Much positive feedback from the meeting of 13 May 2009. It seems most of us had fun.
Opening Bon-Bons included free demo copies (many thanks!) of the latest Stellarium from Barry. A formidable effort.
Many thanks to the five listed speakers (Geoff, Marc, Mark x 2, Michael, Monte) for remarkably clear and professional presentations. Four different speakers gave four extraordinarily differerent views on autoguiding. This format seems to be productive. I won’t attempt to summarize the talks, but just cherry-pick.
It seems that PHD guiding has the unanimous group Seal of Approval and perhaps the place to start.
Geoff and Marc used one approach: off-axis guiding using a prism to take off some of the unused field from the main scope. They both showed how it provides a mechanically simple, tight, robust system, and avoids the horror problems of differential flexure and mirror flop. Geoff showed a proprietary camera with the prism mounted in front of the internal filter wheel, so that a guide star can still be found even with a very dense narrowband filter in place. The user must supply their own guide camera. Marc used a separate prism. He discussed related topics: focussing with a Bahtinov mask, being polar aligned at least roughly, choosing a guide star that is not burned out but showing a gradient from centre to edge, not being too fussed about banana-shaped guide stars. (Reminds me of Space Monkey - a song by Patti Smith about bananas or so).
Mark and Michael used almost the opposite approach to differential flexure: weld everything to everything with heavy metal, till nothing, neither the guide scope nor the cameras nor the focussing tube nor the thin wall of the OTA where it bolts on can possibly move. Mark poetically pointed out that 25 kgs of scope and camera only has to move 7 microns to get one pixel of differential flexure. But they showed that with patience and ingenuitiy, it can be done, and the proof is in the pudding: 0.6 arc sec guiding. An advantage of the separate guide scope is that one seems to get a better choice of guide star, for photographing say NGC 55 or the Helix, in the Outer Darkness far from the Milky Way.
Later, Monte added that he also used a separate guide-scope, rather than the tiny, built-in second guiding chip on the STL, because it can be hard to find a guide star. I have the same experience with the STL, and also use a separate guide-scope. The PlaneWave mirror and camera are bolted on with big bolts, so there is no problem with mirror flop etc. The guide-scope and camera are similarly bolted to each other and to the extreme ends of the main OTA using very heavy aluminium section.
A conclusion might be that the separate guide-scope and camera is more flexible, but prefers either a refractor, or a lockable mirror, or short focal length, and/or considerable engineering. The prism-system with off-axis guider (in front of any filters) is much simpler to get going, but might struggle to find guide stars far from the milky way.
All four speakers reported that it is necessary to be able to handle small amounts of field rotation when stacking images.
The Chair commented on the related issues of backlash and polar alignment. For merely hideously expensive mounts (eg EQ6, CGE), there will be something like 60-120 sec arc of backlash on the declination axis. If you don’t have a battle plan, you’ll get 100 pixel stripes, no matter how good your guiding. And the head just isn’t adjustable to better than about 0.2 of a degree: you tighten the locking nut, and the thing moves again. So there will be some error. More important than size is direction: If your mount is 0.2 deg above and to the west of the pole (easily achievable), there is a sweet spot in the Eastern sky where there’s negligible field rotation but consistent drift. That consistent drift solves the backlash problem. Conversely, if you try photographing along the line of the axis error, you get hideous field rotation and hideous backlash. If you haven’t worked through this issue, you will, by luck, get miraculous guiding on some nights and not others. Mark Trudgett pointed out that the Chair had forgotten to explain how to put the pole reliably 0.1 deg out in the required direction. I use the same method as Monte and others: sky modelling software like T-Point. Any other approaches are welcome.
Michael Samerski and especially Mark Trudgett both took us back in time, with considerable flair, from their current set-up, where they’re getting tight sharp stars and perfect guiding using very economical gear – back in time, to when they weren’t. The economical gear was a great teacher, showing how attention to detail mattered. Michael reported on the astronomical uses of a coke bottle and a kite string in launching a tarpaulin over a street lamp.
I showed two Interesting Objects or Stretch Goals:
After the break, Richard Carmichael showed some new images using his Vixen gear. I think I understood that he also was working unguided for at least some of these. I was momentarily distracted by technical issues and cannot fully report on these, but they looked impressive. I look forward to more, and a chance to chat about the details.
Fred Vanderhaven then confessed under Peine Forte et Dur to possessing an image of the Bug Nebula, or Butterfly Planetary, NGC 6302 in the hook of Scorpius. This is a middling bright object (Mag 12.8), and because it is extremely small (about 1.5 x 0.5 min arc) it is very bright centrally, so it’s easy to get some sort of a first shot at it. But the depth and sharpness and sheer extent of the image that Fred showed were of Hubble quality. I didn’t immediately recognize it for two reasons: Firstly it was rotated 90 deg, and secondly it wasn’t all blurry and grainy and dim. There was eleven hours of H-alpha alone. Fred is to be congratulated on a spectacular tour de force. He’s achieved the kind of detail that normally only planetary photographers get, but on a faint object.
We had a brief discussion on beginnings and endings: we’d seen one begining (Yoda’s Walking Stick – a jet from a proto-star) and two kinds of ends: Fred’s planetary nebula being one, and my supernova remnant being another.
I finished off in a near unsuccessful rush to beat the burglar alarm, showing a 4am Helix in narrowband, and a bunch of other stuff. Michael Sidonio kindly gave a 15 second warning and we had to rush from the building.
Due to our increase in numbers, technical skill, and productivity, there is now too much material to fit in 3 hours every second month. I asked Monte to lead a vote on changing to a Monthly meeting frequency. Very roughly, the vote went 60% yes, 0% no, 40% abstained. This will allow a slightly less hectic pace, an earlier get-away, and more time for everyone to join in, show their slides, have deeper discussion. We are yet to decide whether it’s best to use the Wednesday falling ten days after Wirruna, or to shedule the meeting for the Wednesday closest to the full moon.
As the alarm was going off, we discussed whether next month’s Workshop should be on focussing (a small, manageable task) or Narrowband. We voted for the former. Details and Request for Submissions soon.
A final thanks to everyone for pitching in and cleaning up at very high speed.
Mike Berthon-Jones,
Lost Photon Patrol.