The Quest for an Optimal Telescope
It's been a few years since I purchased my 6 inch f/5 Newtonian from
Discovery telescopes. No longer available from Discovery, virtually the same
model is the Celestron version, shown at left. 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
(shown here) 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
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
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
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
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
Rehab Step 1: Adjust The Tube Length
One option to fix the focuser intrusion into the light path was to remount
the optics into a larger diameter tube. However, I feared that this may destroy
some of the portability I purchased the telescope to achieve. In addition, the
primary mirror mount was designed to also serve as an end cap for the
telescope, and it would be difficult remove the mirror from the existing mount,
as it was glued to the mount. I was afraid I might break the mirror if I tried
to remove it from the mount.
It seemed that the better option was to modify the existing tube, which is
what I chose to do. This consisted of removing the optics and cutting off some
of the rear end of the tube to move the mirror cell forward. I determined how
much by observing some distant targets in hills a few miles away with each of
my eyepiece/Barlow lens configurations. When I found the maximum intrusion, I
measured the amount of intrusion and used this as the measure of how much tube
to remove. This moved the focus further out from the side of the tube, and the
eyepiece tube no longer extended into the optical path. Fortunately, the
eyepiece tube had enough travel to accommodate this extended focal point.
Rehab Step 2: Eliminated The Clock Drive Vibration
No sooner had the focuser 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 obscure 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 has a slip clutch with a manual knob one
can twist for RA adjustment. But it has a synchronous motor that doesn't cause
any discernible vibration.
Rehab Step 3: Get The Bugs Out Of The Spider
With the new, vibration-less clock drive, 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 limb of the bright planet. With six broad spikes bleeding
away, details near the limb of the planet were hardly discernible. You can read
more about diffraction spikes at Diffraction.
The 3 element spider on the secondary was part of a plastic fitting that
also stabilized the end of the thin-walled telescope tube. Being
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,
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
Rehab Step 4: Trap Those Unwanted Photons
After all of the previous steps were completed, I found the images finally
approaching what I'd anticipated. But in looking at the smooth interior
finish of the metal tube that held the optics, I realized I still had another
factor limiting performance.
While the inside of the tube was painted flat black, the smoothness of the
finish still allowed some specular reflection of unwanted light to make it
through to the eyepiece, raising the brightness of the background. Since I
view primarily from my backyard, there are occasions when neighbors' porch lights
are on, providing the stray light impacted my DSO views.
The problem was exacerbated by the shortness of the telescope tube beyond the
eyepiece. An old guideline suggested having the telescope tube extend past the secondary
by the diameter of the tube. So the suggested amount of tube extension beyond the
secondary for my f/5 telescope was 6.5 inches. The actual extension is about 2 inches.
This allows unwanted light from sources several degrees from the target to still
enter the tube.
Again, to replace the tube with a bigger, longer one would impact the portability
I wanted to maintain. So I did the next best thing. I covered the interior of the
tube with black flock paper. Flock paper has a sticky back on one side, and a fuzzy
black surface on the other. Because of the fuzzy surface, there are no longer any
specular reflections from stray light. Stray light just gets gobbled up by the black
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.
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. I've taken a
few lunar and planetary photos with the Newtonian that show good performance.
You can see these at 6
Inch Newtonian Astrophotos.
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. The only issues with the telescope being a Newtonian is the
mirror cleaning (once every few years), and the critical alignment. I now use
a Cheshire eyepiece to do the alignment, which you can read about on the
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.
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.