{"id":28812,"date":"2022-02-02T13:39:18","date_gmt":"2022-02-02T13:39:18","guid":{"rendered":"https:\/\/www.skyatnightmagazine.com\/?p=46987"},"modified":"2022-02-02T13:54:10","modified_gmt":"2022-02-02T13:54:10","slug":"a-guide-to-star-diagonals","status":"publish","type":"rss_feed","link":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/rss_feed\/a-guide-to-star-diagonals\/","title":{"rendered":"A guide to star diagonals"},"content":{"rendered":"

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

Published: Wednesday, 02 February 2022 at 12:00 am<\/p>

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The eyepieces on Newtonian reflectors generally remain at a reasonably comfortable viewing height irrespective of where they\u2019re pointing.<\/p>\n

But as refractors and Cassegrains tilt up in altitude their eyepieces<\/a> get lower and lower, making them harder to look through. This is where a diagonal comes into play.<\/p>\n

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A diagonal is a small triangular attachment, usually incorporating a small surface-coated mirror set at 45\u00b0.<\/p>\n

It bends light collected by the telescope through 90\u00b0 before projecting it into the eyepiece.<\/p>\n

This simple attachment makes it much easier to observe because now you can view looking down into the telescope rather than up into it.<\/p>\n

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Star diagonals are essential accessories for refractors or Schmidt-Cassegrain telescopes: they give a comfortable, upright observing position.<\/span><\/p>\n

However, they do have optical disadvantages, apart from mirror-flipping the view. <\/span><\/p>\n

Cheap diagonals use low quality prisms or aluminium coatings, which can degrade and lose reflectivity.<\/span><\/p>\n

But these days most diagonals feature dielectric coatings, which consist of multiple layers of thin oxides.<\/span><\/p>\n

These can retain 99% reflectivity throughout the life of the star diagonal, and give a higher surface accuracy.

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How to test a star diagonal<\/strong><\/h2>\n

When testing out a star diagonal to see whether it\u2019s worth your hard-earned money, use it to examine both bright and faint objects in the sky.<\/span><\/p>\n

The brightest object in the sky, the Moon, is a great test for how well baffled the interior of each star diagonal is against stray internal light.<\/span><\/p>\n

Fainter star fields can be used to assess the dielectric coating.<\/span><\/p>\n

10 of the best star diagonals<\/strong><\/h2>\n
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1<\/span>

Celestron 90\u00b0 1.25-inch Dielectric Star Mirror Diagonal<\/h3>\n<\/div>

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