best aperture in phot

santiago wrote on Dec 02, 2008

I explored some images of TNOs and determined that apertures about 1 FWHM gives the higher SNR for the object. Should i use that aperture for the photometry? i guess that for reference stars about 100 - 200 SNR bigger apertures should be fine... is there any possibility of setting different apertures for reference stars and for the object? Does "phot" task eliminates surrounding stars that may add flux to the object passing near them?

Thanks,

Santiago.-

---

santiago wrote on Dec 02, 2008

after doing photometry over several images i noticed that in the output line in the terminal sometimes shows INDEF string in the magnitude column and "err" string instead of "ok". Within the first images, it showed all magnitudes and OKs, but after those images, some stars show INDEF and "err" values... i think the sky conditions got better on this late images... and i started to change some parameters to get all OKs but i can't figure out yet... may be more experienced users know what parameter may be affecting... some fwhmpsf value or sigma...

any suggestions?

Thanks again,

Santiago.-

---

Francisco Valdes wrote on Dec 02, 2008

I am not an experience photometrist or fully knowledgable about the tasks. The one thing I am certain about is that phot does not remove contaminating stars in the aperture. The only way to do that is with a PSF fitting photometry task such as in the DAOPHOT package. Normally you need to use an aperture that will not include more than one source. You would need to do things more manually if you want to use different size apertures for different stars.

The INDEF values are due to a failure of the photometry. You would need to read the help and figure out what exactly is resulting in the INDEF values.

Good luck.

Yours,
Frank Valdes

---

Last post on Dec 02, 2008