Monolux 60mm Rebuild

Rebuild of a Monolux 60mm Telescope

After joining an enjoyable astronomy egroup that was devoted to the use of small, refractor telescopes, I was quickly enthralled with the group. It brought back memories of my childhood, during which I often thought of buying a 60mm refractor, but never did because conventional wisdom was that 60mm wasn't big enough. But the views through 60mm telescopes, described by many of the most experienced members, really caught my attention. Was what they described possible?

The problem was, I didn't have a telescope that fit their mainstream emphasis. My only refractor was a 50mm Jaegers Refractor, and I was admittedly amazed at how well it presented those targets. But I was a bit dubious about what besides the moon and planets one could see with a small refractor. Yet you can even buy a 60mm refractor telescope on a computerized mount, like the Celestron 60LCM Computerized Telescope (Black). But is there much to see with a 60mm?

I'd never used my little 50mm telescope for star objects, and wasn't sure it would show much. I frankly didn't think a 60mm would show much either.

But the talk in the egroup enticed me, and I wanted to share in the experience. To do that I figured I needed to find myself a good 60mm telescope. I went for the used telescope route. I was thrilled when an egroup member graciously donated an abandoned, vintage 60mm Monolux of 700mm focal length (f/11.7) to my cause, so that I could use precisely the size instrument that was causing all the buzz in the egroup.

The telescope in received condition

As you can see in the image above, the old Japanese made instrument had seen a number of years and showed some wear. It definitely needed some tender loving care. But the Monolux is considered a good representative of the many Japanese telescope sold in the heyday of small refractor telescopes. Based on that I wanted to give it a fair tryout.

Visible in this image are a few scrapes and scratches, and a couple of holes that were likely used at one time to mount a finder scope. You might also notice that the focuser is of the 0.965 inch variety. It still worked well, but had only about a 2.5 inch travel, and of course only accepted the small 0.965 inch eyepieces.

Here's another view of the telescope as received. Incidentally, the tripod was not part of the donation, only the telescope. Again, you can see a few unused holes in the body of the telescope where a tripod cradle used to fit.

Here you get a better view of the focuser assembly. The holes in the side of the tube were likely used to mount the telescope to its original mount. The telescope's original mount was long gone, but that's not likely a great loss. Refractor telescopes even in the era of this model were still notorious for having mounts that were undersized -- with the exception of a few, like the venerable Unitron.

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The Adventure Begins

I found I was on a bit of a journey when I decided to refurbish the vintage Monolux. I cover a few of details of my efforts on this web page. I imagine that some of the problems I encountered are likely typical of those you might experience should you decide to refurbish a vintage refractor telescope.

First, when I got the telescope, I had but one 0.965 inch eyepiece to use in the instrument. So I purchased a hybrid diagonal that allowed me to use my 1.25 inch eyepieces. The hybrid diagonal has a 0.965 inch snout on the telescope end so that it will slide into a 0.965 focuser. On the eyepiece end it has a full 1.25 inch adapter.

This provided a workable solution for using most of my 1.25 inch eyepieces, but there were a couple of limitations. Since the focuser didn't have a locking screw to hold the eyepiece or diagonal, the heavier 1.25 inch eyepieces would cause the star diagonal to rotate under their weight. I had to be sure the eyepieces were placed in a perfectly up position. Second, the limited 2.5 inch travel of the focuser didn't let me get full use of my Barlow Lens. I could use it on some eyepieces, but not others.

To use the telescope, the first thing I had to do was rig up a tripod or mount of some kind. Fortunately, as shown above, that problem was already mostly solved. I had already made a tripod (actually a pedestal mount) for my little 50mm refractor. I just had to drill a 1/4 inch hole in a 3 inch hose clamp, and use the clamp to fasten the Monolux to my DIY pipe tripod. A 1/4 inch screw of 2 inch length goes through the hose clamp and through the plywood cradle fastened to the pipe fitting tripod. This simple method provides a telescope mount of exceptional stability and ease of use.

My first light experience was a bit disappointing. When I looked at Saturn, I was unable to focus to a nice, crisp image. I could see that it was the planet Saturn for sure, but the image was definitely sub-par. I pointed the telescope over to a moderately bright star to check out the in and out of focus star images. Viewing star images at high power is a common way to test telescope optics. 

On one side of focus the star image looked reasonable normal (a bit oval), but on the other side of focus the pattern was terribly distorted. Something was clearly wrong. Was the telescope a dud?

At the suggestion of a member of the aforementioned astronomy egroup, I examined the telescope's objective. It is an air-spaced doublet with the two objective elements separated by a spacer ring. After trying all combinations of arrangement of the two elements, I determined that as received, the front element had in the past been removed and accidentally replaced facing the wrong direction.

After correcting that anomaly, I tried for another chance at a first impression. Images were much better. At low to moderate power the star images looked fine. But at power in the 100x range, images were still not quite as good as I'd hoped. Another glimpse at a star image revealed a noticeable amount of astigmatism.

Astigmatism is characterized by oval shaped out of focus star images, with the inside-of-focus oval at 90 degrees to the outside-of-focus oval. By rotating the eyepiece and finding that the pattern stayed in the same configuration, I was able to confirm that the astigmatism was coming from the telescope's objective, not the eyepiece. Had the eyepiece been the trouble, the orientation of the oval patterns would have rotated as I rotated the eyepiece.

On another tip from an egroup member I spent a day meticulously rotating the objective elements with respect to one another. After each adjustment, I would re-test the out of focus images by looking at a pseudo star image. My pseudo star image was the sun's reflection off of a power transformer insulator some 1/4 of a mile away. The curved surface of the insulator gave a solar reflection that proved a worthy fake star. I was amazed that this technique worked, and by following the procedure carefully I was able to find an arrangement of the objective elements that virtually removed the astigmatism.

Eureka!

More actual astronomical target observations through the telescope confirmed that it was performing well. Planetary images were crisp and tolerated magnifications of up to 150x, which is about typical for a good 60mm telescope. Moon images were very pleasing. The telescope was also able to split some tight double stars near it's theoretical limit.

The 120x maximum that you often read for the maximum magnification for a 60mm is obtained by applying the general rule of maximum useful magnification as 50 times the objective diameter in inches. Thus, 2.4 x 50 = 120. However, atmospheric turbulence is one of the contributors to this general rule, and small aperture refractors are less affected by atmospheric turbulence than larger instruments. It's not uncommon for quality 2.4 inch (60 mm) refractors to achieve up to 75 times per inch of objective diameter. For a 2.4 inch, that means up to 180x when seeing is exceptional.

With a correctly installed objective with its elements in an optimal relationship, it was time to move forward.

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The Decision To Refurbish

Now that I knew the Monolux had good optics, I decided to go the distance and refurbish the vintage refractor. To start, I went after all those old, unused holes. I filled them with epoxy, and after the epoxy dried I sanded it down to the point of being unnoticeable.

Next I removed the focuser and the objective cell and repainted the inside of the telescope tube with flat black paint. The original black coating was a bit thin in places, and with only 60mm of aperture to work with I didn't need any interference from stray light sources.

Next I sanded and repainted the telescope's outside. Showing no imagination at all, I painted it white. After about 3 coats of paint, the telescope tube looked as good as new.

I then added a 6x30 finder scope. I decided to not re-install the original 0.965 inch focuser. Instead, I purchased a quality 60mm focuser from Meridian Telescopes (alas, they are no longer available). The focuser is a heavy duty focuser of metal construction with a 6 inch travel, and accepts standard 1.25 inch diameter eyepieces. Since the position of the eyepiece with standard 1.25 inch star diagonal was further away from the objective than with the original focuser, I had to shorten the main tube by the difference. When I finished installing the focuser, I re-attached the objective cell, stepped back and admired a fully refurbished, vintage 60mm Japanese refractor, now capable of directly using the more standard 1.25 inch diameter eyepieces.

Above you see a closeup of the refurbished telescope. No more scratches and unused holes. You can see the new focuser in this image, and the new finder scope. The telescope is shown mounted to my good old pipe fitting tripod. The pipe fitting tripod is easy to use, and makes a super steady mount for my new old telescope.

The mount, as shown on the pipe fitting tripod page serves multiple duty. A single wing-nutted bolt is all that must be removed to switch to my 50mm Jaegers refractor, my DIY 60mm f/16.7 Carton refractor, or my Vixen 70mm refractor. By switching tripod heads, I can also use the tripod to hold my ETX90 when I'm in a Maksutov mood.

Since this is a vintage telescope, it is all metal construction, and now with an updated focuser for the larger 1.25 inch eyepieces. I could go one step further with this instrument, and bring it up to the level of the previously suggested computerized model. How? By buying something like the iOptron SmartStar-E 8500B Computerized AltAz Telescope Mount (Astro Blue). I could easily adapt this computerized mount to my pipe tripod and really have a top notch system. Maybe one day I will.

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The Views

I've had a chance to do a little viewing through the 60mm Monolux since the final optics alignment and the refurbishment. Starting with the moon, I've been startled by the clarity. I can routinely use magnifications up between 100x and 150x. Comparing it to a formerly owned Meade 90mm refractor (a 90s era Chinese import), I'd have to say that the image quality in the Monolux is better and the images crisper.

I've looked at Jupiter on a number of occasions. I could easily see the two major belts on Jupiter, and see that the Southern belt is split into two narrower belts. I could make out darker splotches along the Northern belt. I was not able to resolve the Great Red Spot this time around, and I understand that it's been a rather low contrast object. I was, however, able to spot Io's shadow as it shuffled across the planet. The shadow was very tiny, and not always visible, but I could make it out most of the time.

Jupiter 2010 with 60mm Refractor Update. In 2010 the Southern Equatorial Belt on Jupiter seemed to disappear, as revealed in the above photo obtained with my long focus 60mm. This strange phenomenon of belt fading has occurred before, though not often. Because of the missing SEB, I was able to detect the Great Red Spot for the first time with my Monolux 60mm telescope. When the SEB is visible, it merges with the Great Red Spot, making it to difficult a target with the Monolux. Note that while this image wasn't taken with the Monolux, it was taken with my Carton 60mm, a same size telescope.

I haven't looked at many DSO targets with the Monolux yet. I've seen the Andromeda galaxy (M31) and one of its companions (M32), the Ring Nebula (M57), the Pleiades (M45), and the double cluster in Perseus. The Orion nebula is a wonderful target, and the 4 brightest stars of the Trapezium are visible through this 60mm. I was able to just glimpse the Crab Nebula (M1) from my backyard observing site. This is just a tiny fraction of the objects within the grasp of this size telescope, and I list these few as just examples of what such a small telescope can do.

The Monolux does a very nice job on double stars. Star images are always crisp, and at moderate power at most a single, dim ring is seen around the brighter stars. Contrast that with my ETX90, which while having excellent optics, always gives couple of shimmering rings around even moderately bright stars. The central obstruction of the ETX90 Maksutov optics redistributes some light to the first few diffraction rings. While the ETX90 with its greater aperture can split closer doubles, the view is not as pleasing as through the Monolux refractor.

With its light weight low-maintenance, and solid construction, the Monolux is a great telescope for those impromptu or short evening tours of the universe. Since most of my observing sessions fall into that category, my Monolux and Carton 60mm telescopes get quite a bit of duty.

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Summary and Final Thoughts

The reputation of the poor 60mm refractor has taken a beating during the last several years. Such telescopes used to be considered not only fine beginner telescopes, but good supplements to any amateur astronomer's collection. Just look at what a 60mm Unitron goes for now-a-days, if you can find one for sale.

Now, most 60mm refractors are Chinese imports, made to inexpensive standards with lots of plastic, and sold by most distributors as telescopes for kids. They come with ultra-cheesy mounts and poor quality eyepieces. No wonder they don't inspire many youngsters to become astronomers.

But if you want a lot of fun and a bit of a challenge, find an abandoned vintage telescope at a garage sale or on eBay. There are a number of brands you'd be safe with. The following list is not exhaustive, and with a bit of research you can learn of other brands: Tasco (yes, the old ones were good), Towa, Sears, Monolux, Edmund, Unitron, Carton, and Jaegers.