{"id":31739,"date":"2022-05-10T11:03:16","date_gmt":"2022-05-10T11:03:16","guid":{"rendered":"https:\/\/www.skyatnightmagazine.com\/?p=50457"},"modified":"2022-05-10T11:19:11","modified_gmt":"2022-05-10T11:19:11","slug":"a-guide-to-telescope-autoguiders","status":"publish","type":"rss_feed","link":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/rss_feed\/a-guide-to-telescope-autoguiders\/","title":{"rendered":"A guide to telescope autoguiders"},"content":{"rendered":"

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By Ade Ashford\n \t\t<\/p>

Published: Tuesday, 10 May 2022 at 12:00 am<\/p>

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An autoguider is a digital camera with a CCD or CMOS detector at the focus of the guidescope or, in the case of an off-axis guider, at the focus of the imaging instrument after some of its light has been diverted by a small prism.<\/p>\n

The autoguider captures regular snapshots of the sky and a computer compares them to detect star drift caused by the apparent movement of stars across the night sky, issuing corrections to the mount via a cable attached to the autoguide port to keep the mount on track.<\/span><\/p>\n

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M 101 The Pinwheel by Gary Opitz, Rochester, NY, USA. Equipment: Telescope Engineering Company 140 APO refractor, ZWO ASI1600MM mono camera with filter wheel, ZWO LRGB filters, 80 mm Orion Shortube guiding refractor, Orion Starshoot Autoguider camera, Orion Atlas mount<\/span><\/figcaption><\/div>\n

Why use an autoguider?<\/strong><\/h2>\n

T<\/span>oday\u2019s digital cameras possess awesome potential, empowering more people than ever to become astrophotographers.<\/p>\n

But, while it\u2019s easy to produce passable wide-field constellation portraits using nothing more than a DSLR, a fixed tripod and exposures<\/span> of up to half a minute, capturing a detailed image of a typical deep-sky object is a bit more involved.<\/p>\n

As a minimum, you\u2019ll need to use much longer exposures and a lens with a focal length of at least 200mm. <\/span><\/p>\n

Ideally, the camera<\/span> needs to be attached to an equatorial mount<\/span> that has been precisely polar aligned so that it accurately tracks the stars as they move against the night sky \u2013 otherwise you\u2019ll see trailing in your images.<\/p>\n

If you need help picking out a mount, read our guide to the best telescope mounts<\/a> available.<\/p>\n

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Vega and the surrounding stars through a telescope. Unguided, the scene looks blurry and indistinct. Compare this to the one taken with a guided setup at the top of the article. Credit: Ade Ashford.<\/span><\/figcaption><\/div>\n

However, even the most expensive and precisely polar-aligned mount will exhibit unavoidable image trailing with long focal length lenses or long exposures.<\/p>\n

For example, my 25-year-old Vixen GP mount is capable of unattended exposures with a 400mm lens \u2013 but only for exposures up to 3 minutes.<\/p>\n

This limitation is due to the cumulative effects of minute imperfections in the drive system\u2019s gears (termed \u2018periodic error\u2019), flexure in the camera and mount, and atmospheric refraction.<\/p>\n

To maintain pin-point stars, we must resort to guiding the mount throughout the exposure.<\/p>\n

Traditionally, astrophotographers would do this by carefully monitoring a star on the reticle of an illuminated eyepiece in a guidescope<\/a> \u2013 a small scope mounted in parallel with the imaging instrument \u2013 making tiny drive corrections with a hand controller as and when required for the duration of the exposure.<\/p>\n

Fortunately, today\u2019s computerised equatorial mounts free us from such drudgery<\/span> by possessing an autoguide port (sometimes called a CCD port) conforming to the de facto standard set by the Santa Barbara Instruments<\/span> Group\u2019s ST-4 autoguider nearly 25 years ago.<\/p>\n

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With an off-axis guider, there\u2019s no need to attach a separate guidescope to your setup.<\/span><\/figcaption><\/div>\n

What\u2019s in an autoguider?<\/strong><\/h2>\n

An autoguiding system consists<\/span> of the following parts:<\/p>\n