The Quest for an Optimal Telescope It's been a few years since I purchased my 6 inch f/5 Newtonian from Discovery telescopes. In those days Discovery sold primarily telescopes containing their own optics. Now they seem to make only Dobsonians, and have become a retailer for many other telescope brands.

It turns out that the mount and telescope I purchased were made in Taiwan. Discovery imported the unit and added their own optics. In my case, a lime-glass mirror objective.

I chose the unit after months of shopping. I'd alway used telescopes I constructed myself (usually with purchased optics), but I'd never owned a sizable commercial instrument. This wasn't a matter of ego, but originally because I didn't have much money to put into telescopes, then later because making my own telescopes had become a matter of habit.

I found that Discovery had two instruments with prices and features that caught my eye. Those were an 8 inch f/5 Newtonian and a 6 inch f/5 Newtonian, both delivered with an equatorial mount. Calling Discovery, I found that both telescopes shipped with the same mount.

I'm sure a lot of people would question my decision to go for the 6 inch over the 8 inch. A lot of experienced amateur astronomers think one should always go for the greater aperture. But my reasoning was tempered by several considerations.

The Reasoning Behind the Purchase

First, I figured that if the 8 inch unit was even barely adequately supported with the equatorial mount, the 6 inch should be very stable. Second, I'd found during the previous couple of years that I really did a lot more observing when I gave away my old 8 inch DOB, and all I had left was a small Jaegers refractor. It seemed that portability translated to more viewing, for me at least.

So what I wanted was the maximum aperture I could use in a portable package. I considered an 8 inch SCT, but I remember watching a friend of mine set up his Celestron, and could see that while the 8 inch SCT telescope is compact, scope and tripod together aren't that portable.

Looking at the combined weight of the Discovery 6 inch Newtonian and the mount (total of 35 lbs), I thought I'd be able to move the unit around my yard while the scope was fully assembled. If so, it would save me a lot of time and give me a good combination of portability and power.

I'm happy to say that my expectations were met on both fronts. The mount is very stable and easy to use with the 6 inch on it, and I can move the entire unit without disassembly.

The Fly in the Ointment

Sadly, even when properly aligned, the telescope performed poorer on planetary images than I expected. Looking into the problem a bit further, I found that the cause was the barely adequate tube size. The diameter of the thin-walled metal tube that housed the optics was only 7 inches. General ATM guidelines would recommend an 8 inch tube.

The problem this caused was that with short focus eyepieces, the eyepiece focuser extended into the optical path. This greatly reduced the quality of the resulting images.

The Rehabilitation Plan

Plan A was to remount the optics in a larger diameter tube. However, I feared that this may destroy some of the portability I sought to achieve. In addition, the primary mirror mount was designed to also serve as an end cap for the telescope, and would be difficult to mount in a larger tube. Finally, the mirror was glued to its mount, and I was afraid I might break the mirror if I tried to remove it from the mount.

Plan B was to modify the existing tube, which is what I chose to do. This consisted of removing the optics and cutting off enough of the rear end of the tube to move the re-mounted mirror cell forward. This moved the focus further out, and the eyepiece tube no longer extended into the optical path. Fortunately, the eyepiece tube had enough travel to accommodate this extended focal point.

No sooner had that problem been solved than I discovered I had another design issue that was limiting the quality of planetary views. The clock drive had a vibration (more like a hum) that was enough to blur details smaller than perhaps 5 to 10 arc-seconds.

Looking online, I found that JMI had a clock drive that was a direct replacement, so I ordered it. The JMI drive doesn't have the electronic control that my old drive had. It's a slip clutch type with a manual knob one can twist for RA adjustment. But it has a synchronous motor that doesn't cause any discernible vibration.

Images got better, but were still not spectacular. During a Mars opposition I noticed that the spikes caused by the secondary spider vanes were causing a very distracting amount of light extending from the bright planet. With six broad spikes bleeding away, the limb of the planet was hardly discernible.

Getting the Bugs out of the Spider The 3 element spider on the secondary was part of a plastic fitting that also stabilized the end of the telescope tube. Because they were plastic, the spider vanes were excessively thick in order to have the necessary strength. The vanes were in fact nearly 1/4 inch thick. I needed, as a minimum, to replace the spider with thinner vanes. I decided to make a more aggressive modification. I removed the thick spider elements, leaving the supporting ring, and fabricated a thin metal curved secondary holder. The new secondary mount makes a 180 degree loop and evenly scatters light throughout the field of view, eliminating spikes.

How Sweet It Is

Now I finally see through the telescope what I want to see. Planetary images are quite good for an f/5 instrument, and the spike-less views are reminiscent of those through a Cassegrain. And with the f/5 focal ratio, the telescope is a great performer on stellar objects. To add one last optimization, I covered the inside of the telescope tube with black flock paper.

It has been a long road to get the telescope I bought transformed into the telescope I want. The optics made by Discovery are excellent. The mount made in Taiwan is smooth and sturdy. The telescope is solidly constructed, but had design issues that kept it from performing up to its potential.

In summary, I made the following modifications to fix design limitations and enhance the telescopes performance:

What I ended up with is a high performance 6 inch telescope on a very stable mount that is so portable I can move it in and out of the garage and around the yard without disassembly.

It is a good general purpose instrument operating at f/5, which is what I wanted. I didn't want an SCT or Maksutov, though they could be portable in this size range, for a couple of reasons. First, they cost more than twice what this instrument set me back, even with the new clock drive. Second, I don't consider telescopes above f/8 in focal ratio to be that great for general use. Many star objects and comets benefit considerably from a wider field instrument.

It took some time and effort, but in the end I have a pretty nice telescope. In fact, a great telescope. Not great as originally shipped, but being the simple design of a Newtonian, it lent itself well to modification by even someone like me.

Recommendations

I don't know if the other models similar to this have the issues this one had, but there are models much like this still available.

The convenient astro-customized search engine at right browses a number of popular telescope outlets. You can use it to find an EQ Newtonian similar to mine.

Custom Search

You can find models similar to my Discovery, which would deliver the same combination of significant aperture and portability that I find very conducive to frequent observing.

You can also get a similar design with a computerized mounting in at least the 5 inch range. That wasn't available when I was shopping, but it certainly is now.