Apparent brightness / photometric calibration
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Anyone know what is a good source explaining how to obtain the brightness of an object from FITS files for a citizen scientist like me? From scratch that is, like explaining to a fresh green student 😃 .
Or perhaps it's even possible to give the needed steps and actions here in thread?
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Budgieye
moderator
Ummm..... ?
http://www.sdss.org/dr14/algorithms/magnitudes/#nmgy
http://iopscience.iop.org/article/10.1086/324741/fulltext/
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bluemagi
Hi, I have a book that might help you, its called:From Luminous Hot Stars to Starburst Galaxies by Conti,Crowther and Leitherer. Chapter 2? I am not sure this is what you want. Cheers, bluemagi
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Thank you both, unfortunately not really what I'm searching for.
In this case I have a FITS file from DECaLS in Aladin Desktop (or DS9) and I want to measure the apparent brightness of a particular object. So need some sort of nitty-gritty step-by-step student level description of measuring that. A tuturial for sextractor for example or something similar.
Thanks
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JeanTate
in response to Ghost_Sheep_SWR's comment.
By "measure the apparent brightness" do you mean something like "get an estimate of the apparent magnitude (in the band in which it was observed)"?
If you're thinking of using SExtractor, I salute your ambition and determination! 😃 Perhaps a good tutorial is (Benne) Holwerda's SExtractor for Dummies.
How precise and accurate do you want your estimate to be?
One reason I ask is that there are several Q&D methods of getting good, but not so precise or accurate, estimates, using DS9 (and Aladin).
For example, in the field your target is located, find stars with published magnitudes (preferably in the band you're observing in) and use DS9 Regions to get the total flux (or ADUs or whatever). A quick plot will give you a line (take the log, magnitudes remember!) and your object's apparent magnitude will follow with a bit of arithmetic. It helps that the DECaLS response curve is (close enough to) linear (AFAIK). If your object is faint and/or extended, you'll have to do sky subtraction, which is fairly straight-forward if somewhat tedious.
Is this the sort of thing you're looking for?
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Yes I do mean apparent magnitude (no idea why I called it brightness) 😃
I found B. Holwerda's tuturial already and seems like a good starting point, except that I've also read somewhere that SExtractor is for large fields / many objects. But I'm really focused on a single object of interest and if possible get a good mag estimate. So I wondered if there are easier and better methods explained somewhere. And the object is not faint or extended.
Yes I get the basic idea of plotting such a line objects with known magnitudes, and think it might even not be necessary to use stars with published magnitudes since there are also a lot of objects around in DECaLS with given magnitudes that I can use.
But YES that is exactly what I want to do, preferably in Aladin but if need be in DS9, do you perhaps know of a source where this is explained in detail / step-by-step, esp. the arithmetic? (FYI I didn't know you had to use log and don't know why this is necessary).
Thank you very much so far 😃
(Q&D?)
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JeanTate
in response to Ghost_Sheep_SWR's comment.
"brightness" is fine; it's just that it's a bit ambiguous (OK, strictly speaking "magnitude" is too - per unit frequency? or wavelength? - but too detailed for here and now).
I've also read somewhere that SExtractor is for large fields / many objects
AFAIK, it is certainly better in such cases, but it'll work fine for smaller fields too I think.
I have little experience with Aladin, so I really don't know what to suggest. For DS9, well, it's infamous for the lack of tutorial-like material, so if you ever do find something, please let us all know! (This Astrobites might help). If you're comfortable with Python, there's a fair bit of material on how to estimate magnitudes (or similar) based on FITS images as inputs (e.g. on stackoverflow)
The CCDs that are at the heart of most modern cameras have a response that's very close to linear (ignoring 'dark current' and saturation), one photon = one electron. However, the magnitude scale is logarithmic (5 mags = 100-fold difference in flux; 10 mags, 10k). I don't know about DECaLS, but many surveys put a huge amount of effort to produce 'calibrated' FITS, where 1k counts (or ADU or ...) equals the same number of photons (actually flux) no matter what field it's from. If so, then with "a lot of objects around in DECaLS with given magnitudes that I can use", once you've worked out a counts/magnitude relationship you're almost finished (I'm assuming you're OK with estimating apparent magnitude to ~±0.3 mag or so, and not ±0.03). You should probably do a sanity check ... see how changing the size of the DS9 Region you use changes your estimated apparent magnitude ... you may find you'll need to do 'sky subtraction'.
Thank you very much so far 😃
You are very welcome! 😃
"Quick and Dirty" ... maybe I'm showing my age 😦
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That astrobite link also leads to some other tools that might prove useful, so yeah that helps a lot; GAIA - Graphical Astronomy and Image Analysis Tool / AstroImageJ .
Also the object has its magnitude given for three exposures so that's probably useful as a convenient check.
I'm convinced I can figure it out eventually, I'l report back when I'm successful (don't hold your breath) 😃
Again thanks!
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