The Off-Broadway Revue

2025/06/23. It's not really even opening night. It's been a Moony, hazy few weeks, and it's been a busy month, too. Even absent nightly clouds, it's taken a while to find time and inclination to give the co-mounted telephotos decent attention. Tonight, at long last, the 300mm F2.8 Tamron (with the Canon 6D) and the 400mm F2.8 Nikkor (with the Canon R6) are collecting photons together. There's not a lot of sky to work with from the backyard, but there's enough for a sanity check, and there's no reason to haul this kit anywhere until I know it works.

Some preliminary images of Alkaid (30s, F4, ISO 1600) allowed me to confirm that they're at least sort of aimed at the same spot (a laser pointer on the porch represents my only effort at that so far -- if the lenses are 12 inches apart, the dots projected from their respective mounts should also be 12 inches apart at whatever range I care to measure; and at 55 feet, they looked good). Measurements of tonight's images will say how much adjusting is needed. I also have a chance to try various schemes to combine the data.

Tonight I'm not messing with flats or darks, and I'm not even guiding. It's a slapdash, give-it-a-go sort of evening. I took a few minutes of data on Alkaid at F4, then opened up both lenses to F2.8, tweaked focus on each, and pointed to M101. I took a little over an hour of 30-second exposures just to have plenty of data to play with (and on the off-chance that everything does work straight out of the box, that will be enough exposure for a good photo, too). Why not dream a little? Film is free these days.

Bingo: based on the M101 frames, the two lenses are aimed within 17 arc minutes of each other, offset about equally in RA and Dec. That ought to be close enough for all anticipated uses, but I'll need to plot the fields with the 1600MM and MC cameras to be completely sure. If you want to be a perfectionist, nudge the 300 just a little toward the 400 (see the overlayed image below). Things are fine for the 35mm format cameras, but to insure the best possible overlap for the the smaller sensors in the ASI1600 cameras, it wouldn't hurt. Also, tweak the vernier focus cage on the 400mm. Something has slipped, and it won't quite reach a sharp infinity focus. See note below.

Outer frame: 300mm. Inner frame: 400mm.
They're just overlayed, not combined.
FWIW: the R6 image is 5496x3670, the 6D's 5472x3648

Tonight's mistakes: I left the R6 on ISO 6400 rather than backing it off to 1600 in honor of this bright sky. And I failed to press the shutter release attached to the 6D all the way into its locked position, so I only got a single 30-second exposure of M101 through the 300mm. In any event, that was enough to confirm aim. I also dropped that single 300mm frame in amongst a few 400mm frames to see if PixInsight would accept it as part of its star-alignment process. Yessir! There are a couple of YouTube videos out there that had me worried about what might be needed to combine data from the two lenses. There are still some conceptual issues I need to work out, but so far, so good. Even the simplest combination strategy worked well tonight.

Note on adjusting the 400mm's focus cage. The cage consists of two pipe straps, a connecting bar, and a vernier screw. The strap nearer the camera is cinched down on a fixed portion of the lens. Leave that one alone. The other grips the focuser barrel. To adjust the cage and insure that infinity focus is within the range of the vernier screw, first set the screw to somewhere near the middle of its range. Then loosen the strap the grips the rotating barrel. Rotate the barrel to your best guess at infinity focus, and then retighten the strap on the barrel. That's it. [Odd that I have to figure this out every time something needs to be tweaked. It's really simple! Maybe writing it out will help me remember it.]

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