{"id":38924,"date":"2022-12-02T14:28:09","date_gmt":"2022-12-02T14:28:09","guid":{"rendered":"https:\/\/www.skyatnightmagazine.com\/?p=113474"},"modified":"2022-12-02T15:10:03","modified_gmt":"2022-12-02T15:10:03","slug":"the-best-off-axis-guiders-for-astrophotography","status":"publish","type":"rss_feed","link":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/rss_feed\/the-best-off-axis-guiders-for-astrophotography\/","title":{"rendered":"The best off-axis guiders for astrophotography"},"content":{"rendered":"

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By Steve Richards\n \t\t<\/p>

Published: Friday, 02 December 2022 at 12:00 am<\/p>

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Guiding your mount can make a huge difference to the quality of your deep-sky images. This is because accurate tracking enables you to increase the length of your exposures while ensuring you get good, point-like star shapes by avoiding star-trailing.<\/p>\n

Fitting your guide camera to an off-axis guider (OAG) is a popular alternative to the time- honoured practice of fitting it to a separate guide telescope to make sure that your mount tracks objects accurately.<\/p>\n

They also resolve two of the issues associated with using a separate guidescope<\/a> \u2013 additional weight and unwanted movement between the two telescopes.<\/p>\n

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NGC 253 (Sculptor) by David Atti\u00e9, Abu Dhabi. Equipment: C11 XLT Fastar, focal reducer 0,63x, CCD Moravian G2-4000 (internal filter wheel), Skym\u00e9ca Off Axis Guider Skytrack, LodestarX2, Az-Eq6, Baader LRGB filters.<\/span><\/figcaption><\/div>\n

OAGs are small and light, but most importantly, they have a small prism that picks off a sample of the view to find a guide star, through exactly the same telescope that you use for imaging.<\/p>\n

They\u2019re not without their own issues \u2013 it can be difficult to achieve simultaneous focus of the imaging camera and the guide camera, and the reduced field of view captured by an OAG\u2019s prism can make locating suitable guide stars difficult.<\/p>\n

But they do get rid of unwanted movement between the main scope and guide scope, and combat the problem of \u2018mirror flop\u2019 in Schmidt-Cassegrain telescopes, when the scope\u2019s mirror moves slightly during long exposures as the scope tracks.<\/p>\n

If you\u2019d like to know whether an OAG could help you take better images, read on as we put four to the test.<\/p>\n

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An Active Galaxy by Terry Hancock, Fremont, Michigan, USA. Equipment: Astro-Tech AT12RC, Paramount GT-1100S German Equatorial Mount, QHY16200A mono CCD, QHYOAG-M Off Axis Guider, Optolong LRGB filters.<\/span><\/figcaption><\/div>\n

How we tested our off-axis guiders<\/strong><\/h2>\n

All off-axis guiders were tested using the same criteria: build & design, connectability, ease of use, features and guide star quality.<\/p>\n

The features we looked at included:<\/p>\n

Attachment<\/strong><\/h3>\n

We checked how easy it was to attach each off-axis guider to the telescope and to the imaging and guiding cameras. We also tested the rigidity of the connections, as any movement here could result in inaccurate tracking.<\/p>\n

Construction<\/strong><\/h3>\n

Off-axis guiders should be light, to avoid stressing the focus tube, yet robust enough to support the weight of both an imaging camera and a guide camera. The pick-off prism should be of high quality \u2013 adjustable yet held securely.<\/p>\n

Ease of focus<\/strong><\/h3>\n

It\u2019s important that both the imaging and guiding cameras achieve focus at the same point, so we checked that this was possible and noted any adaptors and spacers that were required to attain good focus with both.<\/p>\n

Image quality<\/strong><\/h3>\n

The pick-off prism can introduce optical aberrations as it collects light from the very edge of the field of view where optical distortions are at their worst. We checked the quality of the guide stars to ensure their suitability.<\/p>\n

Prism use<\/strong><\/h3>\n

We noted the size of each prism, as this would affect the area of sky that would be covered, and checked if the prism\u2019s position could be adjusted to allow us to choose a specific area of the sky from which to select guide stars.<\/p>\n

Best off-axis guiders<\/strong><\/h2>\n

Celestron Radial Guider<\/strong><\/h3>\n
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