Prairie Skies, Part 2

The bad news for severe weather geeks like me is that not every night can have a thunderstorm.  So we need to find things to look at in the sky when there's no clouds or lightning to capture our attention.

Truth be told, I was an astronomy geek long before I became a weather geek, but my interest in astronomical observing tended to be visual.  I had spent vast amounts of my youth connecting various cameras to my supply of telescopes, but the results tended towards blurry images and long waits at the photo developers to see what I'd done wrong a week earlier.

But last Christmas, my son talked me into buying a T-adapter to connect his Nikon DSLR to one of my telescopes.  And since then, we've been hooked on astrophotography.  And it turns out that taking good pictures of celestial objects is easier than one might think, once a few basic principles of "what is signal?" and "what is noise?" are understood.  And once we understood these, our photography efforts improved greatly, as the following image shows.

That image is of M31, the Andromeda Galaxy.  It was taken with a box-stock bottom-of-the-line Canon digital camera (the 1000D), through a small telescope (an AstroTech AT66) that really isn't much more in terms of its optics than half of a pair of really good binoculars.  The mount used to guide the telescope can be described as "hopelessly inadequate for the job", yet the picture's quality speaks for itself.  So how did we get such a fine image from such simple equipment?

As SST/TA's resident Nassim Nicholas Taleb fan, I tend to think of this question in terms of Taleb's dichotomy of mediocristan and extremistan.  Mediocristan is the world of statistical variation we are used to, where distributions of such parameters as height and IQ live.  Variations in this world are captured reasonably well by Gaussian (i.e., bell-shaped) probability curves, with symmetrical statistical distributions about some easy-to-determine mean (average) value.

Extremistan is where we find those low-probability/high impact entities that Taleb calls Black Swans.  This parcel of intellectual real estate is characterized by wildly different characteristics from the norm, so that terms like "average" don't help much in understanding overall tendencies, and where much of the domain of interest is as interesting as watching paint dry, so that only rarely are truly spectacular results found.

Here's a good example of how these astronomical prizes are found: this image is taken in the direction of the heart of our galaxy, and two especially interesting objects pop up there, the Lagoon Nebula in the south (down) and the Trifid nebula to the north (up).  Note that there's lots of celestial real estate that's full of stars, but that the bright nebula are rarer in this view, even though this direction for our observation is about as rich an area as we'll find at this end of the universe.

And it turns out that this is a pretty good approximation for looking at the night sky.  If you get out under a good dark sky far from any city lights, and examine the heavens with a substantial pair of binoculars, you'll soon find that most of the sky is populated with just a bunch of relatively nondescript stars, but some of it has some more interesting features, e.g., where the Milky Way traces the shape of our own galaxy.  And if you use more powerful optical aid, then you'll soon learn that just pointing a big telescope at random generally leads to small fields of view that have some stars in them, but nothing all that interesting.

The key word in that last sentence is "generally".  Because if you know where to look, then amazing things can be found, e.g., galaxies, nebula, clusters and such.  These sights are rarer, and their signals do not lie in mediocristan, but live squarely in extremistan, because they run the gamut from small clusters of like-minded stars, to the remnants left over from supernova explosions, to stellar nurseries where new suns are being born, to clusters of galaxies that defy comprehension in their sheer scale.

But the problem is noise... because whether you're looking at these extremistanistic signals or taking pictures of them with expensive imaging equipment, there's noise found in the optical train, either in your own eye (which is notoriously noisy in the dark) or in your camera.  And this noise makes detection of the beautiful signals difficult or impossible.

Luckily for the astronomy geek, the laws of statistics can be used to fix this problem, so that we can use low-cost optical and camera equipment to gain high-value images of celestial objects.  The key is appreciating that noise lives in mediocristan, and in particular, it has a statistical distribution that has mean zero, and that is essentially Gaussian.   What this means to us is that if we take a whole bunch of images of the same object, and then average them, the signal persists while the noise dissipates.

And then we get better results than we deserve.

Here's an example, taken at 4 am last week, just as the winter constellation of Orion was rising right before sunrise.  This first image is a 30-second exposure at high gain from my Canon XS camera taken through an old-school Celestron Schmidt-Cassegrain telescope, and we can see that something interesting is going on here, but exactly what is not yet obvious.

The compound stellar system known as Trapezium is visible near the center of the nebula, and there are hints of interesting color appearing in the frame, but we really can't see much of anything in particular.  But if we crank up the gain in the image, more hints appear of structure and color, but unfortunately, they appear along with an awful lot of noise, as the next image shows.

This is an improvement in terms of seeing the bigger picture of this nebula, but we can do better by taking more exposures, and then adding them up and dividing by the number of exposures, so that the signal persists, but the noise tends towards its average value of zero.  Then we can easily see the structure of the Great Nebula in Orion emerge in the next image of this sequence:

So the key here is to stack up a sequence of not-so-interesting noisy images, so that we can extract a single relatively noise-free picture by taking advantage of the fact that what we seek is found in Extremistan, and what we wish to disappear lives in Mediocristan.  Thanks to that simple fact, we can readily keep what we want and lose what we don't.

And this statistical magic is not confined to big bright objects like the Orion Nebula.  Here's the same result applied to a dim object that barely registers in any individual frame.  This example here is NGC 891 (NGC = New Galactic Catalogue), an edge-on spiral galaxy at the edge of the constellation Andromeda.  The individual images barely register anything beyond a fairly-barren star field, even with a 30-second exposure at high gain through a good-sized telescope:

As always in astronomy, success lies in knowing just where to look, and in the center of this individual frame is a ghostly little needle-shaped apparition better known as NGC891.  If we crank up the gain in this image, we begin to see the shape of this galaxy, but we also see a whole lot of noise:

And if we stack a bunch of these images (in this case, 20 of them, for a total of 10 minutes of exposure), then we begin to get somewhere, and the shape of an entire galaxy emerges for our consideration:

This technique of getting better images than we deserve can be utilized on just about anything found up there, and planets are especially good targets for this imaging method.  In this case, we don't try to save up a lot of photons via a small set of long exposures, because solar system objects are pretty bright.  Instead, we take a ton of short exposures by making a movie of the object, and then working our averaging technique over hundreds or thousands of very-short-duration frames.

Here's an image of Jupiter taken last week.  Not much to see here beyond a Jovian disk suffering from a lot of noise and distortion introduced by the earth's atmosphere... 

But if we stack up about 500 of the clearest frames from a movie of Jupiter taken through the telescope's optical train, a much clearer picture of this fascinating planet emerges:

You gotta love that Great Red Spot, now that you can see it!

Armed with this technique, almost any modern telescope can be connected to the simplest digital single-lens-reflex camera, and the result is remarkable given the simplicity of the equipment involved.  And while it helps to have a good equatorial mount for tracking the stars, even the simplest clock-driven alt-azimuth mounts found on entry-level telescopes can be effectively utilized for astrophotography by taking account of the field rotation that occurs with time, so that the averaging process just requires a tad more alignment work in order to gain the best images.

So thanks to good optics and modern digital camera technology, you too can be an astrophotographer.  All it takes is knowing where in Extremistan the interesting objects are to be found, and then using the Mediocristan residence of the camera noise to improve the signal while getting rid of the noise.

--Cieran

Comments

[Maureen Lang]

Cieran,

Getting these image results from your back yard? I'm stunned! And a wonderful step-by-step explanation of how it was accomplished as well. You may have gone beyond simple astronomy geekdom here into the realm of divine astro-geekdom. Can't wait to enlarge the Orion Nebula, NGC891, & Jupiter pics on my Bravia.

Profound thanks for posting this.

[John Minnerath]

Nice series Cieran.
The sky above is full of wonders for those who will seek them out.
The advent of digital cameras and the processing programs for the images has opened up a whole new world of astrophotography, images not even possible by large observatories not many years ago.
Looking at your work tempts me to submit a series.
I had sent PL one image of a sunrise colored by volcanic aerosols, but I haven't sent in any others.
Maybe when my new camera gets here I'll submit a series of the Sun in Hydrogen Alpha, another new field for backyard astronomers. Or a series on Jupiter, or? :), there is just no end to the list of things to see and photograph in the sky around us.

[Cieran]

John:

You sound like someone who knows what he's doing! Hydrogen alpha images of the Sun? Please (!!!) consider showing off some of those! Give in to that temptation!

I'll be the first to admit that I don't know what I'm doing on this front. I know where the cool objects are to be found (a lifetime of visual observing does produce that side-effect), but this whole "connecting the camera to the telescope" topic is new to me, so if you have some expertise, it would be great to see the results.

[John Minnerath]

Cieran,

Observing that great fusion furnace of the Sun through an Ha scope and seeing in real time the incredible prominences and power of our "star" in action is a mind blowing experience.

Of interest only to those who may be interested in amateur astronomy, my main scope for the past 15 years has been a split ring JMI NGT 12.5 inch in my observatory.

I'm waiting on a DMK camera, a high resolution, specialty thing, that should let me produce some nice Solar and planetary images.
I never though I would leave film and join this digital age, :).

I'll put together a few images today of subjects that can in most cases be seen by anyone with the naked eye and submit them here for consideration.

[optimax]

Cieran,

Your photos are impressive and beautiful black swans swimming in space. I'm glad you know where to look and how to photograph. Not being that familiar with digital photography, I was wondering when you mention multiple exposures whether it is in the camera (same frame) or do you combine the exposures in photo shop, or what ever program you use?

Also admired your lightening photos. I miss the lightening storms that lit up the desert in Tucson, the Pacific Northwest has lots of rain but few lightening storms.

We usually do get a good view of the Perseid but this year was too overcast to see the sky. Last week I was sitting in my backyard on a clear night and saw a nice little fireball flash across the sky showing its smokey trail before it burned up.

[Cieran]

Optimax:

Good question! While I have heard tell of experienced astrophotographers superimposing a wide range of exposures (sometimes even from different telescopes!), I'm a complete neophyte, so I use a sequence of identical settings, e.g., same scope, same night (to help insure some level of consistency in the statistical distribution of noise, as it varies with camera temperature), same camera, same exposure settings, etc.

I'm sure as I figure out what I'm doing, I'll start elaborating, but for now, I choose to confuse one issue at a time!

Finally, some folks use photoshop or GIMP for this. I use a great program called "Nebulosity" to handle the nebula and such, and another program called "Registax" to handle the planetary images. Each has its own learning curve, and I'm glad to say that I'm learning (e.g., as of last week I was getting much better images of Jupiter than the one shown in the article).

Hope this helps!

[optimax]

Cieran,

Yes, it helps. Thank you. Now if I can only apply this to my Yashica D or Minolta SRT101, I can join the 21st century. Hope to see your new Jupiter pictures.

[John Minnerath]

Astrophotography soon gets really involved.
If you have a camera with a removable lens, then with parts Cieran mentioned above, you can attach directly to a telescope. Otherwise you can do Afocal photography or Piggy Back the camera to a scope.
For anything but the shortest exposures a driven mount is needed.
Then there's Polar Alignment, Guiding, Auto-guiding with a special camera also attached, plus computer, on and on.
Here is a good forum to browse through. Quite a bit of information about the basics and techniques involved, one of many dealing with the subject.
http://www.telescopejunkies.com/
One can go for the ultimate or do what I strive for, which is to get photos that are pleasing to the eye and that others will enjoy also. All the while trying to keep the cost of equipment within reason.

[Cieran]

John:

It's true that astrophotography can get involved very quickly, but there's still lots to do with relatively rudimentary equipment. A good polar alignment is pretty easy, and insures that the axes of the telescope's mount point in appropriate celestial directions. I am in the habit of doing that step first, and pretty soon, it becomes a routine process that is both easy and pleasant (pleasant because much of the effort involves just sitting for awhile under the stars).

Beyond that alignment and basic drive step, additional guiding isn't required for many interesting objects (e.g., planets), and while auto-guiding is really useful for long exposures (i.e., many minutes), all of the images I showed here use only the built-in and generally overmatched clock drive functions of my off-the-shelf telescopes with their stock mounts. So you can do a lot with very little.

Not that I wouldn't like more cool toys, of course!

p.s. I looked at your "Visual Observations" document, and it's GREAT! If you want me to convert it to some appropriate HTML with embedded images for distribution here instead of as a downloaded document, let me know.

[John Minnerath]

Cieran,
Sorry about my photo submission here. I made it with MS Works9, not knowing that odd ball program wasn't compatible with anything else in the known universe!
I've since got rid of it and installed MS Office.
I sent PL a version in MS Word, but I don't know if it went through or if he just hasn't had time to look at and replace the uncooperative version I originally sent.

Yes, astrophotography is great, and some cool stuff can be done with a minimum of equipment and effort.

[Charles I]

Those are fabulous. We have a telescope at the cottage but it never occurred to me to buy a digital slr and give it a go. But there's no computers allowed!

Thanks for sharing