My 135 mm Tamron Delusion Ends, It's Plan B now

I really thought I could get away with using my old Tamron lens for imaging Integrated Flux Nebula (IFN), but it's not to be. The number of minuses kept growing as I spent more nights practicing with the setup. I realized that my desire to use the Tamron and was blinding me to the issues that doomed it.

The greatest difficulty was being unable to rotate the field of view in a reproducible manner. This happened every night I used the setup with one night being 45 degrees out of kilter from the others. Keeping the camera orientation consistent between multiple imaging sessions is essential; without this, some of the field will need to be discarded during stacking. It became obvious that the amount of lost field would probably be substantial, resulting in a much retained field of view. The advantage of the wide 135 mm field of view would be lost. I think the cause was the ring clamp I was using to join the lens and camera to the dovetail. To permit manual rotation of the camera this ring had to be loosened and retightened many times, resulting in misalignment.

Image quality was also not what I wanted. There was no reasonable solution for the back focus error and I would be stuck with strong aberration needing to be corrected by BlurXTerminator. I knew some residual aberration remained, and I was concerned that this might show itself during the aggressive processing I would use to draw out the IFN. 

I came to realize that the stepdown ring I was using to produce spikeless stars was causing severe vignetting. While flat frames could somewhat compensate for this, too much signal was being lost -- again diminishing the effective size of the field of view. Using the lens's internal blades to stop it down was an alternative that created large, flaring spikes around the stars. I found this unacceptable.

Plainly, the lens was not up to the purpose and it was time to move to Plan B.

Plan B

The fallback is to use the FSQ-106 + focal reducer operating at a focal length of 387 mm and focal ratio of f/3.65. This California Nebula image used that configuration and gives you an idea of what it can do. The advantages are many over the Tamron: 

• 17% increase in speed (f/3.65 vs f/4)
• A very flat field with modest vignetting  
• An actual rotation ring
• Amenities like autofocus, autoguiding and dithering, easier creation of flat frames, and automatic meridian flipping

This is a much heavier scope to tote around, but the only real disadvantage it has is the smaller field of view. Here's a comparison:

135 mm Tamron field (outer box) vs 387 mm FSQ field (inner box)

The star cluster is no longer in the FOV, which is fine as the IFN is the real target. Does the smaller FOV (about 3.5 x 2.3 degrees) include enough IFN to be worth imaging? This image of Polaris IFN by another imager has essentially the same FOV as my setup will produce. I think there's enough IFN there to make it worthwhile particularly if Polaris can be reduced in size. I'll also compose the image to have Polaris much closer to the north edge of the frame, making more room for the IFN. 

Other aspects of the comparison image are worth looking at.

The scope used was an f/5.5 refractor with a flattener that didn't affect the focal ratio (so far as I can discover). The camera, an ASI 6200 color camera, has a quantum efficiency about the same as my ASI 2600 mono camera. The total integration time was 5 hours and 10 minutes. My thought is that if I want a reasonably deep image with low noise I should try to get at least twice the total time that went into the image. I'll probably use a plan that requires two nights of imaging: 5 hours of luminance one night, 3 hours of Chrominance another night. That's roughly the equivalent of 11.7 hours of one-shot color gathered at f/5.5. 

Yes, I know, that's a LRGB ratio of 5:1:1:1 and instead of the usual 3:1:1:1. I just like working with luminance; one night a few years ago something glitched and I ended up shooting only luminance. The final ratio was 6:1:1:1 and the result was quite nice (in my opinion, of course).

The comparison image was created using 5-minute light frames, which is probably why star colors are muted and Polaris is bloated. I've had much better luck with shorter exposures, and may simply go with 90 s lights. Both of my linked images used 90 s lights exclusively.

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Tariff watch: the Rokinon 135 mm lens (Plan C) is holding steady at $449 and in stock at B&H.

IFN Setup Passes Its First Tests (With Help From BlurXTerminator)

Last night was good for running some tests on my integrated flux nebula (IFN) imaging setup. I was able to take test frames at different exposures (60, 90, 120, 180 and 240 seconds) and test go-to and tracking. Everything came out well, or as well as could be expected and many questions were answered.

Did Polaris throw an internal reflection?

I stretched a 240 s exposure using PixInsight's Boosted Autostretch and there was no evidence of any internal reflections. It's possible something will show up in an image stack, but this suggests even if it does it will be faint.

Was the field suitably free of aberration? 

Here's a visual comparison of the corners and center in one of the 2 minute exposures (click for full scale). This is basically a raw frame; it's only been brute-force flatted and then delinearized with PI's default stretch.

Pretty awful, isn't it? My guess is that the aberration is coming from backfocus being significantly off. How much off, and in what sense? I'd use NINA's aberration inspector to find out, but this is a manual-focus lens. Sadly, my ability to change the backfocus is next to zero. Skip the following paragraph if talk about backfocus bores you :)

My required backfocus should be about 45 mm (44 for the lens plus1 for the filter), and I'm currently at 42.5 mm. The best I can do to increase this is by adding a 0.8 mm thin spacer or whatever the thinnest M42 spacer ring is (probably 5 mm?). Decreasing the backfocus would require a Canon-M42 adapter that's thinner than 10 mm or a filter drawer that's thinner than 20 mm and can be adapted to use 36 mm filters. Increasing the fun is that this lens is actually at heart a Tamron Adaptall lens circa 1980, fitted with a ring to make it compatible with OM-1 cameras. Both date back to the 1970s, so good luck finding things to take advantage of any of that. And yes, I looked into having a custom adapter built, both Canon to M42 and Olympus OM to M42, and they can't do it in a way that works for me (not to mention it would be $$$ if they could).

Fortunately the aberration can be adequately dealt with using BlurXTerminator (BXT). The corner diagram below shows how well BXT fixes things using the very non-aggressive settings of zero for both its "Sharpen Stars" and "Adjust Star Halos" parameters:

The improvement is almost miraculous. All the corners look sharp. So the lens passes this test thanks to processing with BXT.

Can the lens reach a decent focus?

The focus you see is the result of a few minutes of shooting test frames and making very tiny manual adjustments (just like in the old days before I had an electronic focusing motor). I think it's quite good. Yes, I'd love to have NINA do the focusing for me, but that's not going to happen.

I will need to stabilize the focus wheel to avoid accidental movement; a piece of tape should work.

Was there tilt?

A first look at tilt as calculated by ASTAP gave this.

It's not so much tilt around one axis as it is a sign of the aberration. ASTAP considers the tilt severe (see the small text along bottom of image). After BXT has been run the results confirm the improvement seen in the second corner diagram above: 

Star size outside the center is dramatically reduced and ASTAP now considers the tilt to be "almost none." This means I won't need to add the tilt plate or shims.

What about the mount and Go-To?

The mount slewed to Polaris and plate solved without fault. Manually correcting the rotation was simple and fast. The required rotation was only eight degrees, so the riser wasn't necessary and will be removed.

How was the tracking?

The center stars in the pre-BXTcorner diagram were nice and round so I'll assume tracking is close to perfect. The corner stars in the post-BXT are fine, too, so there's no appreciable field rotation in a 2 minute span.  I use PoleMaster for polar alignment, and it provides almost no visible rotation even over a multi-hour session, so I think that tracking should be more than adequate.

In other words, there's no need to add autoguiding or fiddle with PEC.

Were there any composition issues?

The amount of camera rotation required was only eight degrees from the filter wheel's long-axis up position, meaning that the riser wasn't needed. I'll remove it and the system will be a little sturdier.

Oddly NGC 188 was not quite where I wanted it. It's a touch close to an edge than I expected. I'll have to look at the instructions I gave NINA.

Summary

Everything worked better than expected with help from BXT! Basically, the system is ready for field use, although a little more work remains to get it set up for dew control. Because BXT removed the aberration using very nonaggressive settings, I don't think it will damage the IFN.

A major concern was the mount: would it behave itself for a target so close to the celestial pole? It slewed and centered on the target without difficulty. Manual rotation was easy and the riser won't be needed.

I think the ASI-2600 will do fine without dithering, but I may try that anyway with the PixInsight manual dithering tool.

It looks like almost any exposure will serve well; even a 4-minute exposure had nice round stars; I think I'll probably use 2 minutes for all four channels.

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My reading of Hugo Award winning novels is winding down. I skipped back to the 2000 winner, A Deepness in the Sky, by Vernor Vinge, and it was slog to get through. A few times it was almost a DNF (did not finish). I just couldn't connect with the author's writing style, his use of many side characters that were of little consequence, and the too-happy ending that seemed rather forced. For some reason he felt compelled to add a bloated epilog that served mainly to punish readers. Most of the Hugos have been worth reading, but not this one.

Up next is the 1939 Retro Hugo Award winner, The Sword in the Stone by T. H. White. Yes, it's that sword in that stone; the story has spawned a number of adaptations (the 1963 animated version by Disney is probably still the best known.

This will be the end of the Hugos for me, at least for a while. Maybe I'll start another reading marathon when my wife has her other knee replaced.

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It's lithium battery maintenance time: all the batteries have been fully recharged and then discharged to about 50% of capacity. Just before Northern Nights Star Fest I'll be bringing them up to full charge and selling/swapping them at 80% below list price. I'll provide a full description of what I'm bringing to NNSF in an August post.

A Few More Days to Clear Sky?; A Product to Avoid

Spring progresses slowly, but it appears that next Tuesday night (today is Saturday) will finally be clear and suitably dark for testing the things I need to verify about my IFN imaging setup before it goes on the road. In the meantime I've been putting together the traveling kit so that I can have all the cables and such needed for the project ready. It's also been time to top off all my lithium batteries. 

Lithium batteries are remarkably tolerant of abuse. The can abide deep discharges if you can get them to a charger within a day or so. Almost every lithium battery of any reasonable size now includes a battery management system that prevents too-rapid discharge or overcharging. The batteries also tolerate temperatures that I wouldn't! 

Maintaining a lithium battery is easy, too, even in the seasonal sport of astrophotography. At the end of the imaging year, just make sure they're they've been discharged to about 50% capacity, then every three months bring them up to 100% and back down to 50%. When you resume imaging and their use becomes more continuous just keep them fully charged. 

Yesterday I started the springtime full charge of the battery pack I use to power my laptop, and it revealed something I should have known. The pack in question is a pair of 15 Ah batteries in parallel, so I thought to use my 10 A charger.  The pack's only port is a 12 V automotive socket and the charger was sending 10 A through a fused automotive plug. All was fine for a while, but then the charger indicated it had shut itself off and there was a faint aroma of overheated plastic. That's never good.

Inspection of the plug showed the fuse was intact, but that the coiled spring at the base of the fuse was misshapen and the plastic around it had melted. Supposedly 12 V plugs can handle 120 W (10 A @ 12 V). At the point it started to overheat it was probably being asked to handle around 135 W, which evidently was too much for it. I suspect the rating for the plug I was using was well below this; I've seen some that are only rated for 60 W. 

I use one of these plugs to run power to my laptop, another to feed the mount, and a third to power the rest--CCD camera cooling, dew preventer, and USB devices. The total of all these rarely approaches 50 W and is typically more like 25 W, with a little going to the mount and the rest pretty much evenly split between the laptop and camera plugs. Although I've never had an issue with the plugs during imaging this current limitation is something to keep in mind if I ever change equipment. 

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At first glance this Astromania dew preventer looks like a great deal. A 10-inch strap with on-off switch and easily adjustable power control. And the connections are compatible with common 12 V power sockets. What more could you want for a mere $20?

From that buildup you can probably guess it was not quite a great deal. It was a pretty bad one, really.

I connected it to my primary power box and tried it out. I started with the toggle switch off and the power control rotated to the off position. My power box reported 0 W, 0 A. Good. Then I flipped the toggle on. Immediately the Current rose to 2.1 A and the strap started to warm up. In fact, it got quite hot: too hot to wrap around a cameral lens without risking damage. Obviously the controller was somehow defective. With the Controller at it's full-on setting the current was 2.4 A. Were I not a Vine reviewer, I would have returned this immediately. 

But there was another problem, the strap itself. Most straps are very flexible and have no problem conforming to the cylindrical shape of optics. This one won't do that. The plastic lining of the strap is thick and very stiff, and even working it for a while by hand didn't get it to wrap around a 135 mm lens without a few big gaps.

So it was a complete fail and got one of my rare 1-star reviews. Setting aside the problem with the strap the only way this would function is if you could control the current with some other device. And you would want to first make sure the strap connecter would be compatible with your device's strap socket. (It couldn't work with my Pegasus Powerbox or any of my other manual Kendrick controllers, for example. Yes, you could make an adapter for this, with a 5.1x2.5x3 socket on one end and an RCAphono plug on the other. But why bother.) 

Poor Weather & Caregiving: Renewing my Science Fiction Reading; The Tariffs at Work: Rokinon 135mm f/2 Lens Price Update:

Note that this post has little to do with astronomy or astrophotography, so if that's your only interest save yourself some time and move along 😉 

I've been astro-inactive for a while. There haven't been many opportunities for imaging; spring of 2025 in Minnesota has been cold and cloudy at night and sporting occasional snowfalls that recoat the ground. More important is that my wife had knee replacement surgery near the end of March; I've been home acting as her caregiver until she regains her mobility so even if the weather had been cooperative I really couldn't have be going out all night for imaging sessions.

What I've been doing instead is taking advantage of waiting room time by reading recent science-fiction novels. Given my age it's hardly a surprise that most of my sci-fi reading in the past has been of Golden Age authors (Asimov, Heinlein, and Clarke) with some of the 1960s and 70s anthologies of other authors mixed in. Larry Niven features large in these because I like hard sci-fi. I'm not entirely stuck in the past, though. I've read most of Ursula Le Guin's novels, much of Connie Willis's work, and several books by David Brin. And yes, even all three Three-Body Problem novels (more or less).

For My wife's post-op month I decided it was time to see what current sci-fi writers are creating. I decided the Hugo Awards would serve as a good guide and started reading my way through the last decade of winning novels. I hesitated at first given that the Hugos include fantasy but decided blending in some "soft" fiction would be good way to grow out of the hard sci-fi rut I was in.

Here are the first two weeks of intensive page turning summarized in capsule reviews, with a few notes for people who don't want to read certain types of content. The year shown is that of the associated Hugo award.

2024 Some Desperate Glory, by Emily Tesh.

This is the story of a woman born and raised on an asteroid fitted out as a militarized stronghold. It's the last, best hope of humanity to resist the conquest of the Majo, a collection of alien species slowly expanding its influence through the galaxy. 

Tesh writes in a way that challenges you to keep up with the story; her style reminded me of Le Guin's in The Lathe of Heaven. While some reviewers semi-dismissively lump the story into the "space opera" category, I think it has some significant things to say about both technology and humanity. The plot leaned a little too much on the deus ex machina trope, but the pacing kept me from being overly bothered.

Content Notes: AS, SC and a whole lot of violence.

2023 Nettle & Bone, by Ursula Vernon

Characterized as a dark fairy tale, this is pure fantasy with kingdoms, princesses, and spell-casting godmothers. It reads as an adult, grittier take on childhood fantasies. I had a little trouble with what seemed like a sluggish start, but once the author started adding supporting characters the pacing improved greatly, becoming a mix of humor and drama.

And then there's the scene in the Goblin Market, which alone is worth the price of the book.

2020 A Memory Called Empire and 2022 A Desolation Called Peace, both by Arkady Martine.

At one level these explore the ramifications of a technology that permits the transfer of knowledge, experience, and to some extent personality from one person to another. (The method here is strictly technological and not biological as with the symbionts of Star Trek.) The focus is on a woman thrown into the role of ambassador to a galactic empire. She's only somewhat prepared for what happens, and part of the fun and terror is her having to cope with an alien (albeit human) culture. The author's world-building is based on real past human empires and is fascinating to see unfold.  

Fans of Babylon 5 may wonder if the author intentionally borrowed from that TV series. The protagonist comes from a large, rotating space station (some of the descriptions could fit the Babylon 5 station). There are jump gates that permit movement through the empire and the empire itself resembles what the Centauri might have become if not checked by other spacefaring races and their own lapse into decadence. There's even an aged, ailing Emperor with intrigue between those maneuvering to be his successor. Oh, and let's not forget the mysterious attacks by large, shimmering black, and cloaked vessels from far jumpgates. They sometimes scream as they depart.

I don't mean any of that as complaint. If you want an extensive empire with far-flung possessions spanning tens of parsecs, you need to explain some way of getting around that old wet blanket Einstein. Jumpgates, wormholes, warp drives, hyperspace, folded space, infinite improbability drives, or whatever, numerous authors have already explored the territory. As long as you don't steal technology by its proper name (Sheewash Drive, for example) you're fine. The story is more important than the mundane tech bits.

The quality of the writing is what matters, and in these two novels it more than compensates. 

Content Notes: AS, SC

2021 Network Effect, by Martha Wells

This is the fifth entry (and first novel) in a series call the Murderbot Diaries. I've only read the  introductory novella (2017's All Sytems Red) , and it was a fast, breezy, and intriguing read. Network Effect is a full-length novel and it's recommended that one read everything leading up to it before taking the plunge. So I'm making the collection a birthday gift suggestion and it might be my reading material for this summer's Northern Nights Star Fest (if I'm not stuck at home when her other knee is replaced!)

Content Notes: Lots of violence, as you might expect when the protagonist is named Murderbot. 

2019 The Calculating Stars, by Mary Robinette Kowal

It's no spoiler to say this is an alternative history novel. If you don't mind reading about the many injustices inflicted on 1950s women and minorities (both religious and racial) this is an enjoyable throwback novel that in many ways is very remindful of the 2016 book and movie Hidden Figures.

The story combines both hard science and social commentary. The writing is adequate, occasionally rising to excellent -- particularly at the conclusion. This is the first book in a series of three.

My suspension of disbelief was mildly tested by what I felt like was an overstatement of what the described technology was capable of doing.

2016 The Fifth Season, 2017 The Obelisk Gate, and 2018 The Stone Sky, all by N. K. Jemisin

Yes, an unprecedented three consecutive Hugo awards!

These are world-building carried to an amazing extreme. At this time I'm in the third book and would love to explain what's happening, but my understanding of it is a work in progress. Riding along with the characters as they try to understand what's happening to their world is a big part of the fun. 

The story unfolds in a rather unconventional manner, but that's in keeping with the plot. I'd say more, but it's almost impossible to do so without resorting to spoilers.

Content Notes: Quite a bit of violence, much of it like something from a very dark version of Frozen. 

2015 The Three-Body Problem, by Cixin Liu (translated by Ken Liu) 

This wasn't read during this month's blitz, but I'm including it to round out the decade. This introduces a very alien culture and instantiates the Dark Forest answer to the Fermi Paradox. I enjoyed this book, but found the subsequent two novels (The Dark Forest and Death's End) so unreadable that I hopped rapidly through them to reduce the pain.

Content Notes: Mind-numbing content (mainly in book three and the second half of book two.) 

Content Notes for those to whom these things are important:

AS: Alternative sexualities. I found these were important to the plots, but you may feel differently.

SC: Non-gratuitous Sexual Content. Interestingly, it was The Calculating Stars that had the most explicit sex, suggesting that the author was making the story compatible with readers of romance novels. The only sex scene that felt a little out of place was in A Desolation Called Peace. (And no, I'm not going to tell you what chapter it was in so you can just skip right to it.)

All the stories involve spoken vulgarities, and most feature violence of varying degree.

If you're someone who resents anything that seems the least bit "woke," I'd suggest you stick to the Hugo winners from several decades past. Better yet, focus on the Retro Hugo Awards that recognize works from 1939 to 1954. Two of my favorite stories are there, Asimov's 1951 winner Pebble in the Sky (the first sci-fi I read) and Clarke's 1954 nominee Childhood's End. The latter is an amazing story, and it only lost the award because it was up against the equally amazing and iconic Fahrenheit 451 by Bradbury.

Just so you know, my wife's recovery is coming along wonderfully, and I truly hope that my next post will include some astrophotography news. 

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Tariff Watch:

[UPDATED : On April 5 the B&H Rokinon 135mm lens Canon EF version price rose to $479, $30 up from what it was on April 4. It's now up $110 from it's pre-tariff price in early February of $368. That's an increase of 30%. 

But hold on! The price dropped back down to $449 on April 6!  Was yesterday's price hike an error, or a preview of what happens when they need to restock? Wait and see!