Finder/Parfocal Focuser

A Parfocal Eyepiece Holder that can be swapped as easily as a camera --
and costs next to nothing to make...

NOTE: This "invention" can also be used to adapt a favorite old telephoto camera lens
into a small telescope. Just attach it to your favorite old telephoto!!!

Two of the challenges of astrophotography with a DSLR are finding and focusing on the subject. Especially with the small, dim viewfinders of the typical DSLR camera, it can be a challenge just to locate some objects. And the typical focusing screen is way too small and gets only the roughest of focus.


These problems can be addressed in several ways: a parfocal eyepiece is the easiest. However, my favorite camera adapter must be unscrewed from t-mount adapter, or from the scope itself, in order to swap the camera for an eyepiece. And my Newt must be used with a Para-Corr (shown at left), which again requires screwing many turns to mount and remount the camera. (Other alternatives with the newt also involve several manipulations more than I care to make. )

What I really wanted was a parfocal eyepiece holder that could be mounted or removed as easily as my camera--simply using the existing bayonet mount.


I had a device for my old OM-1  that featured the female part of the lens/camera mount on the front of a simple 1.25 eyepiece holder. The adapter worked the same as the front of the camera, and allowed me to leave the T-mount adapter in place in the focusing tube when I swapped the device for the camera. With an eyepiece in the holder, I could find and focus, swap out the device, and swap in the camera to take the picture.  There did not appear to be something similar for my Canon Eos mount (at least at any price I was willing to pay). 

So, I made one from a rear lens cap (whose business end is shaped the same as the front of the camera) and a spare 2 inch to 1.25 inch adapter.

The procedure was exceedingly simple. Simply cut a 1.25 inch round hole in the middle of the rear lens cap (using a razor blade, x-acto knife, and Dremel moto-tool as needed). Then glue it to the back of a 2 to 1.25 adapter (I used simple household GOOP, but other glues would probably do as well).  

To "calibrate" it, focus your camera as well as it can be focused in your scope. Remove the camera from the T-mount. Attach the newly made adapter in its place. Slide your eyepiece in and out until the image also looks as well focused as possible. Tighten the setscrew, and you have a parfocal eyepiece that can be swapped in and out just like the camera. 

To use the device, simply attach it where you would attach your camera. Find and focus your target just as you would if you were  observing visually. Then swap the adapter/eyepiece out and your camera in and take the picture. (Note that I usually go to a computer assisted focusing routing with DSLR Focus or Images Plus before actually taking the picture--but this is be a good idea whenever using a parfocal eyepiece regardless of how it is attached to the telescope.)

Eyepiece Notes: After you get a perfect focus with computer-aided focusing on any given evening, you may want to adjust the eyepiece in the adapter to assure it is parfocal. I use a simple 25 mm plossl for the best compromise of wide field of view, relative brightness, and good focus. If you wanted to concentrate on focus, use a shorter focal length. If you prefer using for finding, opt for the wider field of a longer focal length. With a set of parfocal rings, you could actually swap different eyepieces into your newly made adapter, and get the best of both wide-field and high magnification as needed.

Materials Notes:  Get the tightest fitting rear lens cap you can find. They cost about $7 new, at most camera shops.   You may want to experiment a little to make sure you have a 1.25 adapter of the right height--that is, one that allows your eyepiece to reach focus at some point in its attachment. After you have been in the hobby for a while, one tends to collect 2 inch to 1.25 adapters and a few spare 25 mm plossls that lend themselves to this handy device.



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