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MSL - Astronomical Observations, Phobos/Deimos, planetary/celestial observations and more
vikingmars
post Sep 20 2013, 07:57 AM
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QUOTE (Deimos @ Sep 19 2013, 07:09 PM) *
Many people would love such images. But RMI cannot image everything Mastcam can image. As Emily indicated in a blog post some time back, planners have to consider the implications of certain failure modes. You'd hate to have a minor problem overnight result in Chemcam's optics or detector being damaged by the Sun the next day, before the situation could be resolved from Earth. So if something is in a part of the sky near where the Sun will soon be, more caution may be warranted.

Thank you very much Deimos for your nice explanations and expertise.
I thought that, to avoid being "hit" by the Sun, instructions would be to point ChemCam away after having taken the night pictures. Besides, how does it work for the RMI camera of ChemCam when MastCam is taking Sun images ? Is the RMI CCD closed (or protected) then to avoid being "hit" by the Sun also ? Or is it "offset" by a few degrees from the MastCams alignment ? Many thanks in advance by your answers.

(PS : And as we say at my bank when we have to find workable solutions for huge industrial projects : " Because it's more easy and quick to say "no" : then try to say "yes"...")
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jmknapp
post Sep 20 2013, 10:22 AM
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QUOTE (vikingmars @ Sep 20 2013, 03:57 AM) *
Or is [CHEMCAM] "offset" by a few degrees from the MastCams alignment ?


Per the SPICE definitions, here's how the ML, MR and CC boresights and optical FOVs line up, assuming the rover is level:

Attached Image


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Greenish
post Sep 20 2013, 01:24 PM
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Also, there is a specific position of the focus mechanism (object distance 2.0 +/-0.2m) for ChemCam that blocks light from hitting the sensitive optical components by using the blockage of the secondary mirror assembly. They must put it in the sun-safe position before pointing the mast unit near the sun to avoid damage (this is also why there is an "exclusion zone" that won't allow the LIBS laser to be used between 1.94-2.22m - the same geometry would allow the laser to damage the mirrors).
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Deimos
post Sep 20 2013, 02:34 PM
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Chemcam has no aperture cover. There are two different problems with Chemcam's "sunsafety". One is while in focus (at infinity, or generally >2 m), and one is while out of focus (and 2 m makes the Sun dramatically out of focus). If in focus, the damage potential is higher. While one imagines the aim would always be commanded down below the horizon, there is no guarantee that any specific command will work. Every spacecraft has command failures (due to error or anomaly). Some types might preclude the RSM from being moved until Earth takes over (I have experience with this on MER and Phoenix, and in the archived MSL data, dust devil movies aimed at the arm's workspace may be found). With the rover a century from the next on-site service call, one plays it safe--don't aim where a single command failure can damage an instrument; use on-board software to reinforce that. It is not that such an aim cannot be done, it is that risk-averse people avoid it because of all the other wonderful things Chemcam can do. So, if a target warranted it, people do know exactly how to create a sequence that would image it. But imagine the bar to be high.

While out of focus, I don't know that the CCD is at risk. But a lot of heat is being dumped into the optics (and the secondary mirror assembly). So, expect limits on what can be done. Fortunately, those limits allow many things; in particular, Chemcam is not at risk if the RSM is stationary in any orientation, intentionally or un-. For Pancam, you may have noticed hot-pixels 'move' with respect to the fixed Sun in transit image sequences. For Mastcam, you may have noticed the opposite. Such small changes in the way the aim is commanded, with other plan manipulations, has allowed the science to co-exist with sunsafety. But believe me, there is a lot of reviewing going on for pretty much any command that moves the RSM (see the analyst's notebook documentarian reports).

It took a lot of work to get to the point where MCAM Sun imaging could happen. The first Sun image preceded the first transit by 4 sols. Before that, it took a lot of work to show that any aiming at the sky could happen--but at least that was ready at touchdown (you may have seen early NCAM sky images that were used for attitude determination).
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paraisosdelsiste...
post Sep 22 2013, 07:25 AM
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Sol 393 Phobos Eclipse by Mars shadow. No words to describe it smile.gif

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Phil Stooke
post Sep 22 2013, 02:40 PM
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Great pics! There were two sequences that sol, one with both moons and slightly different lighting on Phobos. These versions are made of several images each, enlarged and merged.

Phil

Attached Image


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... because the Solar System ain't gonna map itself.

Also to be found posting similar content on https://mastodon.social/@PhilStooke
NOTE: everything created by me which I post on UMSF is considered to be in the public domain (NOT CC, public domain)
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fredk
post Sep 22 2013, 04:39 PM
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Yeah, great sequence. Here's another view of Phobos under different lighting again. It's an average of eight frames while it was in eclipse:

I guess the lighting is very similar to just prior to eclipse, except that the nearly-point-like direct sunlight is replaced with a wide streak of light along the horizon.
Attached Image

If you look at the full sequence, it looks like Phobos darkens as it enters eclipse, and then brightens a bit. That really surprized me, since I'd expect the dimming to be monotonic barring unusual cloud formations along the horizon. But if you look more closely, you can see that the hot pixel noise brightens with Phobos in the later frames. So it looks like Phobos brightens because the exposure length increases, and in reality probably dims monotonically.

Edit: replaced with better version (originally I averaged the frames, which led to big rounding errors and hence spurious blocky features - now I've simply added them all).
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tolis
post Sep 25 2013, 08:28 PM
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[quote name='fredk' post='203359' date='Sep 22 2013, 05:39 PM']Yeah, great sequence. Here's another view of Phobos under different lighting again.quote]
ADMIN NOTE: Excessive quoting removed.

Hi All,

The ability of Curiosity to take disk-resolved images of Phobos is impressive and opens up some interesting possibilities.
Consider, for example, that the project is recording transits of Phobos in front of the Sun in order to pin down its so-called secular acceleration in longitude due to tides. Would it not also be possible to measure its position by imaging it while in the shadow of Mars and as it passes in front of bright stars? In that case, a single long exposure would show all stars as trails, except that the occulted star will show a gap due to (invisible) Phobos passing in front of it.
Phobos is moving quite fast against the stars so a minute or two of exposure time should do it. Of course, you would need to know the time accurately, but if it known well enough to allow solar transit timings, it should also do for stellar occultations.

Now, why would this not work?
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fredk
post Sep 25 2013, 10:09 PM
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Stars trail fairly slowly, so it's not clear to me how accurately you could pin down the timing of the occultation by measuring the length of the truncated star trail. With solar transits, the timing is as good as the clock.

The other thing that occurs to me is that we've seen that stars need to be quite bright to show up with good signal to noise on mastcam. So it's not clear how often Phobos would occult a bright enough star.

But maybe it's possible - it would definitely be cool to see an occultation from Mars.
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Deimos
post Sep 26 2013, 02:06 AM
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One reason it might not work is if the sequence timing were different for certain sequences compared to typical images, and a series of images started after the eclipsed Phobos occulted Aldebaran, instead of having a 25-sec R0 image start 10 sec before. Hypothetically. You could get a series of images like on sol 387.

The timing and positional accuracy isn't the same as with solar transits--but the relative value of a near miss vs. capturing the event works out very differently. Imagine the sol 351 Phobos-Deimos event with only the image from right in the middle of the event, compared to only the central image of the annular transit; or imagine a missed transit with nothing visible next to the Sun, compared to Phobos near a bright star.
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tolis
post Sep 26 2013, 11:49 AM
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QUOTE (Deimos @ Sep 26 2013, 03:06 AM) *
One reason it might not work is if the sequence timing were different for certain sequences compared to typical images, and a series of images started after the eclipsed Phobos occulted Aldebaran, instead of having a 25-sec R0 image start 10 sec before. Hypothetically. You could get a series of images like on sol 387.



What I am advocating is really *one* long exposure. To give you an idea of what the result might look like, I attach an observation of an occultation of a star by an asteroid, obtained by John Broughton from New Zealand n 2008:

Attached Image


taken from

http://www.occultations.org.nz/planet/2008...nsteina_Rep.htm


The gap in the star trail near the center corresponds to the occultation. Knowing the absolute start (or end) time and the duration of the exposure one can derive the times of first and second contact.

Some BOTE calculations: stars near the Martian celestial equator would move in the martian sky about as fast as they do on the Earth, ~15 arcsec/s.
The pixel size of the 100mm fl eye on Mastcam is (5.1 deg)*( 3600 arcsec/deg)/(1200 pixels) ~ 15 arcsec/pxl. so stars would drift at around 1 pixel/s. A typical chord length of an
occultation corresponds to the time it takes Phobos to travel a distance equal to its apparent size relative to the stars. I' m guesstimating this to be in the region of 10-15 sec.

I can think of (at least) two sources of uncertainty in planning this observation, clock drift and Phobos ephemeris uncertainty.

I think the latter is of order 2-3 km or 1 sec of time in the along-track direction.

The former needs to be in the order of 10 sec or more to cause you to miss first or second contact.

There may also be sources of uncertainty that I'm not qualified to assess (in addition to messing up the calculations somewhere).
These are the ones that usually bite you in the end..
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jmknapp
post Sep 26 2013, 12:09 PM
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So would Aldebaran be the only bright star near enough to the Mars equator to potentially line up with Phobos?


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Deimos
post Sep 26 2013, 02:31 PM
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I'm not saying there's anything wrong with the occultation idea, or that a second image for dark subtraction is needed. I'm just saying that sometimes there's a 25-sec exposure, and one imagines that it would have been more interesting if started 85 seconds earlier. Maybe something can be inferred from that, maybe not.

Joe: There are a few candidate bright stars, depending on season and on what time of night Phobos passes the star. My bet is there are observable occultations just about every night, but one could question the S/N of most. The sol after the recent eclipse ingress images, Starry Night says there was another eclipse ingress with shadowed-Phobos passing through the Hyades, but that may not have been a good night for planning astronomy.
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djellison
post Sep 26 2013, 03:33 PM
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QUOTE (tolis @ Sep 26 2013, 04:49 AM) *
I can think of (at least) two sources of uncertainty in planning this observation, clock drift and Phobos ephemeris uncertainty.


The shape of the moon itself. Where does 1st contact occur on the moon. Where does 4th occur. That will also challenge interpretation.
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tolis
post Sep 26 2013, 03:53 PM
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indeed, even for a circular disk there are two solutions (north and south) for the same chord length. This ambiguity does not exist
when you can *see* the silhouette of Phobos, as has been the case for the solar transit observations. Still, with enough occultations
there shouldn't be any trouble fitting a well-behaved orbit solution. The other advantage of this type of positional measurement is that,
unlike solar transits, you don't have to wait for the sun to pass through the martian equatorial plane (ie where the
moon would appear to be from an observer at an equatorial site on the surface).
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