Re: Cassini: where are colors and detail?

From: Roger N. Clark (change username to rnclark) (username_at_qwest.net)
Date: 07/11/04 

Date: Sun, 11 Jul 2004 15:52:21 -0600

Chris L Peterson wrote:
> On Fri, 09 Jul 2004 21:08:21 -0600, "Roger N. Clark (change username to
> rnclark)" <username@qwest.net> wrote:
>
>
>>Chris,
>>I think you misunderstand out-of-band response and implications
>>of the response profile of filters. For example, the V-filter
>>used in astronomy, which is one of the fundamental filters
>>used in most fields of astronomical photometry has a peak
>>response at 0.53 microns (similar to the photopic response of
>>the human eye), yet has 0.033 relative transmission (relative
>>to 0.53 microns) at 0.486 microns (only down 15 db)!
>>
>>There is a marked difference in a out-of-band leak, like an IR
>>leak (which the Viking lander cameras had with filter wheels),
>>and wings on a filter. The key is total integrated energy in the
>>wings and/or out of band compared to the main response profile.
>>What is most important is KNOWING the response profile so it can be
>>properly modeled in any scientific interpretation.
>
>
> I'm not mistaking these two different things.

> There is a major difference in
> application between a wide band photometric filter, which is intended primarily
> to provide a standardized response, and a set of well characterized bandpass
> filters which can be combined to produce spectral data.

I agree with this statement assuming you are referring to comparison
of an RGB sensor to a multiple filters in a filter wheel that covers
a larger range. But that is obvious regardless of the filters.
Multiple wavelengths for spectral information is usually better, duh.

> I haven't seen a color
> detector suitable for the latter. Even the 300D, which is the best I've
> evaluated, produces numerous identical RGB values for different wavelengths
> between 450nm and 520nm.

With the filter response curves I have I do not see how this is
possible. While one filter response may be dropping as you change
wavelength toward the red (e.g. blue filter), another will be increasing
and both red and green increasing at a different rate. Thus
R, G, and/or B values continue to change as wavelength is scanned.

> Again, I'm not saying you can't use a color detector for scientific
> applications.

Chris, this is from one of your previous posts:
> They let other wavelengths through that contaminate the data, there are
> ixel-to-pixel non-uniformities that are difficult to quantify, and the Bayer
> pattern results in a loss of resolution and a decoupling of the spatial and
> color information. In short, they are not very useful for any real science.

> I'm only saying that they are, and probably always will be,
> inferior to selectable filter cameras for most uses. Their place is for
> collecting images that aren't intended for rigorous spectral analysis, for
> imaging rapidly changing events, and for tasks like maintenance and navigation.
> I can't imagine a mission with science goals similar to Cassini, or Galileo, or
> the Mars Rovers, to ever exclude a selectable filter camera if favor of a color
> sensor. Someone may make a case for both, but they will need to have a pretty
> good reason given the value of payload space on such missions.

You have constantly waffled back and forth. I entered this
thread trying to clear up some confusion about why cassini
didn't use bayer filters. I responded:

"The main reason not to fly a RGB digital camera sensor
is that it would be too limiting to have only 3 colors. ..."

You responded:
"I said the reason was that color sensors don't provide good data..."

I responded:
"Color RGBG sensors do provide good data, are linear and are great
scientifically as far as they go."

Let me summarize the way I see the use of Bayer and similar sensors.
I do this as a current planetary scientist on multiple
planetary missions, and I am currently a co-investigator on
6 proposed missions, including mars landers (MSL), asteroid
and moon missions. I have also served/serve
on multiple NASA committees that define future science.

1) RGB Bayer cameras are now space qualified and a reputable
    manufacturer is delivering cameras (e.g. Malin Space Systems).

2) The future of imaging from spacecraft is changing. The cameras
    have less role in spectroscopy. That is because of the field
    of imaging spectrometers. Imaging spectrometers go way beyond
    the few to a dozen or so filters in a filter wheel. They often
    go beyond the limits of a single detector. Cassini has 3 imaging
    spectrometers covering ultraviolet, visible, near, mid and
    far infrared (UVIS, VIMS, CIRS). VIMS, for example, has about
    96 channels (independent wavelengths) that it can image in
    compared to the ISS (camera) which has 12 filters. And the imaging
    spectrometers like VIMS image in ALL WAVELENGTHS SIMULTANEOUSLY

3) The combination of 1 and 2 means future missions could well
    carry a high spatial resolution imager, with a few fixed
    wavelengths (filters over the detector) and an imaging
    spectrometer Indeed, the Mars 2005 orbiter is just like this,
    though I don't believe they are using a bayer configuration
    on the camera. The HIRISE camera and CRISM imaging spectrometer
    will pair up to map Mars like never before.

4) Cameras do a lot more than spectral information. Spatial
    interpretations of image data is generally far more important
    than compositional information. which true spectrometers
    do much better.

5) Combination of high spectral and high spatial resolution
    instruments have been demonstrated and proposed for future missions.
    These systems slice a spatial image and reorder it from
    2-D to 1D array so the photons can be sent through a slit in a
    spectrometer. Thus all 2-D spatial and all spectral
    wavelengths are measured simultaneously.

6) It is likely that a higher spatial resolution camera will be
    included on all spacecraft for putting the imaging spectrometer
    data into perspective and stretching that interpretations
    from the spectrometer to higher spatial resolution. That camera
    will be with either black and white, of fixed filters, and Bayer
    configurations have been proposed. This is the case for all
    proposed planetary spacecraft I have seen. Gone completely
    are filter wheel cameras.

The scientific information these systems deliver will be truly stunning
and will open up whole new areas of research and understanding never
before possible.

So, Chris, despite you saying bayer sensors are no good for spectral
data and science, the scientific field is and will continue to move
forward and use them, and I'm sure with great results. Time will
tell and prove who is right, at this point is seems futile to
continue to argue.

Roger

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