Since the first days I could observe the sun through a narrow band solar H-alpha filter, I was wondering what the tuning effect of such a filter would be. It does not matter whether the filter is pressure, tilt or temperature tuned. The effect of slightly shifting the filter central line is the same. With the temperature tuned filter, I have at hands, a comparable test series is easy to achieve.
I know from previous observations that the filter performs best at 58.0C. Therefore I set the test points at 5 degrees steps across the range of possible temperatures. Only close to the 58C I added 2.5C halve interval steps. Each individual image was recorded with a set of 100 frames, from which the 20 best frames were stacked. At each temperature I recorded 2 sets at different exposure settings: one for the surface (granulation, … – unfortunately no sunspots were active) and the other for prominences (only minor activity here as well).
As is obvious from the result, the low temperature settings yield close to no interesting view. Between 55.5C and 63.0C the filter delivers good contrast. Beginning at 63.0C contrast degrades again.
Imaging setup: Telescope: 102mm refractor Camera: QHY183M @ -20C Filter: Solar Spectrum – Solar Observer Series 1.5, 0.5A solar H-alpha filter
Again a series of images of the currently active regions of the sun. All images taken in the mid-morning of 2020-11-30. Unfortunately, these will be the last images for several days, as a persistent cloud system arrived that day…
The prolongued quiet phase of the transition from solar cycle 24 to 25 hasn’t have much to show off. But now, the sun has several sunspots as well as prominences to marvel on! It is really fascinating, that the sun burst into activity within only a few days.
I was lucky to get a brief period of cloud free late morning to image the sunspots 12785 (the spot to the right), 12786 (the large whale-shaped spot to the right, including the tiny spots left to it) and 12788 (the group of tiny spots to the south-east of 12786) in a 2-tile mosaic. On the limb, there is quite some activity too! See the 2 parts with prominences here:
On November 22 on the south-eastern limb of the sun, a stable prominence was visible. The prominence seems to consist of three different prominences (one flame type, one fan type and one arch type). They all seem to origin from sunspots 12785 and 12786. Whereas I assume, that the arch type prominence to the right is most likely from 12786.
The timelapse shows the activity of the prominences from 09:45-12:30 UTC. The clip has 342 individual images. Each image was stacked from 12 frames out of bursts of 300 frames. Post processing and colorization in PixInsight.
These 2 images from beginning and end of sequence show the beautiful structur as well as the changes within.
Imaging with narrow band H-alpha filters for solar imaging (prominences and chromosphere) requires the light beam to be almost parallel before entering the special interference filter called Etalon filter. This is achieved by i.e. telecentric systems, also extending the focal length by a factor of 2-4x. The sensor protection glass and anti-reflection glass of the camera create reflections with each other and the filter surface. Due to the parallel light beam, these reflections create interference patterns, noticable as so called Newton’s rings in the image. It depends on several different factors like sensor construction (micro lenses, …), exact angle of sensor in optical path, angle between sensor and filter / protection glass, … how strong the Newton’s rings influence the resulting images.
It is possible to reduce or eliminate this in post processing. But any minor shift in the imaging train will make it almost impossible to compensate with flat-field images. Fortunately, there is one alternative option: tilting the camera by a few degrees (usually up to 5 degrees), to widen the distance of the Newton’s rings, where they are no longer disturbing.
As these tilt adapters have a quite steep price tag, I constructed and printed one myself. I had to create several versions, until I had achieved a proper stability as well as stray light protection. But finally, I have a working tool 🙂 If you are interested in the design, you find the 3D files and description here: https://www.thingiverse.com/thing:4301757
See how much the tilt changes the resulting image! Hints to the images: – The blurry look in the image with Newton’s rings results from the alignment algorithm locking on the Newton’s rings instead of surface features – the adapter attached to the filter is a prototype without stray light protection. Hence a strip of black insulation tape was used for shade
This prominence occured right after the sunspot region 12579 vanished and rotated out of view. The video shows the development from 09:02:25-12:15:05 UTC+2. Video recorded with 715mm f/7 refractor with 4x telecentric system and SolarSpectrum 0.5A filter on ZWO120MM-S camera. Sequence consists of 490 images. Each image is a stack of 500 frames each. (For the complete sequence, 245.000 images have been processed)
I had the chance to borrow a Hydrogen alpha filter for solar observation. As the weather was just perfect, I set up my 80/600 refractor with the filter and a camera attached. Every 15 seconds I captured 400 frames for later processing. After 4 hours, my hard disk was loaded with 850GB of data…
Now, more than 2 days of stacking, aligning, optimizing, … are over. The resulting image sequence is simply fascinating and beautiful:
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