Short Tube
80mm F/5 Tune-Ups
(Also applies to many other Chinese-made refractors including
102mm)
By Bob Berta
First....the Orion, Celestron, and several other companies have the same scope but in
different setups....some like the Orion are scope only whereas Celestron includes a tripod
and mount for another $100 or so. But the basic OTA is the same....a Chinese made 400mm
focal length refractor with a 80mm achromatic doublet that is air spaced.
Disclaimer....these fixes are all easy user fixes but of course they may cancel your
warrantee and I am not responsible for any goofups you do. But they are worth it....the
improvement in performance can be dramatic!
1. Some examples show a bit of pinched optic defect. To see if your scope has
this focus on a bright star and slightly defocus the image. You should continue to have a
round image that resembles a bull's eye. If the image is not round but tends to have three
semi corners you have pinched optics. The doublet is air spaced on the scope (good) and
the spacing material is three thin shims between the two elements. You can see them at the
edges of the optic when looking at the front of the scope. The pinched optics comes from
having a retaining ring a little too tight. To fix...slide off the dew cap/lens hood....it
pulls straight out. Note that there is a black plastic cell that holds the optics and on
the very front is a threaded plastic retaining ring. Simply unscrew this ring a very small
amount and see if the problem goes away. Note that this can be temperature
sensitive....you may not have any pinched optic defect at warmer climates but in very cold
climates it may show....so you might want to adjust the ring based on viewing temperature.
Pinched optics result from excessive pressure on the lens against those three spacer
shims.
2. The next fix requires you to actually pull the front element from the holding cell.
To do this you fully unscrew that retaining ring at the front that I mentioned in tip #1.
Make sure that the scope is aiming upwards.... You don't want that lens to come falling
out on the floor! Now get a clean white men's handkerchief (100% cotton is best) and place
it over your hand. Place your cloth-covered hand over the end of the scope and invert it
so that the element will come loose from the cell. You may find that the lens doesn't
easily slide out....keep your hand in the same position and gently tap the scope/hand and
all on a hard surface to make it pop loose. You will note that the lens assembly has two
elements and in fact you could actually separate those two elements from each other if you
wish by gently applying a little sliding pressure to the two elements to pop them apart.
While you don't need to do this there are some ST owners that have actually experimented
with pulling them apart and rotating them in relation to each other to get the best image.
Warning.....make sure you keep track of which lens goes in front/orientation/etc. you
don't want to put the lens in backwards! I recommend marking the edge of the lens with an
indelible pen/pencil as to Front Back, etc.....or drawing a little sketch to remind you
how they go together. Now get a black felt tip permanent marker and blacken the edges of
the optics which are now frosted but not blackened. Don't use paint as the paint would
make the lens too tight for the cell. Don't put the lens back in the scope when you are
done yet....go to the next step.
3. Look at the various screws, etc. inside the scope from the now open tube. You will
see a bunch of shiny screws, the end of the focusing tube where it is cut off, etc. Get
some flat black model paint and paint over these items. You might consider removing the
focusing assembly by removing the three larger chromed screws. This will give you access
to the focuser assembly so you can paint the inside and also observe how the focusing
mechanism works (later hopup tip). WARNING.....it is ok to remove the three chrome screws
that hold the focusing assembly on since they thread into thicker aluminum but DON'T
unscrew the three screws that hold the main front plastic lens cell assembly to the scopes
aluminum tube. Those three screws thread into soft and thin aluminum and you could strip
those screws. However....if you do all is not lost....you can drill out the holes to
1/8" and replace with pop rivets which actually make the assembly a stronger and more
durable assembly. Now you can put the scope back together again....to reinstall the
doublet make sure you don't just "drop" it into the lens cell....if you do it
will probably not go in square. Rather place the lens setup on top of your handkerchief
covered hand and invert the scope tube. Use your hand to push up the lens into the scope
keeping it square and making sure it seats fully. Replace the retaining ring making sure
you don't overtighten.
4. The last fix is to remove any excessive slop from the focusing assembly. The
instructions don't mention this but if you look at the very top of the focuser assembly
next to the focus thumb lock screw you will see two tiny holes. These contain two allen
screws. The focusing assembly rides on two plastic rails. The upper one is adjustable via
these two allen screws to take out any slop. Take the scope to a hardware store to find an
allen screw that matches the screw....I don't know the official size. To check for
excessive slop loosen that focus thumb lock and grab the rear of the focusing tube and see
if you can move it in an up and down motion relative to the scope. If so you have a little
too much motion. By the way...a by product of too much slop is that your star finder scope
will have a hard time staying in alignment with the scope image. You will need to adjust
both of the screws a little to get the upper rail to fit smoothly at both ends of the
focuser travel. You may find that it is a little easier to do this with the focuser
assembly removed from the main optical tube by taking those large three screws out I
mentioned earlier. One thing, which some people have tried, is through star testing.
Determine if the optics could be improved with a little tilting of the optics. This is for
advanced users but involves spacing the optics with shims in the lens cell....very much a
trial and error effort but it may be worth it if you see a major misalignment (not likely
in the samples I have seen). Once completed you will have a scope that will no longer have
pinched optics, will have completely or mostly eliminated any purple ghost image around
very bright objects like the moon, reduced flare, increased contrast and sharpness. These
fixes all cost nothing except for a few pennies for paint and felt marker. But if you want
to dramatically improve the images I recommend dumping the stock erecting prism or star
diagonals that come with the scope. The worse is the erecting prism which is excellent for
terrestrial viewing but is no good for astronomy....it will make a line in the image and
at higher magnifications is not very good in sharpness. I recommend getting a higher
quality star diagonal mirror....the one that I recommend as being the best is the Lumicon
Enhanced Star Diagonal at $98. While this is expensive if you ever intend to get other
scopes it can be used on all of them with a big improvement on any stock Orion, Celestron
or Meade stock star diagonal. The difference in sharpness, contrast and light throughput
is pretty obvious. Other than that you might consider a better quality eyepiece although
the stock ones that come with the scope are decent. Where they show their weakness is at
the edges of the image where they get soft. I am not recommending a super expensive
premium eyepiece but any good Plossl would be a big improvement. To show what the scopes
are capable of I routinely use mine with a Lumicon Star Diagonal and Pentax XL 10.5
eyepiece. While I recognize that the eyepiece alone cost about 1 1/2 times what the whole
scope does, it shows what the scope is capable of....sharp contrasty images right to the
edge that rival those I have seen from some very expensive APO scopes! Pretty impressive
for a relatively inexpensive scope. As far as magnification.....the 10mm eyepiece will
give 40x and I routinely use a 5mm eyepiece for 80x which seems to be about the ideal
magnification for a lot of stuff. I have also pushed the magnification to 160x with a
barlowed 5mm eyepiece and the image is still very good....even though that is about the
limit for a 80mm scope. While I have a big fancy expensive scope I find that the 80mm
Short Tube is a fantastic scope....it is portable, easily setup, large enough to show
plenty of interesting items and durable. It has also seen a lot of duty by other users as
a finder scope, wide field astrophotography lens (excellent results by the way!) and
probably tons of other uses. When I go to a star party it seems that the Short Tubes
outnumber all other types by a pretty wide margin.
Bob Berta RKB4@pge.com
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