Living With an Inexpensive Telescope
Do you have an inexpensive telescope? You know the type, bought at a department
store, the type you probably got for Christmas. The ubiquitous 60 millimeter
refractor, or 4 inch reflector, seemingly destined to sit in a closet,
or to be passed around the neighborhood during yard sales.
But it doesn't have to be that way. Say you are short on funds, or like
me, an unrepentant tinkerer. Fact is, many of these scopes have some redeeming
qualities, enough to warrant some investment in simple mechanical and optical
improvements.
Your Typical Inexpensive Scope
The typical 60 mm refractor is a tube with an altitude-azimuth mount. The
eyepieces used are .965 inch in diameter as a rule. It sits on a tripod
of wooden or aluminum construction. Occasionally the scope uses an equatorial
mount. Inexpensive small reflector telescopes almost always have a equatorial
mount, sitting on a tripod of similar construction.
A few months ago, one of our club members mentioned that he had a scope
he was trying to move out of his garage. Generously, he decided to donate
the scope to our astronomy club, The Central Pennsylvania Observers. After
a quick inspection, we realized this puppy needed some work.
At this time I was in the final stages of making a 12.5 inch Dobsonian
reflector, and was looking for a telescope for my son and I to use while
finishing my scope. So I volunteered to work on this little one in return
for using it in the interim. I also wanted to write up my experience, as
I wanted to show how with a little work, a person with a modest scope can
enjoy amateur astronomy as much as a person that has spent a lot of money
on their system. Remember folks, amateur astronomy is not a contest.
The scope was a closed-tube Newtonian of indeterminate manufacture,
(seriously folks, I'm not trying to avoid a lawsuit, there was no manufacturer's
name on the scope) the mirror was 4.5 inches, and the focal length was
8.7. It had a .965 inch focuser and a focal length reducer to shorten the
length of the optical tube. The scope was cradled onto a German equatorial
mount. There was also a novel finder scope that mounted onto the side of
the tube at an angle, and a little flip mirror that introduced light from
the front of the tube into the finder scope.
Checking the Collimation
After taking a initial look through the scope, I decided to take a look
at how well the scope was collimated. Removing the eyepiece I looked into
the eyepiece holder. Immediately I knew what half of the inside of the
optical tube looked like. The collimation was terrible!
I then looked for the adjustment screws. Guess what? There were none!
Fearing a huge problem with a complicated cure, I disassembled the back
of the scope. The mirror was glued to the back of the cell. Not a good
thing. The mirror cell was held onto the back of the scope endpiece with
three screws. This gave me an idea. If I could put slightly longer screws
at these three points, and put a compressible substance between the back
of the mirror cell and the base of the holder and viola, I had a method
of collimating the scope! I used the rubber tips of three pencils. Cheap
for sure, but it worked.
If you are not familiar with the collimation of a telescope, consider
that in a Newtonian telescope, for best image quality, the primary mirror
should reflect the light it receives directly to the secondary mirror,
which then reflects the light directly to the eyepiece. When looking through
the focuser without an eyepiece, you should see the mirror centered in
the view, with a teeny tiny eye looking at you centered in the image of
the primary mirror. It's your eye by the way, so don't be afraid.
If the collimation is off, as it was in the club scope, we have to adjust
the pivot of the mirrors to line them up. After I made the modification
to make my mirror cell adjustable, I simply adjusted the screws to tilt
the mirror in the proper direction. this took about a minute. The secondary
collimation is adjusted in this scope by adjustment of three hex-head screws
on the top of the secondary holder.
Collimation is seldom a problem in Refractor telescopes. If your refractor
elements aren't lined up, you are the proud owner of a lightweight baseball
bat.
Looking at the Sky
After adjusting the collimation, I took the scope out for a look at the
stars. The 8mm eyepiece belonging to the scope was almost worthless. The
image was dim and pretty unsharp overall. Some manufacturers must think
we only want to look at the sky under very high power. I didn't have any
other .965 inch eyepieces, so I cannibalized an old set of binoculars for
one of it's eyepieces, and dropped it in the focuser barrel with the .965
inch bushing removed. This gave me a nice 30-35 mm eyepiece, though it
didn't have a lot of eye relief, since it came from a binocular. The binocular
eyepiece gave a sharp image, and contrast was significantly improved.
What is Wrong with this Mirror?
Training the scope on some stars, I saw the thing was suffering from some
pretty nasty astigmatism. Taking the scope slightly out of focus, the stars
looked like little triangles, with one point of the triangle being the
star itself. After an hour, the temperature of the scope reached equilibrium
with the outside air, the astigmatism problem went away, and stars looked
like the pinpoints of light they were supposed to look like. I was going
to have to do something about that mirror mount though. The problem was
that when the manufacturer glued the mirror onto the cell, they caused
distortions bad enough to twist the mirror during the cool-down period.
Thus is due to the different expansion rates of the glass of the mirror
and the mirror cell material.
Remounting the Mirror
The solution to this is in this case, to remove the mirror from the cell,
and remount it using three small "blobs" of Silicon RTV. If you do this,
place the blobs on the mirror cell about two thirds of the way to the edge.
Space the blobs about 120 degrees apart. Next, place three spacers on the
cell, then place the mirror in it's proper place on the cell. Be sure
that the spacers can be removed when the RTV solidifies. The silicon
rubber will spread out to hold mirror evenly, and the The flexibility of
the RTV will help isolate the mirror from the flexing of the cell. I remounted
the mirror holding clips before the RTV dried.
The Telescope Mount
My next task was checking out the mount. This turned out to be a pleasant
surprise. Unlike so many inexpensive scopes that are cursed with wobbly
and almost useless tripods, this one was really solid. There was no wind
induced wobble at all on a night that was gusty enough to blow my van doors
shut, and a solid whack on the tube settled down in just over a second.
The Equatorial mount was solid and just a very little bit stiff. All in
all, the mount and tripod is probably the best part of this scope.
The Finder Scope
The finder scope was another story altogether. What I originally thought
was novel, and kind of cool, (kewl?) turned out to be dim and hard to focus,
and almost impossible to find stars in. This thing was going to come off
at the first possible moment, and be replaced with a Daisy bb gun led sight.
I seriously don't think this scope is usable with this finder. In fact,
I put the Orion nebula in the scope's field of view, and then tried to
locate it with the finder scope. I did see it, but most of us prefer to
use the finder scope to locate objects to observe in the main scope, not
the other way around.
How it Turned Out
With these modifications, the club scope is a usable instrument, nothing
spectacular, but still okay. Time permitting, I may scrap the tube that
came with the scope, remove the focal reducer, and either add a spider
to the secondary holder, or flatten the glass holding the secondary. I'm
sure that Optical window is plain plate glass, and if the surfaces of the
window are not parallel, sharpness and contrast will suffer. This will
make the optical tube assembly a bit longer, but I expect performance increases.
Things to Consider
If you are considering upgrading an inexpensive telescope, there are a
few things to look for and do that will improve performance pretty dramatically.
There are also some scopes that are not worth messing with. If the scope
in question has a plastic objective lens or a field stop (a circle with
a hole cut in it that obstructs some of the light from the lens) placed
right behind the lens, you probably shouldn't waste your time. Many of
these scopes have doublet lenses however, and optically they aren't too
bad at all, especially mated with a good eyepiece.
Eyepieces
Notice I said good eyepiece. What comes standard with so many of these
scopes is not a good eyepiece. Notice I am using great restraint here.
If there is one thing that will improve the optical performance of an inexpensive
telescope more than anything else, it is the eyepiece. Get a good one.
If you are on a budget, go to the Astromart website. They often have .965
inch eyepieces for sale. You might also consider modifying your scope with
a 1.25 inch focuser, as there is a better variety of lenses in this size.
You also don't want to get an eyepiece that is too short. Anything less
than 10mm is going to magnify the image beyond what is usable with a small
scope. If you have an old pair of binoculars sitting around the house,
you may want to tear them apart and use one of the eyepieces for your scope.
Quality can vary, but they will almost always be better than the eyepiece
that came with the scope.
Finder Scopes
The finder scopes on these things are usually not very good. If you happen
to have obtained a good one, don't play the lottery any more because you've
used up all the luck you're going to get in this lifetime. For the rest
of us, listen up.
A finder should allow you to position your scope's view very near
the object you want to observe. You have the choice of magnifying or non-magnifying
finders. On a small scope there is a lot to be said for non-magnifying
finders. These can be as simple as two holes that line up together, or
a Telrad, which is a LED device that you use a illuminated bullseye to
center on the object you want to see, while looking directly at it. A Daisy
bb gun LED sight works very nicely on a small scope, and is inexpensive
to boot. Some people modify the gunsight to reduce the light output of
the LED.
Another possibility is a small gun scope. Small ones are available at
inexpensive prices at many department stores. These scopes have the advantage
of being easily adjustable. I have a gun scope on one of my telescopes,
and it works very nicely
Mounts
If you have a wobbly mount, you can try a few things. Leather washers can
often smooth out a sloppy alt- az mount, both helping hold the optical
tube, and damping out vibrations. A good heavy grease can help in the azimuth
section of the mount. If these don't work, you might consider a Dobsonian
mount. Dobsonian mounts are Alt-Az mounts that use Teflon bearings, and
can be made for both refractor or reflector telescopes. Making a Dobsonian
mount is beyond the scope of this article, but there are plenty of web
resources that cover the issue.
If you have a shaky equatorial drive, check the gears to see if there
is any chance of adjusting them. The idea is to have a good tight contact
between the ring gear, and the worm gear. This helps to prevent "backlash",
which in it's extreme form shows up as wobble in the scope. If there is
no adjustment, your best inexpensive solution is to apply some more of
that heavy grease. This won't completely cure bad backlash, but it will
help damp out vibrations.
Tripods
Nothing beats weight in a tripod. As likely as not, the tripod that comes
with the scope is a very light affair, and mounting the telescope on it
gives it a high center of gravity. This is not good.
"But Mike" you cry, "I don't want to carry all that weight with me!
What can I do?" Fear not, Weedhopper, you will not have to carry the weight
of the world on your shoulders. The answer to this mystery of life is to
place a hook on the center post or center of the tripod, make a cloth bag
to hang from that hook that can hold local rocks that you pick up at the
observing sight. You can also use bottles of water, your significant other,
whatever is handy. The idea is to just use centered weight. Just don't
overdo it and break the tripod.
What Can I Expect
Let's make no mistake about it, an inexpensive scope is not likely to give
you the views that you can get if you spend thousands of dollars, no matter
what modifications you make to it. In this world we only get what we pay
for, or a little less. But with a few inexpensive modifications, your inexpensive
telescope can give surprisingly good service, and a lot of enjoyment of
the night sky.
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