iraf.net

Hi,

I want to get magnitudes of stars which composed a globular cluster. For that I use a PSF photometry. But I could see that the parameter zmag defined in PHOT task changes results. To determine this parameter in different filter I used standard stars and the formula:

z_mag=magnitude of the standard star found in the litterature + 2.5*log(intensity of the star in pixels measured with SAO Image (the value is very approximate) -2.5*log(DIT)

So I determine a value of zmag for each filter in this way but it is not specific and I am afraid to not understand completely what is zmag... Can someone explain to me how determine zmag with precision? Am I correct in thinking the value of zmag changes with filter?


I thank you in advance for your help!

Best regards,

Johanna.

The 'zmag' value is the photometric zero point to be used, by default it is 25.0. Some discussion of what/how/why this is can be found at

http://www.astro.washington.edu/courses/astro480/IRAF/iraf2_tutorial.html

and elsewhere via Google.

If your concern is that you are seeing only approximate values in DS9 then this is a known issue with DS9 where the displayed pixel values are only the approximations based on the display z1/z2 range. Loading an image directly into DS9 using the 'File' menu will have it display the actual pixel values in the image.

Ok! Thanks a lot!

Best regards,

Johanna.