Interpreting the data from astrometry.net

[Demonperformer]

I have run an image through astrometry.net and the attached image is part of the data provided in the axy.fits file (at least the first four columns are, the other two were calculated because they looked as if they might prove interesting).

I understand the x & y columns.

Flux is explained (https://astronomy.swin.edu.au/cosmos/F/Flux) as "the total amount of energy that crosses a unit area per unit time". This sounds like it should be directly (or at least logarithmically) equatable to what muggles like me call magnitude. But I have failed to find the formula that does this - everything that talks about flux seems to focus on the effects of distance on flux, which is all very interesting, but not what I am after. So my first question is: Does anyone have a handy formula for translating flux into magnitudes?

The fourth column (background) looks a bit more complicated. My initial thought is that this is the brightness (flux) of the bit of sky around the star ("around" being a vague way of expressing what is, no doubt, far more precise than that). Which got me thinking about subtracting one from the other. This would presumably give the brightness of the star in relation to its background, but I got quite a few with negative numbers which seemed to imply they would be fainter than the background and so you wouldn't see them! So I then thought about dividing, which at least got rid of the negative numbers, but I'm not quite sure why I did it other than that - just seemed like a good idea at the time. So my second question is: What exactly is this figure and how does it relate to/what impact should it have on the flux of the star (if any)?

Thanks.

[vlaiv]

3 hours ago, Demonperformer said:

Does anyone have a handy formula for translating flux into magnitudes?

Assuming that you have background removed and you have pure flux of target star then magnitude of that star is equal to

-2.5 * log (F/F0)

where log is logarithm base 10 and F0 is flux of zero magnitude star using the same instrument

In aperture photometry - there are 3 rings that are used to determine needed quantities.

First / most inner ring is zone where star light is concentrated. This is how flux is measured - simply sum of all values of pixels inside that inner ring.

Then there are second and third ring. All pixels that fall between these two rings are considered background pixels and average value is calculated for these pixels. This is background value.

When you want to calculate pure flux for star, you do following:

You sum all pixel values in inner ring and then subtract mean background value (calculated from pixel values between other two rings) times number of pixels that are inside inner ring.

This gives you "pure" flux value.

I don't really know what columns astrometry.net returns, but you can do aperture photometry with AstroImageJ as well and it will give you values like I described them

Different columns represent different values

(Column source - sky for example is source brightness - sky brightness. N_Src_pixel is number of pixels used for source, N_Sky_pixels is number of sky pixels, etc ...)

[Demonperformer]

Thanks for the reply.

I'm going to have to have a think about this and then have a play with the figures I have been given. First impression is that, even though I won't have F0, as that would be a constant, if I can find the factor that equates one star's (flux-background) in your equation with it's recorded magnitude, then the others would be calculable. Even if I were to go to the trouble of photographing a zero magnitude star every time, presumably the figure would be affected by where it was in the sky and so would not be directly applicable to a photograph taken elsewhere. Any magnitudes are really an added bonus for what I am doing, so I'm not too worried, but wanted to explore my options.

Thanks.

[vlaiv]

Does not need to be zero mag.

You can take any reference star near by (so that extinction and LP levels are similar) that you know magnitude of.

General rule - magnitudes subtract while fluxes divide

So ratio of fluxes = difference of magnitudes