Converting an EF Lens into an Astro Short APO

You own those beautiful Canon EF supertele primes but you cannot use them conveniently as high quality, compact, lightweight astronomical telescopes. Sounds familiar? It's possible to buy a ready made "Scope Eyepiece", complete with a 10mm narrow-field eyepiece and an erect-image prism contraption in a single package that bayonets directly onto Canon EF lenses. Reasonably nice for terrestrial use but a thorough pain-in-the-neck for astronomical use. What we really need is a means of using our standard astro diagonals and our much beloved 1.25" barrel eyepieces. Two-inch eyepieces are probably a bit of overkill, since the lenses are designed for 35mm format (43mm diameter image circles) and the back-focus issues get compounded by the long optical paths in 2" diagonals. When you try to attach a diagonal + eyepiece directly onto an EF lens you immediately find out that there is simply insufficient back-focus. So you need a relay lens or a Barlow to extend back-focus. Easiest is to use a standard Barlow. Two of many ways to attach the Barlow to the lens: Get an EF rear lens cap, drill a 1.25" hole in it and epoxy either a plastic (plumbing) or metal pipe with 1.25" ID. Add a set screw for the Barlow and you are done. I used a second method, yielding a more secure attachment for the Barlow and the sometimes rather expensive eyepieces, to yield a very nice short APO with any EF lens. Here's a version using the Canon EF 200mm/2.8L:

There is no problem generating high magnifications. Just use a short eyepiece and, if necessary, even add a 2x tele-extender. If you have managed infinity focus with the lens on its own, you will also do so with a tele-extender. The problem is achieving an ultra-wide view because you have already extended the lens' focal length by a factor 2x. Nevertheless this is not as bad as it appears at first. Most Canon tele-primes have much faster focal ratios than astro APOs. Even the so-called short-tube or rich-field APOs tend to have focal ratios of f6 or slower. So, for example, the lens above comes out the equivalent of a 71mm aperture APO with a focal ratio of circa 5.6, including the 2x Barlow. Using it with an eyepiece that has the widest-field-possible in a 1.25" barrel consequently yields very satisfactory wide field views. I use a Celestron Ultima 35mm eyepiece to fill this role, but many others are available. Just ascertain that it has the widest-field-possible in a 1.25" barrel; basically that the eyepiece has a field stop of 28 to 29mm. With the Celestron 35mm Ultima eyepiece, Ultima Barlow and the Canon 200mm/2.8L lens the FoV is a very ample 5 Moon diameters.

It takes a lot of skill to fabricate a female EF bayonet, so better not even try; simply purchase one. My source is any cheap 12mm EF extension tube that I dismantle. The above is a Kenko, but I also have an even cheaper version (one of those "generic" deals) that is all metal that I use for attaching an astro CCD to any EF lens. Same principle, use the front (female) bayonet and discard the rear (male) bayonet. This Kenko is part plastic (the black ring with the writing) but nevertheless it's strong enough. Here is the Kenko 12mm Extension Tube with the male bayonet removed by unscrewing 4 watch screws:

The 1.25" eyepiece holder is simple lathe work, so any friendly metal banger can make you one if you are not quite up to it yourself. Retain the 3 or 4 watch screws from the extension tube to mate it to the female EF bayonet:

Important !!! Try out your eyepieces and diagonal before settling on how much extension you can accommodate and still be able to focus at infinity. You will have to hand hold the Barlow, diagonal and your desired eyepieces and look through them with the lens focused at infinity. You then measure the gap very carefully. You should err on the short side. If you have a holder that is slightly too short then all you need to do is to focus the lens. If the adapter is too long you will not be able to focus infinity. With my thingies it is 30mm, i.e. the total height of the adapter made by lathe work had to be no more than 30mm, but yours may be different since there is no common standard to par-focalize astro eyepieces. By the way, if you can achieve focus with one EF lens, you'll also achieve focus with any other, with or without tele-extenders. Here is the assembled adapter, together with "thumbscrews" that are used to hold the Barlow:

 And here is a close-up showing the complete chain in use:


The view through the finished APO is excellent and you just twist the bayonet to stick the diagonal onto any other EF lens. Next time I am out observing I'll try out the Canon EF 600mm/4.0L IS lens as a 6" astro APO :-)

It may be possible to purchase a ready-made adapter that will mate an EF lens to a 1.25" barrel, but for some reason this route can be quite expensive (>$150?) and you still have to verify upfront that it will not give you excessive extension that makes it impossible to achieve focus at infinity. The cheapest, lowest level of required skills route, is the method using a rear lens cap as described earlier, with the above-detailed approach as being a reasonable-cost and more robust alternative. In any case, if you already own a Canon (or any brand for that matter) supertele then you might as well take steps to maximize its utility.

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