Starting from a place of relative ignorance, I've decided to try to learn
about some basic photometry - really, I'm just interested in calculating
magnitudes of stars and other objects from photos at this point. I'm
using the Iris software at this point, since I haven't been able to get my
hands on any FITS format images, which I'd like to try with VT's web-based
engine (since it seems much simpler to use than Iris!). Here's what I've
been doing:
- download jpeg photo of constellation (Ursa Major and minor so far, sans
connecting lines and names, of course) from wherever I can find it - some
from Hubble site
- load the images into Iris
- Using the 'aperture photometry' feature, measure several "background"
spots, record the intensity (the sum of the intensities of the pixels in
the circle). Average this out, record as "Background Level"
- Bring the aperture over a star in the constellation, record the
intensity. Repeat for all of the stars in the constellatoin (using same
size aperture each time).
- Subtract background level from each star's intensity, calculate
"Instrumental Magnitude" as -2.5log(intensity)
- Pick one star, find the difference between its accepted magnitude and my
instrumental magnitude, call the difference the "offset," and add this to
the magnitudes of the other stars, giving me "calculated magnitudes."
I've also tried several algorithms to find an "average offest," though
this doesn't really decrease my % difference much from the other offset
method.
This works pretty well (<5-6% difference) with dimmer stars (2-6
magnitude), but not as well with magnitude <2 stars (up to 30% at times) .
Any ideas where I'm butchering the physics? I'm a little suspicious
about a few things in particular:
- should I keep the same aperture size each time, or calculate some sort
of intensity per pixel, as the bright stars often appear larger than the
dim ones?
- am I not correctly calculating/ applying the offest?
- is there any way to avoid stars that are really close to each other, or
to factor them out, or to just calculate a total magnitude for the
pair/trio and check that? Obv., they wouldn't add together, but I'm not
sure how to add them, though I'm pretty sure I have a good idea of how to
start.
Thanks for the help!
jg
Josh Gates
Stoneleigh-Burnham School
Greenfield MA