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.

First light with new solar imaging setup!

This is a report from the 'first light' session with my new high resolution solar imaging setup!

During the bright summer months here in Denmark I stop work on deep sky or planetary imaging and switch to solar. I do H-alpha imaging with a Skynyx 2-2M camera, i.e. the 'lucky cam' technique with thousands of images.

In the past years I have been on a trend towards ever higher resolution. It started with increasing my focal length to get better sampling on a Coronado SM60 setup. Next, I increased the aperture to 100mm by acquiring a Daystar rear-mounted filter (back-to-back comparison with Coronado is here). This year I purchased a second hand 152mm F=900mm achromatic refractor to use with the Daystar. Normally (and safest!) a Daystar is used with a front mounted energy rejection filter (ERF), but on the Daystar Yahoo forum I heard that this could be replaced by a high quality UV/IR filter that is mounted internally. ERF's are VERY expensive for large apertures so this trick was really what inspired me to try working at six inches.

The scope is a Chinese made achromat mounted on a Takahashi EM-200 equatorial mount. The scope mechanical quality is very good for the price level and I have heard that the optics also are good - especially when the light will be filtered and a tele-extender is used with a small chip camera. Super-duper APO's with super-duper H-alpha filters are really a joke! I had made special adapters so that a Baader 2" UV/IR rejection filter could be placed just after the focuser. This is the highest quality filter and nothing less must be used for this kind of application out of safety concerns. The filter is mounted in a way so that it is not carrying any weight. This is important since a very long imaging train causing lots of torque is used. After some T2 spacer tubes I have a Baader TZ4 tele-extender, then comes the Daystar Quantum SE H-alpha filter. The spacer tubes places the TZ4 at the optimal position relative to the telescope focal plane while ensuring that the 3" Crayford focusser is not extended at all - thus minimizing problems with a sagging drawtube. After the Daystar comes a lightweight helical focusser from Borg and then the Skynyx 2-2M camera. The helical focusser enables me to only move the camera and not the entire long, huge and heavy imaging assembly. Everything is screwed securely together (no draw tubes or clamping rings) to minimize sagging. To achieve this is a true nightmare in adapters! Check out the photos of this setup below.

6" solar imaging setup on Takahashi EM200 mount. Note that dew cap is fully retracted to avoid possible tube currents.

Details of the assembly for high resolution solar imaging

After fiddling with all the spacer tubes and adapters in the imaging assembly I had time to try a few shots. I took two 90 second sequences consisting of ~1500 images of active region 11484. First the Daystar was at 6563.8Å and then I set it to the H-alpha wavelength of 6562.8Å. Images below are stacks of the 60 best images followed by some wavelet sharpening:

AR11484 @ 6563.8Å. Total length of this complex is ~120 arcsec.

AR11484 @ 6562.8Å a few minutes later.

I did not bother to remove traces of Newton rings. The images were taken two hours past midday over a low black roof, so the seeing should not have been to good. Finally, I have not checked the scope collimation after it arrived. Still, I estimate the resolution to be around 1 arcsecond - not bad for a first try. I can't wait to play more with this setup over the summer. Let's hope for clear weather during the Venus transit on June 6th!!

M100 processing - Part 5

I have now calibrated and rejected bad data, so the original 1064 image files have been reduced to 96 LRGB light frames. In this post I will reduce this further to just four - one for each filter. I use MaxIm for stacking with the 'Auto - Star Matching' alignment method. I have previously found that bi-linear interpolation produces slightly sharper - but also more noisy - images than bi-cubic interpolation. On the RGB data I'll go for bi-cubic since resolution does not matter so much here. On the luminance data let's see what can be gained by bi-linear/bi-cubic and all/best half stacking:

FWHM of stacked luminance frames:

                         Bi-cubic:             Bi-linear:
All frames:         3.53"                  3.45"
Best half:           3.41"                  3.31"

By 'best half' I mean using only the sharpest 50% as defined by the image FWHM value. The FWHM can be reduced ~6% by using bi-linear and 'best half' compared to using all images and bi-cubic interpolation. However, the SNR of faint regions of M100 is cut in half by doing so and I think this is too high a price to pay. Instead, I’d rather have high SNR and then later try my luck with deconvolution which really demands low noise data. See for yourself below - there isn't much difference visible between the two results!

Enlarged sections of stacked luminance frames. There is no significant difference visible, but the computer tells me that SNR is better on the bi-cubic/all image.

For aligning  images from various filters I use a common reference image from the luminance stack. This reference is of course not included in the stacking process, but it ensures that the resulting, stacked LRGB images are aligned to each other.

Next step will be to experiment with deconvolution on the luminance image, so stay tuned!

EXTRA NOTE ABOUT IMAGE COMBINING IN MAXIM:

I should mention a problem with MaxIm I encountered while combining the luminance data. I have two sets of sub-exposures - one from March 25th and another from March 27th. By mistake I used a different guide star on these two nights and as a consequence the two image series are pretty severely misaligned:

Two sub-exposures that are severely misaligned.

Still, the data from both nights is good so I'll go ahead with alignment and combining. Note that the two nights produced a different background level - one the first I had ADU=1700 while on the second I got ADU=2300. In MaxIm I use the 'Auto - star matching' alignment mode which works very well, then combine using 'Sigma-clip' combine method and 'delta-level' normalization. The result is shown below.

Problem: combined image has a large offset where the sub-exposures fail to overlap.

I spent several days pondering this problem without success. Only when writing this blog post did the correct line of thought come into place - and with that, a solution! I think the problem arises in two steps. First, during alignment, MaxIm sets pixel values outside the original field of view to zero (other programs often chose to use a median edge value). On my images this will result on a lot of zeroes, due to the large misalignment between the two nights. Next, during image combination, this zero-value creates an offset on the combined result, as shown above. The solution is very simple: just activate the 'Ignore black pixels' option on the 'Combine' tab. As shown below this fixes the problem. Of course the background still exhibits a discontinuity in noise level where the two dataseries fail to overlap but this is quite natural and easy to handle later on.

Problem solved: use 'ignore black pixels' option!