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Originally Posted by TrAI
Yes, it is correct that most color press pictures are based on the L2 as the red channel, but the one you have found is not the only set that includes an L4 too. There are several under the raw images section on the rover homepage, but you will have to combine them yourself to form color pictures...
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I have searched through all raw images from both Sprit and Opportunity, and this is the only colour set that combines L4(red) with L5(green) and L6(blue) filters. Moreover, those images that do use L4 are primarily images that show objects on the lander. In the few examples where L4 has been used on Mars based objects neither L5(green), nor L6(blue) are used. I would love to combine coloursets of the Marsian surface and sky that include L4, L5 and L6, but unfortunately so far none have been taken.
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Originally Posted by TrAI
As for why the majority of the sets returned is L2 based, I don't know for sure, but I would guess near IR is more useful for scientific studies, and you get to use the R2 on the right Pan Cam to create stereo images(2 is the only filter that are the same on both cameras). Also you save valuable bandwidth on the downlink by not transferring the L4, I guess...
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By not taking any L4,L5,L6 colourset images of the Marsian surface science is missing out on comparative data. Whilst there may be an argument for maintaining a majority of images with the near IR filter included, abstaining from capturing images that approximate what the human eye would seem highly questionable.
The stereo arguement falls apart if you simply select periodic images to be taken using L2, L5, L6
AND L4. The addition of the one extra filtered image in such a colourset only represents a 25% increase in bandwidth requirement for these selected images. In return not only are more visually representative results obtainable, but science is afforded the opertunity to conduct more comparative studies
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Originally Posted by TrAI
By the way, I seem to remember that the 570nm is yellow sensitive, not red, in fact the chemistry of the cones are not very sensitive to red at all. I seem to remember the peak of this detection is in the blue part of the spectrum, but since only about 2% of the cones have what you can call the "blue filter", the peek for the eye is more towards the green part(I think there were a little more than 30% cones for green and the rest yellow).. The red frequencies is something the brain must compute by comparing how much less the green cones are stimulated than the yellow(the sensitivity does overlap, so the green has some sensitivity even bellow the peek of the yellow..).. But I guess I got far away from the point of this thread...
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In reply to a letter enquiring about the issue of the image discolourations, Dr Jim Bell (associate professor leading the pancam payload element for the mission) said;
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Originally Posted by Dr Jim Bell
The answer is that the color chips on the sundial have different colors in the near-infrared range of Pancam filters. For example, the blue chip is dark near 600 nm, where humans see red light, but is especially bright at 750 nm, which is used as "red" for many Pancam images. So it appears pink in RGB composites. We chose the pigments for the chips on purpose this way, so they could provide different patterns of brightnesses regardless of which filters we used. The details of the colors of the pigments are published in a paper I wrote in the December issue of the Journal of Geophysical Research (Planets), in case you want more details...
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from
http://science.howstuffworks.com/eye3.htm
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In the diagram above, the wavelengths of the three types of cones (red, green and blue) are shown. The peak absorbancy of blue-sensitive pigment is 445 nanometers, for green-sensitive pigment it is 535 nanometers, and for red-sensitive pigment it is 570 nanometers.
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Combining both sets of details it should be clear that the L4 (at 600nm) is by far the better approximation of true colour than the L2, 750nm filter.
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Originally Posted by TrAI
Anyway, I don't think all blue gets such results, but dye is often very different under IR, and I think the calibration target uses a blue dye that is IR reflective... Whether the sky is discolored or not I guess depends on how much IR light you get from it... Also the uncalibrated images often look too much blue, like the ones in the public Maestro Spirit sets.. Then one have the atmospheric conditions on mars, as I understand it the sky is supposed to be bluish when there are little dust in the air, but I don't know how often that happen(Heh, If the sky here on earth so really is blue, why would it be different on Mars... ;-p)
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The example photographs I provided for both calibration targets (DVD and Sundial) were calibrated using L4, L5 and L6 filters. Moreover comparison between the DVD image produced using the L4,L5,L6 colourset and the Sundial using L2,L5,L6 makes it perfectly clear which of the two methods is better suited for approximating true colours.
As you point out there is some debate as to the true appearance of the Marsian sky. How things really look is one of the great mysteries that motivated people to send these rovers to Mars in the first instance. If the correct filters were used we would be a lot closer knowing the answer to that question. We may also use that information when evaluating other details obtained from the images.
Various factors will affect sky colour, but in comparing with Earth you should take account that the atmosphere is only 1% the density of Earths with very little cloud cover, and with a different mix of gasses.
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Originally Posted by TrAI
The argument about the ones who paid getting what they paid for is a little flawed if they use IR since it is better for science, as the whole point of such a mission is to get the most scientific useful data down, not to waste BW to make pretty pictures for the press, that coupled with that its probably just the best sets that is used as press images will limit the sets with L4s I guess...
Hmm, At least that is how I see it.. But I might be wrong in my guesses about the reasons for the use of the L2 instead of the L4...
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Without public support funds will not be made available for further missions - complete disregard for public opinion would be a foolhardy attitude to take. I have already shown that the amount of data required to satisfy the public thurst for a true approxiamation of the colours of Mars is fairly small (I suspect 10 percent of bandwidth or less would be sufficient).
As well as satisfying the desires of the public we should remember that science will be gaining new comparitive data. Moreover by better approximating the true visual appearance of the sky and surface science is more likely to inspire the next generation of scientists.
Finally, given that the images were released to the public as "approximate true color images", it would seem more honest to supply them with images based on the red, green, blue filter coloursets which we know to be a more accurate representation. The general public will assume that the images they receive are the best approximations that Nasa can provide given the limitations of the equipment. The fact is this is just not true - if the coloursets were L4,L5,L6 based they would be far better approximations. Failure to deliver approximations to the highest standard possible can be seen as at best negligence, and at worst dishonest by the general public.
Nasa needs to look after it's image. It is very unwise to disregard public desires in this way - the public did indeed pay for this mission, and whilst they will accept the necessity to ensure the scientific goals are met, they will also expect that some efforts are made to meet their own aspirations.
Phobos