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Old 2018-11-20, 19:05   #1
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Default Outer solar system satellites

Here's an image of Sycorax, aka, Uranus XVII, a 160km satellite of Uranus. It was discovered in 1997 with the 5-metre Hale telescope. My scope has a 0.4m aperture --- 12.5 times smaller. A total of 3090 seconds exposure went in to this one, The scattered light at the bottom of the image is from Uranus; that at the top right is from a 6th magnitude star.

Sycorax is magnitude 20.5 and, despite the image having a signal to noise ratio only 4 or so, I was able to measure its position to about 0.2 seconds of arc.

Sycorax is circled in the inset image. 'g' is a background galaxy and star 'D' has a V magnitude of 20.65 according to the GAIA-DR2 catalogue.
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Old 2018-11-20, 21:32   #2
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<Borat Voice>Very nice!<\Borat Voice>
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Old 2018-11-20, 23:14   #3
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Absolutely outstanding! To those of us who grew up as amateurs when we idolized Palomar as the largest scope in the world, it is astounding that amateurs themselves now can get to 20th magnitude. Just amazing.

Norm
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Old 2019-01-24, 11:15   #4
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Here is an image of Neptune, Triton and Nereid. The first two were over-exposed on a 3-second exposure, the shortest taken. The remainder of the image is a composite of 22 subs totalling 400 seconds exposure. Nereid, circled, was mag 19.4 when the images were taken on 2018-11-27.

I've not yet attempted either astrometry or photometry on Nereid.
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Old 2019-01-24, 14:01   #5
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Quote:
Originally Posted by xilman View Post
Here is an image of Neptune, Triton and Nereid. The first two were over-exposed on a 3-second exposure, the shortest taken. The remainder of the image is a composite of 22 subs totalling 400 seconds exposure. Nereid, circled, was mag 19.4 when the images were taken on 2018-11-27.

I've not yet attempted either astrometry or photometry on Nereid.
Amazing.

Just out of curiosity, what are the two darkish linear features in the upper-right portion of the image? They appear to meet at right angles.
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Old 2019-01-24, 16:14   #6
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diffraction spike?
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Old 2019-01-24, 16:34   #7
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Quote:
Originally Posted by Dr Sardonicus View Post
Amazing.

Just out of curiosity, what are the two darkish linear features in the upper-right portion of the image? They appear to meet at right angles.
The almost vertical one is blooming from a very over-exposed Neptune. The other is the edge of some of the co-added sub-images. Both show up because of the extreme contrast stretch needed to make Nereid obvious.
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Old 2019-01-25, 00:04   #8
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Default Astrometry done

Fitting an accurate WCS to the image was arduous. First all the GAIA-DR2 positions of everything with 10 arcmin was dowloaded and SExtractor used to find the centroids of all the star-like objects. That bit was straightforward if tedious. The hard bit was converting the data into a common fomat which IRAF's CCMAP can use and, especially tricky, matching the two data sets so that (x,y) image coordinates were correctly associated with (Ξ±,Ξ΄) sky coordinates.

That done, the measured position of Nereid is 23:00:46.425 -07:22:55.94, which compares well with the MPC ephemeris of 23:00:46.3 -07:22:59. The discrepancy is 3 arcsecs; the plate scale is 1.42 arcsec/pixel. Nereid is close to the limit of measurability because the exposure time wasn't really long enough. Lesson learned for next time.
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Old 2019-01-25, 15:15   #9
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Very neat- and at magnitude 19.4. A discrepancy of the measured position of about 2 pixels.

Thanks for the link to the CCD discussion. Good stuff.

Norm
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Old 2019-06-17, 11:01   #10
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We returned to a very cloudy La Palma on Wednesday. The clouds lifted last night so I fired up the observatory for the first time since the end of March. The full moon turned the sky blue so it was almost certain that no useful science images could be taken. Accordingly, I played around with updating the control software, attempting to automate finding the best focus, and so on. That done, I tried for a couple of the brighter satellites of Jupiter, despite them being only 2.3 to 2.8 degrees from the limb of the moon.

Cosmetically the images are appalling. However Himalia (aka Jupiter-VI) shows up at mag 14.9 and Elara (Jupiter-VII) shows the predicted movement of 5 arcsec over 17 minutes. I'm rather pleased at finding a 16.5 magnitude moving object that close to the moon.

Now to wait a week for the Moon to get out of the way and then to do the job properly. With luck at least 8 and possibly 11 satellites may be within range when the skies are dark.
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Old 2019-06-17, 22:05   #11
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Quote:
Originally Posted by xilman View Post
However Himalia (aka Jupiter-VI) shows up at mag 14.9 and Elara (Jupiter-VII) shows the predicted movement of 5 arcsec over 17 minutes.
Nice- an excellent use of moonlit skies. And very impressive system. Good Seeing next week!
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