Wrestling a New Telescope to Life

Skyward Telescopes

The Skyward 150 has been a great success. Before this new effort there were three variations available as free opensource designs that anyone can download, 3D print and build. We have a steady stream of them being borrowed every Sunday from the Blue Door Astro Library. The original version has been printed by many people all over the world and is focused on visual imaging. The compact version was made accessible to makers with smaller printers, and the astrophotography version turned out to be a serious powerhouse, creating beautiful images within the budget of many amateurs.

Aperture Fever Strikes

The challenge with both visual and astrophotography approaches to astronomy is that our targets are so incredibly far and faint that we are always struggling to get more of the building blocks of light - photons, to the eyepiece or the camera. The only practical way to increase the number of photons we gather is to "build a bigger bucket" to catch them like rain drops as they come down to Earth. With a 150mm mirror, the Skyward 150 was already gathering about 450 times as many photons as the human eye. Another popular mirror size is 8 inches, or 203mm diameter, which almost doubles the light collecting capability again. It also gives an extra bump to the resolution by about 1.35X which is nice. I decided to follow that route for a new design.

From Mirror to Model

When I started the new design I knew I would use an 8 inch (203mm) diameter mirror. I also knew I wanted to preserve the same "Dobsonian on an Elevated Rocker Box" concept that allows for comfortable for the observer without stretching up or bending down to the eyepiece. This meant using a shorter focal length (fast) parabolic mirror of 800mm focal length. With this selection in mind I began with the piece that holds the mirror in place- the mirror cell. It is important to consider how the mirror will be secured and supported by the mirror cell. A free software called PLOP allowed me to determine that 3 points are more than sufficient to support the mirror without noticeable degradation due to deformation.

I designed the mirror cell and the rest of the telescope in a CAD software called Fusion 360. In order to accelerate the cooling of the higher mass mirror and to keep the air inside the OTA moving to eliminate any laminar air effects and further reduce any condensation I decided to install a 12V fan. With the mirror cell completed I could then determine the position of the smaller secondary mirror that redirects the light into the eyepiece/camera. I will talk about off centre mounting of the secondary for fast telescopes in a future blog post. We decided to use a massive 70mm diameter secondary in order to reduce vignetting on full frame sensors such as the one in our Canon 6D DSLR.

So once the primary, secondary and focuser were positioned everything else started falling into place. The rocker box needed to be expanded to accomodate the larger OTA. I decided to reuse as much of the original Skyward 150 rocker box as possible.

And with the recent success in testing the new Ten Telescopes 2 inch Focuser (TT2F) design I decided to go with that first and use the more expensive aluminum BF20 focusers on a later version. The TT2F is a serious focuser with massive payload capacity suitable for the largest eyepieces and significant astrophotography gear. It is also compatible with electronic auto-focusers such as the EAF from ZWO.

A heavy duty Ten Telescopes 2 Inch Focuser you can download and print!

Everything on the Skyward 200 is bigger than the Skyward 150, including the aluminum extrusions we use as the structural backbone. I increased these from 2020 to 3030 extrusions.

The end effect is a much bigger telescope design.

Skyward 150 and Skyward 200 telescopes compared in CAD software.

Printing the Prototype

With this larger telescope the printed parts take longer. To print one you need a printer with a bed size of 300mm x 300mm. I recommend ABS or ASA filament for the most robust telescope that will last you many years. That means you will need an enclosed printer with a more managed ambient temperature.

In the slicer:

Printing the mirror cell shroud.

Assembly

With all the parts printed we (I got a lot of help from my friend Vasu) were ready to assemble the Skyward 200. It went together without any serious drama!

Testing

Now we need some clear skies to do some testing! Stay tuned for a future post!