The project: from conception to observations
Introduction
Spectroheliography is the technique consisting in acquiring an image of the sun by scanning the sun with a spectrograph.
The idea is not new, it was conceptualized and got its first realization in the early XXth century.
The advent of fast sensor (CMOS) and cheap-yet-good optics has recently allowed amateur astronomers to develop
their own models of spectroheliographs (SHG), to be used on small installations.
In this page I am presenting the evolution of my own SHG project, that is to some extent an inflated version of the now
famous Sol'Ex invented by Christian Buil and Valerie Desnoux. See here for more informations about the Sol'Ex : http://www.astrosurf.com/solex/.
The project: from conception to observations
After having built and used a Sol'Ex in 2022, I started thinking about building a new SHG which would allow to go further in both spectral and spatial resolutions.
- The spatial resolution is largely correlated with the focal length of the telescope. A longer focal length helps in gaining in resolution simply by improving the sampling at the
level of the sensor (number of arcsec/pixel). But a longer focal length also implies a larger sun at the focus point, and therefore unless you are ok with doing sun mosaics
(which I'm not), a longer slit! This subsequently means also SHG optics supporting larger images.
- The spectral resolution depends on many factors, but in particular the focal lengths of the SHG optics
All in all such a SHG requires longer FL optics for the SHG, typically between 100 and 200 mm (or even more), larger (and so heavier) optics,
a sufficiently large diffraction grating (30x30 or 50x50 mm^2).
The other main constrain I had is that I am not equipped at all to do precision mechanics with metal, and in general my ability to build things out of the blue in metal and wood is not to be trusted at all.
But there is one thing that I can do: building 3D model for 3D printing. I'm a regular user of OpenScad, and with that tool I can do design and send to print basically anything whose size does not exceed certain limits defined by my 3D printer.
The problem in this case is 3D prints are made of plastic, and although there are quite rigid plastics, there are limits in terms of mechanical resistance that can lead to problems if the load is too high.
I came up with the following idea: the collimating and camera lens of the SHG should be hold in place by some very simple metal parts I can order to someone well equipped.
This part should also be the one held by the telescope. And attached to that part, the 3D print that would only hold the diffraction grating which is relatively light.
Choice of the optics
- The slit: this was by far the most critical part: long (>10 mm) and thin (<10 micron wide) slits are hard to find. After some discussion on a french forum I got in contact with someone who could provide such slits! More precisely: 12mm-long and 9 micron-wide in a chrome deposited on high-quality quartz.
The enormous advantage of that choice of material is very good resistance to high temperature, which will be helpful as discussed later.
- The optics: vintage telelenses from Pentax Asahi: I took several of them in 150 and 200 mm FL. Even in good condition they are cheap (between 50 and 100 dollar), simply because nobody want anymore optics to be mounted on M42x1 thread!
SHG configuration
Once the slit and the optics are chosen, there is one factor that affects the spectral resolution of the SHG: the angle between the two optics.
An angle of zero degrees is the littrow configuration, to be forgotten for SHG "imaging" that is very intolerant to off-axis aberrations. Large angles (>40 or 50 degrees) leads to large resolution but also luminosity loss due to the grating orientation w.r.t. the optics.
I finally decided to stick with the configuration used for the Sol'Ex: an angle of 34 degrees, which represent a very good compromise between spatial resolution, luminosity,...
Building
I first did a mock-up of the whole SHG in order to estimate the right dimensions of the metal part. Hoping it was not too incorrect I did order it to a french company https://astromecca.fr/,
and only when I received it I started to print the plastic part. Needless to say it took a lot of time and effort.. and a lot of printing time, to reach the point where the proof-of-concept could be used behind a telescope.
TBC
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