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Saturnian satellites.


Xilman
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Although this image is not of a planet, it is of a planetary satellite and I can't find anywhere more appropriate to post it.

Last summer I imaged (the locations of) some satellites of Jupiter and Saturn but have only just got around to processing them. One target was Albiorix, aka Saturn XXVI. Discovered in 2000, it is only 30km across, roughly half the diameter of the M25 motorway. At the time of observation it was magnitude V=21.5.

62 1-minute subs taken with an unfiltered SX814 on a 0.4m Dilworth were stacked on the mean motion of the satellite and the result compared with the MPC ephemeris and the DSS2 images. The stars are trailed; the faintest one nearby is catalogued at g=20.68 in Gaia EDR3. Its trailed image is marked with the asterisk and red arrow. Despite the low signal to noise, Albiorix shows up untrailed in precisely the correct location; there are no stars of comparable brightness at that position in DSS2 and there were no asteroids thereabouts at that time according to the MPC, so I'm reasonably confident of the identification.

Albiorix_annot.png

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That's a really nice analysis @Xilman! Would be interested in knowing what level of detection is represented in this image (i.e. sigma), and what levels in the individual subs are. Just curious. Regardless, really nice work, you've inspired me to try something similar. Did you do this from your UK or La Palma site?

Nigel

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32 minutes ago, NigeB said:

That's a really nice analysis @Xilman! Would be interested in knowing what level of detection is represented in this image (i.e. sigma), and what levels in the individual subs are. Just curious. Regardless, really nice work, you've inspired me to try something similar. Did you do this from your UK or La Palma site?

Thank you for your kind words. The images were taken from the La Palma site because I have only a 250mm Dobsonian here in the UK. It's not suited for imaging and certainly not for objects that faint.

I need to measure the image to get a good estimate for the SNR but I doubt that Albiorix reaches much more than 5 sigma and may be as low as 3 or so.  Whatever it is, the individual subs will have a sigma sqrt(62)  (approximately eight) times smaller and so well under unity. I will try to make those measurements tomorrow and report them here.

Please try for this kind of imaging. Hardly anyone does it and although the orbits of Saturnian satellites are well known, that is not true of other solar system bodies of similar brightness and you can provide data of real value to refining their orbits. If it wasn't for the declination of the planets these days you would have no real problem picking up sub-21 magnitude satellites with your kit in Rutland. It's a pity our summer nights are so short and bright and the planets are so low in our sub-arctic skies.

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1 hour ago, Paul M said:

That's brilliant work. One thing that I'm trying to work out is how you stacked on the mean motion of the object? No reference object in any frame!

How did you achieve that?

Despite your claim, there were hundreds of reference objects in every frame --- the stars!

 

Stage 1: plate-solve the subs; this puts a WCS (world co-ordinate system) in the header of each. I use a local installation of astronomy.net for this.

Stage 2: look up the satellite's ephemeris and, in particular, the sky motions in RA & Dec of the satellite. https://minorplanetcenter.net/iau/NatSats/NaturalSatellites.html provides this service.

Stage 3: use a home written Perl script which tweaks the WCS in each sub's header to subtract off the sky motion which has taken place since the first sub was taken.

Stage 4: co-add all the tweaked subs using SWarp. That utility uses the WCS in each sub to align them.

Stage 5: use the ds9 FITS image viewer to stretch contrast, zoom, smooth, etc, until the target is visible at the location given in the ephemeris.

 

Easy, but fiddly, when you know how!

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17 hours ago, Xilman said:

I need to measure the image to get a good estimate for the SNR but I doubt that Albiorix reaches much more than 5 sigma and may be as low as 3 or so.  Whatever it is, the individual subs will have a sigma sqrt(62)  (approximately eight) times smaller and so well under unity. I will try to make those measurements tomorrow and report them here.

It was remarkably difficult to measure the SNR with the tools available to me. My favourite photometry program, Russ Laher's APT, failed completely. Astrometrica took a bit of persuasion and gave values between 2.7 and 3.4 depending on precise positioning of the aperture and its radius, so I conclude that the SNR sigma is about 3 --- as in the original guess.  That for each sub would be around 0.4.

The MPCReport.txt from Astrometrica contains the line

99999         C2021 05 04.00000 19 52 26.10 -21 42 19.0          22.43G      J22

where the position is 19:52:26.10, -21:42:19.0 and the magnitude is 22.43 in Gaia's g-band. The MPC ephemeris has 19:52:26.3, -21:42:18 and V=21.5. Agreement is satisfactory in my opinion. With such poor statistics and measurements in two very different band passes, one should expect positions and magnitudes to differ somewhat.

Rule of thumb is that sigma=3 is sufficient for identification of a known object, where additional information is available, but 5 is needed for discovery. These correspond to a 0.3% and 3ppm chance respectively that it is a false positive assuming the errors follow a normal distribution.

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Here is Siarnaq, also known as Saturn XXIX. Most of the technical details are as for Albiorix above. It was imaged in 43 1-minute subs at 22:00 UT on 2020-09-13, which is a couple of hours earlier than Albiorix.

This was a tricky one because at the time of observation the V=20.9 magnitude satellite was only a few seconds of arc away from a star 2.5 magnitudes brighter --- a factor of ten in intensity.

The images are (left) stacked on the stars, showing a trailed Siarnaq, and stacked on the predicted motion of the satellite (right), showing noticeably trailed stars and a round satellite, though at poor contrast because of light bleeding in from the star. The quality is not as good as I would hope because there are clear guiding errors on the stars. Two remedies spring to mind. The first is to try again later this year and the other is to deconvolve the image using a bright unsaturated star to estimate the point spread function.

 

Siarnaq_annot.png

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On 04/05/2021 at 12:51, Xilman said:

Stage 1: plate-solve the subs; this puts a WCS (world co-ordinate system) in the header of each. I use a local installation of astronomy.net for this.

Stage 2: look up the satellite's ephemeris and, in particular, the sky motions in RA & Dec of the satellite. https://minorplanetcenter.net/iau/NatSats/NaturalSatellites.html provides this service.

Stage 3: use a home written Perl script which tweaks the WCS in each sub's header to subtract off the sky motion which has taken place since the first sub was taken.

Stage 4: co-add all the tweaked subs using SWarp. That utility uses the WCS in each sub to align them.

Stage 5: use the ds9 FITS image viewer to stretch contrast, zoom, smooth, etc, until the target is visible at the location given in the ephemeris

I was thinking that it is a bit fiddly but just now I've been playing with a feature in ASTAP, designed for stacking on a minor planet/comet. Select "Ephemeris Alignment" and it will stack on the ephemeris position of the selected object. That object need not be visible in individual subs or even the stack. It does have to be an object in its minor planet and comet catalogues which are editable. So I suspect it will be possible to get ASTAP to accept more esoteric objects so long as the MPC has them on file. 

I know, lazy!  :)

I just used it on a recent run of subs of the fast moving comet C/2020 R4 Atlas. It worked quite well but I screwed the final image by inadvertently including some outliers that were outside the field of the main tranche of subs. 

Anyway, thanks for giving me some ideas! :)

 

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32 minutes ago, Paul M said:

I was thinking that it is a bit fiddly but just now I've been playing with a feature in ASTAP, designed for stacking on a minor planet/comet. Select "Ephemeris Alignment" and it will stack on the ephemeris position of the selected object. That object need not be visible in individual subs or even the stack. It does have to be an object in its minor planet and comet catalogues which are editable. So I suspect it will be possible to get ASTAP to accept more esoteric objects so long as the MPC has them on file. 

I know, lazy!  :)

I just used it on a recent run of subs of the fast moving comet C/2020 R4 Atlas. It worked quite well but I screwed the final image by inadvertently including some outliers that were outside the field of the main tranche of subs. 

Anyway, thanks for giving me some ideas! :)

 

De nada.

I've never heard of ASTAP before. Is it a Windoze-only program? If so, does it work under WINE?

My procedure sounds fiddly but it actually takes very little typing (most of which is cut and paste) and not much more brainpower after the first couple of runs.

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2 hours ago, Xilman said:

I've never heard of ASTAP before. Is it a Windoze-only program? If so, does it work under WINE?

It's W10, Linux and Mac. I first found it packaged with Astroberry and continue with it on Windows. http://www.hnsky.org/astap.htm. It's also my Plate Solver of choice.

WINE? I don't do, but I've had some success with ASTAP under G&T's 🤪

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11 hours ago, neil phillips said:

Interesting recording these Satellites. More like professional astronomy work

You make an important point there which deserves deeper consideration. I feel an essay coming on. A later post perhaps.

I find it remarkable that amateurs can now do work which was exclusive to professionals only 20 years ago. Remember that these satellites were not discovered until mid-2000 and then they were found in images taken on the  3.6m CFHT telescope which is located at a superb site on Mauna Kea.

If it were not for the fact that the satellite orbits are very well characterized from 20 years of observation using the Cassini orbiter and ground based professional astrometry, my observations would be useful to refine their orbits. As it is, all I can confirm is that the JPL-provided positions are good to within a second of arc these days.

You could image these objects with your kit, were it not for the fact that Saturn is so far south in the sky right now. You could give it a try in another 4 years or so. The satellites of Uranus and Neptune with a comparable brightness (Sycorax and Nereid at least) should already be possible for you, as are dozens of asteroids and minor planets in the outer solar system.

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11 hours ago, Xilman said:

You make an important point there which deserves deeper consideration. I feel an essay coming on. A later post perhaps.

I find it remarkable that amateurs can now do work which was exclusive to professionals only 20 years ago. Remember that these satellites were not discovered until mid-2000 and then they were found in images taken on the  3.6m CFHT telescope which is located at a superb site on Mauna Kea.

If it were not for the fact that the satellite orbits are very well characterized from 20 years of observation using the Cassini orbiter and ground based professional astrometry, my observations would be useful to refine their orbits. As it is, all I can confirm is that the JPL-provided positions are good to within a second of arc these days.

You could image these objects with your kit, were it not for the fact that Saturn is so far south in the sky right now. You could give it a try in another 4 years or so. The satellites of Uranus and Neptune with a comparable brightness (Sycorax and Nereid at least) should already be possible for you, as are dozens of asteroids and minor planets in the outer solar system.

Yes agreed i think there can be a bridge between professional and amateur astronomy. And what you are doing most certainly fits into that category. And yes when i recorded a storm spot on Neptune around 2016. I did image the faint smudges of Triton. But no useful visual detail could be got from imaging such objects. So the appeal there is limited visually. 

Edited by neil phillips
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I'm still playing with the process of stacking on ephemeris positions!

Last month I took a series of subs of M87, 30 x 120 sec with my 10" Newt and ASI178MC camera. The purpose of that exercise was to see if I could capture M87's jet. I did although it doesn't show in the image below. I originally stacked this data on the stars (obviously!) and then plate solved to identify the fuzzies then ran ASTAPS's astroid annotation tool and I remember it identifying the location of at least two very faint asteroids. I did find evidence of them in severely stretched subs but not much above the noise.

This thread minded me to restack that data on the position of one of those asteroids and it worked. I stacked on (15538) 2000 BW14 at mag 18.1 but (28730) 2000 GU123 also came out with very little trailing.

Full frame stack, burnt out M87 center stage:

image.thumb.png.155fa56be7053c1e0d4dd36998cbf1b6.png

Crop showing the target 

image.png.b227e9d7ce7f929e1f9d5343e8ec38cc.png

And the secondary object showing slight trrailing.

image.png.5485ed1acf5562b5df95190b19090377.png

 

Where this method will come unstuck for planetary minor satellites is that ASTAP calculates positions from the provided MPCORB.DAT file which is great for Sun orbiting bodies but ASTAP won't calculate positions from orbital elements for non solar orbits.

So I see where I can generate ephemerides for planetary satellites on MPC's website but I don't see (yet?) how to plug those into ASTAP. 

It's fun learning though :)

Edited by Paul M
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Can we hope that others might be inspired by these observations to try to pick up other small bodies in the solar system?

As well as the challenge in doing so, there is real scientifically useful data we can produce.

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1 minute ago, Xilman said:

Can we hope that others might be inspired by these observations to try to pick up other small bodies in the solar system?

As well as the challenge in doing so, there is real scientifically useful data we can produce.

I'm a bit slap dash in my approach to imaging, nothing I do is repeatable! :) I realised a while ago that pixel peeping and complex image processing work flows aren't my forte. While it's very, very fulfilling to create my own images of nebulae and particularly galaxies I'll never take anyone's breath away! They might gasp, but for all the wrong reasons 🤣

This explains my fascination with quite narrow objectives, M87's jet (done!), 3C273's jet (failed again this season), animations of fast moving faint comets (multiple successes now) and picking out asteroids in general. I will now most certainly look for more challenging targets including planetary minor satellites! Working on this will very likely be my summer project while I wait for dark nights to return.

It would be great to capture something of scientific value but that would likely come from someone with a tidier mind than mine!

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All I can say is well done! Totally amazing work!

Me, I'm still doing single images with a push pull DOB, so I really dont care there are lists that show when and where.

If you can work out something 30km across at almost a billion miles, I think Galileo would smile!

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This is a great thread, I’m really impressed with your work  @Xilman, on the Saturnian satellite. I’m very much with you @Paul M, I don’t know if I’ll ever produce images worthy of the kit at my disposal, so I really enjoyed the “go deep” challenges this galaxy season, M87 jet (success) 3C 273 jet (maybe), but I don’t know if I can apply the rigour required for techniques  described in this thread. My own challenge for next season will be to attempt a very modest version of the HST deep field image, even though it will obviously consume a large proportion of my available imaging time. It will be fun researching where I am going to point the scopes at...

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2 minutes ago, tomato said:

...  but I don’t know if I can apply the rigour required for techniques  described in this thread.

I am absolutely certain that you can. Whether you wish to do so is an entirely different matter.

Whichever you chose, go ahead and do it. Primarily to demonstrate your ability; secondarily to inspire others to do likewise.

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On 12/05/2021 at 20:23, Paul M said:

I was thinking that it is a bit fiddly but just now I've been playing with a feature in ASTAP, designed for stacking on a minor planet/comet. Select "Ephemeris Alignment" and it will stack on the ephemeris position of the selected object. That object need not be visible in individual subs or even the stack. It does have to be an object in its minor planet and comet catalogues which are editable. So I suspect it will be possible to get ASTAP to accept more esoteric objects so long as the MPC has them on file. 

I know, lazy!  :)

I just used it on a recent run of subs of the fast moving comet C/2020 R4 Atlas. It worked quite well but I screwed the final image by inadvertently including some outliers that were outside the field of the main tranche of subs. 

Anyway, thanks for giving me some ideas! :)

 

Just had a thought (I am very slow on the up-take now that I'm approaching my dotage). With this data one could stack on the ephemeris of Saturn. The satellite is in such a distant orbit that it will barely move with respect to the planet over the course of the observation.

I could provide the subs if you wish to try it out.

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52 minutes ago, Xilman said:

Just had a thought (I am very slow on the up-take now that I'm approaching my dotage). With this data one could stack on the ephemeris of Saturn. The satellite is in such a distant orbit that it will barely move with respect to the planet over the course of the observation.

I could provide the subs if you wish to try it out.

That thought passed between my two brain cells also but I then decided there would be too much relative motion still. Perhaps not if the subs are taken over a short enough period?

I re-read your earlier 5 point workflow and I'm ok with everything but the script to modify the WCS data in the FITS header using the ephemeris data. For a short run of subs it could be done by hand I guess? 

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13 hours ago, Paul M said:

That thought passed between my two brain cells also but I then decided there would be too much relative motion still. Perhaps not if the subs are taken over a short enough period?

I re-read your earlier 5 point workflow and I'm ok with everything but the script to modify the WCS data in the FITS header using the ephemeris data. For a short run of subs it could be done by hand I guess? 

Much easier is to give you the script.

Call it with "off_stack.pl ra_motion dec_motion *.fits" where the motions are in arcsec/hour.

off_stack.pl

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17 hours ago, Paul M said:

That thought passed between my two brain cells also but I then decided there would be too much relative motion still.

I very much doubt it. Siarnaq has an orbital period of 2.42 years and Albiorix one of 2.15 years.  You can work out for yourself how much that is in seconds of arc per hour to traverse an ellipse about 2 degrees across.

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