M13 con RC8...iniziamo a fare sul serio :-)

[pk825]

Sul mio monitor non sembra rosa il centro
Se non ricordo male eXcalibrator usa proprio gv2 per l'analisi del colore delle stelle, forse bisognerebbe fare una maschera delle sole stelle gialle/arancio e lavorarci sopra per dargli una tonalità gradevole senza toccare il resto

[Elio]

...ero con l'ipad, adesso al pc, non è proprio rosa ma tendente al rosa...non mi piace, sicuro che usa G2V? Quindi la riduce astrometricamente prima?

E no, cambiuare colore solo ad alcune no, così mi piacerebbe vincere facile

[pk825]

Sulla carta fa qualcosa di molto simile:

eXcalibrator calibrates image color by using the NOMAD1 or SDSS database to identify stars in your image that should be white. The program then computes the correction factors to apply to the green and blue channels.

What is eXcalibrator?
eXcalibrator provides an easy means to white balance astrophotos by computing RGB correction factors
based on known star color. This method utilizes star color information from the NOMAD or Sloan Digital
Sky Survey (SDSS) database. eXcalibrator uses data in the astrophotographer's final R, G and B FITS
files!
Additionally, eXcalibrator is an excellent tool for calibrating RGB exposures for a given image-train. This
is similar to the long-established G2V method. However, eXcalibrator produces more consistent
night-to-night results. See section 6.
A Word on Other White Balance Methods:
The astrophotographer may choose several other options to determine color balance:
• Use G2V stars to calibrate the R, G & B exposures. Although popular, this method has
two big drawbacks.
1) Extinction for objects at low altitude causes a problem for all color channels...
especially for the blue. The problem varies at different altitudes. This makes
automatic adjustment difficult.
2) Bad (variable) transparency: G2V calibration does not compensate for color shifts
induced by bad transparency.
• Use the integrated light of a face-on spiral galaxy. This method shows a galaxy with its intrinsic
color. However, if there is galactic or intergalactic extinction, the galaxy and foreground stars are
too blue.
• Use the collective light of a star field.
ο Use this method with caution as the general star population is skewed towards the
red end of the spectrum.
ο This technique can produce good results with globular clusters, if there is no
galactic extinction.
• Simply set the background to a neutral gray. Of course, this will not work with an image
completely dominated by a nebula.
• Just wing it by comparing to other images on the Internet.
The Underlying Idea:
Peter Riepe and Harald Tomsik, published in the German magazine 'VdS-Journal' base the idea behind
the eXcalibrator approach on two articles. The goal is to make those G2 stars not affected by interstellar
extinction, white.
Typical exposure times of object images often run so long that all nearby bright G2 stars become
saturated and unusable for color calibration. The unsaturated G2 stars in the image are often faint and
we have little or no information on these stars, so we usually cannot search for them successfully. With
luck, we can identify a faint unsaturated G2 star, but then interstellar extinction may affect this star and
ruin the color balance.
It makes sense to turn towards photometry. Several databases on the Internet catalog the flux of a huge
number of faint stars measured through different broadband filters. The most important filter system is
the Johnson UBVRI system; where U stands for Ultraviolet, B for Blue, V for Visual (Green), R for Red
and I for infrared. For the purposes of color calibration, we focus only on the B, V and R data.
The difference between B and V gives us the “B-V color index," which characterizes the color of the star.
A G2 star displays a B-V value of 0.65mag. Red stars show B-V values above 0.65, while blue star
values range below 0.65. The difference between V and R builds a further color index. The typical value
of V-R of a G2 star in the Johnson filter system is 0.52mag. The Johnson UBV filters are used with the
Cousins RI filters rather than in combination with the Johnson RI filters. This usage leads to the Johnson-
Cousins color index V-RC = 0.36mag of a G2 star.
For color balance purposes, all of this is useful information

Ho riletto velocemente la procedura e mi sono accorto che non l'ho applicata proprio come dice l'autore, solo che adesso non ho tempo di riprovare.
Stasera o domani...

[Elio]

Sulla carta fa qualcosa di molto simile:

eXcalibrator calibrates image color by using the NOMAD1 or SDSS database to identify stars in your image that should be white. [...]

Ho riletto velocemente la procedura e mi sono accorto che non l'ho applicata proprio come dice l'autore, solo che adesso non ho tempo di riprovare.
Stasera o domani...

Ottimo, ti devo passare l'immagine lineare allora, prima di stretch e quant'altro... o devo prendere pixinsight ma 200 euri non ce l'ho