19 replies to this topic

[james7ca]

About one year ago I captured some images showing seven of Jupiter's moons but I was never completely happy with my image of Amalthea. Amalthea is the fifth largest moon of Jupiter but it is MUCH smaller than the four Galilean moons. It's a non-spherical object with a mean diameter of just 167 km (ellipsoid, 250 × 146 × 128 km according to Wikipedia). By some reports it is also the reddest known object in the solar system. Right now its magnitude is 14.8 and it is well positioned for observers in the western U.S. since it reaches greatest elongation from Jupiter each night just as it gets close to the local meridian (its orbital period is almost exactly 0.5 days, so it's pretty much in the same place within its orbit every night, at least for the next few weeks).

 

In any case, I think I've got something that is somewhat better than what I had last year. I used a Tele Vue NP127is with a 1.8X A-P barlow and a QHY5III-178C camera. I combined (stacked) 24 subs that were each exposed for 16 seconds and use PixInsight's FFTRegistration script to do the registration and integration. Below is the result, with the top image the integration and the bottom version with labels and an image of Jupiter that I took on the same morning (not at exactly the same time, but with the same equipment and at the same scale). As you can see the image of Jupiter (and the other moons) was highly overexposed in order to catch Amalthea.

 

If I get any more clear nights in the next week I may try again, since I think I can do somewhat better. Last night was the full moon and it was fairly close to Jupiter and the skies were somewhat hazy, in fact just after I finished the images I had a pretty thick fog bank move in. Also, the next several days may not be any better, because the moon will be very close to Jupiter.

 

I have other sequences to process (different exposures), so maybe I will find something that is better. In fact, I should blink the subs that I have to eliminate the worst of the lot (seeing and tracking errors).

Attached Thumbnails

• 

Edited by james7ca, 12 March 2017 - 11:48 PM.

[RunningMan]

That's really cool!

[RedLionNJ]

Much as it would give me a perverse sense of pleasure in determining this to NOT be Amalthea (there have been MANY mis-identified images in the past), this certainly appears to be a genuine capture to me. Everything (position at the specified time, relative brightness, distance from Jupiter's limb) checks out.

 

GOOD JOB!

 

 

However,  your size estimate may be a bit off - I believe the long axis of Amalthea is around 250km. It's more like a Fingerling Potato than any other shape I've seen.

 

Really nice work, James. 

[james7ca]

RunningMan and RedLionNJ, thanks for the notice.

 

I got the diameter of Amalthea from SkySafari Pro (they say 167 km), but it has a thin, pointed shape so I'm not sure how they report the diameter.

 

Interestingly enough, this is the fourth time in the last ten days that I've tried to capture Amalthea. My first attempt was with my C6 and then I made two attempts using a TPO 6" Newtonian. I knew the Newtonian would be difficult because of the diffraction from the secondary spider vanes but I rotated my optical tube to put the worst of it away from where Amalthea would be. However, I never got anything that even seemed close (although I did record some nearby stars that were actually fainter than the moon).

 

I think my earlier failures happened for two reasons (or maybe three). First, both the C6 and the Newtonian MAY have more light scatter than my refractor. Second, in the earlier trials I was using my Celestron AVX mount and I think the tracking just wasn't good enough to allow detection of such a faint object (at least given the glare from Jupiter). Then (possibly), I may just not have had as good of seeing on those earlier nights. Or maybe it was just my processing (but, that would make four reasons).

 

Then, I broke down and reverted to my Tele Vue NP127is and my Mach1GTO mount (with which I had successfully recorded Amalthea about one year ago). However, this year I increased the image scale by using a barlow instead of imaging at the prime focus of the Tele Vue. I was pretty encouraged when I started looking at the previews from within Sequence Generator Pro. I was actually able to detect Amalthea in the individual subs, or at least in quite a few of the subs (using SGP's histogram stretch).

 

So, I thought this HAS to be good. Then I stacked the images and found that it wasn't as easy as I was expecting. I think I need to revisit the processing, cull the subs, and maybe try to improve the registration. Also, as I said earlier I have some more sequences using subs at different exposure levels (32s and 8s).

 

I did kind of panic when I first looked at the stacked results because I didn't see any field stars. Then I checked SkySafari and found that Jupiter seemed to be in a star-poor region and there weren't any stars brighter than Amalthea within the relatively small field of my camera. But, I need to check on that at WikiSky to see if I recorded any stars that could be at or slightly below magnitude 15 (perhaps near to the very edge of my field).

 

[UPDATE]

I think I clipped the background in my first integration. When I stretch the individual 32s subs I can see quite a few background stars (but none very near Jupiter). Then I checked one of the individual 16s subs that I used for the above integration and there are background stars in that too. So, I need to revisit my processing.

[/UPDATE]

Edited by james7ca, 12 March 2017 - 11:45 PM.

[happylimpet]

Excellent images. My feeling is its more about clean optics than anything else (ie minimum scattered light), as it really isnt that faint. This has been my excuse for my lack of success with my 12". Also the 0.5 day thing screwed me over for ages!

[Stargazer3236]

About one year ago I captured some images showing seven of Jupiter's moons but I was never completely happy with my image of Amalthea. Amalthea is the fifth largest moon of Jupiter but it is MUCH smaller than the four Galilean moons. It's a non-spherical object with a mean diameter of just 167 km (ellipsoid, 250 × 146 × 128 km according to Wikipedia). By some reports it is also the reddest known object in the solar system. Right now its magnitude is 14.8 and it is well positioned for observers in the western U.S. since it reaches greatest elongation from Jupiter each night just as it gets close to the local meridian (its orbital period is almost exactly 0.5 days, so it's pretty much in the same place within its orbit every night, at least for the next few weeks).

 

In any case, I think I've got something that is somewhat better than what I had last year. I used a Tele Vue NP127is with a 1.8X A-P barlow and a QHY5III-178C camera. I combined (stacked) 24 subs that were each exposed for 16 seconds and use PixInsight's FFTRegistration script to do the registration and integration. Below is the result, with the top image the integration and the bottom version with labels and an image of Jupiter that I took on the same morning (not at exactly the same time, but with the same equipment and at the same scale). As you can see the image of Jupiter (and the other moons) was highly overexposed in order to catch Amalthea.

 

If I get any more clear nights in the next week I may try again, since I think I can do somewhat better. Last night was the full moon and it was fairly close to Jupiter and the skies were somewhat hazy, in fact just after I finished the images I had a pretty thick fog bank move in. Also, the next several days may not be any better, because the moon will be very close to Jupiter.

 

I have other sequences to process (different exposures), so maybe I will find something that is better. In fact, I should blink the subs that I have to eliminate the worst of the lot (seeing and tracking errors).

 

How did you determine where Amalthea would be located? Is there an ephemera to locate Amalthea on a daily basis so it can be imaged?

[james7ca]

I use SkySafari Pro on my iPad to check the position of Amalthea, but there are sources online that will provide similar information.

 

Here is one link:  https://pds-rings.se...iewer2_jup.html

 

And here is a simulation image produced by the above online tool for Mar 16, 2017 at 2:42AM PDT (9:42 UT). And, the color simulation with an approximation of the surface of Jupiter from SkySafari Pro. Note that SkySafari may not show the correct positions for some of the smaller moons of Saturn and Jupiter (although it seems to get Amalthea correctly).

Attached Thumbnails

• 
• 

Edited by james7ca, 15 March 2017 - 05:07 AM.

[Stargazer3236]

I have SkySafari 5 Plus and it doesn't show any small satellites of Jupiter or Saturn.

 

I have a satellite generator from Sky & Tel that shows where Phoebe is located, but nothing for Amalthea at Jupiter.

[Stargazer3236]

I use SkySafari Pro on my iPad to check the position of Amalthea, but there are sources online that will provide similar information.

 

Here is one link:  https://pds-rings.se...iewer2_jup.html

 

And here is a simulation image produced by the above online tool for Mar 16, 2017 at 2:42AM PDT (9:42 UT). And, the color simulation with an approximation of the surface of Jupiter from SkySafari Pro. Note that SkySafari may not show the correct positions for some of the smaller moons of Saturn and Jupiter (although it seems to get Amalthea correctly).

That is an awesome online program!

 

I like how it shows the smaller moons in relation to the known moons J1-J4. I will try this out when I start my regular Jupiter imaging sessions as soon as this dang blasted cold weather and snow disappears from up here in the Noth'East.

Edited by Stargazer3236, 15 March 2017 - 12:54 PM.

[james7ca]

I have SkySafari 5 Plus and it doesn't show any small satellites of Jupiter or Saturn.

 

I have a satellite generator from Sky & Tel that shows where Phoebe is located, but nothing for Amalthea at Jupiter.

 

Have you turned on the option to show the minor moons under the Settings menu of SkySafari? I don't know whether this option is only available on the somewhat more expensive Pro version of the program.

 

I've also read that the freeware program Cartes du Ciel also shows the positions of the minor moons.

[Stargazer3236]

I just discovered the CdC aspect of the moons, it's great! But I have a problem navigating in the CdC star charts. I have Star Seek Pro from Orion and it is the predecessor to SkySafari. It doesn't have the moon option and neither does Skysafari 5 Plus. I would figure that Star Seek Pro ($39.95) on iTunes website would have the moons option for the major planets, but it doesn't.

 

What is the actual location of the minor moons in Skysafari? I tried looking for it in the Solar System in the settings.

 

I like the https://pds-rings.se...iewer2_jup.html a whole bunch. I will use that to prepare for imaging those moons when I go out to image Jupiter!

 

I plotted Thebe for tonight at midnight and Adrastea will also be visible.

[james7ca]

So, as a further confirmation I took the simulation produced by the Jupiter Viewer online utility and overlaid that to my image. The simulation locations are given by the black dots and you can see that my image of Amalthea is more-or-less exactly where it should be. When matching/overlaying the images I think I got the scale and rotation slightly off which caused some slight displacements for the moons on the right side (Callisto and Ganymede), but Amalthea looks spot on (better, than I could have hoped, science prediction matches observation).

Attached Thumbnails

• 

Edited by james7ca, 18 March 2017 - 04:28 AM.

[james7ca]

And here is the simulation plot from Jupiter Viewer.

Attached Thumbnails

• 

Edited by james7ca, 18 March 2017 - 04:30 AM.

[Stargazer3236]

I wonder if you overexpose enough, that you can see the rings of Jupiter? With a long exposure time and overexposure, could it be possible to see the rings?

[james7ca]

I suspect that the rings and many of the other close-orbiting moons are all out of reach for amateur-class equipment. I'm pretty sure that most of these objects were discovered by space probes, not by earth-bound telescopes.

 

One of the issues here is that the largest amateur telescopes are likely to be Newtonians and the diffraction from the secondary mirror supports ("spikes") are likely to mask faint objects that are near to Jupiter. In fact, one recommendation for anyone who is using a Newtonian or RC/DK or classic Cassegrainian, rotate your tube so that the diffraction spikes fall outside of the plane of orbits for the moons. I even tried that with my 6" TPO Newtonian but I was still unable to detect Amalthea with that scope.

[james7ca]

I'm back with a new processing on the image of Amalthea. I wasn't that happy with my earlier image because it didn't include any background stars to allow some kind of context on the brightness of Amalthea. So, I processed a set of subs that were exposed for 8 seconds each and with a little more care in the processing I was able to bring out quite a few background stars. The full integration contains 48 subs and these were captured about 13 minutes after my original effort (see the above images that were posted back in March).

 

This is also a wider view and reproduced at a smaller scale and left in color whereas my earlier effort was cropped and converted to grayscale.

 

I've identified a number of the stars (but not all) and it looks like the faintest stars are just below magnitude 15 (NASA Horizons says that Amalthea was magnitude 14.4 at the time when this image was taken). The GSC (Hubble Guide Star Catalog) numbers are from Sky Safari with the magnitudes in parentheses. I did no masking or spot adjustments to bring out the image of Amalthea, but I did place a mask right at the boundary between the large central flare from the optics (just outside of the position of Io) to allow a slightly greater stretch for the background stars. I also did a deconvolution (sharpening) which left a bit of an artifact around some of the brighter background stars.

 

One reason I was motivated to revisit this data set is that I had my 9.25" EdgeHD out last night and I tried again to capture Amalthea since it was near to its maximum elongation from Jupiter during a time when Jupiter was reasonably high in the sky (or as high as it now gets from the northern hemisphere). Unfortunately, I found absolutely no trace of the moon which probably goes to show that the seeing conditions have as much to do with recording this moon as anything else (since under good conditions I captured Amalthea with a 5" refractor, while I've now failed on several different nights when trying to use a 6" SCT, a 6" Newtonian, and now a 9.25" EdgeHD under average or worse seeing conditions).

 

In any case, below is my latest processing effort.

Attached Thumbnails

• 

Edited by james7ca, 24 May 2017 - 06:51 AM.

[Stargazer3236]

 

[old_frankland]

Exceptional work!  And, most informative commentary.  Bravo!

[james7ca]

David and Jim,  thanks for the notice.

 

I may do another round of processing on this, since I think if I create a synthetic luminance channel I might be able to bring out Amalthea and the fainter stars a little more. There are some stars that are in the faint background and I'd like to see how far I can push the limiting magnitude on this data set.

[happylimpet]

Youre right about the seeing....I tried several times before I tried on a night of superb seeing and it just jumped out!