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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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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?

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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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