Changes between Version 11 and Version 12 of PS1_GPC1_Magic_Status_20090628
- Timestamp:
- Jun 29, 2009, 3:11:15 PM (17 years ago)
Legend:
- Unmodified
- Added
- Removed
- Modified
-
PS1_GPC1_Magic_Status_20090628
v11 v12 1 = PS1 GPC1 Magic Status Report of 2009.06.28 = 2 3 (Back up to [wiki:IPP_for_PS1 IPP for PS1]) 4 5 == Magic False Positives == 6 1 7 We have been working to reduce the number of false-positive detections from magic in GPC1 images. 2 8 … … 13 19 First, there is only a single real satellite streak in this image, and it is easily detected. However, there are many false positives. It is clear from their orientation along the camera rows and/or columns that most of the false streaks are due to artifacts in the image associated with the camera structures. A closer examination reveals several types of artifacts. Three regions are marked in the greyscale image. Below, we show some images to illustrates these effects more closely: 14 20 15 1) tiltystreak artifacts: 21 === Tiltystreak Artifacts === 16 22 17 23 [[Image(tiltystreak.artifacts.jpeg)]] … … 21 27 In the longer term (beyond this initial desire to deliver MD field data to the consortium), we will need to decide with a larger set of data which cells / chips are better handled by applying tiltystreaks, and which would be better without it. It may be the case that the choice depends on the conditions, or perhaps which need a statistic to judge if the application improves the image or not (would a simple robust measure of the cell noise be sufficient?) 22 28 23 2) glints: 29 === Glints === 24 30 25 31 [[Image(glints.jpeg)]] … … 27 33 This image is another zoom on the greyscale image above. In this case, there are two glints which are visible starting at the right side of the image and running roughly parallel with the cell edges. There are several such glints visible in the larger image, at least two on each of the left and right sides of the focal plane. We have seen these structures in many images, and it is generally clear what is happening: a bright star is falling on a structure on the edge of the focal plane and reflecting off a roughly 45 degree surface. Exactly what the reflection sources are is unclear. The geometry of the glints is telling us something about what they could be -- we usually (always?) see glints which are parallel to the chip edges, and usually from the top & bottom of the camera, as in this case (keep in mind the rotation). For now, I think we will have to accept these as extra streak sources. With some effort, perhaps we can determine the physical location of the glint sources and determine the stars causing the glints; if they can be well modeled, then they can be masked in the camera stage just as ghosts and other features generated by stars are currently masked. 28 34 29 3) Other structures: 35 === Other structures === 30 36 31 37 || [[Image(xy66a.jpg,550)]] || [[Image(xy66b.jpg,550)]] || … … 36 42 37 43 The other features are persistent star trails which were not corrected by burntool. These structures are seen in other parts of this exposure. I am somewhat surprised that burntool left these behind. It is certain that this exposure was processed by burntool, but I wonder if there was a failure of some sort in the processing or the sequencing. We can double check that burntool has entries from the previous exposures. It might simply be that the decay timescale is not long enough so that these escaped the burntool algorithm. Assuming that burntool was correctly run on the exposures in this sequence, then we will have to accept these type of artifacts until burntool can be improved. 44 45 == Variance of the Difference Images == 38 46 39 47 We have also had concerns that some of the Magic false positives are caused by fractional errors in the variance. We have found that the GPC1 data processed to the difference level has a higher pixel-to-pixel standard deviation in the background than expected from the noise propagation. In the case of the exposure above, the effect ranges from about 15% to 25% (ie, stdev is 15% to 25% higher than expected). We have shown that this effect does not appear in our simulated data analysis, which seems to suggest it is not caused by the software specifically. The last four images illustrate the impact of an error in the variance level of this scale. … … 50 58 Although we do not understand the source of the elevated variance, we can correct for it by measuring it during the difference analysis. The robust statistic can be used to measure the standard deviation of the pixels in the signal-to-noise image, and adjust the variance of each skycell as needed. If we implement this, we need to take care to raise an exception if the predicted variance change is too large -- that could be evidence of something else going wrong. 51 59 60 == Summary == 61 52 62 In conclusion, we have a few areas where we can further attempt to address the causes of magic false positives. There are still a number of camera artifacts that can be masked; we can avoid applying tiltystreak to certain problematic chips; we can check if burntool was run correctly; we can empirically adjust the variance to match the observed pixel standard deviation. However, there are definitely features which will be difficult to address. The biggest concern in my mind is the variable nature of the features and structures we have seen.
