This one is a really esoteric note (8/2/2009) which will mean something only if you, too, are waging a battle against your neighbors' nightlight fetishes, and recognize and can string these terms into a sentence without breaking a sweat: "Baader 7nm hydrogen-alpha filter," "parfocal," and "Epoch 2010."
Lesson One: the marks on the Losmandy G11 polar alignment reticle are labelled for different epochs. It's not just for looks. See, I've been centering everything up and shooting like it was 2000 since 1992. Lately the tracking has been for shit. I thought it was because the bearings had gotten gummy again or that maybe I'd walloped the blocks that hold the RA worm. When sane, I was thinking about cleaning and rebuilding this and that; when insane, I caught myself measuring the price of A-P mounts against the headroom on various credit cards. Then I stumbled into a detailed explanation of how to set Polaris and its field stars in the finder according to the epoch (1990, 2000, 2010). And tonight, when cloud holes were large enough to be tempting, I meticulously aligned the mount using the 2010 marks. And got clean 2-minute subframes at 762mm that are at least as good as 20-second frames have been lately.
Lesson Two: so I wanted to try a Baader 7nm hydrogen-alpha filter to turn off my neighbors' excess nightlights once and for all. I put a Hap Griffin modified Canon 20D behind the A-P refractor and on this moony night, drew a bead on Vega for a sanity check. And discovered the flaw in my plan to do effortless, DSLR H-a shots. You can't see a thing through that filter! I should've known. The modified 20D really is quite sensitive to H-a light, so I pretty quickly walked the focus into something resembling sharpness. But I thought, am I gonna have to use a notebook computer and tethered camera to really focus this? I won't be able to use a stiletto if I can't see the Ronchi pattern and the unmodified 50D can't see H-a light so its live-view feature won't help.
Then came the dawn (unfortunate metaphor): I bet the Baader "Fringe Killer" is close to parfocal with the same company's H-a filter, at least behind a nicely apochromatic telescope like my 5-inch F6 (it's not as apochromatic as later A-P models, which is why I sometimes use the Fringe Killer, but it's pretty close). I put a triangular focusing mask in front of the telescope. I replaced the H-a filter with the FK filter and did my best to produce a symmetric, many-armed star image. Then I replaced the FK filter with the H-a filter and exposed a couple of frames. Bingo! If these two filters are not exactly parfocal, they're close enough to be better than a merely good start.
I'm sweating like a horse; my head hurts; my leg hurts; I hate building and unbuilding the observatory (especially just to run tests) amd carrying optics around, but I found out some stuff that I need to remember and that others' might need to think about, too.
I'll be less crabby tomorrow. Maybe.
August 3. Well, I'm still pretty crabby. But at least I got to play with the lathe to make an aluminum spacer to insure that the fronts of Baader filters don't collide with the front-surface mirror in the A-P Maxbright diagonal. Its dielectric coatings are tough, but let's be reasonable — it's better not to invite metal to glass contact. I turned a 2-inch aluminum ID tube to a length that holds the camera at focus with everything racked almost all the way in then tapped in a set screw. This evening, I tried it with the Baader H-a, the Fringe-Killer, and the Fringe-Killer with a non-modified 50D (into which I clipped an IDAS light pollution suppression filter). All reached focus. That's step one. To the right is a a 30-second focus-check of M57 and environs under twilight skies with a nearly full moon. Promising...
August 4. A little more lathe work: one reason I was having trouble reproducing focal positions last night was that the short, wide-aperture 2-inch prime focus adapter kept shifting in the A-P diagnonal's clamp ring. That never happened before, so it likely is the fault of the new extension ring. Indeed, the A-P clamp ring was grabbing the adapter at a different spot from the one it normally grips. Where this adapter is normally gripped, there's a shallow notch. With the extension ring in place, the A-P clamp grabbed the shoulder of the notch rather than the notch. Things had a way of shifting a mm or 2 depending on exactly how firmly I clamped the ring and exactly how the adapter was positioned when I tightened it down. I chucked the adapter in the lathe and widened the notch so that the A-P clamp ring grips entirely within the notched segment (again). Much more solid, much more repeatable. Cloudy skies again, of course.
August 6. The sky is hazy; the Moon is so close to full that the difference doesn't matter. It's nearly 80F outside, and I can see my breath. That's how humid it is. The SQM says the sky is glowing at 17.4 mags per square arc second. I can see Vega and Deneb and Gamma Cygni and very little else in that part of the sky. I aimed into the center of Cygnus and did a series of 200s exposures with the Fringe Killer and then another series with the H-a filter. Both were intended to verify focus and to see if I can really leave the trailer-mounted G11 in place after polar alignment and use it without a lot of additional fuss. In the first frame of the H-alpha sequence, I saw the nebulosity around Gamma Cygni starting to come through. We'd need a lot more frames for a real try, but playing with this short stack is promising.
At right are the five red frames stacked and tweaked
(total exposure 1000s, 5" F6, 20D-HG, iso 1600, Baader 7nm H-a).
They're slightly cropped to avoid amp glow and assorted other defects.
I tried combining with the color F-K images. It'll work, but it'll take more care than I can give it today. I'll probably wait for better, less-moony data.
Focussing was just a matter of starting with the A-P drawtube at its stop, then backing the fine-focus knob of the Feathertouch out 2/3 of a turn. The camera was mounted in a Maxbright diagonal via the short-wide adapter with the modified notch and the home-turned spacer ring. FWIW, in the full frames it's clear that focus was somewhat better in H-a than in white light (Fringe Killer filter). Put qualtitatively, the difference is 2.8 vs 3.9 pixels FWHM. Look into that.
10/25/2010. My how time flies... Opted to see what the 70-200mm Canon F4 zoom and an H-a filter might do. I stacked a couple of stepdown rings to put the 6nm Baader in front of it and used the latest iteration of my long series of barndoor mounts. Go have a look at this sidebar on barndoor / "Scotch" mounts.
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