Should you make your own telescope?

Making you own telescope is often abbreviated as ATM (amateur telescope making) and it has quite a following of enthusiasts.

Great Astronomers Used To Do It

If you read about the history of astronomy, you will see that many of the great astronomers in the past built their own telescopes.

They sought to improve on what was available and also, to build an instrument that was designed for the work they wanted to do.

You can justify a DIY (do it yourself) telescope similarly now. I like the idea of getting a custom solution, together with perhaps getting more for my money.

Also, the idea of creating something with your own hands, that you then use to explore the Universe, is fantastically attractive to me, at least.

But, Times Have Changed?

When I was first getting into astronomy, it was fairly common to make your own telescope. As a teenager, I was tempted to try, although I did not.

Nowadays though, there is a much wider range of good-quality, commercially produced telescopes, available at fairly inexpensive prices.

The industry has been led by a small number of large companies. They have taken advantage of mass-production techniques and manufacture in the currently low-cost area of China, to provide some very good value equipment to amateur astronomers.

So Why Would You Make Your Own Telescope Now?

I know that many keen astronomers would think that you were a bit bonkers, if you announced you were going to make your own DIY telescope today.

They would tell you to go for the good commercially available stuff, I have just mentioned.

Generally, I agree with them.

However, I would suggest there are two situations when it is perfectly sensible to consider the DIY (or ATM) telescope making approach.

These are:

Firstly, if you want a large aperture telescope (say over 12 inches/300mm aperture) and cannot afford thousands of pounds or dollars to buy one “off the shelf”

Secondly, if you would really enjoy the project of creating a telescope yourself and would get real satisfaction, from using an instrument built specially to your requirements.

I personally fall into the second category.

So How Would You Go About It?

So if you decide to give amateur telescope making a try, how would you start?

The first thing is to settle on building a reflecting telescope, based on a mirror, not any other type of telescope. Don’t think about refractors or hybrid designs (for example Schmidt Cassegrains), these use large lenses and are too hard to make.

The most favoured telescope for DIY build is a Newtonian reflector, on a simple Dobsonian mounting. There are many plans available to show you exactly how to go about it.

Then comes the big decision – will you make your own main mirror?

This is exactly what DIY telescope makers always used to do.

The advantage is that you CAN get a good mirror at low cost, by physically grinding and polishing a blank disc of glass into a parabolic telescope mirror.

The disadvantage is that it takes a long time, is a lot of hard work and requires a high level of technical skill to get the correct profile on the mirror surface.

I have to admit, I’ve never made a mirror, but people do and they seem to enjoy the experience.

I would recommend instead, buying a ready-made mirror set. This is a matched pair, consisting of the large main mirror together with a flat secondary mirror, which is what you need make a telescope in the Newtonian configuration.

Commercially-made mirror sets are not too expensive, although the price goes up rapidly with size.

Most importantly, buying a mirror set should ensure that you have high quality optics as the basis of your telescope.

Then you will need to check your plans and make or buy all the various other components and get started!

Here is a simple online spreadsheet to look at the various items and costs involved. ATM Cost Spreadsheet

Summary

It is perfectly possible to build a DIY telescope and many people do.

If you are keen to own a large reflector and would enjoy the project, it can make a lot of sense.

It has a problem with the slow-motion drive on the declination axis.  The screw thread and rack section drive, no longer mesh properly.  So the dec drive does not work.

We decided last week, to replace the drive with a new worm wheel and worm set, to be supplied by Beaconhill telescopes in Louth, Lincs.

But careful dial-gauge measurement, revealed a run-out on the dec axle (’wobble’), which would jeopardise the smooth working of the worm.

So we were back at  square one.

The final solution remains distant in the future.

But we have found a stop-gap.  

We will re-vitalise an alternative dec slow-motion control, which was fitted to the telescope decades ago..  It should help us get the telescope working, soon.

My new telescope was delivered two days ago, so here are the first impressions.

It’s a Skywatcher 80ED refractor from the Evostar Pro series. This means it has an 80mm diameter, high quality objective lens, made from fluorite glass.

This special glass material is used to eliminate false color as much as possible from observations and keep them sharp.

ED1 OTA Version (optical tube assembly only)

It’s the first high-quality (apochromatic) refractor I’ve owned. Before purchase, I asked experienced astronomers and this telescope was well-recommended for it’s image quality and value for money.

Skywatcher 80ED showing necessary extension tube

I bought the ED1 version – this is the bare optical tube (OTA) only. You can also get the ED2 version. It’s exactly the same optical tube, but comes also with 2 eyepieces, finderscope, diagonal and case.

I already had eyepieces so I decided to save money and just get the OTA, which cost £235 (about $420) delivered.

Attaching To A Mount

First impressions were good.

The tube assembly is a handy size and weight. And with a focal length of 600mm (f7.5), it’s fairly short.

It was easy to attach to the equatorial mount, I already had.

You can see from the photo, I have used the white Skywatcher tube rings it was supplied with, but have attached them to the existing bar on the mount.

Because of this, the rings are rather too close together.

I have retained the Skywatcher supplied (longer) dovetail bar and will use it on the next mount. (I intend to get a much better quality, motorised polar mount for this telescope, in the next few weeks)

First Light Through The Scope

Observing Chair and Skywatcher 80ED

Amazingly, the sky was very clear on the first night, so I did some observing.

First thing to point out, is the focus tube is too short to allow use of an eyepiece, ‘as is’.

You need either an extension tube or a star diagonal, to lengthen the optical path and bring the image to focus.

Remember I mentioned above, the ED2 version comes with a 90 degree diagonal attachement.

I used an extension tube I already had. I have annotated the photo above, to highlight this extension.

Observation

Jupiter is bright, but rather low in the South at present. It made a good first target for the new refractor.

I was very pleased to get a good sharp image of Jupiter, using a 9mm eyepiece. I could clearly see the darker bands on the planet.

The crayford focuser of the scope is very smooth and nice to use.

I really needed higher magnification, but the 9mm eyepiece is the shortest focal length I have at present. With the 600mm focal length of the telescope, it gives only 66 times magnification. (600 divided by 9)

I’m confident I will see clear views at much higher power, so I am well pleased with the new refractor.

I will report progress over the next few weeks.

In recent years however, various hybrid types have become available, which attempt to combine the best features of the two traditional types. But we will discuss more about hybrid designs in a moment.

Refracting telescopes use a main lens made from glass and called the objective lens. This lens bends (or refracts) the incoming rays of light as they pass through it, so they come to a focus. This was the type of telescope that Gallileo famously used back in the early 1600’s to make the first astronomical observations of the Moon and Jupiter.

Reflecting telescopes on the other hand, do not use an objective lens, instead they use a main or primary mirror. The surface of this mirror is specially shaped into a concave curve called a parabola, which has the handy property of reflecting all incoming light rays to a focus.

Let’s look at these two traditional types in more detail.

Refractors

Refracting astronomy telescopes have a main lens ranging in size from about 60mm (2.5″) in diameter and then on upwards. The cost increases quickly, as it is expensive to make good quality lenses, so it is rare to see an amateur instrument with a size greater that 150mm (6″).

Refractors have the advantage of allowing all the light collected by the objective lens, to pass unobstructed through the telescope to the eyepiece at the other end of the tube. Consequently, even small diameter instruments can work well. A decent quality 70-80mm refractor is still an excellent telescope for amateur use.

The problem with refractors however, is so-called chromatic abberation. This simply means that false colour can be introduced to the image you are observing. This happens because the different colours of light within white light, are bent by the lens through slightly different angles, resulting in slightly different focal points.

This used to be a big problem until it was discovered that by sandwiching two (and more recently three) lenses into a doublet or triplet lens, and also by using special glass, these false colour effects in the image could be removed.

Refractor telescopes with colour correction are called achromatic and the best ones, using the multi-element lenses are called apochromatic.

However, the manufacture of these clever lenses is expensive, although the prices of achromatic refractors has come down greatly in the last few years because of Far Eastern factory production.

Reflectors

The reflecting telescope was invented by Sir Isaac Newton in the early 1700’s. As we have said, it uses a mirror to focus incoming light.

This has two big advantages – firstly mirrors are cheaper to make than lenses and you can make them in large sizes, and secondly, mirrors do not have the problem of false colour.

As a result, you can get a much bigger reflector for your money and they have always been very popular amongst astronomers. The largest telescopes in the World all use mirrors.

The disadvantage however, is that you need a bigger reflector to get the same light-gathering power as a refractor because it is necessary to use a second mirror (the secondary) to divert the focussed light beam into the eyepiece. This second mirror within the telescope tube, blocks out some of the light.

Hybrid Telescope Designs

Recently, we have seen the emergence of hybrid designs which combine mirrors and lenses. These catadioptric reflectors are compact telescopes that are capable of producing high quality images, while being available at reasonable cost (typically a few hundred £ or $).

These hybrid telescopes use a lens called a corrector plate at the top of the tube, together with a primary mirror at the bottom. Mounted inside the tube is a secondary mirror. This reflects the light a second time back down the tube and out through a central hole in the primary mirror, and into the eyepiece (or other imaging device).

There are a few disadvantages, but they usually work very well and have consequently become very popular amongst amateur astronomers.