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Collimating a Schmidt-Cassegrain Telescope (SCT)
The Schmidt-Cassegrain Telescope (often called SCT) is a popular telescope amongst amateur astronomers. The telescope gets its name from those who invented the optical system, Barnard Schmidt (1879-1935) and Guillaume Cassegrain (1625-1712).

The SCT contains elements from both reflecting and refractor telescopes. Like a Newtonian reflector, the SCT has a rear primary mirror that focuses light, but also incorporates a front lens or corrector plate that helps to fold the optics, making this telescope very compact whilst maintaining a high focal length. In the centre of the corrector plate, a secondary mirror is located. The light which has passed through the corrector plate, and reflected off the primary mirror is focused on to the secondary mirror before being reflected down to the eyepiece. The eyepiece holder is located at the back of the SCT tube, similar to a refractor, but for the focused light to reach the eyepiece, it must pass through a hole in the middle of the primary mirror.

When wanting to observe stars and planets in particular, the light needs to focus accurately to reveal the fine details. To obtain ultimate focus the SCT needs to have its secondary mirror aligned with the primary mirror, this method is known as collimation. Collimation is also required when using Newtonian reflectors but this chapter simply discusses collimation with SCTs.

See the diagram below showing the light path of a collimated SCT

How To Tell If Your SCT Is Collimated

Using a reticule eyepiece, centre a bright star and defocus the telescope. You should see something similar to the concentric ring pattern below.

If your SCT is not collimated then you may see something similar to the below.

How To Collimate Your SCT

To resolve collimation issues with SCTs, typically you simply need to align the secondary mirror so that the light paths are straight/direct. The secondary mirror is held in place by three screws which can be adjusted to collimate the SCT. The factory fit screws require the use of a screwdriver to adjust the mirror angle. Considering you need to use a star to collimate your SCT and stars are visible at night, using a screwdriver so near the corrector plate at night is not advisable.

There is help at hand so the use of a screw driver is not necessary to collimate your SCT, simply your fingers. The screwdriver less solution you need is called Bob's Knobs which are used to replace the factory fit screws. Once these factory fit screws are replaced, you can use your fingers to collimate the SCT. Bob's Knobs can be purchased from here. Make sure you order the correct thread, if in doubt email Bob who will be able to advise you.

The image below shows a SCT with standard factory screws and another SCT fitted with Bob's Knobs

Fitting Bob's Knobs

When your Knobs arrive from the US, you will have three knobs (thumb screws) and three spacers depending on what you ordered. Unscrew each of the factory screws in turn, replacing with a knob, do not remove all the factory screws at once as your secondary mirror will fall out. Once all the knobs are fitted you might want a friend to help you for the next stage.

Step 1
Know the focal length of your SCT, for an 8inch SCT its 2 metres.

Step 2
Ask your friend to stand 2 metres away from the front objective of the SCT with a clear direct view of the secondary obstruction.

The concentric circles might look similar to the below.

This telescope is out of alignment. The centre obstruction shadow is concentric with the centre obstruction, but the other rings are not concentric

Step 3
Turn the knobs so that the circles your friend sees become as concentric as possible, see below.

Step 4
Take the telescope outside and point the telescope at a bright star and de-focus the telescope.
You might think at this point you have broken your telescope as it will not focus very well, fear not it just needs final collimation which may take you a long time to complete.
Help from a friend will come in useful, one person to turn the knobs whilst the other views the star in the eyepiece and gives instructions of which way to turn the knobs. Adjust the knobs, re-centre the telescope as required due to mirror shift and keep adjusting the knobs until you can see concentric circles when de-focused on a star. When your circles of the de-focused star are concentric, try focusing the telescope. If your stars turn to pin points you have completed a successful collimation process.

NOTE: Do not over tighten each knob, as you tighten one knob, the opposite side will need slackening off.

The image below shows a collimated SCT showing its concentric circles

Article by Gavin Lacey

Mid-Kent Astronomical Society
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