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So I have a photometry data sourced from Solar Mass Ejection Imager's (SMEI) server as an ‘add-on data’ for my point source of interest (a variable star) with a 104 minute cadence. However, it’s not really in the UBV photometry system, in V-band specifically, but rather scales with their SMEI photometry system while processed through a centroid fitting.

Is there a possible way to initially calibrate the photometry data like mapping the point source's nearby bright stars perhaps? And if so, how should I start of?

Attached photo is the 'raw' light curve of the star from the gathered SMEI data.

enter image description here

Please bear with me for I'm just a novice especially with regards to photometry. Guide(s) and response(s) are always welcome and I look forward to.

Clear skies.

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  • $\begingroup$ Welcome to astronomy SE! Interesting question indeed, I am sure there is someone out there to help you soon. Just one remark: I added two tags and a wiki-link to make your question a bit more accessible. I am not quite sure what exactly you are after, but I guess that's because I am not an expert of photometry either. $\endgroup$ – B--rian Mar 5 at 10:54
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    $\begingroup$ Hello. Thanks @B--rian. I'm afraid my question is little bit on a grey area (I'm actually hesitating on posting this ^^). Nonetheless, what I meant with regards to calibration of photometric measurement, is if it's possible to convert or "re-scale" the data into those Johnson systems because; 1.) They do have different system not those Johnson UBVRI and so its magnitude system. 2.) Different analysis (they're using 'centroid fitting technique' which I'm not familiar) thus the light curve of the variable star won't totally 'fit' to the existing data e.g. aavso. $\endgroup$ – CGHA Mar 5 at 11:16
  • $\begingroup$ I think your question is fully on topic, but I am not familiar with the Johnson system and its defintion. Although I know fitting with quite a few techniques (outside of astronomy, mainly), I have not heard of "centroid fitting" either. Maybe you could share links on both topics, please (e.g. as an edit to your post which adds a section "background")? $\endgroup$ – B--rian Mar 5 at 11:19
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If I understand the question correctly, you have raw data from a CCD sensor in arbitray units (which correlate with brightness) and your challenge is to callibrate this intensity for a certain frequency range.

Not knowing much about the SMEI sensor and where on the sky it has been looking at, I struggle to give you a full recipe. If you have for an object whose intensity (time series) from other observational instrument in the desired frequency band, you could use that non-SMEI data to correlate (if not even fit e.g. linearly) with the raw data time series for the same object as recorded by the SMEI sensor. This gives a calibration function which delivers intensity from raw data in arbitraty units.

References which sound possibly helpful

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I have never worked with SMEI but I do have some experience with photometry and, while I do not understand completely the question, I think I can give you some ideas. First of all, did you get the data from here? If so, depending on what you will use it for it might be a good idea to look for spurious variability and remove it (for example, section 2.2 of this paper). Besides that, according to their website, the data should already be in their own photometric system as you stated (when you mentioned raw photometry my brain linked it to starting the whole process of CCD calibration), and I think that the centroid fitting is related to locating the objects on the images (such as some of the outputs in here). Getting the data to the standard photometric system might be attempted by what you are suggesting, and specifically, I can think about the following:

  • As B--rian suggested, you can correlate the two systems, although to do this you would need a good sample of stars with measurements in both systems, and perhaps follow examples such as the photometric relationships for Gaia or this paper.
  • Depending on what type of variable star you are observing, you could consider the physics behind the variability to correlate the different filters. This is an example for Cepheids.
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