Major Update to Stellar Photometry Tool

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Major Update to Stellar Photometry Tool

#1

Post by admin »

Hi folks,

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 :D

Let's start with a screenshot...
Screenshot 2024-10-07 155518.jpg
When the tool is activated, SharpCap will detect stars in each image using the star detection parameters that can be adjusted at the bottom left (actually the max count now does nothing - just writing this post has let me spot small issues with the new version!). Detected stars are highlighted with small circles around them in the image - the colour indicates how suitable they are for photometry measurements:

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
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Jean-Francois
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Re: Major Update to Stellar Photometry Tool

#2

Post by Jean-Francois »

Hello Robin,

Nice tool.

I tested it with some images from the Thymbraeus asteroid (occultation ~ 2 weeks ago).
I did the analysis with the Prism (v10) software.
So I wanted to compare the SharpCap tool with Prism photometry analysis.

Short ... you need some improvement :-) It was not possible to reproduce the same analysis.

Here a summary of the Prism output:
Thymbraeus_courbe_de_lumiere.jpg
The first image is the sum of all the images (fast 5 hours with 127 images @ 120 seconds exposure).
The second image shows the first image of the sequence with the detected trace of the asteroid Thymbraeus.
The third image shows a statistic of all the detected stars (rms vs. magnitude). Some points are "off" the main curve ... they are not visible on all the images (cause by a drift of the images ... bending between the guiding telescope and the Newton tube).
The last image shows the light curve of the asteroid Thymbraeus ... it rotates in ~12 hours and the shape is not spherical.

Note that it is a special case ... the target is moving and very faint (magnitude around 17).
So the threshold on 15% is may be too high.

If you want to play with the images ... then I can upload it.

Regards,
Jean-Francois
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Re: Major Update to Stellar Photometry Tool

#3

Post by admin »

Hi Jean-Francois,

thanks for trying it out!

I must admit that I wouldn't really expect it to work on a moving target given the code as it is at the moment... The code is set up to try to deal with stars moving from frame to frame because the whole image has drifted, rather than because the target is moving. I suspect that at some point the movement would pass a threshold level meaning that the code would decide it was a different star rather than the same one.

Happy to have a look at the data to see if anything might help that situation, but actually a stationary target would be more in keeping with what I'm trying to achieve at the moment.

cheers,

Robin
jld37
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Joined: Sat Nov 27, 2021 7:07 am

Re: Major Update to Stellar Photometry Tool

#4

Post by jld37 »

Hello,
Many thanks for this new feature.
I'd used the first photometric tool and I tried to measure SN magnitude
by comparaison and calculation between the Adu and magnitude of known stars...
And now SC will do the job...!!!!!
Jean-Louis
timh
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Re: Major Update to Stellar Photometry Tool

#5

Post by timh »

That looks brilliant Robin. - will have a go at repeating the variable star study I tried before with the old tool and see how this improves things. It looks far better now and should be much more accurate
Tim
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Re: Major Update to Stellar Photometry Tool

#6

Post by admin »

Hi Tim,

feedback would be very welcome - I'm not really a variable star observer, so I put together what I thought *might* be useful, and did the calculations to the best of my understanding. It's quite possible that things aren't quite right yet, and comparisons with other measurement approaches would help root out any errors!

cheers,

Robin
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