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Basic telescope improvements |
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If you
first want to learn about telescopes I would like to recommend my
webpage here
Here
we go...
You might have read
elsewhere on warnings to avoid department store type of
telescopes, mostly costs around 35$...60$. Not only are the
optics small (small aperture and 0,96" eyepieces) and not
good for showing you much. The diameter of the objective lens
determines how much light can be gathered to form an image. It is
usually expressed in millimeters. Often those telescopes as an
objective lens of 50...60mm, and if your lucky 70mm.
The telescope usually comes on a very flimsy and shaky tripod
that won't keep the telescope stable for aiming and viewing. Such
telescopes have demoralized many would-be astronomers and caused
them to run away from this exciting hobby.
Magnification
The rule used
by amateur astronomers is 50X per inch of aperture or 2 X
objective aperture in mm. This is for quality optics, as the
optical quality goes down so the ability for the scope to give
clear images at higher powers. So with this information we can
conclude that the highest power a 60mm refractor from a
department store can resolve is 125X.
Perhaps you own one of these scopes. My first advice is to
discard it if possible (or use parts of it to build your own),
and buy a quality telescope from a dedicated telescope store. But
I presume, because you're reading this, that you are committed to
making the best use of it. What can you do to make it more
usable? Well, plenty.
1th,
get better eyepieces
Ask any astronomer,
and they will tell you that the eyepiece is half the telescope.
The optics in the eyepiece focus the light from the lens or
mirror into your eye. We'll see later how to improve those cheap
eyepieces.
The eyepieces that are packed with department store scopes are,
unfortunately, even lower quality than the scope itself. They are
often mostly plastic, with optics that will give you fuzzy
images. They have a very narrow field of view, so you get the
feeling you're looking at the universe through a straw. And you
have to place your eye very close to these eyepieces in order to
see anything (called poor eye relief).
2nd,
prevent reflections
The tube of the
telescope was, luckely, painted matt black and had one single
baffle (small ring aprox. halfway in the telescope tube that
limmits the reflections seen from the focuser).
-- Solution one?: do nothing (but
do spray it matt black if it has a shiny entirior).
-- Solution two: try to get your
hands on flocking paper (or black velvet or black felt) and glue
it on the inside. Believe me, it will make a big difference,
surely when you use your telescope in city lights, or when
viewing the moon... It will improve the contrast and loose some
of those reflections that tends to get into the eypiece when
viewing bright objects.
-- Solution three: use another
baffle and flock it (see figure and explanation below)
The inside of the dewcap was shiny black plastic (same as the
ouside): no good!
-- Solution one: sand the inside
down with a coarse sandpaper and paint it flat matt black.
-- Solution two (best one): try to
get your hands on flocking paper (or black velvet or black felt)
and glue it on the inside. It will keep light from reflecting
from the inside the dewcap and getting into the objective lens.
The eyepieces same thing or worse: shiny!
We need to remove all internal reflections inside the eyepieces
themself and (if needed) clean the lenses as well (read about it
here). None of those eyepieces I saw had any baffling or are even
lacking flat black matt paint! Shiny black plastic isn't all that
dark! Just keep it in the light, straight or under an angle, and
you will clearly see the reflections. Any reflections are loss of
light and can cause ghosting or glimmer. I even saw eyepieces
where the chrome part of the eyepieces was chrome on the inside
too!!! The Barlow lens that came with the scope was exactly the
same way: the bottom was chrome and so was the inside! Also, the
entire tube was shiny black plastic. My God, that was a looooong
light tunnel to be seen through the barlow. Completely useless.
3th,
stabilize the mount
Here are some
simple techniques you can try to make the mount more stable.
-- Grease the chrome arm (right side of
the scope and mount) that fits into the opening where the bolt
fastened it.
-- Grease the rotating parts of the top
-- Tighten the wing nuts at the top of the
tripod. This will make everything stiffer.
-- Lower the tripod legs. The lower the
scope is, the less motion can occur.
-- Suspend a weight from the tripod head.
Get a 1-liter box and fill it with sand and with a strong rope or
other means hang it from the bottom of the mount head. The extra
weight will prevent the mount from moving and dampen vibrations.
-- The aluminum legs can be opened at the
top or bottom if you take some care. This will allow you to fill
the aluminum legs with sand. This absorbs vibrations, though you
will have a heavier mount.
-- Try vibration supression pads. These
pads are round rubber discs you place between the ground and the
bottom of the tripod legs. They absorb vibration in the tripod
legs. Commercial pads cost about $50, but you can get results
cheaply by using those kneeling protecting mats and cut three
round parts out of it. Any thick rubber-like material will do.
4th,
remove the slack and improve focuser
The telescope focuser had
enormous slack. It wobbled up and down and left to right. To fix
it I removed the focuser tube by removing the 2 screw beneeth
focuser tube travel.
I stuck "electrician" tape inside the focus tube
travel. Three parts of the length of the focus tube travel and
separated evenly in the tube.
Test if the slack is gone, else stick another tape over it.
Put a little grease on top of the tape.
Remove the old sticky grease from the focuser mechanics and the
tube.
Use a better grease, like for example: Finish Line
Teflon-Fortified Dry 12oz. Lube
You can choose
to keep the telescope as-is or rebuild it into a new one like
displayed below,
but everything described here can be used to improve your
"cheap" telescope without cutting it into pieces
My
modified homemade scope
Born April 20th 2008
Name: RuleScope I
Now, let us
begin to build...
The telescope I
had (got it for 10$ from a fleamarket) was a BlueSky
D=60mm/F=700mm telescope. I will base my homemade telescope from
that type, although you will be able to adjust the modifications
as we get along for your type of "cheap" telescope.
First decide if you like to "destroy" your cheap
department-store telescope and build from it your own Keplerian
telescope. Or do you
"just" want to improve the scope. If you decide to keep
the telescope as-is, then I would like to recommend to above tips
and tricks (and skip the next chapter ;-)...No no! There are a
lot of tips and tricks you can use during the process as how I
build and modified the existing telescope and give it a new
life...
Well, a new life as a Keplerian based telescope that is. What is
a Keplerian telescope? Read all about it here please, but it all comes down to a
slightly improved Galilean telescope. The reason for my decision
to make my own telescope from a cheap department-store scope is
plural:
Ok, we decided to take the telescope appart
the 'blue' one we need, or any other
'magnifying' glass (a convex lens). If you do not
know the focal length just point it at the sun and
measure the distance from the lens to the hole it burned
in the paper (do NOT look into the lens!!!). Second
method is to use the moon at night. The distance from the
lens to the best image of the moon is its focal length.
As this is a simple "magnifying glass" lens and
it's uncoated then we need to reduce the field-stop*read text
(see figure below)
uncovered dewcap
first 40mm field-stop on the
dewcap cover (originally)
second 20mm field-stop on the
dewcap cover
Collimation
Now we can temporary remount everything and start
checking the minimum and maximum focal length using
different eyepieces and distances. Adjust if needed.
We also need to check if the lens is properly placed. Not
only because we've cut a piece from the telescope tube,
but every change you do to the main lens area (like when
cleaning etc.) we need to check the collimation. If you do not know what collimation is
then please read this first. In short: Collimating a telescope
is lining up its optical components (lenses, mirrors,
prisms, eyepieces) in their proper positions.
If you have a cheshire collimation piece it's easy, but
this is a cheap alternative...not as good, but still...
If you do not have a cheshire collimating
tool, I suggest that you make at least a
"collimating cap":
- if your scope has 1,25" eyepieces: out of a
plastic 35mm film canister (black with gray lid). Drill
or punch a small pinhole (+/- 3mm) in the exact center of
the lid and cut off the bottom of the canister. Be sure
the lid is transparent and draw a diagonal with a thin
black permanent marker.
- if your scope has 0,96" eyepieces: use an
protection lid that came with your scope (like one of
your diagonal or the one used to protect the eyepiece
holder). This is of transparent material. Drill or punch
a small pinhole (+/- 3mm) in the exact center of the lid.
Draw a diagonal with a thin black permanent marker.
This device will keep your eye centered of the focuser
tube.
Place the dewcap lid in front of your scope so that it is
completely darkened.
Insert the collimating cap into the focuser in place of a
regular eyepiece and look through the hole while you have
a bright light source behind you. The transparency of the
lid is enough to see the reflection.
<-probably will look
something like this (just not as bright as shown here!)
<-we need to get it like this
(just not as bright as shown here!)
Gently push on ends of the main lens holder till you get
it centered. Sometimes it is needed to regulate the base
of the focus tube traveler. There are three screws that
can be losened so you can adjust it till you get it
right. Tighten the screws.
RuleScope I: Keplerian Telescope Design (using one lens as objective):
The Barlow lens that came with the scope was
exactly the same way: the bottom was chrome and so was
the inside bottom ring! Also, the entire tube was shiny
black plastic. Completely useless. So, use coarse
sandpaper on the inside and spray it flat matt black. Do
not forget the chrome part on the inside too!
A filter like the V-Block filter can do
miracles for your homemade telescope. It effectively
removes the distracting color halos from around bright
objects, greatly improving image sharpness, color,
contrast, and resolution.
You can clearly see that I move
the point of the elevation arm (chrome attachment holder
approx 3 cm from the top of the tube)
Triplet lens detail
The third lens is a simple convex lens ideal of the same
focal length as the doublet, but I had only a 900mm focal
length lens. Try to get your hands on a rubber ring (like
used inside those old tape-recorders) that fits the
diameter 60mm. This ring spaces the achromatic doublet
with the 900mm focal length lens.
So,
what can I expect to see?
With all these
improvements you will get a useable scope, although reducing the
size of the scope like I did does not make it a better scope, on
the contrary. But you know the reasons (see text).
The companies who market department-store scopes promise you
every wonder the Universe has to offer. They claim you will view
images in the eyepiece that look like photos from the Hubble Space Telescope. The
reality is somewhat different, no telescope, even very large ones, will give you a
view that matches what you see in photos. These photos are taken
with cameras set up for hours-long exposures, and use films and
CCD detectors much more sensitive than the eye.
However, we do have plenty enough to see through our
"cheap" telescope...the Moon is a fantastic sight in
nearly any telescope!
You can see Mercury, Venus, Mars, Jupiter, and Saturn easily
through the telescope.
Also some double stars (appear very close to one another in the
sky) of which the color contrasts can be sometimes breathtaking.
The most famous double star is Alberio. A small refractor like a
department-store scope actually works well for viewing double
stars.
Open star clusters. You might try observing the Orion
Nebula(M42), Andromeda(M31), M36, M37, Great Hercules
Cluster(M13)...
Only a few diffuse nebulas are observable in 60mm refractors;
half of them are in Orion. Among the "known ones" (M42,
M8, M17...) we find more discrete nebulas surrounding stars (NGC
2023, NGC 1999....)
Look for some good software (starcharts) that allow you to find
all of this and much much more with your PC. My favourite is Stellarium By
Fabien Chereau. It's
easy to use and shows you the heavons like you are standing there
yourself.
More software can be found here
the homemade scope (rulescope I)
or online
tools
Although only a few
galaxies are bright enough to be visible through light pollution,
two of those exceptions lie near the Big Dipper. The first, M81
(Map 2), is found northwest of the Dipper's bowl, while the
second, M51 (Map 7), is just south of Alkaid, the star at the end
of its handle. Although a low-power eyepiece is best for zeroing
in on each location, a medium magnification, around 80x to 100x,
will probably deliver the best views.
Finally, follow the arc of the Dipper's handle to Arcturus, the
bright orange star that dominates the constellation Boötes.
Northwest of Arcturus, across the border in the faint
constellation of Canes Venatici lies the globular cluster M3 (Map
7). M3 looks like a tiny ball of cotton near a 6th-magnitude
field star through small telescopes. Six-inch and larger
instruments may be able to resolve a few individual points of
light, even from inside a city.
What kind of eyepieces to use
Look for a low power eyepiece to view nebula, open star clusters, and galaxies through. This should be in the 35X to 50X range. The next one you will want is an eyepiece that is in the 75X to 100X range. This would be to look at details in globular clusters, small parts of nebula. Then eyepieces that will give you 50X par inch aperture. This is the high power eyepiece used for planetary viewing when the conditions are correct.
Clear skies!
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