Spectroheliographs at The King's University Observatory

Solar Explorer - SolEx

SolEx - (Buil et al described here) is an outstanding instrument that has been used since summer 2022 at TKUO#3. It is very light weight and can be assembled from a small kit of parts.



The SolEx proved so successful that during the winter of 2023 I designed and constructed a larger version spectroheliograph - SHG V2 (shown below). My design drew heavily on instruments built by Douglas Smith and Ken Harrison who also provided valuable advice during the construction phase. All of the parts for SHG V2 were printed on a low end 3D printer (Ender 3) in both PETG and PLA. A document describing the build which includes FreeCad files is available here.

There are many significant differences between SHG V2 and the smaller SolEx:

  • SHG V2 is substantially larger and heavier than SolEx (1800 g vs 700 g). This is illustrated in Figure 2
  • A larger diffraction grating (2400 l/mm, 30 mm x 30mm)
  • Slit is a fused quartz 12mm long by 9 micron slit from Douglas Smith
  • Collimator and imaging lenses are commercial lenses - (150 mm, f/4 Pentax Super Takumar from KEH camera)
  • The collimator assembly "floats" freely between a pair of mounting plates that attach the SHG body to the telescope mounting flange. This enables easy adjustment of the collimator/imaging lens while on the telescope to achieve proper collimation
  • Camera is mounted with a low profile helicoid focuser to enable precise camera focus.


SHG V2 - like SolEx uses an IMX 178 sensor with a pixel pitch of 2.4 μ and QE of over 80% @ 500 nm. At a chip width of 7.4 mm I am just able to image the entire solar disk at 714mm focal length. Using Ken Harrison's EXCEL - Simspec V1.5 the SHG V2 has the following specs:

  • Hα: R = 62840 and spectral resolution 0.10 A
  • Hβ: R = 28434 and spectral resolution 0.17 A
  • NaD1: R = 43676 and spectral resolution 0.13 A
  • CaH: R = 20442 and spectral resolution 0.19 A
  • Typical scan rates are around 15x sidereal and 300 - 400 fps

Processing Software

I use Douglas Smith's SHG_Main for most reductions, image stacking in AutoStakkert and additional tweaks in PixInsight and Photoshop



Other Images of SHG V2 Figure 1 SHG V2 under construction
Figure 2: SolEx compared to SHG V2 Note the ASI 174 camera was for inti tal testing only. Current camera is a Player-One Neptune (ASI 178 equivalent) Figure 3: SHG V2 attached to Stellarvue 102R

For more information contact me - Brian Martin (bemart1@gmail.com)