What can a field flattener or focal reducer do for my telescope?

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

Telescope stats explained

I have a William Optics Zenithstar 61 telescope. I’m using it with a 0.6x flattener-reducer. What is this doing, and what magnification would I get with a 25mm eyepiece?

Steve says: “The William Optics Zenithstar 61 is a portable, well-made refractor with a short focal length of 360mm making it ideal for wide-field observations.


The multi-coated, doublet optics use low- dispersion FPL-53 glass so the scope is also suitable for imaging deep-sky objects.

As with all refractors, a field fattener or field fattener-focal reducer is recommended for imaging purposes but it is unusual to use such an adaptor for observational use.

There’s no reason why you shouldn’t use one for this purpose as it will allow you to observe even larger swathes of the night sky than the instrument already displays.

The magnification of a telescope-eyepiece setup is calculated by dividing the focal length of the telescope by the focal length of the eyepiece, which in the case of your Zenithstar gives 360÷25 = 14.4x magnification.

This type of magnification is in the binocular realm and will deliver great views of large asterisms, constellations and many deep-sky objects, but planets and the Moon will be very small indeed.

The field of view will be 4.7° across, or roughly the equivalent of nine Moon widths.

Inserting a 0.6x focal reducer produces an effective focal length of 360×0.6, which equals 216mm, yielding a magnification of 8.64x, which equates to a field of view of approximately 15 Moon widths.

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