Eclipse 2012 - Corona photography project

Imaging setup - Borg 100ED refractor,
Nikon D300 camera, Astrotrac TT320
equatorial mount and Induro carbon fiber
tripod. (click for larger view)

This is the second part of my blog essay about the total solar eclipse of 2012. I'll be posting frequent updates on preparations and - once the journey starts - how our progress is going towards the ultimate goal of witnessing this spectacular event.

Besides being a hybrid tour guide / tourist I'll also be an astrophotographer. My project is to attempt medium-high resolution imaging of the inner Solar corona. To this end I'll be imaging with a plate scale of 2.4 arcseconds/pixel using a DSLR camera with a four inch f/6.4 refractor on an equatorial mount. These days most of my spare time goes into preparing this setup for action.

The plate scale, i.e. number of arcseconds pr. pixel is NOT the actual resolution of the final image. This can only be measured on real images and it is affected by many factors, such as optical imperfections, mechanical vibrations, tracking errors, atmospheric turbulence, etc. If all goes really well I'm hoping that the resolution of my setup will be around 3 pixels = 7 arcseconds. This isn't high resolution compared to planetary or deep sky imaging, but it is in the upper realm of corona imaging. Space based observatories are the only routine means for observations of the corona and the best of these (SOHO's LASCO instrument) have a resolution of only 20 arcseconds. The very best ground based images can reach 3-4 arcseconds but this requires equipment and skills that I do not have!

Simulated field of view of my setup.
(click for larger version)

In the next blog posts I will discuss the ongoing preparations, how friends are helping and field testing under the skies.

Eclipse 2012 - Introduction

Totality in Turkey - 2006.

The Moon will cover the Sun (completely!) on November 13-14 as seen from Australia - and I'm going there to see it! I will be working for a Danish travel agency called Viktors Farmor (english translation: Victors Grandmother) as I have done several times before. Viktors Farmor arranges journeys worldwide and has total solar eclipses as a part of their repertoire. I have provided astronomical guidance during their trips to Turkey, Siberia, China and India. This time Viktors Farmor has planned what in my opinion is the most ambitious journey yet and this time I'll try to blog about the whole experience.

Besides general work as an assistant tour guide my job includes providing astronomy themed talks, guided tours of the Southern sky and planning activities related to the eclipse event. I started preparations over a year ago by reading up on Australian and Aboriginal astronomy literature. In recent months I have ramped up preparations of equipment that will come along for the entertainment of our guests and for photographing the eclipse.

In recent weeks the pace has quickened and I am now spending all of my spare time preparing - mostly equipment and talks.

In the coming weeks until departure day (November 1st) most of this blog will deal with technical aspects of the equipment. During the journey I'll post daily updates on our progress and weather outlook. Stay tuned, because this will be quite an adventure!

Tri-band solar imaging - part 6

In previous blog postings I have presented the tri-band Solar imaging technique and discussed related acquisition and processing details while showing a number of example images. So, what can go wrong when doing tri-band imaging? In this blog posting I'll be presenting some of the main problems I have experienced so far.

Tri-band image with no flat field correction

1. Flat fielding is always non-trivial, especially when working at high f-numbers where the shadows of dust particles become smaller and denser. With narrowband solar filters flat fielding by means of the usual sky-flat or lamp sources does not work. When just colorizing single-band H-alpha images there is so much activity across the image that a few dust shadows don't draw very much notice. With tri-band images, these dust artifacts easily become color artifacts which show up more readily on our mental radar. So, take the time to experiment with establising a flat method that works well on your setup. I have used defocussed images of spot-free regions near the central solar disc with reasonable success. Especially with maximum detuning is the need for a good flat field evident since the solar landscape here is more bland than closer to the H-alpha wavelength.

Flat frame with vignetting, dust shadows and Newton rings

2. Newton rings are a particularly nasty problem associated with an extremely narrow filter bandpass. They are caused by interference between reflections inside the optical system and are usually of quite low contrast (typically less than 1% of the total signal). Hence, Newton rings are most evident when the solar filter is detuned to reveal more of the photosphere. The problem is reduced when working with webcam-like imaging devices where drifting solar features are tracked across many frames, hence the rings are often partially averaged out. On my images Newton rings appear as curved bands which easily move across the field of view over time. This mobility is their main curse, since it compromises the ability of a normal flat field to deal with the problem. For the better part of two years I have tried all sorts of acquisition and mathematical tricks to control them without much success. In the end I started doing it manually in photoshop by drawing dark lines in a seperate layer on top of the Newton rings, then blurring and using them as a mask to selectively brighten the regions affected.

Full disc tri-band view, showing effects of filter
non-uniformity (AR1520, July 12 - 2012)

3. Filter non-uniformity exists to some degree for all narrowband filters across their field of view and is in fact one of the main factors determining the filter price tag. The effects can be mildly irritating when working with a single-band image. With tri-band images I experience it much more severely since it causes unwanted color gradients. In fact, with my Coronado SM60 H-alpha filter (a high quality unit made by David Lunt) this problem effectively prevents me from creating decent, full disc tri-band images. See an illustration of the problem at right. At higher magnifications this effect becomes less of a problem since you then more easily can apply the 'background flattening' technique I mentioned earlier.

4. Time: remember to hurry when acquiring the tri-band images! Color artifacts will arise if significant movement has time to occur between frames. Staying within 20-100 seconds should be OK for normal situations, but with very high magnification and/or very fast moving features the restrictions can be much more severe. If you get an X-class flare exploding into your face (like this one!) don't even think about try to get it in tri-band! Micro-flares are also very fast and usually not possible to capture properly - I just accept them as colorful little dots here and there.

Rapid development of micro-flares results in funny colors on tri-band images.

In the next blog posting about tri-band solar imaging I will present a bundle of shots of what I have achieved so far.

Tri-band solar imaging - part 5

This time I'll discuss the kind of image processing involved in the tri-band Solar images previously shown here on this blog.

Frame selection, alignment and stacking (AviStack)

After data aqcuisition you'll have three data sets; one for zero detuning, medium detuning and maximum detuning of the filter. Before these are combined into a tri-band composite the image processing is the same as you would normally do. If you made a movie you’ll select, align and stack the sharpest images with software such as AviStack or Registax. You might also perform flat field correction. Finally, the images may be sharpened and perhaps also corrected for solar limb darkening. You can choose whatever you want to do – the only important issue is that you do the same for all images prior to making the tri-band composite, or else strange color artifacts may appear in the final image. I won't go into further detail here about these steps because they are not particular to tri-band imaging.

Wavelet sharpening (Registax)

Background flattening (MaxIm)

On close-up images, like the one in the example above, I often 'flatten the background' - i.e. adjust the brightness of non-active regions to a uniform brightness level. This deals with any uncorrected vignetting and removes solar limb darkening effects. Background flattening ensures that the maximum dynamic range is used for displaying details of the chromosphere and active regions therein.
I use Photoshop for the remaining process steps where the tri-band image is created. First, all three images are opened and then merged as red, green, blue channels in a color image. Be sure to maintain the bit depths of the original images - I typically work with 16 bits. You can assign the images in whatever way you prefer; I have mostly chosen to use the most detuned image for the red channel, the medium detuned as green and the H-alpha channel as blue. This makes 'energetic' features blue/green or white and more quiescent regions red/yellow. There is nothing 'correct' in this, just do whatever you prefer!

Alignment of RGB levels.

Next, use the move tool to align the RGB channels to each other, while using some distinct feature such as a sunspot for reference. You don't have to do this perfectly, it's always possible to tweak the alignment later on.

Levels/curves details
(click for large version)

The following steps consist of several levels/curve adjustments with the purpose of increasing brightness/contrast and achieving a suitable color balance. All of these steps are really subjective - click the image at right to see details of the method a have chosen to follow for many of my images.

To illustrate that the results can be very different according to taste, look at the image below and see what happens if the hue is adjusted differently!

That's really all there is to it. If you are skilled doing regular narrowband Solar imaging the extra steps involved in tri-band imaging should be no big deal. However, there are limitations and challenges - in the next part of this blog I'll discuss some of these!

The colors of a tri-band image are as much subject to personal taste as regular narrowband images. Above, I  made a major hue adjustment to achieve a completely different look! (click for large version)

Tri-band solar imaging - part 4

Solar features can evolve rapidly and hence the tri-band acquisition process must be completed before these changes become significant. If you don't hurry, 'false' colors will arise that reflect movements of gas rather than changing physical conditions on the Sun.

For high resolution imaging with a webcam or similar device the quest for getting many images to catch moments of good seeing must be weighed against this requirement. Also, the more you zoom in on the Sun and the more active a feature you are targeting, the shorter the available time will be.

It is not possible to pin down a firm limit, but with my targets, preferences and setup (Coronado
SM60, f=1280mm, Skynyx 2-2M camera) I find the total available time to range from 1-5 minutes. Hence, if images are to be acquired for three wavelengths, the available time pr. wavelength is reduced to roughly 20-100 seconds. With my camera the acquisition rate is around 10-25 frames pr. second, hence each series consists of roughly 250-2500 images. Since Solar features change more rapidly in the chromosphere than in the photosphere, the time restrictions are least severe when the filter is detuned the most from the H-α wavelength. Consequently, images with the largest detuning from H-α ‘last longer’ for tri-band work.

Many filters are tuned mechanically in seconds, by tilting or by using a pressure regulator. A fast tuning mechanism is obviously best suited for tri-band imaging. Other filters are thermally tuned which is a slow process that requires several minutes to reach equilibrium between each wavelength change. With such a system, one could opt for ‘dual-band’ imaging where only two images are acquired for the red and blue channels, respectively– one at the H-α wavelength and another at maximum detuning. A green channel can then be synthesized as an average of the two other channels. An example is shown below. For this image a Daystar Quantum SE 0.5Å filter was used with 6562.8Å for the blue channel and 6563.8Å for the red channel. The result is not as rich in color variations, but I still consider it worth the effort.

Dual-band image of AR1195 from April 25, 2011. Daystar Quantum SE 0.5Å H-alpha filter.

Below is another shot - this time a real tri-band image - of AR 1087 on July 10, 2010. This was acquired using a Coronado filter that was detuned by tilting. The colors are obviously better.

Tri-band image of AR 1087 from July 10, 2010. Coronado SM60 filter.

Tomorrow I will discuss the processing flow I use for making these images. A lot of it is standard stuff for Solar work, and the tri-band part is actually quite simple.

Tri-band solar imaging - part 3

In the previous blog posting I posted a question: how will spicules and prominences appear at the solar limb? I am using a color scheme that arranges the most detuned image as red, medium detuned as green and not-detuned as blue. When seen in silhouette against the photosphere spicules and filaments are revealed by their absorption of H-α light. They look dark in H-α images and since this is blue in my color scheme they will appear reddish. When seen against the blackness of space these same features are revealed by their emission of H-α light - and appear bright in the blue (H-α) channel. Check it out for yourself below!

Active region 1193 on April 22 -2011, with many internal color variations. The chromospheric limb appears bright blue since here we see H-alpha emission which I choose to map into the blue color channel. When seen against the photosphere the same features are revealed by their H-alpha absorption and thus appear reddish. Click image for full size view.

Now, take a shot and guess how a flare will appear in a tri-band image! I will show a picture of one later.

I had a hard day at work and blogged too late into the night yesterday, so you'll have to wait until tomorrow for the next posting where I will discuss acquisition aspects of tri-band imaging.

Tri-band solar imaging - part 2

Detuning by tilting - Coronado's T-max tuner

The Sun provides so much radiation that a tiny wavelength interval can be selected to enable viewing of certain layers of the solar atmosphere or certain elements – and still have plenty of light for fast exposures that freeze the seeing. Most notable is the hydrogen alpha (H-α) transition at a wavelength of 6562.8Å. Emission at this wavelength is a sign that hydrogen is being ionized and it comes predominately from the solar chromosphere. Narrowband solar filters can be detuned slightly to maximize contrast or to allow observation of Doppler shifted light from features that are moving rapidly along our line of sight. The tuning range is typically very limited – around one angstrom – but even such small changes lead to dramatic differences in the resulting image. I use a Coronado SM60  H-α  filter that can be tilted slightly using  the so-called T-max tuner; see the photo above.

Rates of change in brightness of various features with filter detuning and how they relate to colors in a tri-band image
(click for bigger version)

The figure above shows a typical scene, captured with this filter at three different wavelengths. The targeted object is active region 11087, which consisted of a small sunspot 20” across and a complex magnetic field which produced several low-level flares during July 2010. At the H-α wavelength of 6562.8Å (corresponding to zero tilt), the field is covered with spicules and contains two dark filaments and a bright region of plage. In all this action the small sunspot almost gets lost, with only the umbra standing out clearly as a dark, irregular spot. At 6562.3Å (half tilt) the contrast of the spicules, filaments and plage decreases. At 6561.9Å (maximum tilt) these features are nearly gone so that the sunspot with penumbra becomes clearly visible. A smaller spot, unnoticed before, becomes apparent towards the upper left.

As the wavelength is tuned away from the H-α transition we are viewing more and more light from the underlying photosphere. What is especially interesting in this regard is that the rate of change in brightness with wavelength is different for various features, depending on what they are. It is not the same everywhere. Hence, if detuned images are used as separate color channels in a composite image various colors will appear that represent different rates of change. This is evident in the tri-band image above where the R, G, B channels are chosen as most detuned, medium detuned and not-detuned, respectively, from the H-α wavelength. In such a color scheme the underlying photosphere appears yellowish, spicules and filaments are reddish, plague is white, etc.

Tri-band image of AR1087 in a quite moment. This active region had a beta-gamma magnetic field and harbored multiple M-level flares during July 2010. Click to enlarge.

Many more colors can be found in this way on the Sun and I'll be showing more later on. For example: how do you think the chromosphere and prominences at the solar limb will appear in a tri-band image as the one above? See for yourself in tomorrows blog posting!