8.5" Newtonian Reflector

This project is to create a telescope all by myself. I started this project for two reasons:

  • I wanted to experience the thrill of building my own telescope
  • Save some money (good quality readymade telescopes are quite costly). Although, in the long term, building your own telescope doesn't save money, because it devours your time, and well, time is money. Anyway, now that I have started it, I am going to finish it.

Plans

Type Newtonian Reflector
Aperture 8.5" (was originally 8.0")
Focal ratio f/10 (was originally f/8)
Mount Dobsonian

Tradeoffs

There are a lot of questions and tradeoffs to consider before building a scope. There are a myriad of choices at every step!

Why build?

Because it's cheaper to build than buy! It's not cheaper in the long run, because you end up spending more time. But it is definitely rewarding. My original analysis:

In Forum (a popular mall near my house), a 4.5" reflector sells for Rs 9,000! A 6" reflector for Rs 19,000!

Online, I can buy a 6" reflector for about Rs 8,000 and an 8" one for Rs 18,000.

Compared with that, I can build an 8" reflector with a Dobsonian mount for about Rs 8,000 or less!

Why Reflector?

Refractive scopes are best but they are costly affairs. And they are beyond amateur telescope makers.

Why Newtonian?

Most easy to build among reflectors.

Why 8.5" ?

An aperture of 6" is too small for observing planetary details and deep sky objects. A 10" is too bulky, and will take a long time for mirror grinding. An 8" is a good compromise.

I settled on 8.5" so that the bordering .5" can be taped off in case of any defects.

Why f/8 f/9 ?

The rule of thumb is: the more the focal ratio the larger the sweet spot of the lens. Images outside the sweet spot, suffer from an aberration known as coma.

A perfect f/4.5 mirror has a sweet spot spanning 2 mm (0.08 inch). An f/8 has a sweet spot spanning ~ 10 mm (0.4 inch). An f/10 has a sweet spot spanning 22 mm (0.87 inch).

Hence a large focal ratio is desirable. However, the larger the focal ratio, the lengthier is the focal length, and consequently the telescope. [ The focal length = aperture * focal ratio ]

So an 8" Newtonian, with focal ratio of f/8 will give a focal length of 64 inches (~ 5 feet), which is a good compromise, I think.

Update: Although f/8 would have been ideal, I ended up grinding my mirror to f/9. I was not getting the required Saggitta quickly due to improper grinding techniques and so I became a bit lazy and decided to go for f/9.

Why Dobsonian?

Equatorial mounts are costly and require alignment before use. They are best suited for photography (which I am not planning to do right now) since that requires tracking on only one axis.

Dobsonians (alt-azimuth) mounts are easy to make, and easier to operate for large scopes (greater than 6"). They are, however, not suited for photography, since these mounts require tracking on two axes. [An exception is when they have a computerised motor control; alt-azimuths are easier to program for tracking!]

Cost breakup

Here is the list of all costs that I have incurred so far in my telescope making. I have split up the costs into two categories; actual material cost and Tools cost. Any money I spend on material that directly goes into the scope is the material cost. Expenses on tools go into the second category.

Actual material cost

Item Cost in INR
Plate glass blank 200.0
Tool blank 50.0
Carborundum 200.0
Pitch tar 100.0
10 inch PVC pipe 1050.0
Rouge (gifted by Vinay) 0.0
Wood (left over from furniture) 0.0
Total 1600.0

The items that I bought off the shelf (from Sharp Vision, Delhi) were:

Item Cost in INR
3 point Mirror cell 500.0
Spider mount 600.0
Finder scope 6x25 600.0
Elliptical secondary flat mirror size: 50mm x 70mm 1000.0
Rack and pinion focusser 350.0
Total 3050.0
Eyepieces Cost
17mm Kelner, 50 degree 600.0
25mm Ramsden 250.0

Total cost so far : 4500

Tools cost

Tool Cost in INR
Hand saw 68.0
Khanas (file) 22.0
Hack saw 20.0