DIY Spectroheliograph Now Available

Christian Buil is a wonder. This Frenchman amateur astronomer has been a driving force behind a number of excellent amateur spectroscopy projects and products such as the instruments sold by Shelyak Instruments in France as well as new 3D printed projects like UVEX and software for spectroscopy like IRIS (image processing) and ISIS (spectral analysis). His web site at http://www.astrosurf.com/buil is a mecca for amateur astronomers interested in spectroscopy, the science of analysing the light of stars (including the Sun of course) through dispersing the light through a prism or grating to split it into it’s component colors.

Now Mr. Buil has released plans for a new spectroheliograph that also (with some minor component additions for guiding) doubles as a spectrograph for nigh time use. The results have been amazing! All images in this article courtesy of Mr. Buil’s website at http://www.astrosurf.com/solex.

Most of our members are familiar with viewing the Sun through a Hydrogen Alpha (Hα) filtered telescope to not only see sunspots like a normal solar filter but also to see surface features and prominences at the edge of the solar disk. A spectroheliograph is an instrument that allows you “tune” to a specific wavelength, say Hα, but also in Hβ, Ca-IIK, Mg-I, He-I, and pretty much any wavelength within the bounds of the instrument.

By “tuning” the spectroheliograph to varying wavelengths it’s possible to traverse the solar layers to get a 3D appreciation of features on the solar disk and how they interact (see below).

As well by taking images with specific wavelengths such as Hα, we can observe the actual motion of the features on the solar disk. For example, if a feature is moving away from us, it’s light will be slightly shifted towards the red end of the spectrum. If the feature is moving towards us the Hα line will be slightly blue shifted. By tuning the instrument slightly to the red, taking an image, shifting slightly blue and taking an image, we can construct a false color image like the one below which show the motion of the gas on the Sun.

The Sol’Ex is a fairly simple device with a number of 3D printed parts pairs with commercial camera to provide the images and specialized software to combine the images produced by the camera into a final solar image.

Spectroheliographs work by imaging the Sun through a slit, which means to take an image of the entire solar disk, it needs to be “scanned” in small slices, and the slices then need to be combined through software to produced a final image. Conveniently if the user aligns the slit to be perpendicular to the motion of the Sun in Right Ascension, by turning tracking off the Sun will pass across the slit of the spectroheliograph and product images.

Optically spectroheliographs are relatively simple – light is collected through a telescope or lens and is then collimated (aligned) by a pair of simple lenses, bounced off a grating that causes the light to be diffracted and “smeared out” into a spectrum, then is focused by another set of lenses onto a sensor. The tilt needed to reflect off the grating gives the devices a characteristic folded shape. The optical components of the Sol’Ex are widely available off the shelf or are available as a kit from Shelyak Instruments (https://www.shelyak.com/) as a kit

for about €400 or about $550CDN. The kit includes a reflecting slit, 2400 gr/mm diffraction gracing, and two doublet lenses. As well two lenses are included to expand the Sol’Ex into Star’Ex, which adds the ability to add a guide camera to the Sol’Ex for night time use as a spectrograph. Guiding for a slit spectrograph is critical to keep the star centered in the very narrow slit.

When observing the Sun through the Sol’Ex the result is a SER format file that contains a series of “slices” of the solar disk in the wavelength selected on the Sol’Ex. The software used to combine the slices was developed in Python by Valérie Desnoux (another luminary in the French spectroscopy world, author of the VSPEC spectral analysis software) which is free and easy to install and operate. Below is an image of the software displaying a single “slice” of the solar disk (the curve of the line is an artifact of the optics and is also corrected by the software). The software can be switched to English if your French is rusty!

Another functon of the INTI software is to remove distortions introduced by the Sun’s image being scanned at too high or low a frequency (producing a rugby ball shaped image) the slit not being aligned with the RA axis. These distortions (prior to correction) can be seen below.

A variety of cameras and imaging software can be used with the Sol’Ex and INTI provided they can produce the required SER image. A Sony IMX178 based camera such as the ZWO ASI178MM or QHY-5III-178 is optimal (the latter particularly due to it’s small size) as they offer the optimal sensor and small pixel size. However most cameras such as my own ASI183MM Pro will work fine but will have black edges where the image of the slit has not covered the entire sensor but these are ignored by the software. The Pelletier cooling offered by the ASI183MM Pro is particularly useful when using the Star’Ex configuration. Obviously as the observations are in a single wavelength (color) a color camera is not useful with Sol’Ex.

The telescope used should be between 200 and 1200mm focal length to get a reasonable disk size (under 200mm the solar disk would be too small to see much detail) where over 1200mm the image size would be very detailed but the exposure time would need to be reduced introducing noise into the images. An instrument of about 420-450mm is needed to capture the entire solar disk in a single “scan”. You can of course arrange multiple scans into a single image.

Mirror based telescopes are not recommended as their aperture is too large to attenuate the Sun’s light sufficiently, causing overheating that can destroy the device, so optimally a small refractor is needed. Obviously aperture directly increases resolution as it increases so the largest aperture possible is still desirable. However with larger apertures such as 100mm+ a neutral density filter is required. For example a filter for a camera lens can be used with some widely available 3D printed adapters.