TJIIRRS: Number 12

Building A Simple 5" Refracting RFT

and a Small Spotting Scope

(March and April, 2007)

Here is a relatively inexpensive and straightforward way to make a small refracting telescope. The objective has short enough focal length that the instrument is probably best used as a Rich-Field scope or for terrestrial observing, though you will probably want an inverting lens for the latter, as the original design will give you an upside-down view. (This is how telescopes generally work, unless you take steps to invert the image.)

Your first step is to acquire a projector lens from an old “Opa-Scope” model 20005 opaque projector. These lenses have 5" clear aperture, and focal length of 18", which will do. The OD of the barrel is 5&3/8". They are available from time to time on eBay, and that’s where I got mine. (If you get two, you can make a very large pair of binoculars.) If you can’t find an Opa-Scope lens, any other lens with similar characteristics and good color-correction will do. Longer focal length is probably a plus; shorter focal length may present problems. Add an eyepiece in an adjustable holder, and there you are. Well, okay, you do have to work up some sort of mounting — the device has a lot of glass in it, and is pretty heavy.

The result may not be very good in formal terms, but it should certainly be a telescope. (Let’s find out by building one.)

On a very hazy basis, I decided to use 2" diameter eyepieces, and acquired two of them. One seems to be another projection lens, a small one, mounted in a 2" eyepiece barrel; it supposedly has 30 mm focal length, but something is not quite right there, as it gives me lower magnification than it should. The other is a regular eyepiece design, a Rini 2" 35mm MPL eyepiece with "T" mount. That gives me magnification of 13 diameters, which is relatively low, and eventually I will probably acquire yet another one, with shorter focal length, or perhaps a Barlow. In the meanwhile, however, I certainly can’t complain. I got both of these eyepieces, btw, from The Surplus Shed, a very pleasant source for optics of various sorts and other surplus items.

[Note: To find the magnification you’re getting, divide the focal length of the objective (in this case 18" or ~457 mm) by the focal length of the eyepiece; 457/35 is just a little over 13.]

I also got lucky, and was able to buy a 2" eyepiece holder on eBay. It’s not a high-quality one, but this is not going to be a high-quality telescope. Besides, if I get sufficiently peeved at it, I can always go find a better one.

The distance from the front of the objective to the leading edge of the eyepiece is 22" or a little less; you should, however, measure this yourself. For one thing, if you want to use this device as a long-reach microscope, the distance will probably have to be somewhat longer. For another, different eyepieces have different requirements, as I discover below...

You can build a tube out of wood veneer (we found a piece on someone’s trashpile that I nearly used for this) or something similar, but a large-diameter piece of PVC pipe will do really nicely if you can find one. I got very lucky, and was given a piece with an expanded region at one end that my Opa-Scope lens fits into, with a bit of “wiggle room”.

Then there is the issue of mounting the eyepiece holder. I chose to use a toilet flange from the hardware store, which is one of several reasons why I originally called this instrument “The Crap-O-Scope”. (Crap[per flange] / O[pa]-Scope [lens]. The name also reflects the fact that it provides somewhat less than professional-level performance.)

Here are some photos of the scope in its initial testing configuration, taken a few minutes after it reached the point at which real testing became possible...


The lens is held in by two plastic spoons and a small screw. Needless to say, this is only for testing purposes, and is a dicey arrangement at best; caution is indicated. (I am actually using a damaged lens that I acquired cheaply on eBay, so as to avoid risking my real objective.)

Note: I should have used black aquarium caulk to attach the eyepiece holder to the toilet flange, instead of white paintable window caulk. Also, I should have painted the inside of the flange before mounting the holder onto it. (I ended up using the cap from a jar of chokecherry jam [very yummy; wish I had more...] to protect the holder from the paint. It worked quite well.) I did use black aquarium RTV to attach the flange to the PVC pipe, and I have added 3 small screws to make sure that pipe and flange do not get any silly ideas about parting company with each other.

Speaking of paint, you should use some sort of matte black paint on the inside of the tube, and also on the back & inside of the toilet flange (or whatever you decide to use as a mounting surface for the eyepiece holder). I used Rust-Oleum® high-temperature black paint, intended for barbecues and gas grills. It is entirely satisfactory for this purpose, and is also the item of choice for the front surface of a thermoelectric cooler that will be used as a laser power/energy sensor. (I can put up a brief page about that, if anyone is interested; if so, please send me an inquiry via email.)

Test Results, Preliminary

(02 April, 2007)

Here is the view from my dining area, out of the window in the next room. You can see one end of a blue house on the right side, about 1/4 or 1/3 of the way up from the bottom:

This photo is cropped from a larger image that I underexposed a little so that the outside world is recognizable; when I tried a “correct” exposure, the camera did its best to compensate for the huge difference in brightness between interior and exterior, the result of which was not viable for this purpose. Please note that this is already significantly larger than what I see with my eye, as I pushed the focal length out in order to get a reasonable number of pixels after cropping. I would guess that you’re looking at ~3X magnification, but don’t quote me.

Here is what I get if I put my G3 up to the eyepiece and adjust the focal length until the camera sees approximately what I see with my eye:

Please note that aside from resizing, this photo is straight out of the camera; I didn’t mess with it at all. (If you want the original pixels, click the little image and then change 12c in the filename to 22c.) Also note that I still have the damaged objective in the scope, and that the G3 was slightly off-center on the eyepiece, as you can see by the skewed position of the vignetting. Despite these issues, it is abundantly clear that this instrument works.

(That evening)

I just got a look at what must be Venus, and it wasn’t too bad. Not much to see, because of the low power, but there does appear to be a bit of a disk. I don’t know whether I will be able to photograph this without a suitable mount (see the section on Mounting), but if the opportunity presents itself, I will try...

(An hour or two later)

The sky has darkened since I wrote that, and I can now see stars, lots of them, ...but only through the scope. I can’t actually see any with my naked eye, and this is only partly because I am not fully dark-adapted. It really is a telescope!

Here is a photo of Venus and some powerlines. Please understand that I had a lot of trouble focusing the camera, and that it was not really possible to hold both camera and scope steady for the 1-second exposure. You should also ignore essentially anything that looks like a star, for example the tiny bright dot just below and to the right of the planet. Even though I had the camera at its lowest ISO setting (50) in an effort to minimize the amount of noise in the image, there are any number of bad pixels in the sensor, and it’s difficult to tell what’s real. (I cleaned up the most annoying of the problems, but did not otherwise mess with the image except to scale it. Again, if you want the full size, change 12c in the filename to 22c.)

(That’s a roof at the top — remember, at least for now this is an inverting telescope.)

You will notice that the image of Venus is not a circle. I went up to a page that lets you calculate the current phases of various objects in the Solar System, and took a look at the current phase of Venus. Sure enough, if you ignore the extra color around the disk the shape is just about right. (The angle is a bit different, but that’s expectable.)

(03 April, 2007, evening)

A nearly full moon is just rising; I don’t know whether I’ll be able to photograph it, but I will certainly make an attempt.

(Somewhat later)

Here is a crop from about the best I’ve been able to do so far:

By eye it looks much crisper, albeit smaller (with the Rini eyepiece the field is just about 3° across), and if I keep it centered there is no noticeable chromatic aberration. (Not to keep harping on this fact, but it was extremely difficult to take these photos, partly because I had to hold scope and camera together and correctly aimed while I tried to focus. I am pretty sure that there was a bit of motion blur, and there were some branches in the way, neither of which helped the contrast or clarity the least bit.) Here’s what it looks like if I just point the camera out the window, with the lens at full wide-angle:

(The camera is actually sitting on top of the telescope tube, which you can see at the bottom of the image.)

(Evening of 06 April, 2007)

It is also possible to take pictures without an eyepiece. This involves pushing the lens out as far as it will go (maximum telephoto), focusing in as close as possible (manually, as that seems easiest for the moment), and getting the camera positioned correctly (I don’t have an adapter). Here is an example:

Considering the fact that I was holding the camera on a rather shaky box for a full 1-second exposure, this really isn’t too bad, though the field is wide enough that it is only useful for rather wide objects unless you have zillions of pixels. (I may have to try getting an actual adapter, once I put the scope onto a mount.)

If I put the eyepiece back in, it becomes somewhat more difficult to focus and to hold everything steady; here is a fairly decent example:

Again, for a 1-second handheld exposure, that seems reasonable.

(07 April, 2007)

Here is another one that I took without an eyepiece, but through the Sony DSC-R1 camera. I opened the lens up to full telephoto, and manually focused as close as I could. I will admit that I tweaked the brightness and contrast on this JPEG, as it was terribly washed out by the camera’s attempt to adjust the exposure. (If you want to see the way it originally looked, click the thumbnail and then change “s1280cb” in the filename to “s1280c”.)

If you want to see the full-size crop, click the thumbnail and then change "12c12" to "22c22". The brightness and contrast are tweaked in that version, so don’t remove the “b” from the filename.

Looks to me like approximately the middle third of the image is of reasonable quality. All things considered, I really can’t complain.


I am now faced with two issues: mounting the objective in the tube (which I expect to do as simply as I can, unless I decide that there is some reason to get more complicated about it), and finding a way to mount the entire scope on a tripod or even an equatorial mount. I don’t actually think there’s a whole lot of point in an equatorial mount, as this is at best a Rich-Field scope and at worst a mere spotting scope for terrestrial use by people who don’t mind seeing everything upside down; but I could be wrong.

I should probably mention the fact that I put three strips of tape on the inside of the tube, up at the front, so that there are now three places that do not have any paint on them. (I am not at all sure how well the paint is stuck to the PVC, and I’m not at all sure how well RTV caulk would stick to the paint, if I decide to use it to hold the lens in place.)

(15 April, 2007)

Yesterday we went thrifting, and found a tripod that seemed like it might be suitable. It set me back $25, so I certainly hope it works. One of the three little arms that tie the legs together for stability was missing, so I replaced it with a wooden one. Here are an overview of the tripod and a close-up of the fix:


This tripod appears to have been designed for some sort of telescopic instrument, but I am not sure exactly what kind. The head is rather strange; it has worm-gear drives on its axes, and is missing at least two of its knobs. It also has some parts with no apparent function. These may relate to yet more missing bits...

I had to figure out a way to mount my scope on the head, which appears to be built to take a barrel that is considerably smaller than my PVC tube. I decided to screw a wooden bar into the mounting area, and hold the scope tube to that with large hose clamps. The groove is only 1&3/16" across, though, and both wood and PVC are slippery, so I had to build a sheet-steel cradle to hold the scope tube securely.


Because the hose clamps will be going around the wooden bar, I have merely glued the cradle to it. This should (we hope) provide sufficient stability. I will, however, be continuing to use my test objective until I am convinced that the configuration really is a viable one. As I say, PVC is slippery, and it will be a while before I really trust this lashup.

(16 April, 2007)

Here is the scope in place on the tripod. Please forgive the spoons — as I said above, I am still in testing mode.

I am rapidly coming to the conclusion that the head on this tripod is a bit too rickety and wobbly; I may eventually replace it with another head that I have here, which is not a very good one but will probably provide a tiny bit more stability. If and when I do that, I will post photos. In the meanwhile, it is certainly a lot easier to photograph things through the scope now. Here is a quick shot of something I can see out my office window:

The chromatic aberration seems to be more apparent with the camera than the eye, and the scene looks better in person than it does here.

(Toward the left edge of the pole, just left of the base of the transformer (which is upside down here), you can see some bad pixels. The sensor of my old Canon G3 has been through the wars.)

Other Considerations

Having noticed the Opa-Scope lens for sale, I decided that 18" was a reasonable minimum for the focal length of the objective; and having built this scope with it, I continue to think that this is correct. Anything shorter than about 18" probably becomes hard to deal with, and focusing gets very finicky. If you can’t find an Opa-Scope lens, just about any projector lens with reasonable aperture and focal length of at least 18" will do. You probably don’t need eyepieces that are 2" in diameter, either. I’m not even really sure why I went with that size; certainly made it harder to acquire a holder. As of 7 April, 2007, btw, I have an adapter, and will soon have a 1.25" eyepiece (15 mm focal length) to put in it. I will report on how well that works.

(13 April, 2007)

When the 1.25" eyepiece arrived I put it into the adapter and tried it, only to find that I couldn’t focus out to infinity. I gritted my teeth and sawed about 5/8" off the end of the toilet flange to shorten the tube; then I tried it again. I can now tell you why I went with 2" eyepieces: they have larger exit pupils, so it’s easier to look through them; because they have larger aperture they capture more light if it’s available; and in general they seem to work better. To be sure, the 15mm eyepiece that I got is one with less eye relief than I really need (I wear glasses), and that doesn’t help. I will be trying it without my glasses, to see whether that makes it viable; but I suspect that I will be looking for either a 13-15 mm 2" eyepiece with decent eye relief, or a 2" Barlow so I can just double the magnification of the eyepieces I already have. I do not like putting more glass into the optical path, but if I can’t find an eyepiece, a Barlow will have to do; at least they’re available.

(24 April, 2007)

I got a Barlow on eBay, and last night I gave it a try. It’s a pretty good Barlow, and I can just about use it with the “30 mm” eyepiece, which unfortunately has a barrel diameter that is a bit too large, and won’t actually fit (I can sorta wedge it into the end, and it stays there if I don’t disturb it) but it doesn’t work at all well with the Rini 35mm eyepiece. It’s pleasant to get slightly higher magnification with the other eyepiece, and I get astonishing eye relief (about two inches), but I have to position my eye rather precisely to get a decent view of the moon, and it is easier to go back to the Rini (without the Barlow) to look at stars.

Moreover, it is painfully apparent that the existing head on the tripod is not going to cut the mustard, and I will be investigating ways to replace it with something else. For now, I will probably use the slightly junky camera head that I have here, but in the long run I may try for something that is better suited to astronomical work.

(15 August, 2007)

Scott Dorsey was kind enough to give me a tripod that was intended for surveying (at least, I think that was the original use), and I have modified it slightly so I can mount the scope on it. I have also put in the “good” lens, with some provision to keep it from falling out:

I still haven’t been able to take decent photos through this device, so I can’t show you what looking through it is like, but here is a lousy shot of Jupiter, showing four of its moons. I have had to do various contrast and level enhancements, partly because there is a streetlight across the way, and it is actually moderately surprising that I was able to take this photo at all. On top of the light problem, it’s a one-second exposure, and my hands were shaking both the camera and the telescope.

I have indicated the positions of the moons with slanted white lines, as the fourth one (lower right) is a bit difficult to see — it is largely lost in the glare from the streetlight. If you check the positions of the moons for tonight and something doesn’t seem quite right, remember that this photo is inverted.

(16 August, 2007)

Couldn’t stand it: just bought a 15mm 2" eyepiece from Knight Owl, on eBay. It has fair eye relief, and will probably serve reasonably well. When it arrives I will probably try some photography with it, and we’ll see how it does.

If you want or need any further details, please feel free to send email.

A Small Spotting Scope

(01 September, 2009)

Scott Dorsey was kind enough to give me a bunch of disused lenses and lens innards, and I have made little spotting scopes from two of them. This one —

— was especially fun because I discovered, while looking for possible parts with which to build it, that the thread of the projector lens innard that I’m using as the objective fits almost perfectly into the thread of the first PVC plumbing fitting. I screwed it in until it stuck reasonably firmly, and left it that way (not glued in place), so I can adjust it if and as necessary. The first fitting is glued into the next one with silicone rubber aquarium caulk. The other end of the second fitting has a very short piece of PVC pipe in it, to hold the final fitting in proper alignment and allow me to glue things together, again with RTV aquarium caulk.

The inside diameter of the final fitting was too small for the eyepiece to fit into it; Mike Nicewonger was kind enough to chuck it up in his lathe and open it up a little for me.

I think this scope gives me about 5X magnification with a 35mm fl eyepiece (from The Surplus Shed), as shown. It doesn’t have a flat field, but it is certainly good enough for everyday (or everynight) use. If I take any photos through it, I will put one here.

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This work is supported by
the Joss Research Institute
19 Main Street
Laurel  MD  20707-4303  USA

Contact Information:

My email address is, where a is my first name (just jon, only 3 letters, no “h”), and b is joss.

My phone number is +1 240 604 4495.

Last modified: Thu Jun 23 15:48:38 CDT 2016