Why you need to let your telescope cool down, and how to do it

BBC Sky at Night Magazine’s Scope Doctor Steve Richards solves your practical astronomy ailments.

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Published: Wednesday, 25 September 2024 at 07:11 AM

Whether you have a reflector or a refractor, you may have heard experienced practical astronomers talk about allowing a telescope to cool down before use.

It is very important to ensure that your telescope is at ambient temperature to achieve the best views, as an imbalance of temperatures will result in disturbed air moving around in the optical tube, which will blur detail.

To add to the problem, air temperatures usually continue to drop throughout an observing session so to some extent you will be chasing the ambient temperature downwards.

Newtonian telescopes cool down faster than refractors. Credit: BBC Sky at Night Magazine

Type of telescope and cooling down

Different telescope designs cool down at different rates, with Newtonian reflectors cooling faster than refractors.

Ideally, Newtonians and refractors should be allowed to cool for about an hour, and Schmidt-Cassegrains and Maksutovs for a couple of hours.

The relatively large mass of the mirror in a reflector holds a lot of heat energy so blowing cool air over the mirror with a fan can help this to dissipate and some Newtonians have a cooling fan built in.

Warm air rises so to help it on its way, placing a Newtonian facing upwards and a refractor facing downwards – but with no diagonal or eyepiece in the focuser – is a good way of speeding up the telescope-cooling process.

Storing your telescope already set up and in a cool, dry place like a garden shed will mean you need to spend less time letting it cool down. Credit: Dennis Gross / Getty Images
Storing your telescope already set up and in a cool, dry place like a garden shed will mean you need to spend less time letting it cool down. Credit: Dennis Gross / Getty Images

Storage

It’s vital to have your telescope at equilibrium with the outside temperature to prevent air currents in the tube from disturbing the view.

Closed tube designs such as Schmidt-Cassegrain and Maksutov telescopes are notoriously difficult to cool down, so it pays to start the process as early as possible.

Store the telescope in a cool area like a garage and set up long before you expect to start your viewing or imaging session.

Remove the front and rear dust a covers at the start of the process and if you have l Newtonian reflector, consider installing an external fan at the bottom of the primary mirror’s cell.

Email your astronomy queries to Steve at scopedoctor@skyatnightmagazine.com and it could get answered in a future issue of BBC Sky at Night Magazine.