This design is based around four ADM Vixen universal dovetail bars: two 14 inch long, one 11 inch long, and one 7 inch long. The telescope is mounted on one 14-inch bar using the included Skywatcher tube rings. The second 14-inch bar is attached to the first with four 30mm long M6 bolts and nuts. The 11-inch bar is attached to the far end with a single 16mm long M6 bolt and nut; this bar holds the grating using an ADM Vixen dovetail plate adaptor (VDPA). The 7-inch bar is mounted perpendicular on the second 14-inch bar with a single 16mm long M6 bolt and nut; this bar holds the camera and imaging lens on a second VDPA. The side of the 7-inch bar is in contact with the end of the 11-inch bar to prevent either from rotating (since both are held with a single bolt). The little 4-inch dovetail bar at the front of the telescope is just a "foot" so that I could assemble things on the floor without it tipping over.
The collimator is a Pentax Takumar 150mm f/4 lens. There is a short M42 extension tube on the lens, followed by an M42 to T2 adapter, then a short T2 extension tube. A ZWO T2 to 1.25-inch filter adapter is screwed into the T2 extension tube. The quartz slit (9 micron wide by 12mm long) is mounted in a newly designed double 1.25-inch filter cell (there is now a second copper disk behind the first to block stray light from the edges of the quartz chip). The collimator assembly looks similar to this one:
The collimator assembly is then attached to a 50mm long T2 extension tube and then a 2-inch nosepiece. This fits into an Altair 70mm 2-inch extension tube, which then attaches to the telescope focuser. The advantage of this design is the telescope focuser only has to support the weight of the extension tubes and the collimator lens, not the whole weight of the spectroheliograph. The ADM dovetail bars are very rigid and do not show any flexure. Also, rotating the collimator at the 2-inch nosepiece allows for the possibility of easy adjustment of the slit orientation.
This design is quite similar to the setup for my 106mm APO triplet, a significantly heavier telescope:
After comparing the two setups, for the 80ED doublet and the 106mm APO triplet, the parts for the 80ED design could easily be adapted to a typical 4-inch f/7 telescope with only minor adjustments to take into account the larger diameter of the telescope tube and greater weight of the telescope. Specifically, the second 14-inch dovetail bar should be attached on top of the first one (rather than underneath) to raise the level of the camera and to allow the mount to grip onto the first dovetail which supports the telescope.
You might ask, where is the box to block out stray light? I decided to try something radically simple and just but a black cotton bag over the end of with the exposed optical elements. This worked surprisingly well, despite the black cotton not being particularly thick. I could even adjust the focus of the two camera lenses through the black cotton. Eventually, I added a second, slightly larger, black cotton bag and this made the inside very dark. I added a piece of black foamboard to the collimator to block out sunlight coming directly down the axis of the telescope, and also to keep the cotton from interfering with the light paths.
The preliminary results have been quite good. For the initial trials I used a pair of 150mm f/4 lenses, a 1800 l/mm ruled diffraction grating (50mm square) and a ZWO 183MM camera. Below are some "first light" images in H-alpha and Ca-H. The results with the 80ED are not quite as good as with the 106mm triplet, but you need to look carefully at full resolution to really notice the difference. The difference between using a 1800 l/mm and 2400 l/mm grating wasn't as much as I would have thought (I can go into the reasons for this elsewhere). Overall, I'm impressed with the performance of the 80ED telescope for this purpose (it has a good reputation for solar work) and most of this build could be done in a day with the parts described here. The only real "machining" required are methods to hold the rotatable grating and to hold the camera and imaging lens, both of which are attached to a VDPA. I mainly used scavenged parts for these but 3D printing or real machine-shop parts could be used.
- H-alpha 80ED SHG. Baader 2-inch 35nm H-alpha filter at the telescope focuser to act as ERF. Stack of 14 of 20 scans. 1.6ms exposure, zero gain. 386 fps, 21x SR scan in DEC. 5 Sept 2023.
- 050923-Ha-600mm-1800g-stack14.jpg (1.27 MiB) Viewed 1433 times
- Ca-H 80ED SHG with 60mm aperture mask. GSO IR block filter (KG3) as ERF. Stack of 20 of 27 scans. 4ms exposure, gain 80 (17%). 241 fps, 13x SR scan in DEC. 4 Sept 2023.
- 040923-CaH-60mm-1800g-stack20.jpg (1.15 MiB) Viewed 1433 times
