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!

Tri-band solar imaging - part 1

In this series of blog postings I will present a method for creating colorful solar images using a narrowband filter. In short, the idea is to tune the filter and acquire three images at different wavelengths. A colorful scene can be created by assigning each image to a different color channel of an RGB image. The resulting color image reveals real spectral changes which
are equivalent to changing physical conditions on the Sun. I call this technique ‘tri-band solar

imaging’.

Tri-band versus single-band H-alpha image (click for bigger view)

An illustration of the resulting view and how it compares to 'normal' colorization of a narrowband H-alpha image is given to the right. The tri-band version displays much more vivid and varying color details and represents sunspot penumbras better than the single-band, on resonance H-alpha image. All you need to make such images is a standard, tunable H-alpha filter and a bit of extra processing work.


Stay tuned to this blog since I'll be posting daily about tri-band solar imaging with the following headlines:

• Background
• A basic example
• Acquisition techniques
• Image processing
• Tri-band pitfalls
• Some colorful examples

I hope that others will find these presentations inspiring enough to try some of their own tri-band work. Please share any thoughts, comments and pictures!

Full disc Venus transit

The Venus transit of June 6th continues to unfold - on my harddisc. I still have lots of unprocessed data to play with and yesterday I did just that. The resulting image is my first of the event featuring a full Solar disc view (full res version HERE).

Venus transit in H-alpha light (Coronado SM60 filter, 70mm ED refractor)
- click for bigger view

The view is a composite of two images made out of 600 frame sequences; one for the solar disc with  2 msec exposure time and another with 27 msec exposure time for the prominences. Acquisition was done at 3:17 UTC when the altitude of the Sun was just 3.8 degrees. When processing the image it was quite evident that the solar disc was flattened significantly by atmospheric refraction. Still the view is not too fuzzy - it helped that I was WAY down at the beach front overlooking the straight between Denmark and Sweden.

Initially, I had set up a bit farther from the water front but as more and more bystanders gathered and flocked around - and in front of - the scopes you could forget about seeing anything. Failing to get anyone's attention to move out of the way I desperately grabbed the entire setup with clock drive running and carried it down to the waterfront. If someone wanted to get in front of my scope here they'd have to get their feet wet. On the picture below you can see me sitting at a table working the computer but the scope I used is completely blocked from view by people! But, hey, lots of bystanders - and not lots of clouds - is something to be happy about!

Imaging at the waterfront - surrounded by an enthusiastic crowd.
Image copyright: Palle Sonne. 

BIG prominence on July 25-27, 2012

Large prominence, July 25 - 2012 (click for bigger view)

We are finally having some great summer weather in Denmark and I'm observing the Sun from our countryside house. This morning a pleasant surprise greeted me: one BIG prominence was hoovering above the Solar limb. Life is good!

(150mm refractor, Daystar H-alpha filter, Skynyx 2-2M camera)

Reprocessed version (click for bigger view)

UPDATE #1: I reprocessed the original image with an emphasis solely on the prominence and keeping the image monochrome. This enables a more dramatic representation of the prominence at the expense of its relationship with the chromo- and photosphere. Still - I like this image more.

The prominence on July 26th - it just got bigger!  (click for bigger view) 

UPDATE #2: On the following day, July 26th, the 'big' prominence just got bigger! It has also changed morphology quite a lot; the internal structure going from 'whispy' to 'clumpy' and the overall shape becoming more 'bridge-like'. Yesterday, the brightest portions were elevated above the Solar limb - today it has assumed the (more common) structure where the brightest portions are also the lowest. The picture to the right was acquired around 10.30am local time, where the Sun had attained a decent altitude (42 deg.) but before the atmospheric turbulence really got started. I tried again at 2pm (Solar altitude 53 deg) and the resulting images were less sharp. This is to be expected, since the image quality for high res Solar work is very much limited by thermally driven, atmospheric turbulence. What a beast! I can't wait to see what it has evolved into tomorrow.

July 27: Prominence has now diminished but has been joined by two others (click for a really big view).

UPDATE #3: On July 27th (the 16th marriage anniversary to Signe, my wonderful wife) the prominence had diminished in size. As a wonderful compensation for this it had been joined by two other impressive prominences. Each of these three were distinctly different: one was dense and bright while good ol' faithful was towering and clumpy, like yesterday. The last one was the most impressive: a very dynamic, stringy structure with a detached upper part. What a show! To capture the entire scene I had to stitch together two shots. Denmark is now due to experience a shift in weather, reverting to more cloudy and unstable conditions. The Solar imaging setup is packed up and ready to come back to Copenhagen next week when my vacation is over.

Here's what the solar imaging setup looks like - in action and packed up in our small VW.

Travellers solar setup

Today I practiced using the small solar H-alpha setup I will be using for our upcoming trip to Mallorca and - later on - to Australia for the total solar eclipse of 2012. I also used this setup for shooting full disc views of the recent Venus transit and that went well. It features a Coronado SM60/BF10 0.7Å H-alpha filter, f=420mm ED refractor on an AstroTrac mount.

This time I wanted to have a go at some more resolution, so I plugged in a 4x powermate. The optical resolution of the 60mm aperture H-alpha filter is around 2.3". The effective focal length of my setup with a 4x powermate is around 1700mm; hence the image scale on the Skynyx 2-2M camera is 0.5"/pixel. I will thus be oversampling the optical resolution by a factor ~4 at 1x1 binning. If needed I can use 2x2 binning and oversample by a factor ~2. I should therefore be able to get close to the maximum possible resolution with this aperture.

Travellers solar H-alpha imaging setup

Setting up is quite fast since polar alignment isn't really that important for this kind of work. In addition, everything is light and small - unlike my other setup which uses a Tak EM200 mount and a 6" refractor. In order to reach focus a 2" diagonal has to be inserted before the rest of the imaging train. The whole arrangement pressed the Crayford focuser and mount pretty hard, but I could still focus and navigate around the solar disc. I am using the Baader Stronghold for holding the scope and enabling fine-tuning of pointing. It was not the completely smooth, solid experience you get with the EM200 mount - but that thing isn't about to sneak on as carry-on luggage anytime soon!

The Sun was pretty quite today and it was late in the afternoon, but I still shot some 1500-image sequences of  a pair of prominences. Stacked best 60 frames using AviStack, wavelet filtered using Registax, and colorized in Photoshop - see results below.

Best 60 out of 1500 frames; 20msec exposure time with gain=3

Best 60 out of 1500 frames; 5msec exposure time with gain=7

Now I just need to foam pad a small carry-on airline suitcase and I am ready for take-off to Mallorca next week!

Venus transit 2012

Had a fantastic transit experience at Charlottenlund Fort, Copenhagen! Will be posting some pictures here as they are processed - stay tuned. Become a follower of this blog to get updates automatically.

Sun had just cleared clouds at horizon.

I'm at work right now and better get back to it!!
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Home from work, get Astrid in kindergarten, go to playground, make dinner, wash floor. I am pretty tired now - I only got two hours of sleep in the past 36 hours. I just got to try and process some data from my new solar telescope. I only got time to try it ONCE before the Venus transit. Just did some basic processing; see result below (click here for full resolution). I am HAPPY! Will play more tomorrow, but now I'll go to sleep with a smile.

Venus with a small protuberance - just before the show ended.

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Had a terrible headache yesterday, which I often do after going all-out for some crazy event. Today I feel fine and I tried to see if the refraction of light in Venus' atmosphere was captured in some of my images. The setup I am using with a H-alpha filter is not optimal for this purpose - it transmits a very small fraction of the Sun's light, forcing me to use long exposure times which in turn will be more susceptible to blurring from our own atmosphere. I think that a white light filter would have been better. Anyway, between third and fourth contact I tried shooting at several (longer) exposure times:

Changing the exposure time to catch a glimpse of sunlight refracted in the atmosphere of Venus

Looking closely at the image with the longest exposure time I can just barely make out a faint arc of light that follows Venus' limb! It is visible for roughly 30 degrees across the north pole of the planet. I tried manually selecting the best images in the sequence, then stacking and tweaking curves. I also applied a DDP filter and unsharp mask to enhance the arc further - see the result below.

Processed image revealing sunlight refracted in the atmosphere of Venus.

This sighting is nowhere near the quality obtained by others in 2004 - I think this is due to the low altitude of Venus as seen from this part of the world. For the ultimate view of this effect check out this image from the Hinode spacecraft!
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Working through all the images I took on June 6th I have now completed basic processing of all the image sequences - 27GB in total, consisting of 24410 individual images. 'Basic processing' means using Registax v.5 to sort, align and stack with a focus on the disc of Venus. The Venusian disc will then be sharply defined, but orbital motion of Venus will cause a slight blurring of the Sun. I therefore do a second processing run using AviStack v.2 and focussing on the solar surface while ignoring Venus. I typically stack 60 images since this (in my experience) is a good compromise between using only the sharpest images and keeping the noise level down in the final result. Each stacked image is sharpened using wavelets, then the two are combined in Photoshop to get both Venus and the solar surface sharply defined. Final touches include colorizing the image and tweaking of levels and curves. My latest result is shown below - taken at precisely the moment of third contact (full resolution is HERE). The resolution, contrast and colors are better than on the previous close-up view shown above - maybe because the first one was made in a zombified state of fatigue..!?

Venus transit - moment of third contact.