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The Minor Planet Observer
Palmer Divide Observatory

2007 Shoemaker Grant Recipient

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MPO Canopus:
Using StarBGone!

StarBGone! is a feature in MPO Canopus that subtracts field stars from images as they are measured. This allows keeping data from many images that you would otherwise have to reject as an asteroid moves through a star field.

Click here for more information about StarBGone! Be sure to note the limitations of StarBGone! It can't subtract out Sirius as a 16th magnitude asteroid passes nearby!

StarBGone! does not require that you align images beforehand. Instead, the setup is part of the lightcurve wizard, which is where you define the comparison stars and target to be measured. Just as with the comparisons, Canopus uses the offset from the "anchor star"
(#1 comparison) to determine the location of the stars used in the StarBGone! process. This makes using StarBGone! easy and straightforward.

Define A Session

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Just as you would before measuring a set of images, define a new session. Nothing special is required when using StarBGone!

Start the Lightcurve Wizard

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On the first page of the Wizard, be sure to check the "Star Subtraction" box. This causes the wizard to display some additional pages that explain and then let you define the stars used by StarBGone!

Set the Comparisons and Target on the First Image

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Again, nothing special is required to this point.

Define the Reference and Subtraction Stars

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The page on the Wizard shown above appears after you've set the comparisons on the first image but before you set them on the second image. In reality, the measuring apertures would be surrounding only one star at a time. They were added here for clarity.

  1. Click on the Reference star on the image. This is the star that will be scaled to match the intensity of the one or more "subtraction" stars (those with which the asteroid crosses over or near). See below for some hints on selecting the reference star.

  2. Click the "Set Ref Star" button. The approximate X/Y position of the star is displayed to the right of the button.

  3. Click on the first subtraction star.

  4. Click the Add button. The data for the subtraction star appears in the list above the button.

  5. Repeat steps 3 and 4 for each additional subtraction star.

  6. Click the Next button to proceed to the page where you define the positions of the comparisons and target on the second image. It is NOT required (or even allowed) to set the position of the reference and subtraction stars on the second image.

  7. Finish the Wizard and measure images as you would normally.

StarBGone! is an "Invisible" Process

The subtraction process affects the "in-memory" buffer of the image and only after you accept the placement of the apertures as each image appears. This means you will not see a modified image with the stars removed. The subtraction results are not saved back to the hard drive, thus preserving your original image.

Selecting a Reference Star

There are several considerations for choosing a reference star:

  1. The star must be on all images that will be measured.

  2. It cannot be at or beyond the point of non-linear response of the camera or to where it shows "chimney-piping" (vertical streaks).

  3. It must be isolated so that no other stars will be included in the calculations to scale it to each subtraction star. A square region with its sides about 1.5x the diameter (or width) of the inner measuring aperture is used for scaling.

  4. It must be brighter than any subtraction star but not to the point where it is overexposed (see #2). The brighter the star, the better the scaling.

  5. The reference star can be one of the comparison stars. It cannot be one of the "subtraction stars."


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This page was last updated on 01/19/11 05:14 -0700.
All contents copyright (c) 2005-2011, Brian D. Warner
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