Canon EF Lens Spotting Scope

Here are my notes on how to turn your Canon EF lens into a spotting scope, ex., Canon EF 500mm/f4.0L IS USM.

Click on any image for a larger version.

This was inspired by Samir Kharusi's web page on Converting an EF Lens into an Astro Short APO. I'm willing to buy a solution rather than machine it myself. Instead of building my own adapter, I did a little web search and ended up with the following. Here are the important notes (standard disclaimers for any errors apply - this is my best record of my experience, your experience may vary).

Hutech Astronomical Products supplies the [Borg Telescopes and Components/] parts needed to mount standard 1.25" telescope diagonals and eyepieces to a Canon EF lens.
For spotting scope or "terrestrial" use, an erect-image diagonal is recommended. I chose a 45 degree angled diagonal for viewing ease. Other options that are not recommended include a non-erect image "star" diagonal for "astronomical" use that has less light loss but is left-right inverted, or eliminating the diagonal altogether with both up-down and left-right inverted.
A Barlow lens is needed since there is insufficient back-focus if the diagonal and eyepiece are mounted directly, i.e., you will not be able to achieve infinity focus without the Barlow lens. Maximum focus distance without the Barlow is about 10 feet (more exact measurement is pending, but regardless) -- not very useful.
A Barlow lens and diagonal from the same manufacturer like Celestron may allow you to eliminate the Barlow barrel. Unscrew the lens from the Barlow barrel and screw it into a diagonal that is threaded for filters.

Parts list:

Make	Part#	Description	Source	Price
Borg	7157	OASIS EOS -> M57AD	Hutech	$139.00
Borg	7364	OASIS M57 -> M36.4AD NEW	Hutech	$30.00
Borg	7314	OASIS 31.7mm Eyepiece Holder SS	Hutech	$20.00
Celestron	94112-A	Erect Image Diagonal-1.25" - 45	Amazon	$32.44
Celestron	93326	Omni 1.25" 2x BARLOW	Amazon	$39.27
Celestron	93230	8-24mm 1.25" Zoom Eyepiece	Amazon	$64.79
		Total parts price		$314.50

The Celestron components above are reasonable entry-level quality parts. The zoom eyepiece is convenient for spotting scope use, however, fixed eyepieces can easily provide improved image quality at the cost of loosing the convenience of zooming. There are higher-quality parts available from Celestron and other manufacturers; they can give significantly better image quality. People with very high quality astro components can reuse diagonals, eyepieces, focusers, etc.

I got the 2x OMNI Barlow as part of the Celestron 94303 Eyepiece and Filter Accessory Kit 1.25 in. for $109.99 from Amazon, and it is also part of the Celestron 94304 Nine-Piece Telescope Accessory Kit 1.25 in. for $60.95 from Amazon. The OMNI Barlow is probably the weakest component in the assembly, and I'd recommend moving up to the Celestron Ultima Barlow or better if you can afford it.

You can use the focus ring of many EF lenses, certainly those lenses with FTM (Full-Time Manual focusing) - Ring USM is wonderful.

The 500mm/f4.0L has a deep inset for the rear element, so that there is plenty of clearance for the diagonal with the Barlow lens screwed in. Please be very careful with other lenses that do not have this deep inset so that you do not damage the rear element of your EF lens. Note that the diagonal does not have to be pushed all the way into the short 7314 eyepiece holder.

Spotting scope full assembly side view - Borg adapter parts, diagonal, and eyepiece:

Spotting scope full assembly side view - Canon EF Lens Spotting Scope

Spotting scope full assembly angled view - Borg adapter parts, diagonal, and eyepiece, Bogen Manfrotto 3421 Gimbal support with Kirk QRC 4 Arca-Swiss clamp:

Spotting scope full assembly angled view - Canon EF Lens Spotting Scope

Diagonal and Borg adapter assembly with 7314 and 7364 part numbers visible:

Borg adapter assembly with 7314, 7364, and 7157 part numbers visible:

Barlow lens unscrewed from barrel:

Diagonal with Barlow lens to be screwed in:

Diagonal and Borg adapter assembly with Barlow lens visible:

Borg adapter assembly with Barlow lens visible - Canon EF Lens Spotting Scope

NoBell Home - gjm - last update 10/11/2007, created 6/3/2007