I have a couple of questions.
Will be grateful to have answers.

When downloading fits image from Image Cutout:
1) Are the fits images calibrated and sky-subtracted in each band?

2) Can I measure my own magnitudes on downloaded image by the following formula?
POGSONMAG = ZEROPOINT + 2.5*log10(EXPTIME) - 2.5*log10(∑data-units),
where ZEROPOINT and EXPTIME are from fits header, while ∑data-units is measured directly from the fits (similar to Pan-STARRS1 description).

3) Are the fits images already stacked? (with different EXPTIMEs)
Can I directly download maximum stacked images in each band?

Many thanks in advance,
Artur.

Hi Artur,

1) Are the fits images calibrated and sky-subtracted in each band?

• The images provided by the cutout service have been calibrated in the sense of being bias-subtracted, flat-fielded, and defringed (where appropriate). But the sky has not been subtracted. (Further comment on the flux-calibration in the next point.)

2) Can I measure my own magnitudes on downloaded image by the following formula?

• Using the formula indicated [Edit: but without the EXPTIME factor; just: ABmag = ZPAPPROX - 2.5*log10(counts)] and the ZPAPPROX keyword (and subtracting the sky), you'll very nearly get the "right" answer (i.e., the result tabulated in our catalogues). The caveat is that the catalogues include the effect of a zeropoint gradient, which is allowed to vary linearly with (x,y) position across a full mosaic image. For images to be included in a data release, the amplitude of that gradient from one corner of the mosaic to the other (about 3.4º away) is constrained to vary by less than 0.18 mag (with the limits being filter-specific and evolving slightly from DR to DR). That equates to maximum zeropoint gradient of 0.012 mag from corner-to-corner in the biggest images provided by the cutout service (600x600"), but the ZPAPPROX keyword currently reflects the gradient-free zeropoint determination, so there could be an offset of the real zeropoint appropriate for the cutout of up to ~0.1 mag (though in practice, the offset will almost always be much smaller than that). At the moment, it's not easy for a user to work out the best zeropoint estimate and gradient for their image (although I think it is possible to do so with a combination of table queries), but your question has inspired us to look at how we can do better.

3) Are the fits images already stacked? (with different EXPTIMEs). Can I directly download maximum stacked images in each band?

• No, the images currently provided by the cutout service have not been stacked. However, that is a feature we are working to include, and I hope we are getting close to a solution that works adequately across a wide range of source types (from stars to galaxies to nebulae).

Cheers,

Christopher Onken

SkyMapper Team

Thank you Christopher for your reply. It helped a lot...
Now, after sky subtraction and using right formula the magnitudes are correctly estimated for my list of galaxies.

Cheers,
Artur.