Hi Fred,

For 20 contiguous pixels you'd need say a grouping of 4 x 5 pixels above the background cutoff. This is a "radius" of 2.25 pixels. That would make the HFR around a pixel so its certainly possible to get some hits with an HFR < 1.

On the other hand with Thresh Multiple set to 2.0 you'll be extracting above the background noise about 95% of the time (2 sigma) so 5% of the time pixels above the background cutoff will actually be background noise (assuming the noise is gaussian). So its certainly possible to get a grouping of 20 pixels > background cutoff that are just noise.

If you zoom is on a star with an HFR < 1 so that you see individual pixels you'll be able to see whether it looks like a real star or not. If its just noise then there are several options:
1. You could increase Min Area.
2. You could increase Thresh Multiple.
3. You could specify a Min Size > 1 so stars with an HFR < 1 will be ignored.

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