Introduction
I'd been thinking about building a new solar telescope since early 2024; in the end I chose a solar Newtonian (with an uncoated primary). At the moment it's usable for photosphere and prominence imaging; I might add more functionality later.
TL;DR: the telescope works as expected, a sample successful session: Solar Newt. 10" 3rd light: great seeing, Hα to UV.
Why?
Apart from taking up an interesting project, Baader no longer sells their ND 3.8 solar film in sizes >20 cm (see e.g. this thread.)
(Other possibilities — like a heat rejection wedge, or a full-flux reflector with sub-aperture ERF, special secondary & liquid cooling — are rather more complex, but in the future, who knows.)
Structure
I knew I wanted a truss build, for airflow and weight reduction. After reading through lots and lots of ATM content, as much as I'd love to have elegant triangles and ball joints, I settled for something simpler: square frames of equal size, connected with square-tubing trusses screwed-on flat (all aluminum). Not a great beauty, but it works (click images for full size):
I cut and drilled everything myself, only had the welding done in a local workshop. The frames use 20x20x2 mm tubing, trusses are 15x15x1.5 mm (20x20x2 mm under the focuser plate). Mostly using M5 screws, except the dovetail profile (M6 + M8 under the primary), which includes small support blocks against squeezing:
Optics
Initially I wanted to dealuminize my SW 250PDS's primary, but in the end got new fused silica mirrors (Construzioni Ottiche Zen). Primary: D = 255 mm, f = 1192 mm (f/4.7); secondary: 67 mm minor axis.
PM cell
This was my first time using a plunge router. I used 2x12 mm plywood (beech), glued with Titebond III (saw it mentioned here on CN). 6-point bottom support, with whiffletree + rollers side supports (as per Mike Lockwood.) Probably an overkill for a mirror this size, but I wanted to tinker a bit.
It's rigid, but a bit on the heavy side (0.9 kg plate, 1.5 kg with all attachments):
Assembly
Precise adjustments of the optical path are of course performed using mirror controls, but the OTA should be put together without gross misalignment. Here's my approach:
- the bottom frame is placed on an adjustable platform and made level:
- the remaining frames are added while controlling their level and concentricity with mini-plumb lines made with threaded needles (there's some wiggle room in the bolt holes for that):
This way the frames end up positioned with 1-2 mm accuracy, which is sufficient (the PM has >20 mm clearance).
SM spider
One can use any kind of secondary mounting for a solar telescope (just without blackening). I was captivated by the wire spider design shown in this thread and decided to build one myself. It uses these locking guitar heads:
Harley Benton Parts Locking Chrome Single R1
and D’Addario PL013 (0.33 mm) steel strings.
The adjustments are a bit non-intuitive at first, but after some getting used to it's very convenient (one chooses 2 opposing knobs, turns them in the opposite directions, the laser moves along the PM very slowly and predictably.)
After assembling and tensioning, the strings would produce a nice, clear tone, which seemed funny at first; but then I realized it means under some conditions I might have long-lasting vibrations of the SM. Therefore I added these little dampers:
Other equipment
For testing I'd put on the small FeatherTouch 1.25" Crayford; now there's a Baader Steeltrack and a motofocuser controlled via Bluetooth (DreamFocuser mini):
The OTA weighs in at 11.8 kg (26 lbs).
Collimation
Initial collimation is performed as with a regular Newt.:
- center SM under focuser
- make SM & PM borders concentric
- adjust SM to send laser beam to PM's center
- adjust PM using a Cheshire
The reflection of Cheshire's annulus in the PM is sufficiently well visible (one just needs to point the Cheshire' disc toward a light source, e.g., a ceiling lamp or the Sun).
Night collimation on a star works fine, at least with a camera (I haven't tried visually with a small exit pupil). Brighter stars are readily visible at lower magnifications (I used 4.3 mm exit pupil for pointing). I can't show a pretty diffraction pattern at this time, the seeing during my night test was mediocre (red long-pass >610 nm filter):
All right, but how does one verify the collimation in daytime? Beside pointing at the Sun's reflection in a ball bearing etc., we can still collimate on a star — the Sun itself! As mentioned in Solar Astronomy, telescope's collimation can be assessed based on the view of granulation. Here's what's worked for me:
- using camera without a Barlow, point at the geometric center of the solar disc (so that granulation is viewed from the optimal angle — straight down). The center can be found by relying on limb darkening; just set the shutter/gain high enough, that the disc's middle is overexposed, and center on that hotspot:
- record a short video and stack it, then sharpen (overly) and assess; if the combination of PM's focal ratio and sensor size (mine is 1/3") is adequate, you'll see where the comatic outer FOV begins. In my case, after the initial laser+Cheshire collimation, it looked like this (I marked the area around the optical axis - as I reckon):
(I left it at that, the images turned out just fine.)
Conclusion: the telescope in its current incarnation holds collimation reasonably well (...at least in the range of orientations adequate for morning solar operations.)
Sample image (Baader Solar Continuum (540 nm/10 nm), Baader TZ-4S, AS!3 and ImPPG):
Edited by Great Attractor, 20 May 2025 - 03:28 PM.
