The Starry Night, 82

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1/15/2012. After visiting with Kathy and Jerry Bevins, we returned home to one of the darkest, clearest skies in months. Seeing was surprisingly good, too. I lined up on M33 to give the guide telescope another try. [Yes, this page does look like a repeat of two pages prior: M33 and IC443, at similar focal lengths. But the methods are more refined and repeatable. I keep shooting the same subjects so I can see whether the latest changes are in the right direction.]

After tuning PHD Guide's parms a little (smaller minimum dec moves; next time re-align on the pole), I got tracking down to 0.14 pixel RMS which is just about one second of arc with a 400mm guide 'scope feeding a Meade DSI. But did the CCD shift with respect to the guider? Not too much, at least not in this part of the sky:



17x300s L, 1x300s RGB
ST80 guide telescope
AT10RC @ ~F5.6

Maybe adding Teflon rails to the focuser made a difference after all. I also tried 600s H-a frames of IC 443, but they were much less satisfying (so maybe adding Teflon rails didn't make much difference?). Keep experimenting; the guide 'scope clearly has its place in the bag of tricks.

For IC 443, I switched from Nebulosity & PHD Guide to Maxim DL5.12. It found good guide stars on the ST2000's 237 guide chip, and the camera calibrated nicely in dual-chip mode (which Maxim supports but Nebulosity and PHD do not)I shot LRGB first, then crossed my fingers and punched in the H-a filter. The guide star stayed nice and bright with both 4 and 6 second integrations. Oncoming clouds allowed me 7, fifteen-minute narrow-band exposures. I tweaked aggressiveness and minimum moves throughout the first few frames. Overall, dec showed frequent 1.5-2.5 arc second excursions but generally stayed within half a second of the target. (Refine the polar alignment! Check the dec gear mesh.) The results benefited from just a little deconvolution to remove the resulting trailing. 900s through the 7nm H-a filter seems just adequate at this shorter focal length. Longer would be better. Seven of these "short" subs left a lot of noise in the sky. 1800s is worth a try, as are a lot more subs. Still, it's clear that I can do narrow-band images with the AT10RC at F5.6 using this configuration:



IC 443
7x900s H-a, 1x300s GB
H-a used as both R and L
On-board guide chip, Maxim DL 5.12
AT10RC @ ~F5.6
Too much noise reduction...


Finally! Successful guiding using the main OTA at significantly reduced FL. One reason this has been a worthwhile goal is that the optical performance of the AT10RC and the A-P CCDT67 telecompressor on this relatively small chip is spectacular. Tonight's exposures were all so long that guiding effects dominate the CCD Inspector plots, but have a look at a 60s exposure from night before last:


ccd inspector plot

AT10RC / A-P CCDT67 / Teleskop-Services collimating ring
60s L frame
CCD Inspector 2


Told you the seeing was dreadful t'other night. The hardware is anything but. So, is this plot cherry-picked? You bet it is. But the other four frames show only tiny differences. The large PSF's (twice as large as on an excellent, steady night) surely disguise some faults. But still... Eventually, I'll need to study some long sequences of plots like this and delve into the next level of refinement. I'd like this kind of performance all the time, everywhere in the sky. Let's make some good photos first.


1/18/2012. You can make a decent strap wrench from a simple extension cord. I wrenched the 1-inch A-T extender off, added more Teflon rails to the focuser (note that rails 0.340" through are considerably too small while 0.345" is just about right). I intended to try a short extension of the CCDT67 in an attempt to get even wider, faster fields than those above (F5 anyone? F5.1?...) but did not have any properly threaded extension tubes on hand. Some things are way easier to buy than to make, so one is on its way from Agena. I'm not saying brands, threads or length until I know at least that it fits; there's enough dubious advice on this site already. [Works fine: it's a 14mm Hyperion fine-tuning ring made by Baader, sports M/F 2-inch filter threads, and note that the telecompressor only threads in about halfway 'cause its M threads are longer than the tube's F threads.]


1/21/2012. An act of Druidic faith amid days or rain and miles of clouds with the Sun still crossing the meridian way below the pinetops (when it can be seen at all). I modified the bracketry under the Lunt solar telescope, confident that the big yellow ball will be back. An ADM cradle will hold the solar telescope solidly on top of the Ritchey where it can be operated remotely. A third Robofocus motor is in my future (1 for the Ritchey, 1 for the refractor, 1 with a drive belt to share between camera lenses and the Lunt). It will be 5-6 weeks before I can try remote solar observing (when the Sun shines on the increasingly settled observing spot), but I can still mount the Lunt on the Meade equatorial by way of a Losmandy DMM plate in the event something too exciting to miss comes along. In the meantime, need to come up with a simple, cheap, toolless arrangement for mounting the focus motor in two orientations (one for the Lunt, one for camera lenses). Sat up late, and I think I have it... No, no! Not like that. Take a lesson from the lathe motor mount and there's an even cheaper way. No! Since I already have these unused parts, I can do it lavishly and cheaply. Idler wheels when used with the Lunt, if needed, will be easy. 1/22: both motor mounts are in place waiting for the motor and for one afternoon downstairs to whittle out one simple fitting to be "permanently" attached to the motor. This is gonna be slick. Just you wait.

While thinking this through, I had trouble Googling up some specs. Now that I have them, here they are: the drive belt supplied with my Robofocus "Mandel" setup is a Gates 90xL037 timing belt. It's 9 inches long (which means it's 18 inches in circumferance -- flatten it out and it reaches 9 inches end to end, hence 18 inches around), with 5 teeth per inch, just about 3/8-inch wide. They're available all over the place from a coupla bucks (industrial suppliers w/unknown minimums and shipping) on up (one off, Amazon prime), in lengths ranging from way too short to way too long. Start with this one, try to make one belt serve both the Lunt and the 200mm Nikkor.



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 LRGB and 7nm H-a filters. A Meade DSI Pro monochrome camera looking through a modified Orion off-axis guider keeps the OTA pointed in the right direction. The imaging camera is controlled via Nebulosity 2; the guide camera is operated by PHD Guide 1.13, both by Stark Labs. The stock focuser on the AT10RC has been augmented with Robofocus 3.0.9 using adapters turned on the lathe downstairs. Maxim DL5.12 performs image calibration, alignment, and stacking; Photoshop CS4 and FocusMagic 3.0.2 take it from there. Gradient Xterminator by Russell Croman and Astronomy Tools by Noel Carboni see their share of work, too.


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                   © 2011, David Cortner