|
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.
|
Ecology T-Shirt Designs
|
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.
|
My Astro T-Shirt Designs
|
The Fly in the Ointment
|
Now for the bad news. First views through the telescope were disappointing.
While views of star objects have always been spectacular through the
instrument, planetary images were especially poor. Images in my 2 inch Jaegers
refractor were at least as good.
I determined that while well packed when shipped, the unit had arrived in
poor alignment. It took me some time to re-learn how to align a short focal
ratio telescope.
|
|
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.
|
My Astro T-Shirt Designs
|
|
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.
|
|
The total length of the curved vane is about the same as the sum of the
lengths of the original 3 vanes, so no additional diffraction surface has been
introduced. In fact, since the new vane is only about 1/16 inch in thickness,
the total diffraction surface is reduced, as well as curved to eliminate
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.
|
Astro T-Shirt Designs
|
In summary, I made the following modifications to fix design limitations and
enhance the telescopes performance:
Moved the primary forward
Replaced the clock drive
Replaced the secondary spider
Covered the inside of the tube with black flock paper
|
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. Check out the
voluminous listings at SHOP.COM  . You'll see 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.
If you're a tinkerer as I am, I'm sure you could find a comparable equatorial
at
eBay Stores
 , and save a bunch of bucks. Performing the modifications I've
outlined will leave you with a telescope that has a combination of features
that's difficult to compete with. I know I won't soon be parting with mine.
| 
