Our friends over at Autodesk have very kindly shared a project they have been working on. The project began due to the purchase of a solar telescope for photographing solar activity. They wanted to design an affordable solution to help with the issue of Newtonian Rings that occur due to the reflection of light between the optical surfaces within a telescope and a camera.
Earlier this year I bought myself a new Solar Telescope with the intention of viewing, and photographing our Sun safely. Looking at the sun visually with a telescope is amazing but my real interest is photographing it. The way we do this is typically to use a special dedicated Astrophotography camera that is capable of recording video at hi frame rates. Recording video allows us to combine typically 500 to 1000 individual frames of the video into a single image using special software that allows us to “stack” those images and obtain the highest possible quality even though we are looking through our turbulent atmosphere.
IMPORTANT NOTE: Never look at the sun directly with the naked eye, or any type of optical device, unless specifically designed to look at the sun directly as you will damage your eyesight irreversibly.
When using specific camera and telescope combinations, particularly with black and white or mono cameras a problem can arise known as Newtonian Rings. This problem manifests itself as a series of concentric rings or arcs that display on the image of the camera. This problem arises due to the reflection of light between the optical surfaces within the telescope and camera.
The solution in most cases is an easy one. Just as if you stood looking at a reflection of yourself in a mirror, if that mirror is tilted “off axis” your reflection will move, and eventually disappear. We can use exactly the same approach by tilting the optical axis of the camera, relative to that of the telescope. This is an already known solution with various options available on the market today to buy. Unfortunately, as with a large amount of Astronomy related accessories, those solutions are expensive.
Having just purchased my new Solar Scope, and having this issue with Newtonian Rings, as were a number of my Astronomy colleagues I set about designing an affordable solution one evening. So, on my patio as the sun went down I started to design a concept for what I am calling an Optical Tilt. Because the tilt angle can vary depending upon the telescope and camera used, my first design was done as a variable device.
To design the device I used Autodesk’s Fusion 360 as it’s so easy to use, and is also cloud based which meant I can iterate through the design regardless of where I am, or which computer I am using. This was a really useful factor as it meant I could tweak the design easily whenever I wished, or as a new idea came into my head. At this point I would like to say that as my day job, I do work for Autodesk as a technical specialist, and have access to a number of different design packages. I also travel frequently and so having access to this design and not having to remember to save the latest version to a memory stick was a huge advantage.
I also wanted to be able to manufacture the device quickly and easily, and thanks to the folks at Ultimaker GB, I had access to an Ultimaker 3D printer.
Given the high costs involved and time required to traditionally manufacture this type of device, I wanted to use 3D printing to create my Optical Tilt. I have access to my own metalworking lathe, but knew the time required to manufacture the Optical Tilt this way would take far longer than 3D printing one. As another consideration, using 3D printing to manufacture enabled me to produce multiple designs of the Optical Tilt. Initially a variable angle version was designed, and later that design was copied and modified in Fusion 360 to produce a fixed angle Optical Tilt meaning quicker print time, less bulk and a reduced optical length which can be important when focusing.
The two versions of the Optical Tilt have proved very successful and due to the ease of changing the design, specifically the angle of tilt within Fusion 360, and the ease and low cost of printing, the fixed angle versions have proved most popular.
You can see here the Telescope and camera setup without the Optical Tilt fitted.
And here with it fitted
Optically the results speak for themselves, as you can see below. The image on the left shows the interference pattern, or banding called Newtonian Rings, whereas the image on the right uses the Optical Tilt and shows no banding.
Having solved this problem now means we can enjoy imaging the sun and recording the myriad of features and events that are happening every minute of every day, and have been for the past 4.5 billion years. As a final thought, look at this image here showing more surface detail of our Sun, and for scale, an accurately sized representation of our pale blue dot called Earth