SCI/TECH: What Color Is Mars, Really?, page 5
Pages: <<  2    3    4    5    6    7    8  >>
ATS Members have flagged this thread 3 times


reply posted on 12-1-2004 @ 07:51 AM by SkepticOverlord
Pancam Pancam uses 1024??2048 pixel Mitel CCD array detectors developed for the MER Project. The arrays are operated in frame transfer mode, with one 1024??1024-pixel region constituting the active imaging area and the another adjacent 1024??1024 region serving as a frame transfer buffer. The frame transfer buffer has an opaque cover that prevents >99% of light at all wavelengths from 400 to 1100 nm from being detected by this region of the CCD. The pixels are continuous, and the pitch is 12 ?gm in both directions. The arrays are capable of exposure times from 0 msec (to characterize the ?readout smear? signal acquired during the ~5 msec required to transfer the image to the frame transfer buffer) to 30 sec. Under expected operating conditions, the arrays have at least 150,000 electrons of full-well depth, and a read noise of less than 50 electrons. Dark current varies with temperature; it is negligible at -55?C and is <1200 electrons/sec at 0?C. Analog to digital converters provide a digital output with 12-bit encoding, and SNR > 200 at all signal levels above 20% of full scale. The detector response has a linearity > 99% for signals between 10% to 90% of full well. Each array is combined with optics and a small filter wheel to form one “eye” of a multispectral, stereoscopic imaging system. The optics for both cameras consist of identical 3-element symmetrical lenses with an effective focal length of 38 mm and a focal ratio of f/20, yielding an IFOV of 0.28 mrad/pixel and a square FOV of 16.8???16.8? per eye. The optics and filters are protected from direct exposure to the martian environment by a sapphire window at the front of the optics barrel. The optical design provides for more than 90% of the encircled energy to be contained in an area equal to 3??3 IFOVs, and 99% in an area equal to 5??5 IFOVs, across the entire range of spectral responsivity of the instrument and over the required operating temperature range for performance of Pancam within specifications (-55?C to 0?C). The optical design allows Pancam to maintain optimal focus from infinity to within about 1.5 meters of the cameras. At ranges closer than 1.5 meters, Pancam images suffer from some defocus blur. For example, at a range of 80 cm (the approximate distance from the Pancam calibration target), the defocus blur is about 10 pixels. Each filter wheel has eight positions, allowing multispectral sky imaging and surface mineralogic studies in the 400-1100 nm wavelength region. The left wheel contains one “clear” (empty) position. The remaining filter wheel positions are filled with narrowband interference filters that are circular and 10 mm in diameter, and that have the central wavelengths and bandpasses listed in Table 2.1.2-1. One filter on each eye has an ND5.0 coating to allow direct imaging of the Sun at two wavelengths. LEFT CAMERA..............RIGHT CAMERA L1. EMPTY................R1. 430 (SP) * L2. 750 (20).............R2. 750 (20) L3. 670 (20).............R3. 800 (20) L4. 600 (20).............R4. 860 (25) L5. 530 (20).............R5. 900 (25) L6. 480 (25).............R6. 930 (30) L7. 430 (SP)*............R7. 980 (LP)* L8. 440 Solar ND.........R8. 880 Solar ND *SP indicates short-pass filter; LP indicates long-pass filter Table 2.1.2-1: Pancam Multispectral Filter Set: Wavelength (and Bandpass) in nm Radiometric calibration of both Pancam cameras will be performed with an absolute accuracy of 7% or better and a relative precision (pixel-to-pixel) of 1% or better. Calibration will be achieved using a combination of preflight calibration data and inflight images of a Pancam calibration target carried by the rover. The Pancam calibration target is placed within unobstructed view of both camera heads and will be fully illuminated by the Sun between at least 10:00 AM and 2:00 PM local solar time for nominal rover orientations. The target has three gray regions of variable reflectivity (approximately 20%, 40%, and 60%) and four colored regions (peak reflectance in the blue, green, red, and near-IR) for colorimetric calibration. It includes a vertical post that will cast a shadow simultaneously across all three gray surfaces at some time within the 10:00 AM to 2:00 PM nominal operating range. The calibration target is large enough that defocus blur will not produce significant degradation of the calibration images. The two Pancam eyes are mounted on a mast on the rover deck. The mast is referred to as the Pancam Mast Assembly (PMA), and also includes several key components for the Mini-TES. The PMA is erected to the vertical position by a deployment actuator at its base. The cameras are located on a "camera bar? with a boresight 180? from the Mini-TES boresight. The rover navigation cameras (Navcams) are also located on this same camera bar, and point in the same direction as Pancam. The boresight of the Pancam cameras is approximately 1.3 m above the martian surface with the PMA in the deployed position. The cameras are moved together by ±90? in elevation using a geared brush motor on the camera bar. The entire PMA head, including the cameras, can be rotated 360? in azimuth by a geared brush motor assembly. A separate geared brush motor provides elevation actuation for the Mini-TES elevation mirror assembly. Hard stops are provided for all actuation axes. The two Pancam eyes are separated by 30 cm horizontally and have a 1? toe-in. This separation and toe-in provide an adequate convergence distance for scientifically useful stereo topographic and ranging solutions to be obtained from the near-field (5-10 m) to approximately 100 m from the rover. Pointing control is <2? in azimuth and <1? in elevation. Pointing knowledge relative to the hardstops is 0.1? over the entire range of motion of Pancam. Pancam will operate primarily during the daytime to obtain high-quality measurements of sunlight reflected off rock and soil surfaces and airborne dust particles, as well as direct solar images using the two ND filters. Twilight or nighttime sky or astronomical object imaging may be possible but has not been committed to by the Project. The required operating temperature range for performance of Pancam within specifications is -55?C to 0?C. Pancam will be commanded by and will return digital data directly to the rover computer. The computer provides the capability to perform a limited set of image processing tasks on Pancam data prior to transmission. These tasks include (1) bias and dark current subtraction, (2) electronic shutter effect correction, (3) bad pixel replacement, (4) rudimentary automatic exposure control capability to maximize the SNR of downlinked data while preventing data saturation, (5) image subsampling and subframing, and (6) image compression using a JPL-developed wavelet compression algorithm called ICER. Pancam telemetry is collected by the rover computer and downlinked according to anoverall priority queue scheme agreed upon in advance by the MER Science OperationsWorking Group. Image data are packetized, with each packet containing sufficientinformation to allow receipt in any order and to allow incremental image reconstructioneven in the event of typical transmission errors and packet losses. Obtained here: athena.cornell.edu... And pasted as text for our members.



reply posted on 12-1-2004 @ 09:04 AM by Kano
RGB pictures can never be 'true color' as it is impossible to re-create every color the eye can percieve using only Standard Red Green and Blue.

A brief explanation can be found here:
hyperphysics.phy-astr.gsu.edu...

To explain visually, we can use the CIE diagram.


Basically using whatever points we pick for red, green and blue, we can only make a triangle, and never cover every possible color visible.

As Dr. Bell stated in his email. The choice for the Red channel in this picture was incoming light with a wavelength of 750 nm. This corresponds to light at the extreme end of the visible spectrum, and the beginnings of infra-red light.

hyperphysics.phy-astr.gsu.edu...

Now, it seems the blue pigment used for the color chip on the sundial (and elsewhere on the electrical connectors) is especially bright at 750nm. I suspect if anyone can get their hands on Dr. Bells papers about the choice of pigments used it would shed some light. But it appears the quirk we see in the photos is a result of the choice of wavelengths which to set as Red, Green, and Blue. Combined with the reflective properties of the pigments used.

Perhaps the longer wavelength setting was chosen to increase the range of colours the camera is able to pick up.

Remember, as someone pointed out, it is not simply a matter of taking a color photo. The PanCam needs to take 3 photos with different filters to pick up the 3 wavelengths. These wavelengths are then assigned to RGB and combined to give the images we see.

It seems therefore it should be possible also to record some images using the L4, L5 and L6 as R,G and B respectively. Which I imagine we will also see as the mission progresses.

Remember shifting the arbitrary points for RGB is just like increasing the size of the triangle shown inside the CIE Chromacity Diagram. Only when it is reconverted to the standard RGB shown by our computers, the triangle is shifted back into its normal position. Thus for the majority of the spectrum the change in percieved colour is minimal. But for colours close to exact 'Red' 'Green' and 'Blue' the shift is somewhat more noticeable. Also the effect of the extra bright blue chip in the near-infra range gives it the interesting pink look.


reply posted on 12-1-2004 @ 09:59 AM by Kano
Pink isnt that far from blue in color space.

Heres a quick way to re-create the effect yourself btw.

In the name of the image on the raw images directory

marsrovers.jpl.nasa.gov...

It shows what filter the image was taken with (yes those black and white ones).

For example 2P126644567ESF0200P2095L2M1.JPG was taken with the L2 filter, which we know is at 750nm.

All the raw images seem to follow this format. A way to re-create the effect seen by shifting the redpoint. (Thats all thats been done, it actually makes the surface seem less red). Is this.

Photoshop-only explanation here.
Download these 2 sets of 3 images.
Series 1.
marsrovers.jpl.nasa.gov...
marsrovers.jpl.nasa.gov...
marsrovers.jpl.nasa.gov...

Series 2.
marsrovers.jpl.nasa.gov...
marsrovers.jpl.nasa.gov...
marsrovers.jpl.nasa.gov...

Now, In the first series the Red component is from filter L4, (600nm) and in the second the filter is L2, (750nm). The green and blue filters are the same for both at L5 (530nm), and L6 (480nm) respectively.

To combine these, we will start with the first series. Open the L4 filtered image in photoshop first. This will be the background Then open the L5 and copy/paste as a layer over the L4 (layer 1), then copy/paste the L6 image as a layer over both (layer 2). Now all you have to do is rightclick on the L6 layer, go to blending options, advanced blending, and make sure only the blue channel is selected (deselect the other 2) this makes this layer the blue channel. Now, do the same for layer 1 (L5) but select the green channel. You don't have to do the same for the last channel as if you havent changed the opacity. Red is the only thing that can show through from that layer.

You should now have a regular, true-colour image of the sundial. As RGB are typically those values.

Now, if we repeat the process with the second series of images. Using the L2 layer as the background (and therefore red channel). We get a completely different looking sundial.

Try it for yourself.

Here are the two processed images:

Series 1:


Series 2:


All this from a little shift of the redpoint by 150nm. You'll also notice it doesn't really change the look of much apart from the extreme colours. I will make a diagram to show how the color-space is transposed.


reply posted on 12-1-2004 @ 10:38 AM by Kano
Ok, explanation of why some colors look out of place on the L2/L5/L6 filtered image.

Firstly we have the full visible spectrum.



We can then map our RGB colorspace onto it:



The curved grey region is the entire visible spectrum. The white triangle is the region of colours displayable by RGB. (The L4,L5 and L6 filters correspond to the points R,G and B).

This is the space recorded by replacing the L4 filter with the L2 filter (ie shifting the Red point by 150nm to the very edge of infra-red).



Notice there is a region recorded that is outside the visible spectrum. (The bottom right corner of the RGB triangle).

Now, when we display the composite RGB image of this back on our monitors. The colorspace that is recorded by the PanCam (with regions outside the visible) is transposed onto the displayable region shown in the first image. Thus a small region of infra-red is now added to the end of the red channel. As it is squashed into the displayable region.

Now, this means anything that is very reflective in the near infra-red spectrum (for example the blue pigment) has a massive boost in the Red channel when transposed. By comparing the L2 and L4 images for the green-chip, we can see the green chip also is quite a bit more reflective in L2 than L4. Thus the blue pigment appears pink and the green a kind of beige.

Also you'll notice that the transposition would actually make the environment look less red. As anything in the true red range (600nm) would be shifted to a slightly shorter wavelength, and appear more orange.


Now, a possible reason for this choice of Red channel. Is that by shifting the red channel to the extreme edge of the visible spectrum, more data can be recorded that would otherwise be missed if red was taken at 600nm (ie anything from 610-750 would be lost). The mission is a geological one, not a picture-taking one. They can also be fairly sure that there is going to be little to no bright blue or green items on the surface. This can be confirmed by using the L4, L5 and L6 filter set periodically. So using L2 instead of L4 as the red point allows more of the visible spectrum to be recorded in the same image. With some slight quirks that don't really effect the mission of Spirit on Mars.

[Edited on 12-1-2004 by Kano]
Pages: <<  2    3    4    5    6    7    8  >>    ^^TOP^^



Russian scientists reach buried Antarctic Lake Vostok
  Posted 6 days ago with 83 member flags
Monsanto quits as GM results announced (EUROPE)
  Posted 7 days ago with 72 member flags
Renowned Geophysicist Says Strange Sky Sounds Are Real
  Posted 1 days ago with 68 member flags
Strange noises reported around North Battleford
  Posted 19 days ago with 67 member flags
Ayatollah: Kill all Jews, annihilate Israel
  Posted 7 days ago with 49 member flags