From: "doodlebun" <gbleser@bellsouth.net>

Date: May 19, 2008 2:58:44 PM MDT

To: Astro_IIDC@yahoogroups.com

Subject: Re: Use of Histogram expand and color balance techniques

--- In Astro_IIDC@yahoogroups.com, Milton Aupperle <milton@...> wrote:

Hi doodlebun and Gail;

On 19-May-08, at 12:25 PM, doodlebun wrote:

My wife and I use Astro IIDC with the DMK21AF04 and Astronomik 

LRGB

filter wheel for imaging Saturn. My wife feels that she gets the 

best

results leaving Histogram expand on to achieve proper brightness 

on

all channels.. I feel that it can give Astro IIDC fits.

Actually it could cause "fits" so to speak, depending on which 

type  

of "Histogram Expand" you use and how high your gains are.

 Astro IIDC 4 has two settings for Histogram expand, "Auto" 

and "Once".

If you leave it on "Auto", then it dynamically adjusts the range 

of  

values for each frame it receives.

If you leave it on "Once", then it's calculates the Histogram 

Expand  

values from the next frame you receive and leaves it that way 

until  

you select "Once" again or Stop  / Start the camera again.

The difference is that in "Auto" with high gains or turbulence, 

the  

dynamic range will change for each frame which causes the 

brightness  

of the image to change each time it is received. This could cause 

the  

"fits" or rapid frame to frame changes in brightness I think you 

are  

talking about.

So the  "Once" method may help improve contrast and at least it's  

consistent frame to frame.

My understanding is the proper way to achieve color balance is:

 Take a bunch of movie frames of an out of focus G2V star using 

the

monochrome camera. Start with some red frames through a red 

filter,

making sure the image is not clipped. Do the same with G and B

filters, taking care to not allow the exposure to change by 

keeping

all "auto" functions disabled in Astro IIDC.

If you check these star colours out at:

http://www.vendian.org/mncharity/dir3/starcolor/

you'll see that a G2V stars like our sun are not really white and  

have color values #fff5f2 which is Red = 255 G = 245 B = 242 (R 

100%  

G 96.1% B %94.9). That makes it yellowish with slight pink tinge. 

A  

better star for pure white would be an F8(V) star #fff9f9 which is  

Red = 255 Green = 249 Blue = 249, which is closer to "white".

 Use photoshop to determine the intensities of the R,G and B

components in each image. From those intensities one derives

the "boost" value given to the dimmer G and B channels of the 

tiffs

generated from stacked Saturn channels.

That sounds do able, assuming your known color star and Saturn 

were  

taken at near the same angle altitude above the ground and 

shooting  

though similar skies.

Basically you image an out of focus star of known colors (i.e. R =  

100% G = 96% Blue = 95%) using the same exposure times for all 3  

filters, being careful not to saturate it to 255 on probably the 

red  

color channel. Ideally you want to get around 55 to 65 % of the  

maximum value (i.e. for 8 bit cameras 140 to 166 values and for 16  

bit 36,000 to 42,600 values) for the brightest.

Then use the measured recorded R G B brightness of the stars  

intensity and then compare that against the expected absolute 

colors  

and adjust gains as necessary to make the two match.

This method means that the RED channel of Saturn is filmed first,

as bright as possible without clipping. Then a green and blue 

movie

is filmed without altering any gain, or any other setting period. 

The

blue movie will be quite dim in the series. There is a strong

tendency to boost gain or exposure time, but it seems that would

throw the whole color balance scheme into confusion.

With gains I'd agree because they aren't usually linear in the 

cameras.

However if you adjust only exposure time, you can also balance the  

color balance that way too. For example if you determined that you  

need to linearly adjust brightness by say Red = 1.00, Green = 1.05  

and blue = 1.15 times to balance using your known star, then if 

you  

were shooting Saturn with Red at say 50 ms, then you need to use 

52.5  

ms Green and 57.5 ms for Blue. Also by adjusting exposure time, 

your  

not increasing gain which boost noise levels. Even digitally  

increasing gain in post processing will increase noise levels, 

where  

as exposure time won't.

Use photoshop to increase the gain in each channel according to 

the

test results from the G2V star. If everything is done correctly 

then

you should get rings of Saturn that are actually "white" like they

are supposed to be.

And are the rings actually "white"?

Here are shots from hubble:

http://heritage.stsci.edu/2001/15/big.html

and shots from Cassini

http://apod.nasa.gov/apod/ap040723.html

http://en.wikipedia.org/wiki/Rings_of_Saturn

I've always thought of them as pale bluish to pale yellowish in  

color, depending on orientation with the sun, as they are ice  

reflecting light back from the sun.

And there will be no reddish bands on the planet

like I have been getting.

Umm, there are reddish and greenish bands on Saturn.

http://solarsystem.nasa.gov/multimedia/display.cfm?IM_ID=186

One thing i have done is too image terrestrial objects of known 

color  

and then adjust the controls visually until they balance on screen  

and what I see. The ultimate means of doing it is to shoot a 

standard  

color chart under fixed lighting with each filter using the same  

exposure time and then adjust each color channel until they 

balance  

out properly. Of course when the image is viewed on different  

monitors with different Gammas and different OS's , it will change 

so  

there aren't any real absolute colors here, we can jst get "close".

Hope some of this helps..

Milton J. Aupperle

Thanks Milton,

 Gail is beaming now. She has been telling me that our sun, a G2V 

star, is yellow, not white. I say that everything on earth is 

illuminated with our G2V star so it only makes sense that the 

internet chatter is full of references to finding a G2V star for your 

color balance. Yet I agree with you that white is R=G=B=255. Color 

discussions can make normal people go crazy. When women consider 

themselves too pale, they put some rouge on. It makes them look 

better even though it's not the "real color". When we image planets 

we put our images through the "anti-cosmetic" treatment.

If our planetary imaging techniques with the wavelet filter Fourier 

Freakouts were applied to images of a person's face, we would 

emphasize every pimple and wrinkle that we could find to bring it out 

from the background. We would all be fired as photographers 

specializing in portrait photography. So the rule simply 

becomes "don't turn your planets into false-color clowns with details 

that arise from amplification of noise rather than real planetary 

features".

  One wonders just how dim light is on Rhea? Here on Earth the flux 

at noon is 1000 J/s/m^2. Surely you could read a newspaper through 

your space helmet? Whip out a color chart on Rhea and see how much 

saturation remains in the dimness.  Here on Earth our software 

histogram adjusters can fully illuminate a shadow scene on Pluto.

  Last night we had lost 1-2 magnitudes from the sky due to smoke. 

Almost as bad as the smoke in Glacier National Park in Aug 2001 that 

resulted in Gail and I leaving for Calgary..and Banff. Photoshop 

removed the haze in my Canadian Rockies images so well I began to 

believe that all photos of ultra-clear vistas must get the Photoshop 

treatment.  

Gail is using Photoshop to fix the yellowing we got in last nites 

Saturn images.

 And before I say goodbye, do you recommend the 4.04 upgrade for us 

monochrome planetary (never lunar) imagers?