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MSL - Astronomical Observations, Phobos/Deimos, planetary/celestial observations and more
paraisosdelsiste...
post Aug 7 2013, 05:25 PM
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It seems Curiosity taken some pics with different exposure times at the sky during the Sol 351:

Attached Image


From the pointing data... could it be Regulus?
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fredk
post Aug 7 2013, 06:28 PM
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Regulus looks good. To bring out the fainter stars, I subtracted two ML frames, took the absolute value, and then did a linear contrast stretch. That supresses noise ("fake stars") and any real stars are visible as pairs of points, with the same separation distance and orientation for all real stars. I then generated a chart of the Regulus region at the same scale as ML (15 degrees per 1200 pix). The scale was set by hand so isn't exact. I then adjusted the one free parameter (overall orientation) until I found a match.

Here's an animation flipping between the chart and the ML difference image. There are two or three stars visible apart from Regulus (circled in the ML frame). The scale's out a bit but the fit is otherwise very good, considering there is only one free parameter here:
Attached Image

The stars apart from Regulus are Leo eta-30, Subra, and possibly Leo 31.

And here's a similar difference of two MR frames. You can see a few real stars (paired streaks, same orientation and separation). These stars are too faint to show up on my chart...
Attached Image

Crazy that Regulus is very close to the Sun now as seen from Earth, so we'd normally never see it this time of year.
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fredk
post Aug 7 2013, 07:05 PM
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Update to the Leo pics with one new ML frame. Now the stars are more obvious. All three previous candidates confirmed, and one new star, Leo rho-47, also spotted right where it should be:
Attached Image
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paraisosdelsiste...
post Aug 7 2013, 07:05 PM
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With the MastCam Left frames and with the advice of Fredk I made a new version of the star frames taken during Sol 351:

Attached Image


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fredk
post Aug 7 2013, 11:23 PM
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Here's a much improved attempt at extracting stars from the sol 351 ML frames. This time I took the two (positive-valued) differences of the two frames, carefully registered them (which entailed correcting for a bit of frame rotation between the two frames), averaged them, and then applied a filter with a kernel tailored to the star streaks (19 pix wide, 1 pix high). These steps were interspersed with various contrast stretches along the way.

The result now starts to look like an image of the night sky, with many stars visible. But the faintest stars I can see are only around magnitude 5.5 or 6, so they would be quite easy naked eye from a dark site. We know that mastcam was not designed to image the night sky!

It's also interesting to note that the direction of star trailing is not very different from what it would be near Regulus as seen from Earth, even though the celestial poles of Mars and Earth are quite far apart. This is because the two poles are roughly in a straight line (geodesic) with Regulus.
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atomoid
post Aug 8 2013, 12:30 AM
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QUOTE (mcaplinger @ Aug 6 2013, 04:46 PM) *
..I'm not sure what your retrograde reference is to..

Relative to the background of stars (not horizon), both moons should be moving in the same direction. However, FredK's animation and the source files show apparent retrograde motion of Deimos relative to those stars. The time span of the sequence is only about 2 minutes which works out to something like 30 miles which doesnt seem enough for any sort of parallax effect. joes mapper says camera bearing 80.39 (E), elevation 58.64 so they arent even at zenith to mazimize that effect, I tried out a quick sim in Stellarium based on the Bradbury landing coordinates and one of the stars in the pair above could be HIP53525 with the lone one below perhaps HIP54084. But maybe these apparent 'stars' are dead pixel artifacts fixed on the frame that are masquerading as stars so there's really no apparent retrograde motion of Deimos (or perhaps most likely ive just lost my mind)

and thanks for the grazing schedule link, oh the things you can miss when you are away on vacation, looking forward to next week!
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mcaplinger
post Aug 8 2013, 12:47 AM
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QUOTE (atomoid @ Aug 7 2013, 05:30 PM) *
But maybe these apparent 'stars' are dead pixel artifacts fixed on the frame...

Yes, almost certainly. Remember that exposing for the Moon/a moon is like exposing for any other (admittedly dark) sunlit object, so these exposure times were fairly short.


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Disclaimer: This post is based on public information only. Any opinions are my own.
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iMPREPREX
post Aug 8 2013, 01:38 AM
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We're getting there with the moon shots!


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atomoid
post Aug 8 2013, 02:34 AM
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It looks like we might eventually get the full sweep between 3:02 and 3:04

So these must be stars in the 2:58 image centering on Deimos.
Then after the close in exposure optimized shots of the transit, we pull back out with long exposure and see the stars again in the 3:05 image
The stars in the long exposure shots appear to correspond perfectly with the stars in the subframe transit shots, so it appears Deimos is going the opposite direction as Phobos relative to the background stars!
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Deimos
post Aug 8 2013, 04:29 AM
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Deimos and Phobos both orbit in the same direction, as you know. Deimos is far enough out that it (like our moon) has an orbital period longer than the planets rotational period, and its apparent motion is in the same sense as the stars. But it lags them, and does not change directions with respect to the horizon or the stars. Remember, Deimos is moving many times faster than the rover in an inertial, Mars-centered frame. Phobos is closer and faster, so it crosses the sky in the opposite direction, but just moves faster in the same direction with respect to the stars. So if they appear to move in opposite directions with respect to something, I would speculate the 'something' is not a star. Warm/hot pixels tend to be much more observable than stars for cameras optimized for landscape photos.
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fredk
post Aug 8 2013, 04:44 AM
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Adding to Deimos's comments, both those MR frames you pointed to (and all the MR frames from that sequence) have the same bearing/elevation, as Joe's site shows, so they're pointing at a fixed direction in the sky, not following the stars. But the frames are minutes apart. Stars would have moved many pixels between the frames. So those are all hot pixels.

Stars are pretty hard to see with mastcam. Compare the raw Regulus shots with my heavily processed ones - how many stars can you see on the raw shots? And with short enough exposures that the stars wouldn't trail, it would be extremely hard.
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Zelenyikot
post Aug 8 2013, 11:42 PM
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Stellarium show it is Regulus. Sol 351
Attached thumbnail(s)
Attached Image
 


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atomoid
post Aug 9 2013, 01:36 AM
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So many hot pixels, some even masquerading as star streaks but in the wrong direction (not sure if hot pixels explains those as they should be stationary in the frame) and the only star (Regulus, as Zelenyikot points out) in these shots is the the streak going lower left Sol 351 03:09 to upper right Sol 351 03:11 (though its possible to notice a couple other extremely faint ones in similar motion).
gotta love those hot pixels though, beautiful starry colors and patterns.
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Deimos
post Aug 9 2013, 02:05 AM
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The hot pixels that streak in the wrong direction are bleeding down columns due to saturation. That is intrinsically colorful on a Bayer-pattern-microfilter CCD, though bland or uneven on a non-patterned CCD. The Regulus images are aimed a bit differently, so any star streaks would be somewhat different, but not column-oriented.
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Gerald
post Aug 10 2013, 10:37 AM
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The 17 Sol 351 Mastcam Right Phobos images available by now can be processed in a way to get a glance at the dark side of Phobos:
Attached Image

Processing roughly sketched:
- Calculating with double precision floating point rgb values,
- magnifying twice, interpolating subpixels,
- registering images to Phobos, iteratively by brightness barycenter of shrinking region (reducing Deimos effect), then by least squares of image deltas.
- averaging each pixel pos over main mode (accepting 20/256 delta from average) of all frames to filter out Deimos, stars, artifacts, and to reduce noise,
- subtracting background (estimated to 27/256 grey value),
- using logarithm to get bright and dark colors within 8-bit rgb range,
- additional manual stretching (brightness, saturation).

Some chromatic aberration seems to get visible this way.
Some processing artifacts (bright pixels) seem to be induced by (undefined) logarithm of 0 for pixels below 27/256, can be fixed.

I've been trying to suggest, that long exposure times can be simulated successfully by registered image sequences, and that motion blur can be limited at the same time.
So I'm optimistic, that comet observations will become possible.
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