (BITPIX=-32). When integer numbers are preferred (a litle bit reduced numerical precision is acceptable), set the output level to the half of range for the integer data-types (for BITPIX=16 is range 65536 and half about 30000. In the case, any photometric information is lost!

It is strongly recommended correct setting of FITS_KEY_FILTER (environment variables) because the filter should be usable for later processing.

Gain estimate

Flat-fielding can be used to estimate of gain of a camera. The estimation determines the standard deviation σ of residuals between output mean and a single flat-field frame. σ ≈ 1 is near to one for correct value of gain. This is basic property of Poisson distribution which has bounded value of number of detected particles n and its standard deviation σ by relation σ=√n. The relation can be used for only when other sources of noise are negligible. Therefore, the ideal value of n is about half of full well capacity of detector (not ADU!).

The gain g is determined by the relation

g = 1 / σ²

The estimated values of standard deviation σ and gain are printed in the output of the routine as last two columns of the table:

...
 Filename, mean, correction,     std.dev., gain:
flat_V_000001.fits: 2.08154E+05  -16.34081     1.92080     0.271
flat_V_000002.fits: 1.84904E+05  -16.02431     1.89552     0.278
                                                 [σ]        [g]
...

Determination of gain is so slow that one is computed only on request by using of --gain-estimate switch (verbose mode is invoked).

When gain keyword is available in the header, the values are pre-scaled by the gain so results will realtive against to the value. To suppres use of the value, set gain by providing -gain 1.

The estimation of value of gain is just indicative and the check by some alternative method (factory provided, …) is highly recommended!

Input And Output

On input, list of frames representing the flat-fields is expected. Optionaly, both bias and dark can by pre-applied on every frame.

On output, just the flat-field frame is created.

Parameters

-bias file
bias frame
-dark file
dark frame
-xdark x
dark frame multiplicative factor
-gain g
provide value of gain (rather than FITS keyword)
--gain-estimate
Try to estimate gain of every frame. The estimate importantly slow-down flat-fielding. Please, pre-set gain on the unit by using -gain 1
--saturate s
Set saturation level. All value out of range: 0 < flat < s, in input files are rejected from any processing. This switch is useful mainly for over-exposed flat-fields. If unused, the value is get from FITS header (by FITS_KEY_SATURATE, see). If the keyword is not found in the header, and frame has number type of integer, the maximum value 2**BITPIX is used, otherwise maximum value of the first frame on input is used (which practically switch-off saturation bound check).
--level x
the mean level of output frame (default: 1.0)
-B bitpix
set numerical type of output images (default -32)
-o filename
save to the output file

Also see Common options

Examples

$ munipack flat -o f_R.fits -dark d10.fits f10_*R.fits

If one needs frame with natural light-level and 16-bit depth, one is using

$ munipack flat -B 16 --level 10000 -o f_R.fits -dark d10.fits f10_*R.fits
f10_1.png
A single flat-field frame.
autoflat.png
A scaled robust mean of flat-field frames.

See Also

Light Curve Tutorial, Photometric corrections tutorial, Averaged bias frame, Averaged dark frame.