Photometry: time between images; number of comparison stars

Hello everybody,
the last weeks I used Siril to photometry Betelgeuse with a slightly defocused 35mm lens (no tracking).
Usually I take a couple of lights, then some darks, then some lights again and so on. So, there are some minutes between each series of lights.

Global Star Alignment without the “Translation Only” mode works flawlessly and I can do measurements on all lights without much noise (even less noise with 0.9.10 than with 0.9.12).
But as far as I understand one should use “Translation Only” for photometry.
Unfortunately, “Translation Only” only works for the first series of lights.
In my case the first two series (in 0.9.10 just the very first series, so there is improvement),
the later lights produce just noise, because the “measurement circles” are at wrong positions:

0.9.10

0.9.12

I hope you can fix this, as Siril works perfect without “Translation Only” mode :slight_smile:

Nice to have:
For multi-star photometry and calibration it might be useful to have more than six compstars per run. I do not know how other programs handle this. Maybe you could add some more colors or stay with your color scheme and use one color more than once. To distinguish the individual comparison stars, it would be cool to select a star by its number in some kind of legend and highlight its plot. :slight_smile:

To calibrate my DSLR for B-V measurements, I did some measurements in G and B of the same stars.
I think to measure on the new layer you have to use the “Clear all” button and start again with picking the stars.
It would be nice to do these measurements at once, e.g. with checkboxes for R, G, B, but I’m not sure how many users need this.

Clear skies!

Please, could you share your sequence?

Indeed I would like to try. You have many way to do. You can try to align image with one star algorithm (with the follow star activated).
It generally works fine.

Will see what I can do, but it will take a bit, it’s 400MB :wink:

Ok, gzip did a good job and I found Firefox Send, but it seems like I can’t send you a private message. So, you will get an email!

Edit: There is a problem with your free-astro mail address. The file will expire after 5 downloads. Not sure if I should post the link here publicly. Maybe you have a different mail address?

You can send me a private message through pixls :wink:

Well, I tried that! Found the Messages tab and had a look on your profile, but I have no idea how to send one… Perhaps it works if you send me a PM?

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Doesn’t work for newbies like me :pensive:

OK, I’ll give it a try, but remember: 5 downloads only :wink:

Download in progress.

Please try again. Does it work now?

Yes :+1:

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Hello.

I quickly did it (with experimental version of Siril) and that works fine. Should work with 0.9.12.


On this screenshot you can see how I make a selection around betelgeuse to align on one star.
It works.

In fact I’ve redone this after applying an ubanding algorithm. It was even better.
Then, after my aligned and unbanded sequence I’ve exported the resut in order to get a new aligned sequence.
That will be the sequence where I will try photometry.


In this screenshot you can see the light curve I have as a result.
You see that the light decreases.

But now, results are poor because your images have some issues and especially a lot of banding. You also have a huge move between your frames and that leads to an accuracy loss because some stars are too close from the edge at some time.

But, it works.

Thanks for your efforts.
I certainly don’t photometry often enough and my camera is not the best, so thank you for the hint to the ubanding algorithm.
I’m playing with the one star alignment (OSA) a bit now, but honestly I’m not getting nice results yet. The computation also takes much longer than global star alignment (GSA), but I will keep trying.

Again my thoughts about the GSA, maybe I am missing something:
“Translation mode” off works perfectly (huge moves between frames don’t matter!), is fast (!), pretty easy to use (no selections and picking stars for alignment needed) and processes all images during the magnitude measurement of a specific star.
For me this means that Siril knows where in the individual images the stars are that I want to photometry (of course it knows!). But for photometry the image rotation should simply not be applied. Therefore Siril should still be able to identify the stars you want to photometry in the non-rotated image. I think there is still a problem right here (see pictures in the first post).
Of course all roads lead to Rome, but I think the GSA road is quite close to being the most comfortable one. :wink:

So you can do global with translation only. That don’t produce new sequence. But you can export result and work on it.

EDIT: There’s maybe a bug… wrong sign in the translation … At least on the 0.99 version. Need to check with 0.9.12
ping @vinvin

Hmmm in fact this is difficult to say because images have rotation. So global registration with translation only won’t work.

EDIT, again: I can confirm. No bug :D.

Here a better analysis.
The curve is nice :).

You did this analysis with OSA, right? I’ll try it tomorrow.

You see, I’m somewhat obsessed with the GSA method for the reasons mentioned in my last post.
GSA with “Translation only” could be used for photometry. This won’t work, if images have rotation (and images with 35mm lens have rotation). I think I got this.
GSA without “Translation only” computes translation and rotation angles for each image. This is easy and works perfect but is not ok for photometry.
Maybe I’m imagining things too simply: Why not define a layer of measurement circles (V, 1, 2, 3, 4, 5, 6 and perhaps even more) on the reference image and translate and rotate this layer according to the results of GSA (without “Translation Only”) and apply these rotated layers to the non-rotated images? Then, the non-rotated images are used for photometry (which is fine) and the measurement circles should match with the stars we want to measure.

I’m not sure to understand. Sorry.
But OSA is so easy to use. Produce no new images and almost never fails :).

In photometry we want to avoid to apply rotation in order to avoid interpolation that could modify raw signal.

Ok, it’s late in the evening :wink: Will have a look at OSA tomorrow.

My last words before I have to take a nap:
GSA without “Translation only”: layer of measurement circles (V, 1, 2, …) is fixed and the images are rotated in such a way that all stars match >> Works perfect, good for stacking, but bad for photometry, agreed!
My idea: Why not keep the images unrotated, but rotate (and translate) the layer of measurement circles (GSA without “Translation Only” makes all necessary calculations of translation and rotation already now!) >> Good for photometry since all images are not rotated.