The Quest for an Optimal Telescope
It's been several years since I purchased my 6 inch f/5 Newtonian from
Discovery telescopes. No longer available from Discovery, virtually the same
model is the
Orion 9827 AstroView 6 Equatorial Reflector Telescope
The 6 inch f/5 Newtonian is classified as a Richest Field telescope. A Richest or Rich Field telescope is designed to reveal the maximum number of stars to the observers eye. Short focus telescopes of moderate aperture are designed to do this, and the f/5 Newtonian is one of the more popular and capable options for this type of telescope.
It turns out that the mount and telescope I purchased were made in Taiwan. Discovery imported the mechanical parts and added their own optics. In my case, a lime-glass mirror objective.
I chose the unit after months of shopping. I'd always 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.
6 Inch Newtonian on EQ Mount
I found that Discovery at the time 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. The difference in price at the time was only about $100. 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 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 C8, 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.
Considering Scope and Mount Combination
The decision I made considered the mounted telescope as well as the telescope itself. I chose a more moderate telescope because I knew that the mount that came with it would be sturdy, given that it was also used on bigger telescopes.
Interestingly, I've read a number of articles where amateur astronomers
have been making the same calculation when considering a couple of other
very popular telescopes. It turns out that the
Celestron NexStar 6 SE Telescope
The Flies in the Ointment
For a time, I did find some things I didn't like. 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 as shown below. Notice that the illustration shows the focuser extending past the edge of the primary mirror, thus blocking off some of the mirror and creating a more complicated and destructive diffraction pattern. This noticeably reduced the quality of the resulting images.
Rehab Step 1: Adjusted The Tube Length
Focuser FOV Intrusion
In the diagram above, a simulated view into the open end of the telescope is presented. The white disk represents the mirror. In this example, the focuser tube is racked in to where typical short focal length eyepieces come to focus. As the illustration shows, this results in the focuser tube extending into the light path. This causes a more complex diffraction pattern and thus lower contrast and resolution images.
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 that I'd 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, as shown in the above diagram. 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 and 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 once done 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. I could see the planetary images become sharper just by turning off the motor.
German Equatorial Mount w/Clock Drive
Looking on line, 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 built in. But it has a synchronous motor that doesn't cause any discernible vibration.
You can see the small clock drive that I was able to obtain from JMI in the image above. It has worked perfectly for years, and with the built-in clutch I can make small RA adjustments with the slow motion controls without loosening the RA clamp. As you can also see, the equatorial mount that came with the telescope has a small polar axis telescope I can use to get good polar alignment.
Rehab Step 3: Got The Bugs Out Of The Spider
3 Vane Secondary Holder (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, shown above, 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. There was also potential of missing dim components associated with bright stars, because the bright star may throw obscuring spikes.
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.
Thin Curved Secondary Holder
I removed the thick spider elements, leaving the supporting ring, and
fabricated a thin metal curved secondary holder, as shown above. The new secondary mount makes
a 180 degree loop and evenly scatters light throughout the field of view,
eliminating spikes. The old spider had such thick vanes that nearly 8 pct of the light entering the telescope was being blocked. With the thin metal curved spider, only about 1.5 pct of the light is blocked. So in addition to eliminating spikes, the curved spider increased by a few percent the brightness of the images.
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 had 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.
Rehab Step 4: Trapped 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 stray light that impacted my DSO views.
The problem was exacerbated by the shortness of the telescope tube beyond the eyepiece. An old ATM 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 fuzz.
Rehab Step 5: Put Eyepiece Holder in Tray
The tripod for the telescope has a utility tray that is suspended between the tripod legs. This tray acts as a tripod leg brace and as a tray for holding eyepieces, etc. The tray has a lip of about 1/2 inch high around the edge to keep things from rolling off of the tray.
Basic Eyepiece Tray
Above you see the tray in the original configuration. The tray is low enough in the area between tripod legs to make for a sizable tray, and one easy to reach. It works pretty well, but tall eyepieces if sitting on end are easy to bump over. And if tall eyepieces or accessories are laid flat, they tend to roll. So I wanted something that made a more sure stable support for eyepieces and accessories.
Inverted Tray Insert
So I made a wooden eyepiece holder that could sit in the tray and provide holes to hold eyepieces and Barlow lenses. The illustration shows the tray not in place but upside down. Notice that at the extremities of the triangular insert are about 1/2 inch thick stand-offs
. These, when the tray is inserted with the stand-offs on the bottom, hold the tray up from the bottom of the tray, giving a deep enough hole to hold the eyepieces and Barlows securely.
Tray Holder In Place
The picture above shows the eyepiece cutouts inserted. This provides 1.25 inch holes to put several eyepieces. Insterted into place, the simple modification holds eyepieces securely so they can't accidently be bumped and knocked over. Of course, it could have been made to hold 2 inch eyepieces with larger holes, or a combination of 1.25 and 2 inch eyepieces.
Rehab Step 6: Replaced the Finder
Rifle LED Sight
The last improvement is one of a subjective nature. The telescope came with a perfectly fine 6x30 finder telescope. But because the main telescope is rather short, I have the tripod set pretty low so that I can often view objects from a sitting position. I do this by loosening the cradle clamps that hold the telescope to the tripod, rotating the telescope within the cradle to put the eyepiece in a convenient position, and re-tightening the cradle clamps.
But because the tripod was set pretty low to accommodate this comfortable viewing arrangement, the finder was too low to conveniently view through. And the finder being a telescope, I had to position myself close to the eyepiece. So I replaced the finder with an LED rifle sight. It was given to me, so I figured the price was right. The rifle sight is much like many of the LED telescope finders, lacking mainly a potentiometer for adjusting the LED brightness.
The LED finder sits a bit further out from the telescope, and I don't have to be positioned so closely to the finder eyepiece to be able to spot the red LED and use it for pointing the telescope.
Not Perfect -- But Definitely Not Bad
Now I finally see through the telescope what I want to see. Planetary images are quite good for an f/5 instrument, as illustrated in the Tycho crater image taken through the tuned up Newtonian. 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 wanted. 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 6Inch Newtonian Astrophotos.
In summary, I made the following modifications to fix design limitations and enhance the telescopes performance:
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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 it being disassembled.
It is a good general purpose instrument operating at f/5, which is what I wanted. I also have an SCT and Maksutov, and they too are portable in this size range. But I don't consider telescopes above f/8 in focal ratio to be that great for general use, especially star target hunting. 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. Good but 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 Collimation web page.
Addendum
Newtonian on Dobsonian Mount
This configuration workes well for me. I can easily lift the telescope off the cradle by the handle on the cradle box, and then move it and the rocker box to my observing location. The combination is much lighter than the same telescope on the equatorial mount. Observing is done from a sitting position, a real plus for me.
Needle Level used for Elevation Indication
Azimuth Indication on Base
Star Pointer Sample View
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. There seem to be more 5 inch options than 6 inch.
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.
for example. And you could always mount something
like that to a pipe-fitting base for stability. 
is a mount of this type -- relatively inexpensive
and easy to use.
have entered the hobby in a big way, particularly
because of their convenience and their surprising affordability.
,
or husky enough to easily handle something like the
.
In fact, Dobsonian telescope models up to 15 inch
are commercially available, and ATM designs handle up to 30 inch
telescopes.
.
is a currently available telescope
much like my purchase.
model. You can still see what looks like a fork
mount on the new ETX 90, but it operates in the altazimuth orientation.
