1. CCD Image Processing …okay, I’ve got a bunch of .fits files, now what?
SSP 2017

2. First, what is a .fits image?
The extension stands for “Flexible Image Transport System”.
The file is literally a 2-dimensional array (in other words, an n × m matrix), whose entries are a 16-bit binary integer. How many possible values are there for a given entry?
a b c. 65,536 d. 524,288

3. The .fits image file
The units of each entry in the matrix is “ADU” (= analog-to-digital units), a somewhat arbitrary conversion of voltage information from each pixel in the CCD to a value. The values themselves are called “pixel counts”. The software (like DS9 or MaximDL or any other “fits viewer”) converts pixel counts to grayscale, and the array becomes an image.
The illustration shows a row within the .fits matrix as a histogram; the vertical axis is pixel count translated to grayscale value.

4. The underlying assumption in a .fits image file about what it measures

5. Also in a .fits file
The .fits file also contains a header, which is the metadata. Metadata is “Data about the data”; in our case, it means information about the observation (date, time, position in sky) and the camera (bits per pixel, pixel length of x and y axes).

6. There are two goals of the OD project
Astrometry – the determination of a precise position of an observed object (used in OD, submitted to MPC)
Photometry – the determination of a precise light intensity of an observed object (submitted to MPC)

7. CCD image processing
When you take an image with a CCD, you get a lot more (or sometimes less) than you bargained for. For example, "hot pixels" come from charge leakage, "dead pixels" don't hold any charge, and if you illuminate the CCD evenly, some pixels are slightly brighter or fainter than their neighbors, so you have to calibrate the image.
uncalibrated image
calibrated image

8. CCD image processing
In addition, calibration removes the effect of vignetting, the falling off of image intensity towards the corners and sides of an image. It is caused by obstructions to the aperture or even shading of the camera by optical elements in the telescope. We can’t “fix” the problem but we can compensate.
Intentional vignetting
Unintentional vignetting

9. Calibration – Dark Current
Since the CCD silicon atoms have easily ionizable atoms, any source of energy may release electrons that are indistinguishable from electrons released by the action of photons from space.
All these “other” electrons are called the dark current. One way to remove their effect is to lower the temperature of the CCD, which reduces the ambient energy that ionizes the atoms. In the photos on the left, note that none of the dots visible were due to captured photons.

10. Calibration – Dark Current
Of course, it is not feasible to remove “thermal” electrons completely, so you take a “dark frame” image, which is taken for the same length of time and same temperature as the “light” exposure with the shutter on the CCD closed. So do this while you are at your observation session.
A dark frame: Note that it is not completely dark.
The dark frame will be subtracted from the corresponding light exposure to compensate for dark current.

11. Calibration – Bias
You may have heard the term “bias” in the context of electronics – bias is the separation of charge (usually accomplished by “doping” a silicon substrate with other atoms with more or fewer valence electrons) in an electronic device.
What a bias frame does, then, is to account for the inherent “current” that is in the CCD itself. A bias frame is a dark frame of zero exposure length.

12. Calibration – Bias
Corollary – A dark frame already takes into account the bias, so we don’t need to take a separate bias frame. You only really need to if your dark frame will be used on different light exposure time images.

13. Calibration – Flat-field frames
A CCD chip is not homogenous; different parts of the chip will respond to light by releasing different numbers of electrons. Moreover, the CCD chip may have uneven illumination due to dust on various optic components. A flat-field frame, therefore, is an image taken of a uniform background so that the CCD reaches about half of its saturation level.
A flat-field frame is actually the combination of five flat-field exposures taken sequentially in an effort to remove random signals. Note the “dust donuts” on the image above.

14. Calibration – Flat-field frames
• You still have to take dark frames for each flat-field image you take; flat-field images are subject to the same problems as light exposures.
• The flat-field frame values (corrected for the dark frame values) are divided into the raw light exposure image values to yield the calibrated light image values
C = corrected value for a pixel
R = raw image value
F = flat-field image value
D = dark frame value
m = average value of (F–D)
G = the “gain” of the pixel

15. The basic procedure for calibrating your
The basic procedure for calibrating your .fits images and finding your asteroid
make sure that you have a series of light, dark and flat-field images, clearly named.
stack (median combine) all the light images of a series together; do the same for all the dark images in a series and all the flat-fields. (There are many stacking methods, but median combine negates the effect of cosmic ray streaks on the image)

16. The basic procedure for calibrating your
The basic procedure for calibrating your .fits images and finding your asteroid
3. calibrate the raw light image by subtracting the dark frame and dividing by the normalized flat-field frame (the MaximDL software does this as one step)
4. align the calibrated images of the three light series of one night in order to “blink” your images to determine if your asteroid is in the image