How can I derive limit magnitude in 5sigma/2arcsec ?

I have the limit magnitude in 5sigma which derive from the magnitude with SNR reach 5. But I can not understand what the meaning of dividing 2arcsec.

  • $\begingroup$ Do you mean arcsec^2 ? $\endgroup$
    – Mike G
    Oct 15 '20 at 16:14
  • $\begingroup$ No, just 2arcsec. May it means the diameter. $\endgroup$
    – GHC
    Oct 18 '20 at 5:56

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