A few weeks ago during the full moon I did an update to the PEC curves on my mount (not the story here). Then, not wanting to waste the remainder of a night of clear weather I decided to go after some targets that would not suffer (much) under the bright, full-moon skies. I ended up taking some images of both Uranus and Neptune using a Stellarvue SV80ST2 80mm refractor with a ZWO ASI178MM camera. I didn't expect to record any surface details, but I thought it could be interesting to image the moons.
So, job done and when looking at the images I wondered whether it would be possible to record more of the moons using a larger scope or with longer integration times. I decided that Neptune was a no-go since after Triton (magnitude 13.6) the next brightest moon was Nereid with a current magnitude of 18.7 (possible, but maybe not easy under the full moon or my normal red/orange zone light pollution).
However, Uranus has four moons that are all fairly easy and I did record all of those with the SV80. Then I started wondering about the moon Miranda that was discovered in 1948 using McDonald Observatory's 82-inch reflector which was at that time the second largest operational telescope in the world. Right now Miranda has a magnitude of 16.5 (according to JPL's Horizons website) which doesn't sound that difficult until you consider that its apparent separation from Uranus currently varies from only 6.5 to 9.4 arc seconds. So, best case you have a 16.5 magnitude object that is 9.4 arc seconds from the center of an object (Uranus) that is almost twenty-one thousand times brighter than Miranda.
Next night I was lucky to have some more clear weather and I tried my Celestron C6. Didn't find anything other than the four brightest moons but on researching my next attempt I noticed that Miranda was going to reach its maximum elongation from Uranus on the morning of September 21 as both were passing through my local meridian. By a near miracle that morning was also clear and I set about imaging using a 9.25" EdgeHD and my ASI178MM camera with a Baader Semi-APO filter (the latter somewhat like a strong luminance filter). During this session I was actually able to see Miranda very fleetingly and intermittently on the preview images from within my capture application (or, at least I believed that I could see it -- certainly not easy).
Okay, so long story somewhat shortened below you will see one of the images that I created from a selected stack of 32 subs that were exposed for 4 seconds each using the lowest read noise gain setting on the ASI178MM camera (with the 9.25" EdgeHD at f/10). The moon Miranda is definitely recorded (close to and immediately below the overexposed image of Uranus) along with the moons Ariel, Umbriel, Titania, and Oberon (with respective magnitudes of 14.1, 14.9, 13.9, and 14.1). The image scale on this is approximately 0.3 arc seconds per pixel (a 67% reduction on the original).
The image capture was done using SharpCap with image processing in PixInsight (image selection, registration, and adjustments) and Photoshop CC2019.
Because confirmation of a capture like this can be, well, somewhat controversial I've included a "finder" chart that was created with an overlay of a plot done with the Uranus Viewer website and my image. The correspondence between my image of Miranda and the plot positions produced by the website seems to be perfect. I've also marked several field stars ("A" through "E") that were also recorded as those can be used as magnitude references.
This kind of magnitude check is really required for two reasons. First, you need to eliminate the chance that what you've recorded is actually a moon and not some brighter star that just happened to be close to Uranus (more on this in a following post/image). Second, you need to make certain that your image has recorded to a magnitude that would actually include the target moon (in this case, Miranda, at magnitude 16.5). If you can't find field stars that are similar to or fainter than the target then you can be pretty certain that what you've recorded isn't a moon.
Here are the estimated magnitudes for the labeled field stars (as determined using the website WikiSky.org on images based upon the Palomar/AAO Digitized Sky Survey II or DSS2):
D: 16.6 (the star USNO A2 0975-00500312, about the same magnitude as Miranda).
E: 17.5 (the star USNO A2 0975-00500283, one full magnitude fainter than Miranda).
The bright star at the top of the image is TYC 0637-1209-1 which has a magnitude of 10.4 (this star was used as a reference point for the overlay done on the Uranus Viewer plot).
You need to look at this "finder" chart at full size and perhaps when using a display in a darkened room, since it is an overlay of my image and the plot from the Uranus Viewer website. What you should see is a dark dot in the center of each bright moon. That dark spot is from the Uranus Viewer plot that was aligned and scaled to match my image (using the field star TYC 0637-1209-1 as a rotation and scale reference).
*** Updated some of the text for greater clarity and for typos at 2PM PDT ***
Edited by james7ca, 23 September 2019 - 04:01 PM.