Newtonian astrograph

The goal was to build a high quality 10″ Newtonian astrograph, able to compete with the ASA astrographs of comparable diameter in image quality. The design has been optimized for the Paracorr coma corrector, with the goal of illuminating an APS-C sensor.

  • Main mirror:
    • 10.03 inches diameter,
    • 0.78 inches thickness
    • f/4.51,
    • FL=45.1 inch=1145mm,
    • weight 4.5lbs
    • yellow spot for collimation 
    • Grade-A Pyrex glass, fine annealed
    • Manufacturer: Meade (starfinder), refigured, re-polished, recoated.
  • Secondary mirror:
    • 3.1″ (78.74mm) minor
    • 4.38″ major
    • 0.75″ thick
    • 1/20 wavelength,
    • 0.15″ offset
    • Antares Optics.
  • Spider design:
    • CNC machined, custom made.
    • Astrosystem secondary enclosure.
    • 3/8″ mounting stand
  • Tube: currently aluminum, to be updated to carbon fiber at some point.
    • ED=12″
    • ID=11.82″
    • wall thickness=0.09″
    • 45″ long,
    • focuser hole center at 39″-6″
    • focuser hole diameter: 2-3/8″
    • Manufacturer: Parallax
  • Corrector: Paracorr type 2, 2″, Televue.
  • Focuser: Moonlite, 2″.
    • Model CR Newtonian Focusers.
    • Drawtube = 2.37″ travel compression ring drawtube.
    • Knob/Motor = 1″ knob  Single Rate (Standard) + MoonLite motor.
    • Curved Adapter for 12 inch tube.
  • Main mirror collimation cell: Aurora Precision.
  • Collimation: Catseye and holographic laser (Howie Glatter).

Here were the design constraints:

  • The astrograph was designed to be mounted on an equatorial mount.
  • Operating the astrograph needed to be a one man operation. For an equatorial Newtonian, 10 inch (primary diameter) is as big as I am confident to handle alone. 12 inch might be possible with a very short f/ number (like f/3), but it was too close to my limit in terms of weight and volume. Now that the 10″ f/4.5 has been assembled, I hesitate to set it up for a single night and realize it will need to be permanently setup in an observatory. Single man operations are possible, though.
  • f/ number: fast number make for fast, light and short telescopes but the price of the primary increases very rapidly as the f/ number decreases, and the collimation tolerances become rapidly unforgiving. Long f/ number make for very long telescopes. I decided the sweet spot for this 10″ project was f/4.5. I would have been equally happy I think with f/4.
  • Coma corrector. Unlike many ATM projects where a telescope is build first, then the builder looks for a suitable coma corrector, the coma corrector was chosen first, and the telescope was built around it. For fast f/ numbers, ASA offers excellent coma correctors. For this project, with a longer focal length, I went for the Paracorr type 2, 2″. The optical design of the Paracorr type 2 is very poorly documented unfortunately, so for modeling I used a design akin to the Paracorr type 1, as describes in Smith/Cragioli/Berry Chapter 14.2.
  • Sensor size to illuminate: the choice of the sensor size is done at the same time the coma corrector is chosen. Going full frame is certainly cool, but full frame camera are very expensive, and on a 10 inch a full frame sensor imposes severe constraints on the central obstruction and coma corrector. So I decided the goal should rather be to fully illuminate an APSC sensor with almost no vignetting. This allows for a smaller central obstruction, less weight on the secondary assembly, and still full illumination of the smaller APS-C sensor. For anything bigger than 12 inches (maybe that 16 inch I see in the future), it would make sense to design the scope directly for full frame, with a 3 inches Wynne corrector.

I calculated the secondary offset with both Atmos and manually (the annexes of Texereau’s book are still one of the best sources) and…. found slightly different results.

I did some point spread diagram with the approximated coma corrector in Oslo Edu. Design file below:

Atmos design and spec of the astrograph.
V curve of the system for focusing.
Image and corner quality: no coma is visible.
More corner quality.
Field curvature: an excellent flat field, could be improved with finer collimation.
Plate solve to verify the exact focal length, with the paracorr installed.
M76 test image. The diffraction spikes on the bright star are imperfect due to a vane being slightly bent. This led to the redesign of the spider system.

Conclusion: the system performs very well, and according to design. I am currently redesigning various parts to increase performance and reliability, as well as setting up an observatory to take full advantage of this imaging beast.

PSF and MTF of the 10″ f/4.5 system, as calculated in Python.
The mirror cell, from Aurora, with the back of the mirror.
The Moonlite focuser, here with a 3D printed (TPU) dust cover.
The spider (from Astro-system), but with the questionable plywood parts replaced by CNC aluminum.
The scope itself.