On 5/28/2010 9:16 PM, David Goldsmith wrote:
On Fri, May 28, 2010 at 8:31 PM, Anne Archibald <aarch...@physics.mcgill.ca>
wrote:
On
28 May 2010 23:59, Wayne Watson <sierra_mtnv...@sbcglobal.net>
wrote:
> That opened a few avenues. After reading this, I went on a merry
search with
> Google. I hit upon one interesting book, Handbook of CCD astronomy
(Steve B.
> Howell), that discusses PSFs. A Amazon Look Inside suggests this
is mostly
> about h/w. I tried to figure out how to reach the scipy mail list,
but, as
> once a year ago, couldn't figure out the newsgroup GMANE
connection. This
> search recalled to mind my Handbook of Astro Image Processing by
Berry and
> Burnell. It has a few pages on the PSF. In the ref section for that
> material(PSFs) there's another ref to Steve Howell that may be of
use: Astro
> CCD Observing and Reduction Techniques, ASP, Pacific Conf. Series,
vol. 23,
> 1992. There are further Berry and Burnell refs that may be
applicable.
Ah, sorry, I've been at an astro conference all week, I should have
expanded that acronym. PSF is short for "Point Spread Function"; the
idea is that with an optically good telescope, a point source anywhere
in the field of view produces a blob of characteristic shape (often
roughly a two-dimensional Gaussian) in your detector. The shape and
size of this blob is set by your optics (including diffraction) and
the atmospheric seeing. A star, being intrinsically a point source,
produces a brighter or less bright version of this blob centered on
the star's true position. To accurately measure the star's position
(and often brightness) one usually fits a model blob to the noisy blob
coming from the star of interest.
I should note that this requires you to have more pixels than you
"need", so that even a point source is spread over many pixels;
without this it's impossible to get subpixel positioning (among other
things). Older consumer digital cameras often lacked this, since it
was difficult to put enough pixels on a CCD, but fortunately megapixel
mania has helpfully ensured that no matter how sharp the focus, every
feature in your image is smeared over many pixels.
> I probed IRAF, SciPy, and Python, but it looks like a steep
learning curve.
> The SciPy tutorial page looks like overkill. They have what looks
like very
> large tutorials. Perhaps daunting. I did a quick shot at pyraf, a
tutorial
> page, but note it has a prereq of IRAF. Another daunting path.
Wait, you think SciPy has too many tutorials? Or that they're too
detailed? Just pick a short, easy, or sketchy one then. Here's one
that's all three:
>>> import scipy.stats
>>> scipy.stats.norm.cdf(3)
0.9986501019683699
That's the value of the CDF of a standard normal at three sigma, i.e.,
one minus the false positive probability for a one-sided three sigma
detection.
> Well, maybe a DIY approach will do the trick for me.
I haven't used IRAF yet (though I have data sets waiting), and I do
understand the urge to write your own code rather than understanding
someone else's, but let me point out that reliably extracting source
parameters from astronomical images is *hard* and requires cleverness,
attention to countless special cases, troubleshooting, and experience.
But it's an old problem, and astronomers have taken all of the needed
things listed above and built them into IRAF. Do consider using it.
Anne
Plus, if you're in the field of astronomy, knowing py/IRAF will be a
*big* gold star on your resume. :-)
DG > a planet or galaxy.
I'm what might be called an advanced amateur. At the moment, I have no
aspirations to really bulk up on IRAF, For my particular task, I have
just about everything, software and methods, I need to complete it.
It's pretty formidable, but within reach. Probably (erratic) hardware
issues hold me back more than anything. The examination (identification
on a plate) of stacked frames as above is a plus, to get away from
having to identify objects in a field by hand and eye. I'd like to
automate the process, since there's a lot of data involved,. If I can
pull this off, I'll probably get a gold star just for making the work
available to amateurs and possibly professionals. :-)
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