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Field of view

Field of view

What field of view should you aim for ?

A1. For many deep sky objects (nebula etc), a relatively low 'magnification' (= wide field of view, short focal length) is required. Whilst light gathering is also important, field of view (short focal length) is often the over-riding consideration, so it's not unusual to find that your 'main' 8" or 10" reflecting telescope is best employed for autoguiding whist the short focus 'add on' 4" refractor is actually used to take the images !

Tip - a 'focal reducer' can be used on a long focal length scope when you want both maximum light and maximum field of view.

A2. For planetary imaging, to resolve the tiny images into 'proper' discs with actual features, high magnification and thus long focal length is typically more important than light gathering power. In this instance, the 'main' scope is often used together with a 'Barlow' (to increase focal length). To increase magnification even further, 'eyepiece projection' is used.

Tip - even if you intend to 'focus on DSO' you should always obtain a Camera <> focus tube mounting assembly of the sort that will support BOTH 'prime focus' (DSO & Moon imaging) and 'eye-piece projection' (planetary imaging).

One note of caution for the DSLR user - with eye-piece projection the eyepiece fits INSIDE the adapter .. this means it's possible for the eyepiece to protrude into the Camera body. If the camera mirror 'flips up' a hits the eyepiece you are going to have a very expensive repair on your hands (a Canon dealer will typically charge £150 to replace the mirror).

How do I get focus when using eye-piece projection ?

Start by positioning the camera, T ring & extension tube on the telescope. Then remove the camera by pressing it's 'bayonet release' button (& not by unscrewing the T ring). Now set your eye 'in the same position' as the camera sensor (typically about 30mm back from the bayonet ring). Then you adjust the focus 'by eye' & lock the focus control. When you re-attach the camera in place of your eye the focus should be close enough to start using a Bahtinov Mask, ideally with a camera that has a 'live view' mode.

TIP. For planetary imaging you will be using the highest magnification possible. This means an eyepiece focal length of 10mm (or even less). The smaller the focal length, the smaller the 'eye relief', which is a measure of the depth of field, so the more 'exact' you will have to position your eye w.r.t. the camera sensor position. It is thus generally much easier to use a 2x Barlow with a 20mm eyepiece rather than a 10mm eyepiece on it's own.

TIP2. Specialist 'Planetary Imaging' eyepieces are now available and quite affordable (when purchased from China, via eBay). These TEND to have longer eye relief than the more normal Plossl type (you should check before parting with your money)

For my guide to Photographing the Moon, click 'Next >>' (in the Navigation bar, left)

Next page :- Using your Camera - (Canon DSLR)