NH at Jupiter, Planning the Jupiter encounter Options

[john_s]

I think the Jupiter encounter deserves its own thread.

I've just been taking a first look at the Jupiter encounter geometry. You can do the same using Mark Showalter's excellent on-line ephemeris tools at the PDS rings node, which by good fortune happens to include a New Horizons ephemeris (calculated over a year ago) for our actual launch date, January 19th. We'll have an updated ephemeris soon, but this one's already good enough for planning. As Roby72 noted in the Star 48 thread, the satellites are (annoyingly) all on the opposite side of Jupiter at closest approach. We'll still get good views of all sides of Io because Io rotates in only 1.8 days and we'll be pretty close to Jupiter for that long. We'll get fairly good coverage on Europa too, for the same reason. But we won't get very close to Ganymede or Callisto. Luckily Io is our highest priority satellite target and Europa is next, so we'll do OK.

[hendric]

Will you be able to let us know which movies and KODAK_MMTs the team selects?

Also, I was hoping you could give us some technical details on your technique to recover the moon image data? I've got a solution:

The readout time is too long on the CCD, causing potential saturation at the lowest exposure times.

1. I assume you can take 0-s frames for measuring bias on the CCD?

2. What about moving the target up/down on the CCD to vary the amount of time for the average exposure? For example, if the CCD took 100 ms to readout, a 0-s exposure with an object at the middle of the frame would give an average exposure of 50ms. And as you move the object closer to the top, the exposure length would get shorter.

3. There would a small increase in brightness as you went down the rows, but so long as you're not totally saturated at the bottom you should be able to subtract out the increase since CCDs are linear.

This would even work to get photos of Jupiter, you would just have to take slices of it and mosaic the parts that don't get overexposed. Hopefully the inter-frame time isn't too long.

Of course, images with multiple bright objects far apart wouldn't be possible, unless the FoV could be rotated to get the objects near the top.

[ugordan]

I think the problem John is trying to describe is image smear during readout phase, not because the exposures are too large.
They can select a short enough exposure which won't bleach out the moons, but that exposure is short compared to the readout phase. LORRI has no shutter so light falls on the CCD continually. As a result, while the image is being readout light from the moon is still coming in and the end result of this (CCDs readout one line at a time, shifting the entire image in the process until the last image line is shifted into the A/D converter) is what I imagine will be a good image of the target with sort of a vertical motion-blur effect overlaid on it.

That is the gist of the problem with imaging the Galileans -- removing the unwanted "motion-blur" effect. Obviously, the images will need to be carefully processed to remove this linear smear. What John Spencer seems to be saying is that they can actually subtract the blurred part, but only for a small, well-defined target such as a moon or Jupiter when it's still far away.

I can't wait to see the results, both from the imaging perspective and image processing perspective they should be interesting indeed.

[john_s]

They can select a short enough exposure which won't bleach out the moons, but that exposure is short compared to the readout phase. LORRI has no shutter so light falls on the CCD continually. As a result, while the image is being readout light from the moon is still coming in and the end result of this (CCDs readout one line at a time, shifting the entire image in the process until the last image line is shifted into the A/D converter) is what I imagine will be a good image of the target with sort of a vertical motion-blur effect overlaid on it.

That is the gist of the problem with imaging the Galileans -- removing the unwanted "motion-blur" effect. Obviously, the images will need to be carefully processed to remove this linear smear. What John Spencer seems to be saying is that they can actually subtract the blurred part, but only for a small, well-defined target such as a moon or Jupiter when it's still far away.

That's exactly correct. For small targets (or, we think, for any image where we can arrange for a number of complete rows of dark sky in the field) we can probably get unsaturated images even at low phase angles, so we are interested in "Kodak moments" at low phase angles as well as high phase angles. And yes, we'll be sure to let you know which "Kodak" images make it into the final plan, though we won't know for a while, probably for several months. Integrating the Jupiter observations is pretty complicated (our biggest issue right now is finding the best times for the necessary Earth-pointed tracking periods, when we can't take data), and any "Kodak" observations will be the last things we add to the timeline, after we've optimized all our science observations.

John.