The Starry Night, 142

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Opals in the sky with Lucy

I'm getting back to a project that would benefit from some serious image scale, so I added a barlow to the camera and put it to work at an effective focal length of about 3.4 meters. There was a ton of Moonlight out there tonight, so I chose a bright target that would reward imaging through narrowband filters. M1, the Crab Nebula, takes on these gorgeous opalescent colors when imaged in the light of hydrogen (shown as green) and doubly-ionized oxygen (blue), and ionized sulphur (red). I used 15 passes of Richardson-Lucy deconvolution to tighten up the point spread function (and to license the title of this post). PSFs were reasonably small to start with, but even 2.8 arc-second (FWHM) stars look messy at 0.44 arc-seconds per pixel. That said, the components of that double star in the center of the Crab Nebula are separated by only 4 arc seconds (the dimmer of them, lower right, is the pulsar itself, the remnant of the star whose wreckage the Crab Nebula is).

Between twilight and Moonrise, I slipped in 45 minutes of exposure through a clear filter. After Moonrise, I used narrowband filters for a couple of hours followed by a quick flat session. I left the telescope collecting darks for the rest of the night.


M1 in Hubble palette
9x300s Clear (45m)
4x900s Ha (1 hr), 2x900s O-III (30m), 2x900s S-II (30m)
AT10RC @ F13.5
ST2000XM @-30°C


Some weeks back, here, I found that with the Barlow mounted to the SBIG/A-P snout, the camera would focus behind the Maxbright diagonal's 4.1 inches of optical path. But using the diagonal costs IR flux (the diagonal acts as a cut-off filter at around 700nm), introduces an additional reflection and an additional point of rotation. For some targets, every photon is sacred, and reflection and rotation issues frig my ability to anticipate and set the orientation of the camera and its guide chip. So I replaced the diagonal with a 3.5" Blue Fireball extension tube from AgenaAstro. Works great, though flats suggest there may be some nasty internal reflections going on (flocking material is around here somewhere, but the flats worked well this time out). Guide 9.0 said I'd find a good guidestar with the camera oriented near 180-190°, and there it was. Best focus is with the stock focuser extended to 3.78cm.

The best guiding I saw was with 20 second integrations for a 9.5 magnitude star. Guiding oscillated a bit between runs of sub-arcsecond corrections and less frequent excursions of several seconds, generally in RA. (Remesh the gear and keep the load slightly out of balance.) After some experimenting, I settled on an aggressiveness setting of "7" for both axes. All in all, it doesn't feel as rock solid as when imaging at F5 or F8, but the results say this works just fine.




Except where noted, deep-sky photos are made with an SBIG ST2000XM CCD behind a 10-inch Astro-Tech Ritchey-Chretien carried on an Astro-Physics Mach1GTO. The CCD is equipped with Baader wide- and narrow-band filters. The internal guide chip of the CCD most often keeps the OTA pointed in the right direction (I'll let you know when an OAG or guidescope takes its place). Camera control and guiding are handled by Maxim DL 5.12. The stock focuser on the AT10RC has been augmented with Robofocus 3.0.9 using adapters turned on the lathe downstairs. A Canon 6D and a modded 50D find themselves mounted on an Orion 10" F4 Newtonian or carrying widefield glass on an iOptron Skytracker. Beginning in May 2013, PixInsight has taken over more and more of the heavy lifting -- alignment, stacking, gradient removal, noise-reduction, transfer function modification, color calibration, and deconvolution. Photoshop CS4 et seq and the Focus Magic plugin get their licks in, too.


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                   © 2015, David Cortner