How to Set Up A Dobsonian Telescope

---

Here is the great amateur Astronomer, John Dobson with one of the telescopes that bears his name.  The Dobsonian telescope is a simplified Newtonian reflector on an alt-az mount.  One of the reasons for their popularity is that the simple mount and basic optical tube mean that the vast majority of the telescope's cost goes directly into the optics.  It is therefore possible to obtain a telescope with good optics, a large aperture and a long focal length for a relatively modest cost.  

While Dobson popularised this type of DIY home built telescope throughout the 20th century, there are also a number of excellent value commercially available telescopes using this design.  

Given their large light gathering ability and manual mount, these telescopes are best suited to visual astronomy of planets and some brighter deep sky objects.  They also work well for planetary imaging.  But in general, their long focal length and manual mounts mean they are not suitable for longer exposure astrophotography.

The basic structure of a Dobsonian telescope uses two mirrors to focus light through an eye piece.

The Primary Mirror is a parabolic mirror that focuses light towards the centre of the telescope at a focal length equivalent to where the eye piece is located.  

The Secondary Mirror is typically a flat mirror that reflects light from the Primary Mirror into the eye piece.

The dual mirrors of Dobsonians are intentionally adjustable and it is important to ensure they are in alignment to get the best possible view from your telescope.

I find the mirrors on my Dobsonian go out of alignment everytime it is moved so collimating is a standard part of my setup routine every night before the telescope is used.  

The first step to setting up a Dobsonian (or any telescope) is to assemble the telescope at least an hour before you intend to use it.  

Especially if you live in a cold country like the UK and store your telescope in doors like I do where there may be a 10-20 degree drop in temperature outside.

This will give the telescope enough time to acclimate to the ambient temperature, expand or contract to a stable length and remove any convection currents inside the optical tube.  From a practical perspective, it is also much easier setting up in the day time rather than stumbling around in the darkness.

I typically use a platform of some sort to ensure there is a solid and flat base under the telescope.

There are a number of ways of collimating your telescope.  They all basically use the same principal of projecting light through your optical train to ensure the light is being reflected and focused onto a central point in the middle of the eye piece.

You can use a Cheshire eye piece which you attach to the eye piece of your telescope and shows the reflection of a target on the rear of the viewing hole.  By adjusting the mirrors until the target is central, you can align the optics of the telescope.

I find a laser collimator to be very quick and effective.  Rather than looking through the Cheshire eyepiece, it projects a laser through the eyepiece and onto your Primary and Secondary mirror and reflects back onto a target.

Given that these laser collimators produce their own light, they are also quite handy for performing collimation at night time.

The first step of collimation is to ensure the Secondary Mirror is aligned properly.  This means it must be square on to the focuser and at 45 degrees.

First, securely fix the laser collimator into the focusser and turn on the laser.  It is important this is secured as firmly as you fix an eye piece so that you are measure the same optical path.

Ensure the "target" on the collimator can be viewed from the base of the telescope; this will make it easier to adjust the Primary Mirror later.

Look down into your optical tube and you will see the laser, a few reflections and a small circle or dot in the center of the Primary Mirror.  

Depending on how your Dobsonian is constructed the Secondary Mirror will be adjusted either with a screwdriver or by turning bolts.

As you adjust the Secondary Mirror fixtures and the orientation of the mirror changes, you will notice the laser will change position.

The Secondary Mirror is aligned when the laser is centered in the small black circle in the middle of the Primary Mirror.

Once the Secondary Mirror is aligned, you will need to ensure the Primary Mirror is properly aligned.

Go to the base of the telescope and there should be some adjustment screw that will change the angle of the Primary Mirror.

It will take a bit of trial an error but you will notice the position of the laser on the collimator target will change as the orientation of the Primary Mirror changes.

The Primary Mirror is aligned once the laser is reflecting into the centre of the target

Now that the telescope is collimated, you are ready to remove the laser collimator, add an eyepiece or a camera, adjust the focuser and begin to view your target.

Enjoy!