RE: [APML] Machholz, Cassiopeia, and Double Cluster

["Andrew Clegg" <andy@w4je.com>]

Hi Ray,

Thanks! Your questions about flat-fielding are right on, and they are
questions that I wonder about as well. The flat fielding does seem to
subtract at least some component of the Milky Way, unfortunately, but
exactly how much (i.e., what would my final shot look like if the Milky Way
hadn't been partially subtracted) is not easy to say. I apply a 100-pixel
median filter to the original big scan (the 2410x3212 version). That seems
to be the largest radius that PS allows, although if I'm wrong, let me know!
That still leaves some visible components, like some of the nebula and the
overall effect of the Milky Way, so I then apply a large gaussian blur (100
pixel radius I think; I'd have to check my notes). This leaves basically a
version of the original scan devoid of all stars and features other than the
apparent vignetting. Although as you suspect, some component of that
"vignetting" is surely the Milky Way running through the middle.

I do the subtraction in Matlab instead of PS just because I get better
results. I am a novice with both programs, but using subtraction in PS
leaves me with residual field fall-off, plus it can only be done on an 8-bit
image. In Matlab, I can read in as a 16-bit image, then convert up to a 24
bit/pixel image, do the subtraction, then convert back down to 16-bit per
pixel, and write back out as a 16 bit/pixel TIFF file for further processing
in PS. The result seems to be an absolutely flat field, although perhaps
with some of the Milky Way subtracted as well!

As you point out, there are multiple models of field illumination to deal
with. Theoretically, I should be doing a division to get rid of vignetting
and a subtraction to deal with gradients. I haven't gotten that far yet. I
wondered if it would be reasonable to build an empirical model of the
vignetting of my lenses at various aperture settings, and then divide my
shots by the appropriate model to compensate for the vignetting. One of
these days I'll try to take some "flat field" images of an illuminated card
or the open daytime sky, and see if I can build a set of models.

I have used IRAF in years past (and AIPS--the radio astronomy package, not
the commercial amateur astronomy package), but that was for radio astronomy
data. How are you finding IRAF for processing astrophotos?

Hope your skies clear soon. Machholz is fading!

Andy


-----Original Message-----
From: astro-photo-bounces@seds.org
[mailto:astro-photo-bounces@seds.org]On Behalf Of Ray Butler
Sent: Wednesday, February 09, 2005 6:44 AM
To: Discussion of Film Astrophotography
Subject: Re: [APML] Machholz, Cassiopeia, and Double Cluster



Andy, that is sweet! How I wish I could get the skies to clear here -
I'd be doing exactly the same with my 645 and 6x6 cameras + E200...

I note that you flatfielded by subtracting the median/blur. Does this
partially (undesirably) flatten out the Milky Way as well? Do you find
it hard to adequately flatfield when the Milky Way runs through the
image, and/or there is a gradient in the sky background due to
increasing skyglow in one direction? A true flatfield should be able to
disentangle these two additive effects from the multiplicative effects
of the system vignetting.

This is a problem I'm tackling at the moment with my own processing
algorithms in IRAF. I think I've developed a good iterative approach to
tackle this sort of scenario, but I haven't had a chance to hone it
fully yet, thanks in part to some computer troubles.

Ray "nursing a very sick laptop back to health at the moment" Butler


Andrew Clegg wrote:

> Good weather in the Shenandoah Valley this weekend. I shot Machholz
> piggyback with the 6x7 on Saturday night, and developed the film while
> watching the Super bowl last night. Scanned the film this morning (using
the
> new Microtek ScanMaker i900) and did some processing in Matlab and
Photoshop
> at lunchtime. The results are at www.w4je.com/machholz.htm. Comments
> welcome!
>
> Clear skies,
> Andy
>
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--
Dr. Ray Butler (ray.butler@nuigalway.ie || ray@physics.nuigalway.ie)
Lecturer, Dept. of Physics || Computational Astrophysics Laboratory
National University of Ireland, Galway, University Road, Galway, Ireland.
Tel: +353-91-524411 ext. 3788   FAX: +353-91-525700

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