Monday, December 21, 2020

M42: November Orion, Revisited

 


(Click for a bigger view!)

I went back and re-processed data from 2020/11/22, used for this image. This time, I included the 10-second exposures, which is how I got the detail in the core of M42 that was blown out in the earlier pic.

I stacked 95x10sec exposures (with 15x10sec darks and the same flats I used for the earlier pic), then took that image and the 47x30sec stacked image, adjusted them so their histograms were similar, then stacked those two. It worked remarkably well; if you look closely at the two images, this one not only has more in the core, it has more detail in a few places, some of the stars are sharper, and importantly, the noise levels are substantially lower.

I think the optics & mechanics I used could still give a little more, if I shot at 200mm instead of 145mm focal length and took more lights, and especially more darks and flats. But I think this is pretty close to the limits of the data I have at this point.

Sunday, December 06, 2020

Nebulosity Processing: A Few Comments on the Orion Photo

 Yesterday I posted a photo of Orion that I'm pretty happy with. I included basic technical notes, but here are some additional anecdotes on the image and processing.

Min Yun asked about the filter (digital signal processing, not glass) I used. This was done with Nebulosity 4.4's "Std. Dev. filter (1.5 - typical)". The documentation is a shade vague, but I believe this discards everything more than 1.5 s.d. above & below the mean, then averages. In my first processing attempts I used straight "Average/Default", which would be mean. Perhaps it's partly because I got better, rather than just the difference in the filter, but this looks quite a bit less noisy than my earlier efforts. Min suggests I try 50th %ile, just using the median. Turns out the software supports that. Hmm, I should give it a shot.


Here's a view of one of the raw images, zoomed in around the Horsehead and Flame.


Look at the tool on the right, and you'll see I've set the display to rescale color in the range 2500-2800. This is after demosaicing (turning the raw black & white pixel values into a color image), but with no other processing done on it. If you look closely at the histogram,  you'll see three peaks, which (left to right) are green, blue and red. The current scaling basically hides all of the blue and green. The reds on this camera accumulate counts much faster than either blue or green, which I've spent a LOT of time investigating. When I first saw these, I assumed it was skyglow from city lights, but the same thing is true for my dark frames, so it's the sensor. I don't know if there is some camera setting I should be adjusting, or if I should be tweaking the raw import function. Nebulosity supports a modest number of Canon models, but doesn't have a direct entry for the EOS 7D. I tried them all, some were different, none were better. (As an aside, I would find a better histogram tool, that showed three separate curves, useful; most imaging programs, like GIMP and Photoshop, have that as a default. I think the author of this software does a lot of his work one channel at a time, before combining colors and doing finishing work in another tool.)

Below the bright star you can see the Flame Nebula, standing out above the noise. In the center of the image, you can maybe, maybe, see a dark spot emerging? That's the Horsehead.

Here's the exact same region of the image, after stacking 47 exposures and using 13 dark frames to reduce the floor.  (Note the differences in the histogram and in the B & W sliders. The values are different both because of the dark removal and averaging, and because the above one was 14-bit Canon raw, and this is 16-bit FITS.) Now we have a view!




Next we do a power stretch, to bring up the details on the bottom end of the intensity scale. (n.b.: this is also necessary to keep the Orion Nebula, M42, in a reasonable range; clamping the values like the image you see above turns M42 totally white. M42 is to the right and above, out of frame in this zoomed view, but is important to the final image.)



And now we're pretty much there! I did one final tweak in GIMP to raise the middle of the dynamic range while keeping the top and bottom the same, which gave the image you saw yesterday.

Here's a close-in zoom of the Horsehead, after all the processing. (Click on it to open up and look closer.) This is the number of pixels I've got there, and you can still see some noise. You can also see how good my alignment and mount are, as well as limits of the optics and seeing, by looking at the stars. The really oblong one at the horse's neck is probably an unresolved double star, which you can tell by comparing it to the surrounding stars. The others show drift toward the lower right of about two pixels, which I calculated elsewhere is about the amount of movement in one thirty-second frame.



All of this took me about twenty hours of learning and fiddling and watching YouTube and reading docs, though it sounds simple once I summarize it like this. There are a lot of tutorials on the web on astro image processing, but figuring out which ones to watch is a chore in itself!

Min asked if I'm doing anything about cosmic rays. Not particularly. The dark frames handle "stuck" (or "hot") pixels as well as the current that accumulates even when no light is coming in. With only 47 light frames and set at ISO 1600, you can see that there is a ton of pure noise; I think most of that is thermal/shot noise rather than cosmic rays. A lot of serious amateur astrophotographers have adopted cooled CCD cameras, that work at -10C to -30C. I'd like to have one, but they seem to be expensive, and no doubt are a learning curve in their own right.

To return to the "red accumulates faster" thing, here's M42 from one of the raw images without any adjustment.





Oh, and the final interesting thing from the shoot itself: a train of high-altitude, very dark satellites flew smack through the middle of M42 while I was shooting! Here's a view from the same raw image above.





There are about five of them, moving very slowly and obviously very dark. It took them about 3-4 minutes to cross the area of my frame, about 5 degrees. That's the amount of time it would take an LEO satellite or the ISS to cross the entire sky. They must be thousands of km up. I kept the frames they cross in my total stack, but their trails are tossed out by the stacking algorithm.

Hope you enjoyed this tour through my brain, and my last couple of weeks. See you in the stars, or at least online. Stay safe!


Saturday, December 05, 2020

The Importance of Polar Alignment

 One of the major innovations in amateur astronomy since I was a kid is the regular use of polar alignment scopes. They existed, but were beyond my budget and patience as a fifteen-year-old. Now they are standard features on a lot of mounts, including my Kenko Sky Memo S.

So, how good does your polar alignment have to be for astrophotography?

Let's say you're off by one degree, toward R.A. 0, for simplicity. That means that if you're aiming at something at the equator (where the effect will be the worst), your aim will be a degree low at R.A. 0 and a degree high at R.A. 12h. The maximum slew rate will be when you're looking at R.A. 6h or 18h.

How fast is that slew rate? Let's see...that up and down cycle will be two pi per 24 hours...max slope of a sine wave is 1, but gotta match those units...one radian per 3.82 hours...so that would be a max rate of one degree per 3.82 hours. That's 15.7 arcminutes or 942 arcseconds per hour. About one arcsecond of drift every four seconds.

Is that a lot? Well, my professional astronomer friends tell me the seeing at sea level is about 1 arcsecond, at best. It's also about the diffraction limit for a 100mm objective, roughly. So worst case, this would limit our exposure time to four seconds, if our alignment is off by a full degree.

But whether we are working near that limit depends on focal length and camera resolution, too. My Orion photo I just posted was about 3k*5k pixels in the original, covering about 5x7 degrees (before I cropped), shot at 145mm f.l.  I calculated that one pixel in that image is about 6 arcseconds.  (All of this is very back-of-the-envelope, so call it 4 to 8 if you're picky.) So, a 1 degree misalignment would drift by 1 pixel in 24 seconds (well, 20-30, give or take). Since I was stacking 30-second exposures, I only needed to be within one degree!

Very roughly, eyeballing my raw images, over about 25 minutes I drifted by about 90 pixels, about two pixels per shot. A little more than I'd like, but with the stacking I did, not noticeable. In fact, zoomed all the way in, the best stars don't really show any distortion in that direction.

So, I thought I had the polar alignment really nailed during that shooting session, but I might have been off by a full degree. (As it happens, I had to allow rotation as well as translation in the alignment of my stack, which may be due to polar misalignment.)

So I'm in pretty good shape for 30sec exposures with the Canon 70-200mm zoom lens. But if I'm going to shoot long exposures with the new C90 Mak, f.l. 1250 and f/14, I'm going to have to do a lot better than one degree. (I'm also going to need a lower-vibration mount.)

Of course, serious astrophotographers shooting at long focal lengths usually guide their scopes, for exactly this reason, using a longer-f.l. scope and a lighted reticle eyepiece to keep the imaging rig pointed right. It's pretty arduous work. But technology is making this one easier, too, if you've got the budget!

Orion: M42, M43, Horsehead and Flame Nebulae

M42, M43, Horsehead and Flame Nebulae in Orion

This is the same data I've been working with for a few weeks now. The two bright stars on the left are two-thirds of Orion's belt, and the bright nebula on the right is the main part of his sword hanging down.  (The image is rotated 90 degrees from what we usually think of as "up" with Orion.) Specs:

  • Camera: EOS 7D
  • Lens: Canon EF 70-200mm f/2.8
  • focal length: 145mm
  • f/2.8
  • ISO 1600
  • Mount: Kenko SkyMemo S equatorial tracker
  • Exposure:
    • lights: 47 frames, 30 seconds each
    • darks: 13 frames, 30 seconds each
    • flats: 10 frames using above setup, but I don't really trust them
    • bias: none (should I? given how much noise is the problem here...)
    • First aligned with rotation (two-star alignment), then stacked using std. dev. 1.5 filter, throwing out extreme points
  • Field of view: approx. 5x3 degrees as cropped
  • Image size: 3338x2225 as cropped
  • Resolution: very roughly, 6 arcseconds per pixel, smallest stars are about 4x4 pixels, so 25-30 arcseconds resolution, I guess, but the brighter stars are about 20x20 pixels
  • Faintest stars in the image: good question! I wish I knew.
  • Software: Nebulosity 4.4.3, GIMP 2.10
  • Date/time: 2020/11/22, about 2:00 a.m. local time
  • Location: Arakine Dam, Chiba, Japan
I have learned a ton about CCDs and image processing, but I have probably learned more about this specific camera/sensor and this specific piece of software than about the principles. Because I had so much learning to do, this photo is probably about 20 hours worth of sitting in front of the computer working.
I'm also sitting on 100 frames at 10 seconds, which might add some detail in the middle of M42. I don't know if it's possible to combine them effectively with this or not. I would still like to make the Horsehead more vibrant, but M42 is so bright, that everything I've done to try to brighten the Horsehead turns M42 into just a wash of white. More to learn, yet, but I think this is pretty close to all that this data has to give.
I'm having fun...

Tuesday, December 01, 2020

C90 Mak: Saturn


 Best shot I got out of a couple of dozen.

Prime focus, 90mm objective, 1250mm f.l., f/14, ISO 800, 1/10th of sec. C90 Mak lens, EOS 7D camera.

Kenko SkyMemo S equatorial tracking drive & accompanying wedge and tripod, roughly hand-aligned using a level and compass (no line of sight to Polaris, and set up before dark, anyway). Vibration is a big problem at this f.l. Most of my shots, even with a 1/10th second exposure, were smeared.


Saturday, November 28, 2020

C90 Mak First Light: the Moon!

 

From my first session with my brand-new Celestron C90 Mak. f.l. 1250mm, f/14, so it's not bright. And yet, with my EOS 7D set at ISO 800, this is 1/400th of a second, and even half that might have produced good results.

It was also cloudy; this was snapped between the clouds, and actually with a little haze still present. Kenko Sky Memo S equatorial mount and clock drive, with alignment only guessed at using a compass, and yet the tracking worked reasonably well -- about as well as I ever used to manage with my old Edmund Scientific reflector, but not nearly as good as I managed last week using the polar scope.

If I'm doing the math right, a 90mm objective observing 600nm (orange) light has a diffraction limit of 8 microradians, or 1.65 arc-seconds. The moon here is about 2700 pixels across, smallest features are 5 pixels or less, call it 1/500th of the moon's diameter. 1/500th of 1800 arc-seconds is about 3.6 arc-seconds, so I might actually be close to the maximum theoretical resolution? Off by less than a factor of two would be pretty good. At first glance, I didn't think this scope was anywhere near that good. There does seem to be some scattering I can see in some of my test images, which definitely adds noise/reduces contrast.

Definitely still a lot of learning to do to get the best out of this new tool, but not a bad first effort.

Friday, November 27, 2020

Beginner's Guide to Working with Nebulosity 4

 


I've been getting back into both photography and astronomy, so, astrophotography. The picture above is the Orion Nebula, shot on Nov. 22, 2020, and processed with Nebulosity 4 software as well as Gimp. It represents my best effort so far, in optics (improved focus and exposure), mechanics (polar alignment, tracking and vibration), and image processing. I think the raw data I took that night still has more to give, so I'll keep working on it. Click on the image for an expanded view!

Nebulosity 4 is astrophoto software that can control some digital cameras and provides a great many editing features for the resulting images. I acquired it because I liked its image stacking features the best of three or four tools I tried on the Mac, but I'm gradually using more of the features. I don't have any real plans to shift to full laptop/software control in the field right now, but you never know.

It turns out that, despite the more-expensive-than-a-game-but-ridiculously-low-for-professionals price of US$95, documentation is a little sparse, so it has taken me a while to kind of grasp the expected workflow, including learning about darks and flats and biases. Most frustratingly, after learning about those concepts, I spent quite a while trying to understand the basic mechanics of working with Canon CR2 raw image files. So, I'm collecting what I've learned so far, in this blog post.

Resources:

  • Really, the first thing to look at is the Nebulosity 3 manual, since there is (AFAICT, as of 2020/11/26) no separate Nebulosity 4 manual yet. (Frustratingly, I kept the link directly to that PDF, but not the page that linked to it, and now I can't find the web page -- an indication that the website needs some love? Ah, found it again -- it's under "Downloads", but you have to scroll down.) In that document, on p. 29, you'll find a nine-step formula for processing images -- exactly what I was looking for, and which took me hours to find. It's the basis for what I write below.
  • This YouTube video by Alex Cardenas is fantastic. It's a near-perfect tutorial on how to do the stacking in Nebulosity, once you have your set of frames ready. However, Alex was working from separate sets of R, G, and B files, whereas I'm working first from JPEGs and then from Canon raw files.
  • This info on Canon CR2 raw image files was a big help in learning about what's going on in the raw files themselves, and what needs to happen to turn them into color images. In particular, Section 4 of that shows how pixels are laid out on the sensor chip, which helps you understand what you're seeing if you are looking at the whole raw file in black and white. Armed with this info, I was able to figure out what operations needed to happen, then the next step was to learn how to make them happen in Nebulosity (see the first bullet point in this list).
  • This presentation by Craig Stark from 2014 is good enough to be useful, but it's long, almost two hours. The best part I've seen so far (I'm about halfway through) starts at 22:10, discussing the basic linear math of what he calls "Classic dark subtraction". I'm sure there are other good sources on the particular topic on the web.
I'll put more about my process into another post.

The image at the top:

Orion Nebulae M42, M43, in the sword hanging down from Orion's belt. The image is turned sideways; the two bright stars to the left are two-thirds of the belt.  Around the lower one (left as we usually think of it), Alnitak (zeta Orionis), there is visible also a bit of nebula.

  • Camera: EOS 7D body, APS-C sensor
  • Lens: Canon EF70-200 f/2.8L USM lens, set at 145mm f.l.
  • Settings: ISO 3200, f/2.8, 30 second exposures
  • Mount: Kenko SkyMemo S equatorial drive and leveled tripod
  • Images: high-quality JPEG
  • 47 light frames
  • 15 dark frames (camera is old, with a lot of stuck pixels, so the darks really help!)

The focus may not have been perfect, but was pretty good; achieved using simulated exposure and digital zoom to set focus, and at max zoom could see quite a bit of vibration from the mount.

I'd like to reshoot with zoom set at 200mm, and possibly stopped down to see if that reduces some of the coma, but fundamentally I think this is pretty close to the capability of this lens, sensor and mount.  Raw images are also pretty noisy, should try ISO 1600 or even 800.

Shot around 3am local time, about 3.5 hours before sunrise.  Shot at Arakine Dam, Chiba Prefecture; perhaps the best dark within two hours' drive of our house, but you can still see quite a bit of sky glow from Chiba city and Tokyo.  The Himalayas it ain't.

Stacking done with Nebulosity, which seems to be excellent for this task.  A little more flexibility in adjusting the light curves would be nice, but that's easy in Gimp once the alignment and stacking are done.  For this image, I simply set a black level floor of around the sky glow, no other light curve tweaking; cropped in Gimp.

Sunday, October 04, 2020

The Organizations I Work With, Or, Where my Time Goes

 If you're waiting on an answer to an email from me, or I owe you a document, or for some reason my inability to get something done is inconveniencing you, I apologize.  I really shouldn't spread myself so thin, but the fact is that there are a lot of things in this world I care about. Worse, as a professor, I don't have a boss. The great thing about not having a boss is that nobody tells you what to do. The terrible thing is that nobody tells you what not to do.  There's no one to defend you: "That's a great project, but Rod's busy. He's available the middle of next year, or you can find some else."

As a prof, we have essentially four major duties:

  • Teaching: one of the big, obvious ones.
  • Research: the other big, obvious one. This includes both doing the research yourself, and managing the research (budgets, schedules, purchasing, hiring, etc.).
  • University service (running the university): the amount of this varies depending on your position, how useful you are (making yourself useless/unreliable gets you out of some of this), and the structure of your institution.
  • Community service: participating in your community, defined however is appropriate for you. Might be the literal community around your campus, might be running a journal or a conference.
For me, and for most of us, community service means working with colleagues in our own and other universities, companies, government labs, and government committees,  to further the field as a whole. In some cases, this benefits your own research projects, in some cases it's much more indirect.
My primary research area, as you probably know, is quantum computing and quantum networking, but I also care about computer networking in general, and distance education and educational technology (though I have no formal training in the latter).
So, here are most of the organizations I'm working with these days (as of 2020/10/1). Some of these are internal to Keio, i.e. a structure for doing research.  Others are external.
  • AQUA: my own quantum computing & quantum networking research group at Keio's Shonan Fujisawa Campus. Truly, the center of my professional life.
  • RG: our larger lab on campus, an umbrella for managing over 100 undergrads in a broad variety of computing areas.
  • AQUA @WIDE: there is also the AQUA working group inside of the WIDE Project. We generally hold a small meeting during the semi-annual WIDE Camp, or run a tutorial during the semi-annual WIDE Kenkyuukai, things like that.
  • WIDE: I'm a WIDE Project Board Member. I do much less for WIDE than most of the other board members, but even so I do quite a bit.
  • KQCC: the Keio Quantum Computing Center, where I am Vice Center Chair. I supervise and participate in a good fraction of the research, but the Founder and Center Chair have actually done most of the heavy lifting on paperwork, recruiting member companies and partners, hiring, etc.
  • CCRC: the Keio Cyber Civilization Research Center. Rather than a driver, I'm a participant here, but this is important work, as well.
  • SOI-Asia and AIII: my involvement here is small, but I do what I can. We're working with some of the SOI-Asia partners to share our MOOC on quantum computing, including translating the MOOC into important languages in Southeast Asia. I also have one Ph.D. student working on technology in language teaching, and this is one of her primary "homes".
  • AINTEC: I'm on the steering committee for the Asian Internet Engineering Conference.
  • JFLI: I'm Keio's representative to the Japan-France Laboratory for Informatics.
  • QIRG: I'm co-chair of the Quantum Internet Research Group, part of the Internet Research Task Force (IRTF).
  • IRSG: being co-chair of QIRG puts me on the Internet Research Steering Group. This means I should be doing a lot more for the IRTF as a whole than I have been.
  • QITF: here in Japan, we are standing up the Quantum Internet Task Force, bringing together most of the researchers in Japan who are working on quantum repeaters.
  • WQRN: I'm part of the organizing committee for the Workshop for Quantum Repeaters and Networks.
  • TQE: I've joined the editorial board of IEEE Transactions on Quantum Engineering, for the moment as an editor for the special section but probably more work coming up.
This doesn't even list the campus committees and program (department) duties, etc. It also doesn't even begin to address handling my research projects -- all that's just lumped under "AQUA" up there at the top.
And in a normal year, I travel to visit collaborators in Thailand, Paris, U.S., Hyderabad, etc., not to mention the conferences and meetings. I often feel depressed and overwhelmed by work, like I'm not getting anywhere near enough done. But then I look at this list, and wonder how I ever get any sleep and time and home.

[Edit on 20/12/3: Add JFLI]