Building an Eyepiece Holder for Canon EF Lenses Using a Rear Lens Cap
For Finding and Centering Astronomical Objects for Astrophotography with Parfocal Eyepiece

Finding and centering astronomical objects in the view finder of consumer grade Digital Rebel or 20D cameras or older film cameras (e.g., EOS 650) can be challenging. A sturdy (flexure free) eyepiece holder that can hold a parfocal eyepiece is a useful aid to astrophotography and can be constructed from a Canon EF lens rear cap and threaded 1.25 inch eyepiece holder.
The example I photograph below uses a Televue 25mm Plossl eyepiece shown in position that is parfocal with the sensor/film plane of EOS cameras (A 19mm panoptic worked fine too). The eyepiece holder will not work with guiding eyepieces (e.g., 12.5mm Orion) or ST-4 as the working distance is too large.

Items needed to make a parfocal 1.25 eyepiece holder: 1) Canon rear lens cap E (Canon # 2723A002- $7); 2) Eyepiece holder unscrewed from cheap 1.25 inch star diagonal (If you don't have one, you can purchase a threaded eyepiece holder [e.g., Borg part 7317 for less than $25]). The cheap Celestron 1.25 star diagonal I took the part from came with the OTA I purchased and costs $33 new at the time of writing. I have two other cheap diagonals that came with used telescopes or guidescope in my junk part box that are similarly constructed.

Tools I used to make the eyepiece holder: 1) 3/8 inch drill and 5/16 inch drill bit; 2) Grinding tool (I used a Dremel power tool with grinder attachment).

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Parfocal eyepiece in eyepiece holder in place of camera for object location and centering. Canon 20D back in place and in focus for astrophotography Parfocal eyepiece and eyepiece holder on Canon EF 300mm f/4 L lens.
Unfortunately, the working distance of the eyepiece holder is too long to use with my Orion 12.5mm guiding eyepiece with reticle or my ST-4.
The constructed cap and eyepiece holder is at least as sturdy and flexure free as the star diagonal from which I salvaged the eyepiece holder.
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Use a Canon lens cap rather than a cheap generic lens cap. The Canon lens has a wider contact surface with the lens that helps keep if from coming loose from the lens or T-ring bayonet mount. Inside detail of Canon (left) versus generic lens cap for EF lenses. Cheap 1.25 inch star diagonal. Eyepiece holder unscrewed from star diagonal.
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Eyepiece holder on white paper. Trace around the holder and then cut out the paper to make a hole guide. White paper hole guide on inside of lens cap to make tracing for hole. Pencil or marker will work. I used garrish fluorescent orange paint (not recommended) to make a negative image of the guide to provide contrast for photography of black on black parts
Drill successively around the inside perimeter of the guide to remove the plastic core. Grind the edge of the hole to gradually and evenly enlarge it until the tip of the threaded eyepiece (but not the threads) will pass through the hole. Screw the threaded eyepiece holder into the lens cap. The metal threads will cut threads into the plastic. Screw it all the way into the cap to form a sturdy, flexure-free lens to eyepiece adapter.
Top view of cap and eyepiece holder.

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© 2004, William L. Castleman
Initially posted 19 December, 2004