this is something that I have been working on in the background for a while, and although it's not finished yet, it's now complete enough to let people have a play with it, make suggestions, find bugs and generally give feedback helping to shape how the final version might look.
The old photometry tool was pretty basic - just gave you a readout of total star brightnesses on the display, in both ADU and electrons (if e/ADU data was available). The new tool does a lot more
Let's start with a screenshot...
- Screenshot 2024-10-07 155518.jpg (495.24 KiB) Viewed 1314 times
Red - unsuitable - peak saturation below 15% or above 85%, so either very faint or close to being saturated
Green - suitable - peak saturation between 50% and 85%
Orange - sort of OK - peak saturation between 15 and 50%
If you have plate solved then some stars will have an additional outer blue circle - those are stars that SharpCap has matched up against the plate solving star catalog and can assign a rough v-magnitude value to those stars based on the plate solving catalog. Note that these magnitudes are purely an estimate - the Vmag value is estimated from Gmag and BP-RP by the Johnson-Cousins relationship given in table 5.8 (https://gea.esac.esa.int/archive/docume ... tml#Ch5.T8), and they are only stored to a precision of 0.05 magnitudes anyway.
Anyway, to actually see something useful happen beyond circles round stars, you need to select a target star and at least one reference star. Click on the radio buttons next to 'Target', 'Ref Star 1', etc to enter star selection mode and then click on the star you want to select as the target or reference stars. For reference stars, SharpCap will auto enter the Vmag value mentioned above as the magnitude of the star (if the data is available), but you can also manually enter an exact magnitude.
SharpCap will now calculate an estimate of the target magnitude for each frame based on the relative brightnesses of the target and reference stars and the known magnitudes of the reference stars. The graph will show how this varies with time, and also error bars on the measured brightness.
You can save the data obtained so far as a CSV file using the 'Save Data' button, or wipe the data using the Reset button.
Note that you can change target or reference stars without resetting - SharpCap saves the measurements for all detected stars since the last reset, so if you change the target star then a new graph will pop up based on previous measurements, not just starting from the time you changed the target.
Another trick is to use the 'Select by Coords' button to enter the co-ordinates of the target you want to measure (rather than trying to find it in the image). SharpCap will find and select that star (if it's in view and a star has been detected at that position) and will also auto-select three nearby reference stars to measure against.
Drift of the image while measuring shouldn't be a problem - with every new frame, SharpCap matches up the stars in the new frame with the ones it is already tracking, accounting for any drift/rotation in the same way it does when live stacking. Of course, sometimes a particular star might not be detected in every single frame, so there may be gaps from time to time in the graph if the target star is not always being detected.
I am attempting to calculate the errors in the magnitude measurement properly - when e/ADU data is available from sensor analysis, SharpCap will calculate the number of electrons represented by the star image and use the poisson distribution standard error for each measured brightness. This is then carried through to the calculated target magnitude using the standard rules for combining uncorrelated errors. I'm not 100% convinced that this is quite right though - the error bars seem small relative to the random variation in the measured brightness from frame to frame, so need to check that through.
Along with this update to add more photometry functionality, I've added a catalog of about 800 or so variable stars (selected to be short period, relatively bright ones).
Unfortunately clear skies have not materialised since I got this all into a workable state a few days ago, so I have been unable to test on a real observing run yet, just processing previous images...
Anyway, I think this feature can best be described as 'work in progress'. Enough of the parts are now there for it to do stuff and calculate magnitudes, but the user interface and user experience (how you interact with the UI) needs more work. The calculations may not be quite right yet (particularly the errors as mentioned above). If anyone cares to go out and try it out, I'd be happy to hear any feedback
cheers,
Robin