| Cleaning the Optics of the Edmund Astro-Scan Telescope
For over 30 years now Edmund
Scientific Company has produced a
very novel instrument.
The design is as simple as possible,
using a short focus 4.25 inch parabolic
mirror with an f-4 focal length the
telescope is capable of breath taking
wide field views of the night sky.
Low power views are best. With a
typical magnification of about 16 x the
milkyway is incredible under truly dark
condition's and the 4 inch light grasp of
this telescope places it far a head of
the competition, in size and weight.
The brilliant red plastic body is made of
a high impact plastic, that will certainly
hold up to some rugged use.
The telescope maintains a perfect
balance no matter what position it is in.
Low power means a finder scope is not
needed. A handy strap is attached for
This is an award
I bought an Edmund Astros-can from a guy in Upper St. Clair, just outside of Pittsburgh, PA recently for $ 60. The front
optical window was pretty dirty, and the diagonal too; so I took it apart for a cleaning. The red tube that holds the optics
is glued together. No real way to open it up. But you can remove the snap ring that hold the window/diagonal assembly
together. Tools used were a pair of diagonal pliers that can bend at various angles and a small regular screw driver.
The optical window has become covered with dirt and water spots
over the years. The front surface may be cleaned with care, but
the inside of the window & diagonal are also in need of cleaning.
Click on image to enlarge.
In any telescope, especially those with a corrector plate (Schmidt -Cassegrain)
it is best to mark the window, or corrector near its edge with a line using a
sharpie permanent pen and draw the line until it contacts the tube housing. This
will help to hold all the optical components in collimation to some degree. So this
is the very first step.
Since I decided to give this "telescope" to my daughter Stormie, the job was
done in her room and washing the optics in the kitchen. This first picture
shows the window before it was removed from the telescope.
Upon inspection the Astro-scan appears to be a robust instrument. With proper
care, and maintenance perhaps it can last a lifetime. Looking down the front of
the tube at the optics you'll see a series of molded plastic rings, that actually
serve as light baffle's.
Baffling an optical instrument improves contrast and prevents light from
scattering; a feature often found in more expensive telescopes.
However, it is the baffle ring design that gives us the impression the tube does
not easily come apart. There is a large metal snap ring that holds the optical
window in place. The ring is held in place by these molded rings. The trick is
how to remove the snap ring from the tube; and not to damage the sensitive
Removal of the sap ring with diagonal pliers
I used a pair of diagonal piers and tugged on the corner of the snap ring.
Using leverage; simply raise the end of the snap ring and put inward and up.
It took only seconds to remove. Just a word of advise -- Do Not scratch the
surface of the optical window. Touching it with diagonal pliers could
easily scratch the window, so be careful!! (It maybe best to use a cheap tool
here, since they are made of mild steel. A brand such as Craftsman or
Snap-on use harden steel and will increase the possibility of damage.)
With the snap ring successfully out of the way, I removed the focusing tube.
I placed my right finger through the hole and "pushed out" the optical window, safely
into my left hand.
At this point it was easy to see the dirt and film that accumulated on the back of the
optical window. The diagonal mirror that is attached to the window was also in need
of a good cleaning.
Overall it was a good decision to perform a well deserved cleaning of this old
telescope's optics. Inspecting the 4.25 inch primary mirror still in the red tube
revealed it was in excellent condition so there was no reason to remove or clean it.
Pictured below are the major components I removed from Edmund's famous
instrument --- the Astro-Scan.
Once the window was inspected, it was time to clean it.
No attempts were made to separate the optical window from the
diagonal, and hence the window as washed first.
The window is made of plate glass ( as indicated by the green color you
can see at the edge). This is also known as regular window glass
but for an optical window it's been checked for flatness and strain.
A mixture of pure distilled water and a drop of liquid dish washing
detergent was placed into a pyrex dish and filled about an inch from the
bottom. By holding the diagonal carefully I simply swished the window
around a bit. After a time the window was rinsed and hand dried.
The procedure was repeated. You can also use cotton balls and continue
cleaning while the window is submerged. However, don't attempt to rub
the surface until you are reasonably sure most of the dirt is off or
sleeks and scratching may occur. You should change the cleaning
water at least once if the window is dirty, before working the surface with
cotton. Rinse well, don't be afraid to waste water.
Dry the window with clean cotton rag's, repeat any additional cleaning if
needed. Look for spots, etc.
A clean telescope window is a
In this image we see that the
greenish edge described above
indicating that the window is
ordinary plate glass.
We also see that the diagonal
mirror is very nicely attached to
the window by a piece of acrylic
plastic cut at a 45 degree angle.
Note the center screw that goes
through the diagonal holder and
attached to the other side of the
window. ( You can see that the
optical window is not perfectly
round as indicated by the straight
edge on the left side.)
The mark on the optical window
visible just under my little finger
was used for alignment purposes
as described earlier.
To clean the diagonal was also an easy task
I used an old plastic child's drinking cup with
the same mixture of distilled water and dish
washing detergent and agitated the mixture
as much as possible.
Diagonal mirrors represent a different situation
then the optical window. The secondary mirror
being a first surface mirror over-coated with a
thin film of aluminum means the surface is very
delicate. Additionally, this aluminum surface is
somewhat porous and can deteriorate over time. Luckily for us it is not exposed in open air
(the optical window proves some protection from this.) and although somewhat dingy it will
clean up nicely.
Back view of optical window/diagonal assembly.
The diagonal looks to be glued in place.
Click on image to enlarge.
Be sure to use distilled water for cleaning optics, especially a diagonal or any front surface mirror. Regular tap water
has an undesirable mineral content that can leave water spots on what maybe otherwise pristine optics!
Do not rub the diagonal mirror with cotton if at all possible, unless it is submerged then only sparingly!!
Once you have cleaned and dried your secondary, it's time to reverse the process and return the optics to their housing.
Examine your optical tube, and align the mark on your optical window with the
corresponding mark on the tube wall. Any line that is on the snap ring is meaningless.
Also look in the area of where your optics will be seated. Look for any reside of the glue
used to bind the upper and lower portions of the telescope tube together. If you believe
that such glue residue will not allow the optical window/diagonal assembly to lie flat in line
with the optical axis, then an extra step must be done.
Scrape away any signs of glue that would interfere with the assembly sitting square on
with the primary mirror. In an examination with the picture on the right, it is easy to see
how excessive glue could poise a problem. Scrap away only where window sits.
Looking down the tube at the primary mirror with
optical window removed. Notice excessive glue to
right of image. - Click to enlarge.
With the window assembly properly
orientated, & the snap ring installed
the telescope takes on a new
Images are much "cleaner" than before
and the contrast is much higher.
Brilliant Jupiter is now in a darker sky,
with evidence of the belt system visible.
The four moons are like little jewels.
While still mostly a low power
instrument, the Edmund Astro-Scan
just might be a perfect instrument to
take on a trip, especially to remote
Well, it's been a lot of fun to work on...
but the telescope was really bought
for my 7 year old daughter Stormie.
....And how does she like it...?
A picture is worth a thousand
Information and images by Al Paslow -
can be used for educational and instructional
Window assembly in simple dish washing detergent.
Click on image to enlarge
Remove fousing tube push window out with finger.