2/07/2015. My 180mm F2.8 Nikkor is an old soldier of a lens, literally: it was surplused by the Naval Technical Intelligence Center. The fellow who bought it put it on eBay with a warning that the lens "certainly is ugly" and a promise that the glass is pristine. He was right about the glass, but the mechanicals are not so much "ugly" as "severely functional." It's been "accurized" --which, if it were a pistol, would be mean it had been tweaked and adjusted within a micron of its life, key components replaced and improved. I trust it means the same thing for a lens. The outward signs of this engineering legacy were off-putting to photographers and especially to collectors: a finely divided scale had been unceremoniously bolted to the focusing ring. "Accurized Model No N180-02" it says. That plate aligned with another small plate that appears to belong under the left-right click adjuster for a rifle sight. The result is that the focusing ring can be read and preset to a tiny fraction of a turn. Four socket head screws of uncertain purpose have been tapped through part of the outer body. If I were betting, I'd say they provide an infinity stop of Gibraltan authority. All that sounded good for military intelligence and for stargazing alike. It was priced right (about 12 cents on the dollar compared to a new lens of the same make, speed, and focal length), but it was old enough to cause some worries.
The 180mm F2.8 is venerable lens; it's been manufactured for decades and has been through many, many editions. Somewhere along the line, some of the elements began to be made using "Extra Dispersion" glass. Lenses which incorporate it ("180mm F2.8 ED") are legendary performers under the stars. Earlier models, not so much. Mine was made earlier. I hoped that the implied fine-tuning made a difference, and, in any event, the price was so good that I wouldn't be out much of it didn't. Also, if it couldn't cover the full 35mm frame of the Canon 6D well, it could surely perform competently over the 2.9x smaller (measured by the diagonal) chip in the SBIG CCD.
The Canon was easier to set up for a quick test, so before the Moon rose tonight, I did just that.
The Pleiades (and friends)
Click the image for a closer look.
Canon 6D, 180mm F2.8 Nikkor @ F4
15s, ISO 6400
Light pollution, haze, airplanes, and trees (just a twig, upper right): you can see some of my ordinary problems here in the Lucid East in that quick test. But you can also see tiny stars over almost all the field and nebulosity already nicely captured around Merope.
Look at those stars! Look at the one down in the lower right corner. Deep in the corners, there's some chromatic aberration (which you probably can't see even if you look closer at the web version). At F2.8, blue haloes are a problem, but I may have missed focus just a bit. Nevermind that. Here at F4, star images are lovely. Louis, I think this is the beginning of a beautiful friendship.
2/08/2015. I made an alignment guide for the SkyTracker based on a simplified version of the diagram Michael Covington provides as a PDF and mounted it on the back of the tracker. Then I lined up carefully and took 136, 30-second exposures of the Pleiades under pretty decent backyard skies.
136x30s (72m total)
6D @ ISO 3200
180mm F2.8 @ F4, cropped
That begins to bring out some of the extensive nebulosity through which the Pleiades travel. The close-in stuff is real, of course, as is, I think, the more distant clouds at 10 o'clock and 8. The relatively small-scale variations amid the relatively darker field around the cluster are probably real, too. It's been casually flat-fielded with my blank computer screen (which is surely suspect) and desperately needs some darks. I'm not thrilled with the complex diffraction pattern produced by the diaphragm blades at F4, so I need to take a really close look at how stars behave near focus at F2.8. One alternative would be to use a stop-down ring intended for filters to act as an aperture stop -- it should produce tighter stars without the points.
I cropped the frame for light gradients, not for star quality. In removing obvious gradients, I surely removed some real nebulosity, and I think some gradients remain disguised as nebulosity (down at the bottom, especially). Unless better calibration makes a world of difference, I think this is about all I can do from here. A trip to the Parkway is in order. [More fool I. See next page!]
2/09/2015. On the way: step-down rings, not that I want to mount filters (often) but to serve as aperture stops. The 180mm F2.8 and 135mm F2.0 lenses have 72mm objectives. Stop them to 52mm and you get F4 and F2.8, respectively, without the crunchy, sparkly iris-blade diffraction. Step-down ("stop down"?) rings are cheap, so a 58mm version is also on the way (F3.5 and F2.5) because just maybe the images clear up and the worst of the moderate full-aperture hot spot goes away without tossing an entire stop of speed. A 48mm version is coming too. With that, I can put 2-inch narrowband filters up front. (Yes, it is deja vu all over again: I did this for the 77mm objective of my 70-200mm some years ago. But step-down rings are small things, and they hide well.)
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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