Introduction

When dealing with faint asteroids in a crowded star field it may be worth the effort to perform a labor-intensive analysis that greatly fades the star field while retaining the asteroid's brightness. The procedure to be described is recommended for only those peoiple who love image analysis. When an initial set of 30 images are used the final image affords a limiting magnitude improvement of 1.1 magnitude (SNR greater by 2.9). This improved SNR is modest, considering that it is based on a 30-image initial set, but the most important improvement is the 100-fold fading of the star field. Thus, even when an asteroid passes close to (unsaturated) stars this objective procedure promises to make use of the asteroid's "information" in each image. This procedure is NOT recommended when the star field is uncrowded or when the asteroids are bright enough to be seen in each image.

Links internal to this web page:
    Star-Subtract and Asteroid Track-Strack Procedure
    Sample Images
    Related Links

Star-Subtract and Asteroid Track-Stack Procedure

    Calibrate 30 images

    Align all using stars

    Median combine groups of 3; call these "small groups" and use filenames a, b, c, ... h

    Avg small grps a & b, c & d, etc, creating "large groups" ab, cd, ... gh

    Perform PinPoint plate solutions for a and e

    Perform "star subtraction" of all small groups using appropriate large grps
        a - gh = a_ss
        b - gh = b_ss
        c - gh = c_ss
        d - gh = d_ss
        e - ab = e_ss
        f - ab = f_ss
        g - ab = g_ss
        h - ab = h_ss

    Choose blank region in a_ss and note x/y location (such as 600/400). Place "white dot" there.

    Calculate interval between small grps (eg, 5.6 min)

    Calculate asteroid pixel velocity from ephemeris (eg, 0.53 "arc/min * 5.6 min / 1.09 "arc/px = 2.7 px/min)

    Calculate "white dot" pixel location for all images (eg, x = 600, 603, 605, 608, ... 619 for a_ss, b_ss, etc)

    Place "white dot" at each of these x/y locations

    Median combine the first 4 small grps using the white dot for alignment, creating abcd_ssdot

    Repeat median combine for last 4 small grps of images, creating efgh_ssdot
        (Both abcd_ssdot & efgh_ssdot should retain the plate solutions associated with "a" and "e")

    Try blinking these two images and try to see the asteroid from its motion.
        (Each image has SNR twice as great as one of the 30 initial images, and the stars should be greatly faded)

    If you can't see the asteroid, average the last two images created using the white dot for alignment, creating a-h_ssdot
        (This image should retian the plate solution information associated with image "a" above)

    Calculate the predicted location for image "a" and search a-h_ssdot at that location for the asteroid. If you see it there
        you can measure its coordinates and magnitude. Note that this image has SNR ~3 times greater than fora single
        image from the initial 30 images. Also, the stars should be greatly faded.

Sample Images
 

Figure 1. One of the initial images after dark frame and flat frame calibration. The moon was only 20 degrees away which accounts for the bright band on the left. FHWM ~3.5 "arc. Limiting magnitude ~18.5 (which is ~0.7 mag worse than normal because of the full moon's proximity). Two circles (right side) show the locations of asteroids based on an ephemeris. Since the asteroid magnitudes are 18.9 and 20.0, which is fainter than this image's limiting magnitude, all features within the circles are stars. [Meade LX200GPS, SBIG ST-8XE, blue-blocking filter, 60-second exposure, tip-tilt image stabilized using an SBIG AO-7; 2007.12.31 UT; Hereford Arizona Observatory]

Figure 2. Final image after star-subtraction and asteroid track and stack using an initial image set of 40 1-minute exposures.
 
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This site opened:  January 2, 2007.  Last Update:  January 17, 2007