Re: [APML] Blue response on astrophotos

[Ray Butler <ray.butler@nuigalway.ie>]

Carlos Milovic F. wrote:

> Hi Ray
> 
> 
>>[...] Whether you do some
>>manual, nonlinear 
>>"curves" adjustment (which personally I disapprove
>>of...!) or a linear 
>>transformation of the dynamic range above the
>>background level (a lot 
>>more objective and scientific) is up to you.
>>
> 

Hi Carlos,

> I don't see your point. Film is by no ways linear, and
> you can easily see that if you divide by a flat image.

Absolutely. "Flatfielding" is one of the standard things I do with my 
scanned film images, and I know it's not as easy as with a CCD for that 
very reason. And the characteristic response _curves_ (!) of films 
published by the manufacturers show by definition that film ain't linear.

However I was not claiming that the _film_ is linear; I was only saying 
that my personal preference is for the _digital image processing_ to be 
linear. In fact, no: that statement is not quite accurate either. I 
wouldn't really care if the processing is linear or nonlinear - only 
that it is exactly reproducible if someone else were given the same raw 
data, or if I came back to the raw data some time in the future and 
wanted to recreate the results again. You just can't get that 
reproducibility if you are interactively and subjectively dragging 
points around curves with a mouse, linearly or not. You can't script it 
(automate it) either, and that is another reason why I steer clear of 
GUI-based software like Photoshop for my image processing. With 
scripting you can batch process, or easily rerun the whole shebang with 
some different parameter or option, without putting all the time in again.

> By the other hand, were are not here making
> scientificaly useable images, 

Are we not? I'm always leery when someone claims to speak for everyone.

Well, I guess most astrophotographers are not setting out to make 
'scientifically useable' images, and I am certainly not trying to do 
that either. Although I've seen plenty of other amateurs' work that is 
genuinely worthy of that label, especially when it comes to celestial 
transients like comets, meteors and aurorae.

My goal is a different thing - approaching the image processing in the 
kind of systematic, objective, reproducible way that my scientific 
training has instilled in me. And I still do this for fun by the way! 
(For work, in between teaching, I use 1m/1.5m/2.5m/3.6m/4.2m/6m 
observatory telescopes around the world, all with CCDs or our own 
imaging photon-counting cameras, plus Hubble and a host of other 
archival facilities. Totally different world to backgarden 
astrophotography with my own modest gear paid for out of my own pocket). 
But yes, it still is fun to develop my own scanned-photo image 
processing pipeline based on Linux and IRAF. First of all there's the 
technical challenge. Ask any programmer about the satisfaction of 
nailing those tricky algorithms. Plus, there's no pressure other than 
the self-administered drive for satisfaction and pleasure. My photo 
results are completely personal and evaluated in aesthetic terms, even 
if the methods employed were objective wherever possible. It's a 
completely, utterly different buzz to the cut and thrust of competitive, 
peer-reviewed research.

 > in fact, with film and
> our equipment is almost impossible (how can we
> characterice the response of the film? 

You know, that is actually doable, if you are willing to expend a lot of 
effort correlating published photometry with your own measurements. 
There's no way it is worth the effort for science, but it might make a 
fun technical challenge for someone. I haven't tried it myself! (;-))

> And what about the vigneting pattern?). 

I have largely cracked that issue, as have some others on APML. Still 
chasing down the perfect solution, if indeed there is one. Still 
enjoying the chase.

> Also, there is the problem of the scanninf process... 

Why is that a problem? It is easy to control dynamic range etc. with a 
good scanner, and if you really want to, you can calibrate scanner noise 
and linearity as easily as calibrating any CCD. I'm not sure what you 
meant here, in referring to a problem.

>So, I can't see the point on
> avoiding the use of nonlinear curves adjustements.

Well in addition to the main reasons I've given above (the fun of the 
challenge, the reassurance of the rigour, the productivity benefits of 
automation), there's also a few more things. One is the curious 
satisfaction which comes from knowing that I'm just being plain 
different, ornery, not following the herd, beating my own path, and 
other cliches! Another is the clear-conscience feeling that one is not 
subjectively massaging data. That might seem like a "so what?" issue to 
the majority of folks, who don't consider their photos as "data" and 
neither do I really, but I just find it very hard to escape my training 
in the scientific method! A further reason is that I save money on 
commercial software; Linux and IRAF are free. Automation and the 
statistical basis of some of my tools also mean that consistency (of 
background level, colour balance, dynamic range) from image to image can 
be excellent if desired. That's ideal for making mosaics etc. And yet 
another reason - there have been elements in my astrophoto pipeline 
algorithms which I subsequently found useful for my research pipeline 
development...and vice versa. I find it quite cool that I have managed 
to blend and cross-over professional research and amateur image 
processing within the same platform. I bet the IRAF developers would be 
pleasantly surprised at that too.

When I feel I've got things working pretty well I may consider writing 
up an article or releasing my pipeline as an IRAF-external contributed 
package. For free, of course.

The bottom line is that I would hope that whatever APML people do, they 
enjoy it, as I do. In advocating my own approach I am not passing 
judgement on what others do. There is a vast difference between saying 
"what you are doing is WRONG!" and saying "have you considered this? it 
works better for me". I always strive to do the latter.


Ray "who has too many exam scripts to mark and if he had any sense he 
wouldn't be writing long messages like this" Butler


> =====
> Regards,
> 
> Carlos Milovic F.
> -------------------------
> Astro & Photo - CMF
> http://www.astrophoto.vze.com
> -------------------------
> Visita "AstroFoto", el foro de astrofotografía en español
> http://espanol.groups.yahoo.com/group/astrofoto
> 
> 
> 		
> ______________________________________________
> Renovamos el Correo Yahoo!: ¡100 MB GRATIS!
> Nuevos servicios, más seguridad
> http://correo.yahoo.es
> _______________________________________________
> Astro-Photo mailing list
> Astro-Photo@seds.org
> http://seds.org/mailman/listinfo/astro-photo

-- 
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


_______________________________________________
Astro-Photo mailing list
Astro-Photo@seds.org
http://seds.org/mailman/listinfo/astro-photo