DSLR Focal Reducer

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Recently Paul Russell conducted a review of  the Williams Optics 0.8X Focal Reducer and Field Flattener, Meade and Celestron focal reducer, and the Mogg Adapters T-0.66FR2 as used on the Orion ED-80. The text of his report is available

A number of people have been interested in using DSLR camera with the new Orion 80mm ED Apochromatic Refractor. George Tucker contacted me and asked if I could have a look at making a focal reducer for this setup. The scope has an 80mm aperture and FL of 600mm giving f/7.5 focal ratio. It has a 2' focuser and gives very good performance for the price by all accounts. What's nice about the ED80 is that Orion have been able to produce a scope that gives performance that approaches the top of the class at a price that doesn't. As one reviewer of the ED80 put it:

"In the battle of who makes the best compact, multipurpose telescope an aspect has been lost in all the rhetoric - it is possible to buy a telescope that does not claim to be the finest in the world, but is one heck of a good balance between aperture, focal length, overall optical performance and cost"

Company 7 Orion 80mm ED Review

I think this concept fits well with the whole motivation behind use webcams as imaging devices and the Mogg Adapter product range. So far the missing link for the ED80 is a reasonable priced Focal Reducer. One that may not be perfect but delivers good quality at a reasonable price. The alternatives such as the Televue 0.8 TV range reflect the pinnacle of quality and price. The opportunity is to develop a focal reducer that's performance equals that of the ED80 at the same reasonable price.

A suitable focal reducer would need to be mounted in 2' tube with a T-Thread mounting at the camera end. Standard T-Rings can then be used to mount all models of DSLR camera. The 2' mounting will enable a larger lens to be used which would accommodate the larger DSLR imaging chip reducing vignetting effects. The focal reducer will work equally well with any scope in the 400 - 800 mm focal length range and is not limited to the ED80.

Design and Development

The first step is to make some proto types and do some testing. I sent a couple of proto types to Chris Venter. Chris has developed a very useful DSLR camera control and focusing program caller DSLR Focus, and was keen to help out. The initial focal reducers was designed to give a reduction of 0.6. You can see the results of this below with the Eta Carinae images. I think this is a bit to much reduction as there is significant elongation of star images as you move towards the edges of the full frame. Some elongation is evident in the full frame image of the Jewel Box which was taken without a focal reducer. The Televue reducer image of Eta Carinae show reasonable elongation.

I think we were a bit optimistic trying for a 0.6 reduction. This reducer is right on the limit of available inwards focuser travel of the ED80 and the elongation of the star images is a little to strong to be useful. A much better result is achieved with a 0.75 reduction unit. Although there is some merit to the higher reduction ratios as they reduce exposure times for dim objects. They might be very useful for galaxy hunting. A variable ratio focal reducer between 0.75 and 0.66 would give you the best of both worlds. This will enable you to use it at 0.75 when you need the full frame and 0.66 when you need the extra help with exposure time and can crop the image down a little.

The focal reducer has a 48mm filter thread on the scope end so you can fit your favourite 2' filters when required.

Comments - Suggestions - Questions:

If you have any comments, suggestions or questions please email me.


 The part number will be F2-VFR-T. Variable focal reduction fro DSLR Camera between f/0.75 and f/0.66.


Orders are now being taken for this adapter. Go to my order page at: order page.


The variable focal reducer costs US$75.

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Test Images


Image (Click image to see full size)


Eta Carinae, Canon D60, ED80, 5 x 60sec at ISO800, focused with DSLR Focus, Focal ratio f/4.7 using prototype Mogg Focal Reducer (f/0.62).

Eta Carinea - full frame of above image. You can see a large amount of star elongation. A reduction ratio of 0.6 is bit to extreme.

Eta Carinea - full frame using Televue 0.8 focal reducer. Canon D60, ED80, 5 x 60sec at ISO800, focused with DSLR Focus, Focal ratio f/6.

Jewel Box - full frame Canon D60, ED80. You can see that in the corners there is a small amount of elongation

Omega Centauri - Canon D60, ED80, 5 x 60sec at ISO800, focused with DSLR Focus, Focal ratio f/4.7 using prototype Mogg Focal Reducer (f/0.62).

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