ST5000 Blob Mode

Change Log
Version Date Comments
1.0 14-Dec-2010 Initial Release

This section describes "blob mode". When acquiting stars, the ST5000 expects well-formed star-like targets, and rejects non-conforming objects such as blobby fibers, nebulae, the moon, planets and so on. You can invoke "blob mode" to change this behavior. In blob mode, the brightest thing is acquired and tracked. The normal star-recognition tests are bypassed.

How Blob Mode Works

Using Blob Mode

How Blob Mode Works

Here is how blob mode works. In normal tracking, a 7x7 (pixel) track box is used. It is centered on a star-like object, and a centroid computes its precise (sub-pixel) location. In Blob Mode, imagine a track box that can be any multiple of the normal one. These values are allowed:

You select a box size that is large enough to enclose your blob. The acquisition goes like this: the track box starts in the upper left corner of the image. We sum up the number of counts enclosed by the box. For a 21x21 box with just a background of 20-ish counts, this sum would come to about 21*21*20 ~ 8800. Then the box moves one width to the right, and a new sum is calculated. The brightest one is remembered. Continue left to right, top to bottom, and you end up knowing the location of the brightest track box. This is probaby not centered on your blob, but will contain more than half of it.

During tracking, the track box is dropped down on the previous location of the blob, and a centroid (a flux-weighted average row and average column) is performed using the whole box. The new position is the value of the centroid. No shape-checking is done, no edge-finding, no enforcing of anything, just a centroid over the whole box. If the box starts off not centered on the blob, the centroid will "walk it in" in a few iterations.

The optimal box size is such that it just encloses the blob; too small, and it will "walk around" over the surface of the blob. Too big, and the tracking noise goes up as non-blob pixels contribute to the centroid.

Using Blob Mode

Blob mode is invoked by a command "blobmode on" or by using the Digital Out Bit 4 (between Acquire and LIS_TST) toggle switch on the GSE suitcase. Turn it on before invoking the acquire, and leave it on until you stop tracking and go back to Idle. However, if you are in an normal-mode "aqcuire loop", with continually failing attempts to acquire, then flipping on blob mode will probably succeed in getting you out of the loop and tracking.

You need a recent copy of the Java Graphical User Interface (GUI). This is compatible with both the MarkIID and Mark IIDG trackers. The blobmode commands are on Uplink1 and Uplink2. The blobsize selecter is on Uplink2. The blobmode state and selected blob size are on the main GUI in the Digital Inputs, Software Inputs, and Camera panels.

We tested blobmode by turning on the room lights while using our star simulator. We saw this. The bright cores are the usual star-like pinholes, but the room lights enter via scattering to form large (~20 pixel) blobs. Selecting a blob size of 21 worked nicely. The track box panel looks like this. In order to display the blob, it is "down sampled" into the usual track box. That is, for this case of 21x21, groups of 3x3 pixels are averaged down to one value. In this example, track box 0 represents 21x21 camera pixels instead of the usual 7x7.