A Young Moon

2025/04/28. I set up in the community lot with the idea of adding Markarian's Chain to the folio and maybe adding some data to the Coma Galaxy Cluster imagery. But first, this: I was curious how long it takes to set up, so I timed how long it took to load the car (8 minutes) and how long to set up, including an adequate polar alignment (from parked to first data 19 minutes). I caught the very young Moon (29.5 hours) low in the northwestern sky and aimed at it while it set and twilight faded from the sky. Earthlight grew more and more prominent the darker the sky and the lower the Moon until just before it reached the treetops, it looked like this.

400mm f2.8, ZWO 1600MC, -15C, gain 139, 10x2 seconds

"Late, late yestreen I saw the new Moon,
With the old Moon in her arms..."
(Samuel Taylor Coleridge, "Dejection: An Ode")

I tweaked focus on 66 Arietis very near the southern limb, and then did a series of 180, 2-second exposures. Dang. I thought 180 would be plenty, but the sequence ended just as the Moon touched the treetops and it took 5-6 seconds to start anew. That leavs a "jump" in an otherwise smooth and hypnotic animation. I've yet to manage to fix it well. In the meantime, this is a stack of 10 frames, aligned on the Moon, with a single image of 66 Arietis retained from the middle of the set. Here's the view at the same moment looking the other way, courtesy of Guide 9.1:

The scale is different since I did a fairly tight crop for the Moon photo.

You might want to stop right here
because this one is about
to get wordy and full of notes
that probably only matter to me

unless, you, too, need to rebuild the power
cord to an Astro-Physics drive computer
or you have just bought a
300mm F2.8 Tamron 60B without the
supposed-to-be-included clear filter. In
either of those cases, do read on.

After Moonset, I swung to Regulus and refocused critically. Adjusted the E-W axis of the camera, slewed to NGC 4438 to produce the composition I wanted of Markarian's Chain. Found some guide stars, and calibrated. Nope. No calibration. I tried various settings, still no joy. Then I realized I was out of practice: the RJ11 cable to the CP3 was not connected. Connected it. And bang, dead CP3. Things flickered for just a few seconds and then went well and truly dead. Tried everything. Decided the cable was defective. Drove home, got the spare. Still dead. Tore down the 'scope and called it a night, thinking the CP3 must be at fault (CP2's go for about $50 used; used CP3's cost around $400; current models are about $1300 new). Unlikely as it may seem, the issue turned out to be broken connections in both cables. New cables run $92 and now use different connectors (Anderson poles), so fixing these is the most practical ticket. I fixed one the next morning with due (paranoid) attention to polarity and a fresh plug. I built in strain relief worthy of Billy Ray Williams. I'll examine the other one right soon.

Now "right soon" has come and gone, and the second cord is fixed, too. I disassembled the 12v plug assembly where the first failed, but all was solid there. A quick look inside the locking 5.5x2.1mm connector at the CP3 end turned up a broken solder joint on the white (center) wire. Resoldered it. Do I trust these cables enough to hang thousand-dollar jaunts into the western wilds on them? I do not. I'll lay in a few common 12v power plugs to get the job done at least temporarily in the event these prove unreliable. Some locking connectors are on the way to remove a little of the suspense that might otherwise result. [Nope: 2.1x5.5mm is too long for the locking ferrule to engage the CP3 connection. I cannibalized one of the two cables to double the length of the ferrule, but apparently the connection does not reach. Thinking... There are 2.1mm "short" connections that might work. But for now I'll rely on the repaired cables.]

5/11/2025. If I were to co-mount two fast telephotos fitted with the ASI1600MC and ASI1600MM cameras, I am pretty sure that PixInsight could stack images from both lenses, maybe straight up, maybe after some format massaging. Another 400mm would yield a mighty light-grasp, but it would also require some exacting independent aiming capability, would be heavy as sin, and expensive all around. Also, duplicating the existing hardware would give me no additional framing options.

I've been thinking about this for months and dismissing it as extravagant. But this evening I caught KEH offered a 300mm F2.8 Tamron ED-IF, the later 60B version which is extraordinarily well-reviewed. It looks like it might be a sleeper, overshadowed by the vastly more expensive Canon and Nikon versions. KEH's Tamron is rated "bargain" class which is scuffed, functional, with good glass and cheap. Introduced in 1983-4, the 300mm F2.8 Tamron was available from B&H, Honest Abe's Focus, 47st, et al for just under $1k. In the autumn of 1984, B&H had the best price: $939, but the first version of this fast Tamron 300 is said to have had problems. The "Super Performance" model 60B came out soon after and incorporated a different design and a couple of exotic elements. By 1988, it was twice as expensive as the original lens. Abe's, for example, offered it for $1,895. Thirty-seven years later, adjusted for inflation, that's $5,100, but nowadays a manual focus super-telephoto is no one's idea of a hot ticket. I paid $205 w/free shipping.

It will need a few pieces: at the very least an Adaptall mount for Nikon or for T2. Maybe a 43mm drop-in filter. Before co-mounting it with the 400mm, I aim to try it out on the lightweight mount which it will ride much more comfortably than the 400mm does (2kg vs 5kg and way less bulk). It will also be more foregiving of uncorrected tracking errors and less than perfect polar alignment. For co-mounting, I'll need long Arca clamps and probably another focusing rig like the one I built for the 400. A long Losmandy D plate with those clamps mounted on it can be cobbled up in the downstairs shop. As for alignment with the other co-mounted optic, close ought to be good enough since the field of view in the 300mm is generous compared to the 400mm.

(5/14:) Uhm, well, not that generous. When I actually fired up Guide to compare fields of view, they're not as different as I expected. The lenses will need to be aligned to within 15 minutes of arc or better. It will take some attention. I'll rig up something to allow the aim to be adjusted and set. Here are both fields centered on M13:

So there's the plan: give it a try standalone; then confirm that PI can combine images with the 400mm; then that I can combine 1600MM and 1600MC images; then get serious about co-mounting it. Also, make sure I can run two instances of my preferred, simple imaging software, otherwise that may need to be swapped for more potent code. As for the dual mount and a means to aim the 300 -- I have ideas.

Why is it worth all this? Because the effective aperture of the 400mm and 300mm lenses working together is Sqr((400/2.8)^2 + (300/2.8)^2) which comes to 178mm or 7.0 inches. If I co-mounted twin 400's, their effective aperture would be F2.0; this arrangement yields F2.24. Every 60 minutes with both shutters open is equivalent to 94 minutes of exposure at F2.8 which doesn't sound so spectacular, but the ability to gather luminance and color separately also comes into play as does the ability to supplement the MC's images with narrowband data. In one use case, I could put the 1600MM behind the 400mm and get deeper with the improved efficiency right there while also mounting the 1600MC behind the 300mm to contribute color data and some additional luminance. In another scheme: put the 1600MM and its filter wheel behind the 400mm to collect narrowband data within a subset of the 300mm's field. Ha-as-luminance or boosting O-III would work. Some pretty faint stuff ought to be in reach. Consider that six hours with the combined rig would be as thirty-six (a day and a half!) at F5.6. And, to keep things simple, I need to provide for putting the R6 and the 6D behind these lenses.

To that end (5/19), it is worth adding a direct Nikon F to EOS R adapter. With that I could put the R6 behind one and the 6D behind the other and do some very deep stuff without the falderall of cooled cameras and a computer.

One more worry (will 5/19 ever end?): A 43mm UV filter belongs in the 300mm light path. There are lots of plausible options (K&F, ProMaster, Hoya, etc). Get it working before working on perfection.

5/19/2025 (still!). One specialty adapter (Nikon to EFW to 1600MM) overshot its destination late last week (New Jersey to Orlando via Florence, SC) courtesy of FedEx. As of today, it is on its way back from Florida and ought to be here tomorrow (done!). S'OK, I won't have all the hardware ready for it in any case. Today the Adaptall (Tamron to Nikon) arrived. After spending a couple of hours trying to do it right (I am 99% sure that the flange of the Tamron is slightly deformed), I pushed the Adaptall as close as I possibly could to its proper orientation (which is very close indeed) and then applied some SuperGlue (!) to help it stay there. So far, so good. My long glass is all Nikon anyway, so this should work with any foreseeable imaging train. If I ever need to remove it, acetone and heat will do the trick (probably). A Nikon to EOS-R adapter is also on the way, but it will take some luck to have it here in time for a looming clear spell.

5/20/2025. How did I put the 400 on the R6 when last I did that? Here's the trick (one of thousands): you have to attach the Nikon lens to EOS adapter to the lens first. Then attach the EOS to EOS-R converter to the effectively already converted lens. If you try to make the EOS to EOS-R converter into a one-step Nikon to EOS-R adapter by attaching the Nikon to EOS adapter to the EOS to EOS-R widget first, things jam up. At least they do today. Go figure. If you followed all that from that paragraph alone, you're a finer man than I, Gunga Din.

5/21/2025. I'm still collecting odd bits to make the best of this bargain lens, some for their own sake, some for my co-mount ambitions. Today, a 112mm lens cap, 2 USB cables, and a thin 43mm drop-in UV filter to fill in for the missing Tamron original are on the way. (Users long ago on an archived thread suggested that the thin ProMaster 43mm UV filter will fit the drop-in holder and serve well to replace the original Tamron glass; I found one for 1/3 the price of a new example, who knows what fraction of the original filter's price.) That's 11, 22, and 20$ to add to the cost of putting this lens to work. Add in the Adaptall to Nikon adapter, the ZWO Nikon to EFW adapter, and a Nikon to EOS-R adapter (about $80 all together), and we're comfortably above half the price of the lens (which actually says more about how cheap the lens was than about the cost of these "accessories"). I'd add a deep, 112mm metal lens hood for improved heat transmission from dew straps, but I haven't found one yet. That's a low-priority item.

I'm still fiddling with the design of the mounting bar for the two lenses. I've scrabbled up two, 120mm Arca clamps, decent plates, and a mounting clamp for the guider. None of those are hard or expensive to replace if they turn out to be needed in their original applications.

Vega rising.

Time out: turns out the 43mm drop-in filter is absolutely required to reach infinity focus. That's Vega and Epsilon Lyrae rising above the trees. No, the best available focus is not that bad, but it's bad enough. The lens won't focus at infinity without the filter (at least on the R6). Discussions online suggest that absent this filter, similar lenses focus only out to about 4km which seems about right. Aimed at Vega, it looks like it's going to be lovely, sharp, and bright as the stars come to focus, but then they just don't quite get there. The good news from the evening is that the SWSA is plenty of mount for this mighty-but-smaller-than-the-400mm lens.

Click the image if you can't read the text.

51 years ago, this was the bonus question on Dan Cross's 2nd semester physics final. I've always remembered it, as well as the cafeteria arguments it inspired. I effin' nailed it. And tonight, that recollection allows me to preserve my equanimity.

5/23/2025. FWIW, for typical photographic filters, the offset for rays passing near the edges of the lens works out to about 0.37x the thickness of the filter (those passing through the center are not deflected at all; the ones in between, by something in between). The wizards at Tamron have allowed for all that, and the lens misbehaves in the absence of that plano-plano "lens" element. The glass in the Promaster HGX UV filter is only 1.3mm thick. We'll see if half a millimeter is enough to get sharp stars. (It was available, the price was right, and it was specifically mentioned as fitting into the rear lens adapter.) Some filters are thicker (e.g., Hoya's UV filters are "often" 2.5mm thick), though it is a hard spec to find in particular cases. Many makers' rings will not fit into the limited drop-in space (Hoya's is said to require some filing), and not all permit stacking (K&F rings apparently do). Some creative shop work could be in order.

Here's a thought: why don't we try the fix already in hand before ginning up alternative solutions?

The ProMaster fits the holder; the holder with the filter fits easily into the lens; and the focus shifts appreciably -- maybe enough. I'm going to need a star to be sure. If the shift is enough, it might not be much more than enough, so I may yet need to think about thicker glass, stacking etc. to have some room to spare for the astro cameras, etc. If it's not enough, there are alternatives. Hoya's glass is (probably) thicker, especially in older filters which are said to use 2.5mm glass; the K&F is the one KEH offers as an "accessory" for this lens, but it may not be any thicker than the ProMaster is.

Sshhh. Look at a star before chasing any farther down this rabbit hole, damnit.

Best focus, Barrett Mountain. ProMaster UV filter. 720 pixel crop, full size.



Infinity stop, beyond Barrett mountain. ProMaster UV filter. The happiest I have ever
been with a fuzzy picture.

Bingo! The sky is totally covered, but from the community lot, I can see lights on Barrett Mountain, about 15 miles to the northeast. The lens with the ProMaster UV filter in place focuses beyond that. That is, with the lens at its infinity stop, I have to focus closer to produce the smallest point spread. That should do it. It's close, but it does the trick. BTW, monkeying around with the Tamron's "baseball stop" can, somehow, not only put a detent within the focal range (say you're watching 2nd base but expect a play at the plate) but also prevent the lens from reaching its most distant focus even if you turn past the detent. So if you get out there and can't quite focus, don't panic until after you've messed around with that preset ring. And now that I know that a simple drop-in filter can address this "can't reach infinity" thing, I'll shop up another, thicker filter because I'd like to have some slack for this and other optical trains. (Done, an older Hoya is on the way. Its mounting ring will probably need to be filed or machined down. If the intertubes are right, this basic Hoya should move the focal plane twice as far as the ProMaster does. That would be, in tech-speak, plenty.)

Late night update: a few stars appeared between the clouds. I took aim at Lyra and found the following PSFs (FWHM in pixels) using a stationary camera, ISO 6400, and 1-second exposures:

No filter, at the stop: 15.7
ProMaster, at the stop: 3.42
ProMaster, best focus: 2.65 (first attempt)
ProMaster, best focus: 2.67 (second attempt)
ProMaster, best focus(1), and BlurX: 1.58

Diffraction spikes around Vega told me that in my hurry to shoot between the clouds, I'd used F4 rather than F2.8, but still, this clearly shows that the infinity focus is just a little this side of the infinity stop and therefore accessible. There are some blue haloes around Epsilon Lyrae A and B for BlurX to work on (see if turning the filter around matters). One-second exposures produced about nine arc seconds of trailing at Vega's declination. That comes to about two pixels in the R6 with this lens. Flatness and tilt are really good. Expect to endure more about that when I have better data to measure. I've reduced trailing and noise for this one which seems pretty promising to me (it reaches about 12th magnitude between Eps Lyrae 1 and 2):

Head of Lyra, 300mm Tamron F2.8 with ProMaster UV filter at F4
Canon R6, 1 second at ISO 6400, BlurX.
Resampled full frame. Click it for 1280-pixel rendition.

5/26/2025. Surprise! Hoya's "HMC Multicoated UV(c)" filter in a slim frame fits without modification. Maybe it produces a little more shift -- hard to tell under all these clouds -- and no, it cannot be casually stacked with the ProMaster and still fit through the 300mm's drop-in slot. The filter holder can be inserted with either side of the filter facing the sky. That may matter to haloes and such unless these filters are multicoated on both sides (as they appear to be). And, finally, the it-is-so-still-a-bargain 300mm now has a tight-fitting lens cap.

5/29/2025. Let's get on with the comounted super-telephoto rig. In the interests of keeping Version 0 simple, just drill 5/16 holes for the 300mm's mounting bracketry and forego any threaded adjustments. Basically: drill the holes for the 400's Arca clamp; do your best to align the 300's, then nudge and tweak until aligned. Version 1 can have push-pull adjusters etc. First just get it made. Then you can have "fun" getting the computer to control two cameras and even more fun merging their data.

5/30/2025. And... done! I've kludged this around an old DMM Losmandy plate without the complication shown. It's a strictly K.I.S.S. design. The 1/4x20 bolts pass through 5/16 holes so that both long Arca clamps can be loosened, adjusted, and cinched down again as needed. I've provided no adjustments other than that. My intent is for this to be a set and forget bit of gear. A casual check with a laser pointer says we're close, certainly within the range of adjustability. Other adjustments would be possible by moving the Arca shoes on the two big lenses, but I would prefer to leave those alone. The smaller mount for the central 50mm guide-scope could be done better. It's offset with a long-ish bushing to give the thumb-lock much more than enough room to turn. A shorter bolt and a shorter offset would do no harm (but also little enough good). The bolts holding the long Arca clamps are not countersunk, so they limit the ability to move this mount within the A-P saddle. That can be fixed if needed in just a few minutes with the drill press and four carefully chosen bolts. And, of course, when guiding is not demanded, a third lens and body could be mounted there. The DMM is on the order of 14 inches or 355mm long, which is safely above the minimum needed to accommodate two lenses. Some of the freeboard is illusory since I've mounted the clamps as far inboard as possible (their thumb screws extend beyond each end) to produce the shortest overall assembly with an eye toward ease of packing the dual-lens kit into a traveling case. Now for some clear skies. I intend to try out this arrangement with Canon bodies before taking the show to Broadway.

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