13 replies to this topic

[KpS]

Weather cooperated. Hot and dry air from Africa brought us a few sunny days. This time, no Sahara sand, the sky remained transparent to the horizon. That day in Prague was over 37°C. The limited view allowed me to start imaging half an hour after the western elongation of Thebe. I decided for a five-second exposure, raw16. A total of five hundred images divided into ten groups have been stacked and form ten frames of animation. Master dark frame was created from video of the same length. Thebe is just above the noise level and is gradually disappearing. Without animation it would be impossible to say with certainty that it is the Thebe. Gif (originally grayscale) is color-mapped for greater clarity. Unfortunately, the background stars are missing, the brightest are over 17 mags.

 

Photos from the field of astronomy is generally expected to be beautiful. I have to say that in this case the animation shows only somehow visualized results of extreme processing. If someone is interested in the first frame of the animation, here it is before processing.

 

 

[KpS]

I went back to the video with Thebe. Reducing background noise can only be achieved by increasing the total exposure time. However, Thebe is moving, so the only way to do this is to register all the images on her. Movement near elongation is not linear. I took all the necessary shifts from the JPL ephemeris. Amalthea and Io move relative to Thebe almost in unison, Ganymede flees away.

 

 

[james7ca]

Karel, this is a great report and I'm surprised that there haven't been more comments. As far as I know this is the first detection of Thebe here on CN. During my previous posts on Amalthea I questioned whether it would even be possible to image Thebe since it wasn't discovered until/by the Voyager space probe in 1979. I think I estimated needing a quality nine inch refractor (to reduce light scatter) or a notably larger reflector (all with good seeing and transparency).

 

In fact, the last time I searched on the internet for Thebe (2018-2019?) I could find NO earth-based images of Thebe, but there was one report from the Pic du Midi observatory where they used a one meter telescope to record both Amalthea and Thebe. They were doing an orbital study on these two moons since they are so seldom imaged and they had to use some special techniques to capture both of these moons. However, the report did not show an image of Thebe, only Amalthea and not really that good of an image (IMO).

 

In any case, the reason I found your post is that last week I was revisiting Amalthea (which I've captured before) and noted that both moons would be at maximum elongation at around 11PM PDT on Friday, July 31. It was a pretty rare alignment with six moons all on the west side of Jupiter. The alignment looked like the following:

 

 

[ JUPITER ]   < Amalthea >  < Thebe >  < Io >  < Ganymede > < Europa >  < Callisto >

 

 

I captured the event with my Celestron 9.25" EdgeHD and Amalthea showed up easily and when I looked for Thebe I could just barely detect something in the correct location but I don't think it was nearly as good as what you've shown here. I imaged at f/10 with a Baader red CCD filter and a ZWO ASI183MM Pro camera. I'm still working on the data set to see if I can do better before I post my images. I also tried a second time on Saturday night (since Thebe was again at maximum elongation) and I think I could detect something that could be Thebe (this time on the east side of Jupiter), but certainly not at all convincingly.

 

Anyway, you've posted a very impressive result and I'd like to know what software you used to create your presentation (I mean as used in post #2 to stack a tracked version of Thebe).

 

I've been using PixInsight combining its FFTRegistration result (I've also tried AutoStakkert!) with the CometAlignment tool in an attempt to track Thebe over an extended period of time (like you've shown in post #2). I found a technique that seems to work but it required multiple, manual steps and I'm not really sure it is giving reliable results. My method seems to work, but I'm seeing some inconsistencies and besides as you noted the apparent movement of Thebe is not really linear and the CometAlignment tool assumes straight-line linear. Given the latter, I'm considering using the CometAlignment tool in a piecewise fashion, breaking up my total sequence into several separate tracks.

 

Interestingly enough, my image of Thebe also shows that Io and Thebe seem to track fairly well together while Europa showed significant movement.

 

Although I've used NASA's Horizon website to get the ephemeris for Thebe I'm not seeing good agreement on Thebe's position given some of my other tools (SkySafari and the online Jupiter Viewer).

 

In fact, Jupiter Viewer seems to throw an error when you try to plot Thebe, possibly because it isn't using the latest ephemeris and thus can't plot that moon's location. As for SkySafari,  I didn't really expect it to be as accurate as the Horizon website, but the differences are pretty large and in any case I'm not sure that Thebe's orbit is that well known and I'm sure it changes month to month (slightly, but when you're working at sub arc second scales those small differences can matter).

Edited by james7ca, 03 August 2020 - 11:48 AM.

[KiwiRay]

Thanks for resurrecting this, James.  This was an impressive achievement, and from your review of the history of Thebe imaging (essentially no history), it certainly deserves greater attention.

[Kokatha man]

Karel, I don't know whether I just clicked the "Like" button just now...or did so last year when you posted it - it's excellent! Perhaps you should have made the title "My best ever image..." & folks would be queuing up to read the thread..!!!

 

A great feat!     

Edited by Kokatha man, 03 August 2020 - 08:43 PM.

[KiwiRay]

Karel, I don't know whether I just clicked the "Like" button just now...or did so last year when you posted it -

That's funny - I had the same experience.  I was about to apologise for missing it last year, then noticed I'd already clicked like and couldn't remember when I did that.  It could be that until James replied today that I didn't appreciate the significance of this achievement.

[Kokatha man]

That's funny - I had the same experience.  I was about to apologise for missing it last year, then noticed I'd already clicked like and couldn't remember when I did that.  It could be that until James replied today that I didn't appreciate the significance of this achievement.

Ray - start worrying, believe me..!

[sunnyday]

I am very happy to see this thread  surface once again.
I just learned things, which
is not trivial.
thanks to kps and james 7 ca.

[John Boudreau]

This is indeed an impressive feat, and I have no idea why I hadn't noticed this post last year.

 

An all to belated, 'Very well done, Karel !'  

[KpS]

Thank you guys for the warm comments and also for all the likes.

 

james7ca, thank you for reviving this one year old thread and also for the interesting comments. I look forward to your results.

Probably the most accurate ephemerides not only of the planets but also of their moons are provided by JPL Horizons. Personally, I use Cartes du Ciel (CdC Planetarium). The positions of the planets and moons can be based on JPL ephemerides once downloaded (a few GB). In addition, CdC allows you to view faint moons.

For the initial registration of images, it would be ideal to have a sufficiently bright star in the field. Unfortunately, that was not the case. So I used Jupiter and AutoStakkert. The software used to stack a tracked version of Thebe was Iris (Christian Buil, astrosurf.com). It's such a Swiss Army knife. For example, it can shift images with subpixel precision. Very useful was also the decomposition of images into wavelets. The transition from ten animation frames to the image registered on Thebe requires only nine shifts. I did them manually.

[james7ca]

Karel, thanks for the feedback. So, you did the shifts manually, that's just what I didn't want to hear.

 

PixInsight has a number of useful tools to aid with this process, but unfortunately none of the tools that would be really helpful will accept a batch of files (that is, except for the CometAlignment tool and the FFTRegistration script).

 

However, if I can get the Horizons data into Cartes du Ciel and then plot a high scale overlay for Thebe then I think I will have a process that will be mostly automated. I've also considered trying to use AsmCoder8088's Tycho software but that would require me to purchase a new video card for my PC and I don't think that software is really sensitive enough to pick up something as difficult as Thebe. Besides that it is designed to look for asteroids and a planetary field might confuse the software.

 

Thus far I've been able to produce at least one image where Thebe seems apparent, but it's a pretty ugly result and it really doesn't pass my personal test for being a clear and compelling detection.

 

In any case, I'm still somewhat amazed by the fact that I was able to get something even under my red zone light pollution and a gibbous moon and with Jupiter so low in the sky. That said, the glare from Jupiter is the biggest issue, so maybe the light pollution and the moon don't really add much to the difficulty (but it certainly can't help).

Edited by james7ca, 05 August 2020 - 03:43 AM.

[moonwatching ferret]

 This is giving me ideas lol how do i fing=d a epheremieidies with Jupiters smaller moons

[james7ca]

 This is giving me ideas lol how do i fing=d a epheremieidies with Jupiters smaller moons

Already mentioned several times:

 

Although I've used NASA's Horizon[s] website to get the ephemeris for Thebe...

 But, here is the link:  https://ssd.jpl.nasa.gov/horizons.cgi

 

Also, Jupiter Viewer can be useful (but may have issues with Thebe, as was previously discussed):  https://pds-rings.se...iewer2_jup.html

 

In any case, given Karel's result and my recent experience on Thebe it shouldn't be a terribly difficult target given a large enough scope and with good seeing (where the latter certainly favors those in the southern hemisphere).

Edited by james7ca, 05 August 2020 - 02:06 AM.

[KpS]

So, you did the shifts manually, that's just what I didn't want to hear.

I agree, the word manually does not sound too good. But I only needed nine shifts. After entering Thebe's position in an Excel spreadsheet and calculated the necessary shifts, I did these using the command line in Iris.

 

Yes, the glare from Jupiter is the biggest issue. Or more precisely, the problem is the photon noise that remains after subtracting the glare. About a month before capturing Thebe, I wanted to get an idea of the sky brightness around Jupiter. At a distance of 54 arc seconds, I measured about 13 mag per square arc second. The light pollution in the big city is much lower.