Instructions for Star Subtraction Method
for Processing Asteroid Image Sets
Bruce L. Gary, Last Updated 2014.12.31

This web page describes my method for processing asteroid image sets using "star subtraction." I have found that it provides better quality results for all star fields, especially those that are crowded with stars and especially when the asteroid is faint. For uncrowded star fields and bright asteroids the benefits are small but still present. There are two phases: image processing (with MaxIm DL 5) and spreadsheet processing (using Excel).

Image Processing Phase

I will assume that there are several FOV image sets. For each FOV, do the following for as many images that can be loaded into MaxIm DL as possible (without using virtual memory (hard drive supplement to RAM); for me that's ~ 150 images).

  1) Calibrate images.
  2) Star align (preferably using 1 star alignment for a star near the FOV center). Note any poor quality images. Review using animation tool, to be sure all images are aligned well.
  3) Save these images to a directory branching off the directory containing the raw images (such as ...\StarsPresent)
  4) Median combine all (or some) of these images. Record it (in the directory just created) as FOV#1_mcALL.
  6) Select 1st image; run the Batch Process tool. Start recording (click the red circle).
  7) Run the PixelMath tool, check "Subtract" and enter FOV#1_mcALL in the Image B box. Make sure 100% is selcted for both images. Set Add Constat to 1000. Click OK.
  8) Stop the Batch Process recording. Use the Batch Process tol to select all other images (not the 1`st one, or the FOV#1_mcALL images), & click OK to process them. All images should now be "star subtracted."
  9) Save them to a directory (such as ...\StarsSubtracted). This shouldn't be necessary, but it's a precaution in case you have to repeat some of the following.

10) If the asteroid is so faint that you might not find it easily, run the animation tool and note asteroid x,y locations for images near the beginning and end of the image set. 
11) Add the artificial star (in upper left corner) to all images
12) Run the Photometry tool.
13) Check "Snap to centroid" & "Act on all images".
14) Highlight the first image with the asteroid's x,y location. Select "New Moving Object" in the middle -right window.
15) Set the photometry aperture to nominal values: "Large Rings" & 4x6x24.
16) Find the asteroid at the noted x,y location and click on it.
17) Highlight the image for which you have the late asteroid x,y location; select "Mov 1" in the middle-right window.
18) Find the asteroid & click on it.
19) Highlight an image somewhere in the middle and verify that the asteroid's photometry circels are on it.
20) Select "New Reference Star" in the middle-right window, and click on artificial star.
21) Click "View Plot" and inspect the plot for outliers.
22) For any suspicious outliers, click on the plotted outlier to see its image highlighted and displayed. Sometimes the "snap to centroid" feature will be influenced by a nearby defect and cause the photometry circles to be un-centered on the asteroid. If this is detected, click "Back" (from the LC display), uncheck "Snap to centroid" and nudge the photometry circle pattern so it's centered on the asteroid.
23) This is optional: view all images and verify that the photometry circle pattern is centered on the asteroid; if not, nudge it. 
24) Save to a CSV file in a directory branching from the FOV#1 directory. Use name such as FOV#1a-StarsSubtracted 6x6x24.
25) Change photometry dimension to 6x6x24 & save CSV file. Note which looks better: 4x or 6x.
26) Close photometry tool. Unload the star-subtracted images (Alt-F E).

27) Load the image FOV#1_mcALL.
28) Invert it (using Screen Stretch window tool). Create a 255-bit version using the Stretch tool (selecting "Linear" and "Screen Stretch" and "8 bit"). Save it as a JPEG image file.
29) Switch to Windows navigating program and print this JPEG "finder" image.
30) Load the "stars present" image set from the ...\StarsPresent directory.
31) Add the artificial star to all images.
32) Run the Photometry tool.
33) Highlight the "early" image for which you have asteroid x,y location info.
34) Be sure to check the "Snap to centroid" box.
35) Select "New Moving Object" and locate the asteroid. Click it (using 4x6x24)
36) Highlight the "late" image for which you have asteroid x,y location info.
37) Select "Mov 1" and click the asteroid.
38) Highlight a "middle" image and verify that the photometry circle pattern is over the asteroid.
39) Select "New Reference Star" and click the artificial star.
40) Temporarily increase the photometry target circle size to, for example, 8 pixels radius (this will help in selecting "check stars"
41) Select "New Check Star" in the middle-right window.
42) Identify a check star (near the beginning of the asteroid's path), verify that it's not saturated and that it has SNR > 100, and click it. Note this star on the printed JPEG "finder" image, labeling it with the number assigned to it by MDL. 
43) Repeat the above step for at least a dozen more stars, and preferably ~ 20 stars. Don't worry about a low SNR when stars are withn the sky background annulus; a useful guide is to select stars with a maximum count between 10,000 and 40,000. You can adjust contrast/brightness in a way that helps identify these stars.
44) Re-set the photometry circles to the nominal values 4x6x24.
45) Click "View Plot" and don't be bothered by the asteroid outliers.
46 Save the CSV file with a name such as FOV#1a-StarsPresent 4x6x24 (in the same directory as the previous CSV file save).
47) Change the photometry signal aperture radius to 6 pixels and save the CSV file (e.g., FOV#1a-StarsPresent 6x6x24).
48) Close photometry tool. Unload the stars-present images (Alt-F E).
49) Re-load the FOV#1_mcALL image, and do a PinPoint Astrometry plate solve.
50) Make the Cross Haris visible, and place the cursor over the image center.
51) In the Information window, select "Astrometric". Read the RA/DE coordinates, and note on the printed finder image.

49) Switch from MaxIm DL to Excel, & load a template spreadsheet for asteroid use (something that I can provide).
50) Update the target RA/DE coordinates in the spreadsheet (1st page)
51) On the 2nd page ("Import") import the CSV file "FOV#1a-StarsPresent 4x6x24"
52) To the right of this imported data, import the CSV file "FOV#1a-StarsSubtracted 4x6x24"
53) For info purposes, compare the RMS noise of the two Mov1 columns. To be fair, delete RMS entries corresponding to the "StarsPresent" data that are affected by background stars (i.e., RMS > 0.5). Make a note about the ratio of RMS values. Typically I get ratio = 2 to 5.
54) Copy the SrarsPresent data to the 3rd page ("STD").
55) Copy the Mov1 column of the StarsSubtracted data to the 3rd page as replacements for what was there in the Mov1 column. This is the crucial step, for it assures that the best mag's for the asteroid are used while preserving the Check Star mag's (made with the same photometry aperture settings).

From this step the rest of the analysis is the same as usual. For example, the finder image is used to extract APASS BVg'r'i' mag info from C2A, which puts the asteroid mag's on a calibrated scale. An entire additional tutorial could be written on the use of my spreadsheet for processing asteroid data.

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