SGBA14 Notes: Telescope Making

Convener: Akkana P.
Notetaker: Kelley N.

shallowsky.com – presenter’s website, where other resources and links are available sjaa.net – has loaner scopes, free if you’ve paid the membership fee sonotube Chabot space center.
Why telescopes? Because astronomy is awesome! Right now, galaxy M82 has a supernova in it.
[pictures] It has been all over Twitter. (M82 is called the “exploding galaxy” because of all the filaments coming off of it, which are hard to see with a personal telescope. It’s possible if you take your telescope to a dark area.)
The moon is an excellent object for a hobbyist to view. [pictures] The moon pictures shown are pencil and charcoal sketches made by people looking through a telescope. Attendees took them for real photographs. [pictures passed around, are on printed pages]
Presenter started with store bought telescopes, then eventually decided to try making her own after she learned that a lot of people do it.
There are two important parts: the lens (which captures the light) and the mount (that the telescope sits on). Then there is the tube, etc.
In a reflector, a mirror captures the light and directs it to the eyepiece. (diagram drawn on sketchpad). In a very large reflector, the observer is seated at the point where the light converges; in a smaller one, a secondary mirror (which is about 1.5” in diameter in the demonstration telescope) directs the light to the eye.
Don’t ever be shy about going to a star party! People love to show off their telescopes. There is an etiquette, but people are friendly.
As a guess, a nice 6” starter telescope costs about US$100-$150. The Dobsonian reflector shown (has a $35 mirror)…
Inventor John Dobson was a monk. He wasn’t allowed to have a telescope due to monastery rules. He sought to make a telescope, but had trouble with the mounting system because existing mounts were complicated with counterweights, etc. So he designed what is now known as the Dobsonian mount. He found that great tubes can be made of cardboard tubes intended for concrete work. Generally, length of tubes gives more power, while shorter tubes give wider fields. Eyepieces – you will typically want 3-4 for different magnifications. The eyepiece then goes into a focuser. You can spend as much or as little on this as you want.
Demonstration of the action of the mount on the demonstration scope. Showed altitude and azimuth. Fuzzy parts of velcro used to give it stability. Friction is very important to hold the parts of the mount so that it will maintain its adjustment. /there is a certain type of formica (Ebony Star) that is popular among astronomers for telescope bearings. Melamine edging is also popular.
It’s not critical what you use, what you need is special principles.
[demonstration scope mount passed around for attendees to see. Uses a Teflon pad and ebony star.]
There are places where all these items can be ordered specifically for telescope building.
Demonstration of presenter’s design for breaking down the scope for transport and reassembling it. It involves a bungee cord.]
The mirror is about 6”, the tube is about 8”. This is for temperature reasons; when you take a warm scope out of a warm car and set it up on a cold hilltop, there can be distortions if there’s not enough room for parts to expand and contract. Showed the mount for the secondary mirror, and pointed out that you need to be careful with it. Her “classier” scopes have what is called a “spider” which attaches the mirror in multiple places.  Described the technique and criteria for alignment.
Reflector vs: refractor: reflector uses a mirror, refractor uses a lens. Diagrams on pad.ddd
Demonstrator showed a scope she bought at auction, called a “Yeager Supefinder.” It’s to attach to a much larger scope for spotting. Has about a 5” lens. For a five pound scope, you need a solid tripod.
You should use an equatorial mount, presenter has one in the car. [diagram drawn on pad] described how a clock motor can be attached to an equatorial mount. With no clock drive, you will need to continually “bump” the scope to maintain focus on the same object.
Film cans just happen to be the same size as telescope eyepieces! In the digital age, we will suffer for eyepiece containers 🙂
Refractors are more likely to be mounted on equatorial mounts because of their weight. Reflectors are more likely to be used with Dobsonian mounts because they are so cheap and easy to build.
You can even build your own mirror. Presenter’s recommendations are to start with a tube, use a hand held saw, drill and router. To be useful, a mirror has to be ground to the tolerance of 1/8 of the wavelength of yellow light. Optics are another matter.
To grind your own mirror, start with a telescope blank. You use another piece of glass, grind in a Z-shape, keep rotating it, and somehow this “magically” creates a spherical curve. [diagram drawn on pad] Presenter speculates about how it works. A more complicated series of things you can do changes it to a parabolic curve. There are classes and workshops available.
Presenter described how she found a drum case that was the perfect size for her broken down telescope to take it to Hawaii. Also, how she made a case out of an old pair of her husband’s jeans because his legs were long enough.
Aluminizing the hand grown mirror. Fascinating process to watch!
Speculates that her hand built scope cost about the same altogether as a store bought scope of the same size, but her parts are of higher quality.
Interactive demonstration of the reflector. Astronomy scopes are set up to focus at infinity, so not much can be expected when using it to look at near objects in the daytime. Session attendees were allowed to look through the scope. The presenter described why all objects viewed through the scope will be upside down.