Bolton Astronomical Society

"Zooming out" with a focal-reducer
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Author:  rwilkinson [ Sat Dec 05, 2015 11:33 am ]
Post subject:  "Zooming out" with a focal-reducer

Long focal-length telescopes such as Maksutovs and SCTs are great for high-magnification planetary observation and imaging, but for many other objects a wider field-of-view is needed, which requires a shorter focal-length telescope.

But rather than swapping to a shorter optical tube, how about "zooming out" with your existing 'scope?
Well, this is actually possible - by fitting a focal-reducer lens between the 'scope and camera. We've recently purchased one of these for use with our Atik Infinity camera system:
This type is designed for use with small-sensor imaging systems (the Infinity has a 2/3"-diagonal CCD). A bigger lens would be requred for a DSLR imaging system: these are available, but are rather more expensive (>£100).

Here are some examples of our little Revelation Astro reducer in use with the Infinity camera, trained on a familiar local landmark:
File comment: C8 SCT at f/10
clock_f10.png [ 454.06 KiB | Viewed 2142 times ]
File comment: Using the focal-reducer to achieve f/5.6
clock_f6.png [ 475.25 KiB | Viewed 2142 times ]
File comment: Adding an extension-tube behind the reducer to achieve f/4.2
clock_f4.png [ 460.76 KiB | Viewed 2142 times ]

The reduction-factor of the lens depends on its spacing from the sensor. Fitting the lens right onto the front of the camera's nose-piece produced an f/5.6 system, but then adding a 15mm spacing-ring behind the reducer opened it up to f/4.2.
I found this on-line calculator from Wilmslow AS:
which helps in working out what can be achieved (the Revelation reducer-lens has a focal-length of around 95mm).
The "in-focus" parameter is a measure of how far the focus-adjustment needs to be retracted to achieve focus. The moving primary mirrors in SCTs & Maksutovs give them a larger focus-range, but refractors have a more limited accommodation, so less focal-reduction can be realised with them.

The benefits of using a focal-reducer include:
  • the larger field-of-view makes it easier to locate your target. and allows larger targets to be imaged
  • a brighter image allows shorter imaging-times
  • the reduced image-scale means that tracking errors are less significant

So compared with my C8's native f/10, operating it at f/4.2 gives a 5.7x larger field-of view, a 5.7x brighter image and reduces my tracking-errors by a factor of 2.4.

Bear in mind that the ultimate image-quality with a focal-reduced 'scope will not be as good as with a telescope expressly designed for this shorter focal-length. But if you only have the one optical tube, using one of these lenses can give it a new lease of life to image a wider range of objects.

Author:  rwilkinson [ Thu Dec 24, 2015 10:02 am ]
Post subject:  Re: "Zooming out" with a focal-reducer

Here's an example of over-doing the focal-reduction (from f/6.9 down to f/4.1):
File comment: SW80ED reduced to f/4.1
panstarrs_10min_f4.png [ 349.27 KiB | Viewed 2070 times ]
Note the vignetting effect and how the outer stars are distorted.
The wider field-of-view does make it a lot easier to find a target when star-hopping, and you could always crop out the sharp central area around the target.

For comparison, here's the same 'scope operating at its native f/6.9:
File comment: SW80ED operating at native f/6.9
panstarrs_15min_f7.png [ 420.21 KiB | Viewed 2070 times ]
Note that the image is much sharper across the field, but not as bright - even with 900-sec total exposure time compared with 600-sec at f/4.

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