Astroimage processing

Re: photographic magnitudes??

chipdatajeffB — Fri, 21 Mar 2008 02:33:18 GMT

NP, HTH!

These online resources really help us all get more out of our observing time, eh?

Re: photographic magnitudes??

MoFoYa — Fri, 21 Mar 2008 02:16:04 GMT

thanks to the simbad db i found one of the faint stars in my image and was able to identify its magnitude and type.

turns out that it is a 15.88 mag. T Tau-type Star!

thanks again Jeff!

Re: photographic magnitudes??

MoFoYa — Thu, 20 Mar 2008 20:02:28 GMT

thanks Jeff! that helps a lot.

i have read that the photographic limiting magnitude is roughly 2-3 mags fainter than visual. i plan to check this estimate against what i have recorded.

Re: photographic magnitudes??

chipdatajeffB — Thu, 20 Mar 2008 19:05:42 GMT

If you're using something like an SBIG camera to make the photo, you can measure the magnitude photometrically. A CCD imager "integrates" the charges from individual photons interacting with the chip's "charge wells." You can use the raw image to "count" photons in this way if you know the chip's quantum efficiency. Software like MaxIM DL often includes routines to estimate magnitudes very accurately in this way.

Otherwise, what I do is look up the field of view on SIMBAD or other DBs that are linked to the Digital Sky Survey or other faint-magnitude source.

With SIMBAD (just Google that) you can look up a known star by name or SAO designation, etc., and display a field of view. Other stars in that field of view which already have a designation can be identified and then looked up in the DB to find their magnitudes.

Estimating magnitudes based on other known stars in the FoV is an interesting exercise, and it can train your eye. It's a little different when working with photos, because you don't know the linearity of the imager or film used to make the photo. But one way to do it is to enlarge the image and count the pixels in the diameter of the two stars, then extrapolate the unknown one from the known one. This method is reasonable if you have more than one known magnitude in the FoV and they're several magnitudes apart. Comparing their measured pixel-widths to the differences in magnitude can give you a better scale to go by for extrapolation if you have several stars to use as a reference, instead of a single star.

Variable-star observers use this method "live" and can estimate to within about two-tenths of a magnitude reliably, once they have the experience.

photographic magnitudes??

MoFoYa — Thu, 20 Mar 2008 16:19:23 GMT

i wasn't sure where to post this.

does anyone know how to determine the magnitude of the faintest star visible in a digital photograph? i've recently taken some images and tried to compare them to some charts that i have, but the charts do not show stars to faint enough mag. i've looked around online and found the same issue.

my motovation for figuring this out is purely fun. i'm just curious to know what mag. i've been able to detect. i was hoping for more detaled charts if anyone knows where i can download them. if not, there must be a way to calculate the magnitudes from comparing to known stars in the field right?

either way, any advice is welcomed. TIA