This program calibrates images from the Messenger mission's MDIS instrument. The input image is corrected for the following, in order, during calibration:

• Step 1: Bias removal.
• Step 2: Linearity correction.
• Step 3: Remove the hot pixels.
• Step 4: Readout smear correction.
• Step 5: Uniformity (flat field) correction.
• Step 6: Conversion to I/F
• Step 7: PSF correction
• Notes
• References

Step 1: Bias removal.

The signal level of AMICA is determined by a combination of an electronic offset which defines the "zero" level of the analog-to-digital converter (the bias) as well as any additional signal due to thermal electrons (the dark current). In general, the bias offset depends upon the electronics and CCD temperature. The bias for AMICA was empirically modeled as a function of time: The constants B₀, B₁ and B₂ were chosen to fit the curve to the data as closely as possible with the following values:

Step 2: Linearity correction:

The DN level stored in raw images is approximately proportional to the number of photons detected in each CCD pixel over the exposure time. Linearity is a measure of the degree to which the CCD response is proportional to the incident flux. Linearity was tested during the pre-flight and mission phase of the mission. During the pre-flight phase, DN values taken at different exposure times showed a linear intensity with an incidence flux between 1000 DN and 3500 DN. By curve-fitting the pre-flight data, the following equation was obtained:

Step 3: Hot pixel removal:

During the mission the CCD was exposed to intense radiation from cosmic rays. This resulted in some pixels with an aberrant dark current value that was higher than their neighbors (the hot pixels). The number of hot pixels increased during the mission. This made accurate measurement of faint objects such as stars difficult. The following hot pixels were identified and removed from the output images during calibration by setting their values to ISIS::Null:

Step 4: Read-out smear:

The AMICA instrument is shuttered electronically. Images are exposed for a certain exposure time in addition to the vertical charge-transfer period of 12.288 milliseconds. It is during the charge-transfer period that a read-out smear in the vertical direction is produced. The read-out smear is predominant in images with exposure times on the order of 100 microseconds. Originally the mission plan was to correct for this smear on-board the spacecraft. However, an anomaly in one of the reaction wheels on October 2, 2005 made this impossible. As a result of this, some of the images returned by the AMICA instrument are smear-corrected, while others are not. This necessitated creating a smear model to remove smears from images taken during the descending and ascending period. The read-out smear brightness I_smear for unbinned images is modeled from the observed images as follows:

Step 5: Flat-field correction:

Performs a correction for pixel-to-pixel variation in CCD response and vignetting (reduction of image brightness near the periphery compared with the center). Flat-field images for all bands were acquired using an integrating sphere at NEC Space Technologies, Ltd. at room temperature (around 30 degrees Celsius). A flat-field image is one which has constant uniform brightness everywhere. AMICA was pointed into the integrating sphere to acquire images of a field that is known be be spatially uniform to an accuracy of approximately 2%. The correction is accomplished by dividing each pixel of the output image by the corresponding pixel in the flat-field image.

Step 6: Convert units to I over F:

This converts the raw DN values to I/F (radiance) units. The formula is as follows:

Step 7: PSF Correction:

The point-spread function (PSF) is a model describing the two-dimensional distribution of light emitted from an infinitely small point source. For point light sources, there is a focused and an unfocused function, which when combined give a full description of the light distribution. The focused function was determined empirically to have the following form:



The variable r is the distance in pixels from the optical center of the point source. The values of alpha for the different filters are listed below:

The unfocused component of light distribution is modeled as a sum of Gaussian functions: Both the focused and the unfocused components are calculated for each pixel in the image, and subtracted out of the final calibrated image. A_i and sigma_i are constants derived empirically from images of Itokawa and the Moon and enumerated below:

Notes

1. Any in-flight dark current noise for this mission was buried in the read-out noise (~ 4 DN). When observing Itokawa, the image intensity had a range of 1000-3000 DN. Therefore, the dark current was considered negligible, and is not accounted for in this calibration application (see Ishiguro (2010) ).
2. The derivation for the formula for smear-removal in the case of unbinned images does not appear in any of the references given below. Questions concerning the derivation steps for this formula should be directed towards the USGS.

References:

1. Ishiguro, Masateru, et al. "The Hayabusa Spacecraft Asteroid Multi-band Imaging Camera (AMICA)". Icarus 207(2010) 714-731.
2. Ishiguro, Masateru. "Scattered light correction of Hayabusa/AMICA data and quantitative spectral comparisons of Itokawa". Publ. Astron. Soc. Japan (2014) 66(3), 55 (1-9).

Output units

The output units of the DNs will depend on which calibration flags are turned on.

• If IOF=TRUE, output units are measured in I/F
• Otherwise output units are measured in W/(m² micrometer sr)