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.

<Borat Voice>Very nice!<\Borat Voice>

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

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.

[QUOTE=xilman;506754]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.[/QUOTE]
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.

diffraction spike?

[QUOTE=Dr Sardonicus;506759]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.[/QUOTE]The almost vertical one is [URL="https://en.wikipedia.org/wiki/Charge-coupled_device#Blooming"]blooming[/URL] 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.

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 (α,δ) 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.

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

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.

[QUOTE=xilman;519446] 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. [/QUOTE]

Nice- an excellent use of moonlit skies. And very impressive system. Good Seeing next week!

Caliban (Uranus XVI)
 

The latest in the series is Caliban, aka Uranus XVI. Like Sycorax it was discovered by the Hale 5m back in 1997.

This was really challenging because it shines at magnitude 22.2. A three hour exposure produced a barely visible image of the satellite. The signal to noise ratio is about 3 and extreme contrast stretching was needed to make the satellite visible. The included image is the best I have managed to produce. More details are available at [url]http://www.astropalma.com/Projects/Satellites/caliban.html[/url]

Wow- Congratulations on that one- Mag 22.2. Incredible.

Norm

[QUOTE=xilman;535958]This was really challenging because it shines at magnitude 22.2.[/QUOTE]I would say so! Good grief. At magnitude 22.2, "shines" just doesn't [i]sound[/i] right.

Let's see here. Mr. Sun is magnitude -26.7. So halfway between that and Caliban's 22.2 is magnitude -2.3. That's brighter than the night time stars, but not a whole lot, so -- what? Jupiter or Saturn, maybe? So this object is about as many times dimmer than Jupiter or Saturn, as they are dimmer than the sun.

It reminds me of a light bulb in the movie [i]Mr. Hobbs Takes a Vacation[/i], of which the title character says something like, "if you turn it on, the whole room goes black."

[QUOTE=xilman;500593]Here's an image of Sycorax, aka, Uranus XVII, a 160km satellite of Uranus.[/QUOTE]Two years later I had another go at Sycorax. Same telescope, different camera and a longer exposure.

Sycorax is much easier to see this time. The star marked with an asterisk is mag. 19.03. A number of background galaxies are in the frame but I have not yet identified them. My guess is that they are mag 18.5 -- 21.5. Sycorax itself is magnitude 20.7.

That is an impressive magnitude. HST is often quoted as getting to 29 or so.

[QUOTE=Uncwilly;579772]That is an impressive magnitude. HST is often quoted as getting to 29 or so.[/QUOTE]HST has several advantages
[LIST][*]It has an aperture six times that of mine, which corresponds to 36 times the light grasp, so there's 4 magnitudes for a start[*]It has a dark sky: no light pollution, no airglow from molecular recombination reactions[*]It has diffraction limited optics which put all the light from a star into a disc 50mas across; mine is seeing limited and rarely does better than 2000mas. An area difference of 1600 times makes a big difference to the contrast![*]The extreme deep field has an exposure time of 23 days; I have yet to reach 3 hours, about 200 times shorter. The Hubble image reaches mag 31.2 for a 5-sigma detection.[/LIST]My current limit for 5-sigma is probably about 22.5 from a single night's data.

Nereid and (16095) Lorenball
 

Here is the third largest and brightest satellite of Neptune. It was the second discovered, after Triton, and the second easiest to image from these parts. Proteus is in second place in the first two categories but orbits so close to Neptune that it is extremely hard to see with earthbound telescopes.

The prominent asteroid trail is of (16095) Lorenball which happened to be in the district at the time --- 2020-09-14 02:00.

2200 second exposure made from 38 subs.