From: Milton Aupperle <milton@outcastsoft.com>

Date: May 19, 2008 2:06:06 PM MDT

To: Astro_IIDC@yahoogroups.com

Subject: Re: [Astro_IIDC] Use of Histogram expand and color balance techniques

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