In 2023 I had the chance to borrow a Shelyak LHires III from a university for half a year. After I returned it I considered getting one myself. Christian Buil's 3D Sol'Ex/Star'Ex had attracted my attention as well, but I feared the plastics in it, wondered how that would affect stability. My initial plan was to make an aluminium version of the Star'Ex, but then news reached me about 'this guy from Vietnam' who had designed an aluminium version. This guy is Minh T. Nguyen, who had started MLAstro, and produced these magnificent Spectroheliographs.

I wanted to use a spectroscope for Pro-Am work through the AAVSO, to get spectra from comets, and perhaps do some fun stuff as measuring the rotation speed of the Sun, Jupiter, and perhaps the orbital speeds of the Jovian Moons.

A spectroheliograph, however, is a spectroscope meant to be used for imaging the Sun in selectable wavelengths, but being closely designed after the Sol'Ex/Star'Ex, it can be converted to a spectroscope for deep-sky spectroscopy by adding calibration and guide units, and another slit.

The SHG700 I received came from batch 6 and arrived in October 2025. In order to adapt the SHG700 to deep-sky work, I added a Shelyak Alpy 600 calibration module to it (the white flat box in figure 1). This module has a NeAr (Neon-Argon) calibration lamp, a tungsten lamp for creating flats, and it can block incoming light from the telescope so that darks can be taken. Controlling the calibration module from data-acquisition software (CCDciel in this case) can be done using a switch like the SOS Electronics 12V four channel relais board in combination with NOYTIO USB Relay Controller Switch Driver (see figure 2).

MLAstro already offered an aluminium guiding module, so that was ordered together with the SHG700. As guiding camera I decided to use an old ZWO ASI1600MM Pro Cool that I once exposed to direct sunlight, which caused a permanent ghost image of the Sun on the chip, rendering it less suitable for deep-sky imaging. The advantage of this camera is that it has a large imaging chip, which is beneficial to plate-solving. It sees the sky through the reflective surface of the spectroscope's entrance slit.

The SHG700 guide module is meant to work with a mini camera like the ZWO ASI120MM Mini, that had to be mounted a ZWO Helical focuser to the guide-module. For deeps-sky imaging this can cause two issues: The helical focuser may not be stable enough to ensure a fixed position of the slit on the guide camera chip during long sessions (during which the orientation of the spectroscope changes), and cameras with larger chips like the ASI1600MM Pro Cool will not get the right back-focus distance when using the helical focuser as that focuser is too long.

I thus created another solution: instead of the helical focuser I mounted a low-profile 2" to 1.25" adapter on the SHG700 (for parts see figure 5). The camera is equipped with a standard 1.25" nose-cone that fits the low-profile adapter. Downsides of this solution are twofold as well: the adapter only has a single clamp-screw as a result of which the camera could wobble some 6 pixels, way too much to be useful. By adding a second screw this issue was completely overcome and the slit now remains stable well within a pixel. The other issue is that this solution is not a focuser, but by using spacers of varying thickness (one home-made 1.25" x 2.5mm aluminium spacer and a set of Baader T2 spacers) this was also overcome.

Originally the SHG700 is equipped with a 7µ wide reflective entrance slit. For solar imaging this is fine, even better: the narrower the better as one would only need one column of pixels to scan the full solar disk. For deep-sky spectroscopy, however, we want to properly sample the incoming spectrum. As science camera I had chosen the ZWO ASI2600MM Pro, a camera with a APS-C size chip and 3.76µ pixels. As the SHG700 has the same optics on both sides of the reflective grating, the slit is projected as wide on the imaging chip as it actually is. Being 7µ wide, the image of it would be just under 2 pixels, too low to satisfy the Nyquist criterion.

I had thus ordered a Gen2 14-20-26-32 µm slit (with 25 µm central hole) from Shelyak. This slit was meant for the Star'Ex and a 3D-print slit holder is available. Sadly enough these 3D designs do not fit the SHG700, but then I am not a fan of plastics anyway. So I had one printed as example, created a drawing for an aluminium version, and created that in my workshop (see figure 4). Meanwhile Minh is working on an aluminium slit holder for this slit as well, so hopefully he can deliver these soon.

First light was done on a tripod by simply aiming the SHG700 to the sky at daytime. Despite it large sensor, but thanks to the excellent optics in the SHG700, the spectrum is razor sharp all over the field of view (see figure 5, click here for the full version in jpg-format and here as tiff)! This was way better than I hoped fore and it turned out to produce a 100nm wide spectrum when using a 2400l/mm grating in combination with the ZWO ASI2600MM Pro. Combined with the 20µ slit a resolving power of around R=15000 can be achieved at the H-alpha wavelength (see figure 8).

I had also ordered a 300l/mm grating from MLAstro for full visible light observations, but that delivered a 1000nm wide field of view in this set-up (see figure 7). So a new grating is ordered, will post the results here once it is tested.