Technical Background

Images of the Sun in Hydrogen Alpha by Michael Caligiuri

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The Sun's Photosphere and Chromosphere

Photosphere: This is the 100km thick inner layer of the solar surface where light, escaping from the Sun's shell, radiates. Images of the photosphere reveal features such as sun spots, spicules, filaments, ribbons, and grannules. Filaments can form as ribbons across the solar disk and rise up through the chromosphere. To learn more about the solar photosphere, click here.

Chromosphere: This is the second of three main layers of the Sun's atmosphere, roughly 2,000 kilometers thick. The chromosphere sits just above the photosphere where coronal ejections, prominences and solar flares can be observed.  Prominences are filamentary clouds and arches of hydrogen plasma suspended above the photosphere by strong variable magnetic fields.  Unlike the spinning iron core of the Earth which generates our protective magnetic field, the sun's magnetic field is generated by the flow of hot ionized gases in the sun's convection zone.  This "flow" produces electrical currents which act as a magnetic gyro.  Many prominences remain captured by these random  magnetic fields for days.  A coronal mass ejection (CME) is an unusually large release of plasma and magnetic fields from the chromosphere. For more Information on the solar chromosphere, click here.

About the Images

Equipment: Images of the sun's photosphere and chromosphere were captured using a Lunt 100mm solar telescope, mostly from the backyard. These scopes are equipped with a hydrogen-alpha filter, passing light with a wavelength centered on 656.28 nm having a bandwidth of 0.7A. This Lunt 100 telescope is equipped with a DSII double-stack etalon module, narrowing the bandwidth even further  from 0.7A to 0.5A.  This increases the details of surface (photospheric) features. Harmful emissions are further blocked by an 18mm blocking filter. A high resolution video camera is used to record dynamic changes in the layers of the sun's photosphere. The current configuration uses an FLIR Grashopper (USB3) camera instrumented with an Sony IMX174 monochrome CMOS chip. Attaching a Tele Vue Powermate 2.5x barlow between the blocking filter and camera increases resolution even further with an effective focal length of 1785 mm.  All images, with the exception of full disk captures, have an image scale of 0.66"/pixel.

Solar H-alpha telescopes are equipped with etalons for tuning the centroid of the narrow band of light they pass. Tuning the etalon changes the pressure within a sealed chamber.  Changing pressure alters the refraction index of the light waves passing through the chamber.  Controlling the refraction index allows the sun's broad spectrum light to be "tuned" around 656.3 nanometers to reveal sunspots, filaments, prominences and plasma ejections. The current configuration has two etalon tuners to sharpen the Ha zone even further.

Data Acquisition: Video images are acquired using frame rates ranging from 42/sec (full frame) to over 150 f/s (for selected regions of interest). Generally, I'll collect 3000-5000 frames per file. A 42 f/s 3000 frame exposure takes 68 seconds. Typical imaging sessions involve collecting 10-15 video files from different regions of the sun over a 1 hr period of time. Conditions and even features can change noticeably over an hour or two, so multiple files are created to capture these changes. With files approximating 6-8 GB in size, file management and image back-up can be a challenge. Currently using FireCapture v2.4 for data acquisition.

Processing Workflow: Video files are automatically analyzed to select the sharpest subset (usually 10-15%) of frames. These frames are then registered using several hundred alignment points and stacked to create a single 16-bit image file. Combining many single frames improves S/N.  Stacking is done using AutoStakkert 3.0. The image then undergoes sharpening and further processing such as inverting, colorizing, more sharpening, and noise reduction in Photoshop CS.