The Image History tool will give you insight, at a glance, as to which recently captured images might be bad. Each series is defined by the target, binning and filter and contains data for HFR and Star Count. Regardless of the originating event, a history item will be grouped together if all three of these items are identical. Using the drop down box at the top, you can select any series and view a chronological display of HFR and Star count data. In an ideal world, both HFR and Star Count would remain relatively flat. In reality, there are many environmental factors out there that aim to destroy your images... temperature drop, clouds, guiding, differential flexure, etc. If you see a significant deviation from a typical straight line, this might be an indication of a bad image. To inspect an image, simply double click on the list view or click on the graph point representing the image you would like to inspect. If you decide it's bad, simply click the "X" icon on the image or right click the image in question and choose "Mark as Bad". This will prepend the image with "BAD".
To clear all image history, right click almost anywhere and select "Clear image history". You can also clear just the current series by clicking "Clear current series".
Thus tool has a dockable module counterpart for convenient viewing:
Possible example of bad image from temperature drop (loss of focus):
Notice the drop in star count with a simultaneous rise in HFR.
Possible example of bad guiding on a single frame:
Notice the star count was relatively constant, but HFR increased. The next frame returns to normal.
When you are imaging in a star rich area, the number of stars that can be detected is capped at 300. In this case, the graph will appear as such:
Notice the star count is constant, but bad images are still easily detectable by referencing the HFR data. In this case images 7, 8 and 10 are bad (this graph is from real data).