Ali sent some FITS files on Slack. Images are
-rw-r--r--@ 1 cstubbs staff 58M Sep 24 06:53 CU2A6058.fits
-rw-r--r--@ 1 cstubbs staff 58M Sep 24 06:55 CU2A6042.fits
-rw-r--r--@ 1 cstubbs staff 58M Sep 24 06:55 CU2A6050.fits
-rw-r--r--@ 1 cstubbs staff 58M Sep 24 06:55 CU2A6081.fits
my first comment is that we need to be sure the analysis pipeline appends a date+time stamp when the FITS files are made. I think the reduction scripts I wrote long ago do that, if not we need to make sure we do that.
Images look like this:
This image 'stretch' (mapping of intensity values to greyscale) shows more clearly that the main sources totally stand out in the frame:
Here is what I see:
- there is horizontal striping that we can get rid of by subtracting a median column from each column in the frame.
- There are 7 bright sources and each of them has a ghost to the left, presumably from an internal reflection in the lens
- There is a source of stray light in the bottom of the frame that's bright smudge
- There is what looks like a lens flare ghost as an arc emanating out clockwise from the bright-smudge source, that overlaps one of the sources.
- In all 4 frames, the sources of interest don't shift much.
Sources of interest are at (roughly)
| x | y |
|---|---|
| 748 | 3841 |
| 763 | 1580 |
| 1503 | 3083 |
| 3731 | 3854 |
| 3769 | 1626 |
| 4502 | 878 |
| 5227 | 2374 |
Bad smudge ends at y=500 and we don't have a source below y=500.
Here is what I would do to process these frames and pull out the centroids of interest:
- Find and kill the stray light source. But we can't go back in time and do that, so instead just cut off y<500 from the frames, to make things cleaner
- compute the median of all columns and subtract that from each column, to get rid of the horizontal streaking
- Either smooth the image with a Gaussian kernel with sigma~3 pixels to account for residual Bayer pixel artifacts and fragmentation of the PSF
- run a source detection code with a fairly high threshold of significance, and then select out the bright sources that have centroids near where we expect them.
- compute mean of all 7 centroids, call that the 'average position' of all 7
- subtract that average from each of the 7 positions, which is differential motion of each spot.
- I'd also compute the mean intensity for all 7 spots, and take the ratio of each spot's intensity to that mean. This will let us look at scintillation as well as image motion.
- Put the results into some sensible data structure(s)
For reasons of time I'm going to skip steps 1,2,3 and run Source Extractor on the four frames, with a tweaked configuration file that requires high significance for detection.