Wednesday, September 26, 2012

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.

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