Modifying a low cost Barlow lens for use with the Pentax PF-EDAII spotting scope (1 Viewer)

[billb9430]

In another thread in this section, Bjorn, who uses a Pentax XW 14 mm eyepiece in his Pentax PF-65EDAII spotter, posted a question about whether or not folks would recommend obtaining a Pentax 10 mm XW eyepiece for occasional higher power views. In my opinion, IF cost, weight, and bulk are NOT considerations, the shorter focal length Pentax XW’s are a good solution - - but since the PF series of spotters take standard 1-1/4” eyepieces, a VERY low cost and light weight alternative solution is to get an appropriate Barlow lens to use WITH the 14 mm XW.

Agena AstroProducts (and likely other suppliers, too) sells the GSO 1.25” 2x Achromatic Barlow lens (#GS2BL) for $33.00 U.S. For folks with normal (or corrected) vision, my PF-65EDAII focuses the image of an object at infinity using just the 14 mm XW at about 5/8 of a turn of the focusing knob from full “in” position. Using the unmodified GSO Barlow with the 14 XW in the scope will allow focus of the image for infinitely distant objects for folks with normal vision, but to do so, the focus knob must be nearly to the full “in” position. This does not allow optimal “in” adjustment for folks who are even a little bit myopic without their glasses. Fortunately, there are a couple of “work-arounds” that WILL enable better use with this Barlow. (Clearly closer objects will focus OK if the infinity objects will focus, since there is a lot of “out” travel available.)

The GSO Barlow screws apart into three pieces: a top section to hold the eyepiece with securing thumbscrew, a 1.25” diameter connecting barrel, and a lower cell containing the Barlow lens. The first “work-around” requires no mechanical modifications. Just detach the Barlow lens cell from the bottom of the GSO unit and screw it directly into the bottom of the Pentax 14 mm XW. The threads are the same (metric, .60 mm pitch). This yields about 40x, as compared to 27x with the 14XW alone. I determined this magnification with my scope by measuring the exit pupil of the combination with a caliper and magnifier and divided the scope aperture (65 mm) by that value. This arrangement focuses the image for an object at infinity with the focusing knob about 7/8 turn from full “in” position – so even more “in” focus is available than with the XW eyepiece alone! The “Barlow factor” when the cell is attached directly to the 14XW is about 1.48x rather than 2x. The disadvantage of this method is that to change power, you need to screw the small Barlow element onto the end of the eyepiece while out in the field.

The second “work-around” method is to replace the middle threaded barrel section of the GSO Barlow with a shorter one. Surplus Shed in Blandon, PA, USA has anodized aluminum barrels of the correct diameter and thread pitch, needing only to be cut to length. This replacement barrel, Surplus Shed Item No. M2104, sells for $4.00, U.S. If you are not familiar with cutting tubes, you may want to order two in case you spoil one or for further experimentation. Of course, you could also just cut down the original GSO barrel, but then could not return the Barlow to its original function for use with other telescopes. The original and Surplus Shed barrels are about 30 mm long and I cut mine to ¾” (19 mm) for use with the Pentax PF scopes. I have used one shortened on a lathe for several years, but did not post about it before because most birders don’t have access to a lathe to make their own. In an attempt to make this modification possible for more folks, I recently made another “cut down” GSO Barlow WITHOUT using my lathe. You’ll need a scriber, a hacksaw or jeweler’s saw with a fine blade, a fine cut mill file, and a vise. Instructions follow:

1. Wrap a piece of paper or thin card stock around the 30 mm long barrel so as to form a perfect circumference with the edge of the card 19 mm (3/4”) from one end of the barrel. I used a 3 x 5” note card and taped it to the barrel so 19 mm was covered and then taped the card to itself to hold the wrapped card in place. Using a sharp scriber, mark a line around the barrel along the paper edge. Remove paper. See photos 1 and 2.
2. Holding the barrel crosswise in a padded vise, use a fine hacksaw or jeweler’s saw to begin cutting on the line. Do not go through the barrel wall, but once the saw kerf is established, loosen the vise and rotate the top of the barrel a little towards you. See photo 3. Retighten and with the back of the saw blade in the established kerf, extend the cut forward a little with each stroke, keeping on the line. Have patience, stay ON the line, and continue this procedure a bit at a time until a shallow groove extends all the way around the barrel. Then continue lightly cutting and rotating until the 19 mm section is about to part off. To conveniently finish the cut without pinching the saw blade, screw the end of the nearly cut tube onto the upper thicker eyepiece-holding section of the Barlow and use its screw to secure the whole thing to a thick 1.25” piece held in the vise. I used an old, discarded eyepiece barrel for this piece. You can now continue sawing around the tube to finish parting off the 19 mm long piece with light saw strokes. See photo 4.
3. Using a fine file, smooth any irregularities to complete your new short Barlow midsection barrel. See photo 5.
4. Screw the eyepiece holder part into the end of the shortened barrel and the Barlow cell into the other end. Done!

With my PF-65EDAII, the 14 XW eyepiece in this modified Barlow focuses the image of an object at infinity with the focuser knob ¼ turn from full “in” position and gives 52x with a Barlow factor of about 1.9.

By the way, this moving of the position of the Barlow lens is not a new idea. When Frank Goodwin first popularized Barlow lens use in the US over 60 years ago, his lenses were a friction fit in the tube and came with instructions and a little stick to push the lens cell into position to provide different magnifications. Edmund Scientific sold the Goodwin Barlow after Mr. Goodwin’s death. Edmund’s Goodwin Barlow instructions are still available for download on their Anchor Optics site. Check it out.


I’m pretty sure that the image quality using the Barlowed 14 XW is not as good as would be provided by using just a single, shorter FL Pentax XW eyepiece, but I am unable to see any difference in casual viewing. Close equivalents to the two Barlowed XW 14 magnifications are a 10 mm XW (giving 39x rather than 40x) and a 7 mm XW (giving 56x rather than 52x). Another benefit of having the GSO 2x Barlow is that with the lower lens cell removed and the original connecting barrel in place, the empty Barlow tube will function as an extension tube for your 14 XW, allowing closest focus for the PF-65EDAII to move from 5 meters to just 2.5 meters – in case you want a really magnified view of a hummingbird at your feeder.

A GSO 1.25” Barlow shortened and used as described for the PF-65EDAII also works similarly with other eyepieces. The method of screwing the Barlow element cell directly onto the eyepiece can also be used in the larger Pentax scopes of the PF series, the PF-80 and PF-100. Unfortunately, it is more complicated to modify the GSO Barlow to use the second method for those scopes. I’ve succeeded in getting the PF-80 and PF-100 to focus the image of an object at infinity with the focus knob about 1/8 turn from full “in” position - but only by trimming both the top of the upper eyepiece-holding part to allow the eyepiece to sit closer to the Barlow AND trimming a bit of the excess rim off the bottom of the Barlow lens cell so that the lens itself will sit closer to the objective. These modifications would be difficult for those without access to a metal lathe. Additional difficulties with the larger scopes arise because the full aperture appears to be slightly diminished in them, so calculating magnification is harder. The PF-65EDaII objective is not vignetted at all, so one can just use 65 mm and divide by the measured exit pupil to find magnification. (By the way, is anyone else bothered by the fact that the focus knob on the PF-65EDaII turns the opposite way for close focus as compared to the larger PF scopes?)

For those who have a PF-65EDAII scope and are considering purchasing a single Pentax XW eyepiece, to be used with this Barlow method for higher powers, an XW 20 mm might be even more versatile than the 14 XW, with magnifications of 19.5x, 29x, and 37x. Spending just $33 for the Barlow and $4 for the extra tube, you can’t go too far wrong.
- Bill

 

[DRodrigues]

Well done Bill!
It's another version of the barlow I mentioned in http://www.birdforum.net/showthread.php?t=152519

 

[billb9430]

IGNORE earlier post – NEW Pentax Barlow MOD here

I suspect it is typical of problem solving that after working along one line for several years with some success (in this case, adjusting the length of the three segments of the GSO Barlow), suddenly a much BETTER solution presents itself with a completely different approach!

Yesterday I was pondering the role of the little pin that protrudes in the bottom of the eyepiece “well” of the Pentax PF spotting scopes. I even measured its position, finding that its top was 2 mm from the well bottom on the PF-65EDAII, 6 mm above the well bottom on the PF-80EDAII and 7 mm above the well bottom on the PF-100ED. I had always thought the function of the pin was to protect the window (or prism face) that sealed the well bottom and made the scope watertight. Upon lighted inspection, however, the “well” had a nicely defined constricted metal rim at the bottom ABOVE the glass “window”! . . . Then why have a “pin” at all, since it keeps the Barlow from seating all the way to the bottom of the eyepiece “well”? I do not have a Pantax Zoom eyepiece, but the possibility occurred to me that it might be just to secure the Pentax zoom eyepieces from rotation as the power ring is turned when the collet is not quite firmly tightened. A PM to David Rodrigues in Portugal, brought his rapid reply that indeed, his Pentax zoom had notches in the bottom rim that would catch on those pins. (As the pins protrude from the wall near the bottom of the well, the PF-65 comes in at the 12 o’clock position while the PF-80 and PF-100 have their pins at the 6 o’clock position.) Thanks to David’s confirmation, it was easy for me to see a “new way” with the Barlow, as likely you do now, too.

Since the bottom aluminum cell of the GSO Barlow has a thick rim around the glass element, if a groove to accommodate the pin is made the length of the cell, the properly aligned Barlow will be able slide past the pin and seat all the way to the bottom of the eyepiece well!

After removing the glass Barlow element, I machined the groove using a 1/8” end mill with a milling attachment on my lathe, but believe this could also be done by hand with the edge of a rectangular file. Once grooved and glass replaced, the Barlow cell was re-assembled to the other original, unmodified parts of the GSO Barlow. See three photos attached.

In brief testing, the 2x image of the Barlow-augmented Pentax scopes seemed parfocal, or nearly so for ALL the Pentax XW and XL eyepieces I tried. Additionally, this was the case for the Agena 20 and 15 mm SWA’s, a home modified 25 mm military Erfle, and a Televue 13 mm Nagler. In brief testing, I did not find ANY eyepieces that came to focus in the Pentax spotters that would NOT come to focus with the addition of the GSO 2x Barlow in ANY of the three Pentax PF spotters!

This low cost ($33.00 U.S.) GSO Barlow hopefully will prove to be a useful tool for birders using the Pentax PF spotting scopes who occasionally need higher power than their standard eyepieces provide. - Bill

 

[DRodrigues]

Nice work Bill!:t:
It would be nice if someone would produce commercially a Pentax scope friendly barlow version as you showed is possible.

 

[billb9430]

Adapting a 2x Barlow for your Pentax PF scope without machining

For those able to do milling, modifying the GSO 2x Barlow to enable focus at infinity in all the Pentax PF scopes (65, 80, and 100 mm) is simple, as described earlier in this thread. However, most birders do not have access to machine tools. Here’s how to do the same modification without specialized tools.

Besides the $33.00 Agena GSO Barlow ( GSO 1.25" 2x Achromatic Barlow Lens # GS2BL ), you’ll need a hammer, cold chisel, old dull utility knife disposable blade (a new one would work, too, but no better), thin rectangular file (about 1/8” or 3mm thick, with teeth on its edge), vise, padded jaws for the vise (thin wood strips will work) and (optional but recommended) a bench grinder or Dremel tool with small grinding wheel and a “Vise grips” pliers.

1. Unscrew the bottom black aluminum cell from the GSO Barlow. Your task is to file a slot lengthwise in the cell, about 1/8” wide and 1/16” deep (3 mm wide and 1.6 mm deep) to allow the Barlow to slip past the little pin near the bottom of the Pentax spotting scope eyepiece “well”.

2. Before clamping in the vise and filing, however, it is prudent to carefully remove the glass Barlow element. [If you want to gamble on this, leaving the glass in while you work, skip down to step 3 – I wish you good luck but do not recommend you risk possibly breaking the Barlow element.] If you have an adjustable retaining ring spanner, use it. Most folks won’t have one, though. The retaining ring is likely tightened too much to remove with your fingernail and using a small screwdriver means a slip could ruin your lens. If you are like me, probably the disposable blade utility knife in your toolbox has a somewhat dulled blade from its last use. Take this opportunity to put in a new blade and rather than throwing out the old one, we’ll make a ring spanner wrench to fit the Barlow ring from it. The thickness is OK, we just need to remove the “points” from the double-ended blade and make the length of the back of the blade a little over an inch or about 25 mm to just fit across the retaining ring and catch in the notches. I suspect you could grind the blade down using only a grinding wheel, but it would be tedious to do so. Rather, clamp the blade in your vise (not padded) with one triangular end protruding above the jaws and use your hammer and chisel to strike a sharp blow to its base. See photo 1. The blade is brittle and should shear off level with the vise jaws. Wear eye protection while doing this. I like to place an old leather glove over the blade and chisel to “catch” the point when it breaks off. If you have a bench grinder or Dremel tool grinding wheel, hold the blade tightly in a “vise grip” locking pliers and carefully grind to smooth the edge off square. Then measure over 25 mm, make a mark, and place the blade back in the vise with the other triangular end of the blade “up” to shear off to the correct length. Use the chisel and hammer, as before. I left a little extra metal to be taken down with fine grinding on a bench grinder or using a Dremel tool grinder, but with care you may be able to get the sheared end close enough to the proper length to serve as your retaining ring spanner without the necessity of grinding. Put a layer of masking tape over the blade, since you will hold the cutting edge in your fingers when using the spanner. See photo 2 for the finished spanner blade. By putting the back of the blade in the ring notches, you can likely now unscrew the retaining ring. See photo 3. If it doesn’t come out easily, hold the blade in your vise grips to get more torque. For really stubborn cases, heat the Barlow cell gently with a hair dryer or heat gun to expand it and the ring should unscrew. Using tissue or lens paper, carefully remove the Barlow element and put it safely aside. It is a cemented achromat and the thinner element goes to the bottom of the Barlow when you reassemble.

3. Now that we do not have to worry about the fragile glass optic, temporarily reassemble the cell to the Barlow barrel and mark the position on the cell where a groove will engage the pin in your scope so that the eyepiece-securing thumbscrew at the top of the Barlow will be in the most convenient position for you when the Barlow is in the scope. Then remove the cell and clamp it crosswise in your padded vise jaws so that the cut location is upward and that edge of the cell extends barely above the vise jaws. Using very light strokes of the file edge and guiding with your thumb, begin to file the groove across the cell. Try to keep the file level. Work carefully and clean the file teeth every few strokes with a wire brush or file card to minimize aluminum “pinning” that clogs the teeth. Continue this careful filing until the depth of your groove just reaches the threads on the upper part of the cell. Once these threads show the slightest glint from the file touching them, your cut is deep enough. See photo 4. Check to see that the groove is of uniform depth all the way across the cut and smooth any burrs raised so you won’t scratch the inside of the Pentax eyepiece “well” when inserting the modified Barlow.

4. Clean filings from the cell and reinstall the Barlow lens (thin element down) and retaining ring, making it just finger tight with your homemade “retaining ring spanner”.

5. Re-assemble the cell to the rest of the GSO Barlow. See photo 5.

6. When inserting the Barlow in your Pentax scope, take care to align the groove you cut with the pin so that the Barlow will slide past the pin and go all the way to the bottom of the eyepiece “well”.

Done!

 

[billb9430]

Replace your GSO Barlow metal thumbscrew with a Nylon one

Folks accustomed to the nice collet type eyepiece-securing mechanism of the Pentax PF series scopes may not like the marks that the metal thumbscrew of the GSO Barlow discussed in this thread can leave on their eyepieces. I always change mine out for a nylon thumbscrew so eyepieces used in the Barlow don’t get little screw-tip marks on their lower barrels.

The GSO 2x Barlow metal thumbscrew is metric thread, M4, with 0.7mm pitch. Nylon or plastic replacement thumbscrews with that thread are available in most countries locally or by online order. Unfortunately, such metric thumbscrews are harder to find locally in the USA. An alternative that works well is to run an 8-32 N.C. tap into the M4 threaded hole in the top of the Barlow. The two screw threads are close enough that the re-tapped hole will hold a standard 8 – 32 N.C. nylon thumbscrew securely. A threaded length of about ¼” (6mm) is needed for the Barlow, but it is easy to cut down a longer Nylon screw if necessary.

The replacement Nylon or other plastic thumbscrew won’t mar your eyepiece barrel, yet will hold the eyepiece securely. -Bill

 

[billb9430]

USA source for M4 nylon thumbscrews

You're probably getting tired of my Barlow monologue - sorry for this added post, but just discovered this.

For those in the USA, where metric nylon thumbscrews are hard to find, AgenaAstroproducts not only sells the GSO 2x Barlow ($33) featured in this thread, but also markets a package of four M4 nylon thumbscrews for $2.50. You may want to order them together if you want to replace the Barlow's metal screw and don't have nylon metric thumbscrews available locally. - Bill