> From dpeach78@netscapeonline.co.uk Fri May 24 20:03 MDT 2002 From: "Damian Peach" Subject: Colour in Jupiter images. Date: Sat, 25 May 2002 01:32:44 +0100 Hi all, Following a meeting of the BAA Jupiter section staff last weekend, one of the things we discussed is how colour's are presented in Jupiter images, and how processing affects them. We suggested supplying a very lightly sharpened or unprocessed RGB composite image, specifically for the judgment of stubble colour shades on the Planet, that sharpening routines affect. This "chrominance image" would provide accurate colour information on the Planet, provided good RGB filters are used, and images are correctly composited. I attach an example using one of my own images from March 27th, 2002. A lightly processed RGB composite image is presented, each frame carefully "weighted" before composition into the final composite. The high resolution colour image is an LRGB processed image for comparison, to show how colours are affected. The RGB, i feel does represent accurately the colour's i see visually. I would be interested to hear opinions on observers supplying such an image with there normal images but just used for colour judgments. Best Wishes Damian ******************************************************************* Damian Peach. ALPO/BAA Jupiter Sections. BAA Saturn Section. E-mail: dpeach78@netscapeonline.co.uk Homepage: http://www.the-planets.co.uk/ ******************************************************************* BAA Saturn Section:=20 http://members.netscapeonline.co.uk/dpeach78/index.htm BAA Jupiter Section: http://www.the-planets.co.uk/jupitersection/index.htm ******************************************************************* > From sherrodc@ipa.net Fri May 24 20:30 MDT 2002 From: "Clay Sherrod" Subject: Re: Colour in Jupiter images. Date: Fri, 24 May 2002 21:28:48 -0500 This is an absolutely excellent example of the same principal that I have been touting over here for years; the image on the left (RGB) is more representative of not only visual perception and perhaps true color, but also more useful to determine gross morphological changes as they occur on large scope on Jupiter; often in high resolution images, such as your excellent example on right (LRGB), the overall parameters of such a large change or "happening" are missed....the old "...missing the forest for all the trees...." principal. A wonderful example, Damian, and one that I plan to use in the classroom to elude to many issues that we study...thanks. Clay ---------------------------------------- Dr. P. Clay Sherrod sherrodc@ipa.net Arkansas Sky Observatory www.arksky.org > From egrafton@ghg.net Sat May 25 09:42 MDT 2002 Date: Sat, 25 May 2002 10:37:55 -0500 (CDT) From: egrafton@ghg.net (Ed Grafton) Subject: Re: Colour in Jupiter images. Hi Guys The accurate reproduction of color is a most difficult issue. There are many issues that conspire to foil one's attemps to consistently standardize the chrominance especially when comparing data from one scope/CCD to another. Here are a few of the issues: 1) The bandpass of the filter sets have gaps and the gaps vary in width. Some of the data color data is missed in many filter sets, and to a different degree among filter sets, SBIG included. All else being equal, comparing color data from different filters is guesswork. Also most (amateur) filters being used are dichloric insteade of dyed glass. The dichloric filters have the characteristic of changing the % of transmitted light depending on the F/ ratio of the scope( the angle of incidence of the light cone). The same CCD/filter set will not yield the same color results on a 6 inch f/8 and a 8 inch f/6 scope. 2)The weights for each filter should be standardized for proper color balance. This can be done by measuring the flux through each filter set on a solar analog star to determine the proper ratios with the idea being that the stars are a white light source( sort of). This also takes into account the quantum eff. of the chip, the filter set being used and the f/ ratio of the scope ect. 3)Atmospheric extinction is another source of color error. The atmosphere is a filter in itself and is dependent on the altitude of the object as to the degree attenuation by wavelength. This is why the sun is orange on the horizon as much of the blue light has been absorbed by the atmosphere. Proper color balance should have the weights modified as a function of the altitude of the object. This is not to say that the attenuation is the same at 45 degrees in the Arizona desert as in the swamps of S. Florida due to the degree of water vapor in the air( or even in Houston before or after a cold front moves through modifying the transparency of the atmosphere and not linearly so as a function of wavelenght). 4)Non linear processing of the color data will change the color balance. For LRGB images it would be most accurate to leave the chromance unprocessed as far as non linear filters are concerned. 5) Post processing in Photoshop, ect. Although the color can be enhanced in PS the result is a artist rendition as far as color goes. I have frequently used PS to enhance the contrast between features such as making the festoons have a great deal of blue contrast and therefore more easily seen. Standardizing one's own images for color should be quite fesible but there are issues that are not manageble. Ed Grafton > From rmiles@baa.u-net.com Sat May 25 10:13 MDT 2002 From: "Richard Miles" Subject: Re: Colour in Jupiter images. Date: Sat, 25 May 2002 17:17:10 +0100 Folks, Further to Damian's original mailing and Ed's valuable contribution, there is one other factor that needs to be taken into account. The present discussion relates to the colour of images as depicted on a good computer monitor (usually CRT type). However, when images are reproduced in hardcopy form, e.g. in a paper publication, the colour reproduction process will very likely alter the appearance compared to the CRT view. Depending on the technology used for hardcopy reproduction, it is sometimes necessary to apply some sort of transfer function to the original image so that once it has been reproduced it is displayed optimally compared with the CRT view. Unfortunately it is not possible to generalise how this is done precisely since the degree depends on the technology of the hardcopy reproduction. Suffice it to say that further image adjustments may be needed on a trial and error basis prior to final hardcopy printing. My two penneth, Richard Miles