Maintenance, collimation,
flocking, baffling, cleaning |
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Collimating Reflector telescopes |
M E N U |
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Collimating a Newtonian telescope
Collimation
is the process of aligning the mirrors of your telescope so that
they work in concert with each other to deliver properly focused
light to your eyepiece.
Conclusion: collimation is a way of improving the performance of
a telescope considerably. Often, no acceptable result can be
achieved at high resolution without an irreproachable
collimation; image processing is incapable of compensating for
the damages caused by a misalignment. Collimation is not a
superfluous technique destined for opticians and purists. The
images given by a misaligned telescope can be as awful as a 30$
toy-store scope. It's why some people who would not want (or do
not know) to collimate their reflector to turn to a refractor of
moderate size.
Collimation is the stumbling block of reflectors; it is certainly
the main cause of the mediocre reputation of SCTs. These
telescopes are extremely sensitive to misalignment; their
collimation can be destroyed by a fraction of turn of a
collimation screw or just by moving them in the back of your car,
a bump etc...
Colimating using a Cheshire
collimation piece
This is a front view of the Cheshire. It has an mirrored
surface set at a 45-degree angle and a hole in the mirror
and in the top so you can look into the scope through the
mirror.
1. Insert a Cheshire sight tube into the focuser, and
rack the focuser way out.
2. Shine a bright light into the mirrored surface of the
Cheshire. This will reflect down the tube and onto the
inner surface of the corrector plate.
3. Look into the Cheshire and you should see the shadow
of the secondary mirror against the ghostly reflection of
the corrector plate.
4. Also, look for the faint shadows of the
Cheshire’s crosshairs on the reflection.
5. Carefully adjust the PRIMARY adjustment screws to
bring the shadows of the Cheshire crosshairs behind the
actual crosshairs, and the shadow of the secondary behind
the actual secondary. NOTE: I have found that just
tightening the locking bolts is often enough to move the
shadows.
Star collimation
By observing out-of-focus star images, you
can test whether your telescope's optics are aligned.
Place a star (polar star if possible) in the centre of
the field of view and move the focuser so that the image
is slightly out of focus. If the seeing conditions are
good, you will see a central of light (the Airy disc)
surrounded by a number of diffraction rings. If the rings
are symmetrical about the Airy disc, the telescope's
optics are correctly collimated.
needs collimation - 
correctly aligned
Aligning the Secondary Mirror
Point the telescope at a lit wall and
insert the collimating cap into the focuser in place of a
regular eyepiece. Look into the
focuser through your collimating cap. You may have to
twist the focus knob a few turns until the reflected
image of the focuser is out of your view. Note: keep your
eye against the back of the focus tube if collimating
without a collimating cap. Ignore the reflected image of
the collimating cap or your eye for now, instead look for
the three clips holding the primary mirror in place. If
you can't see them (Fig.1), it means that you will have
to adjust the three bolts on the top of the secondary
mirror holder, with possibly an Allen wrench or Phillip's
screwdriver. You will have to alternately or loosen one
and then compensate for the slack by tightening the other
two. Stop when you see all three mirror clips (Fig.2).
Make sure that all three small alignment screws are
tightened to secure the secondary mirror in place.
Fig.1
.
Fig.2
Aligning the Primary Mirror
Find the three locking screws at the back
of your telescope and loosen them by a few turns.
Retighten the locking
screws when after the collimation.
Ok, now run your hand around the front of your telescope
keeping your eye to the focuser, you will see the
reflected image of your hand. The idea here being to see
which way the primary mirror is defected, you do this by
stopping at the point where the reflected image of the
secondary mirror is closest to the primary mirrors' edge
(Fig.3).
When you get to that point, stop and keep your hand there
while looking at the back end of your telescope, is there
a adjusting screw there? If there is you will want to
loosen it (turn the screw to the left) to bring the
mirror away from that point. If there isn't a adjusting
screw there, then go across to the other side and tighten
the adjusting screw on the other
side. This will gradually bring the mirror into line
until it looks like Fig.4. (It helps to have a friend to
help for primary mirror collimation. Have your partner
adjust the adjusting screws
according to your directions while you look in the
focuser.)
After dark go out and point your telescope at Polaris,
the North Star. With an eyepiece in the focuser, take the
image out of focus. You will see the same image only now,
it will be illuminated by starlight. If necessary, repeat
the collimating process only keep the star centered while
teaking the mirror.
Fig.3
........
Fig.4
Still not clear enough?
I would like to recommend the movie made
by Andy's Shot Glass website: http://andysshotglass.com/Collimating.html
This movie will show exactly how collimation is done.
Collimating a refractor telescope
What
do you see visually when your refractor telescope is out of
alignment? Usually not too much other than continued good images
if the misalignment is slight. Many refractor optical trains are
permanently aligned and that alignment, while often not perfect,
is usually pretty good. However when there is appreciable
misalignment you may notice flaring to one side on stars across
an entire field, this becomes more and more noticeable as
magnification is increased. Images are just not as sharp, stars
are not pinpoints. You may also notice additional colour faults.
1. Centre and square the focuser. This is not usually possible with commercial amateur instruments, and should anyway be unnecessary in a well-made instrument. If it is both necessary and possible, the simplest way is to remove the objective lens with its cell and place a pair of cross-hairs over the aperture of the telescope tube so that they cross in the centre. Centre the focuser and use a peephole sight tube to align it on the cross-hairs. You can make simple sighting tube for a 1.25" focuser with a 35mm film can with 1mm holes in the centre of the base and the cap.
2. Centre the objective lens. If this is possible, it is usually achieved by rotating an eccentric ring in the cell. This may require a special tool (e.g. a peg spanner) and you may have to slacken grub-screws before the ring will rotate. You can mark the centre of the lens with a small disc, such as is produced by a hole-punch, of acid-free tissue which is dampened with clean water to enable it to be stuck to the glass. The tissue disc should be aligned with the peephole sight tube.
3. Square on the objective lens. There are often either push-pull screw pairs or single screws with springs for this purpose. Fix a small light source (e.g. penlight or LED) such that it is a few centimetres outside the focuser and exactly along its axis. The light will be reflected off the surfaces of the lens elements. Introduce a thin piece of glass, such as a microscope slide cover slip, at 45º between the light and the focuser, so that you can see the reflections off the objective. When the objective is squared, the reflections will be coincident
4. Star test the telescope.
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 then you can skip
this...but this is a cheap alternative...not as good, but
still...
"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. Replace the lid.
- 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.
Baffling a refractor telescope (or eyepieces)
How
to Place Baffles
If
you want to enhance your viewing further, you may want to
consider adding baffles to the inside of your telescope tube.
Baffles are washer-like rings that help to block stray light.
This has the effect of increasing contrast and preventing stray
light and reflections from distorting the image seen in the
eyepiece. If your telescope already has baffles, you will want to
make sure they are properly placed.
There are web sites out there that will show you how to
calculate precise hole diameters and distances from the end of
the telescope for your baffles. But for all practical purposes,
the inner diameter of the baffle (the diameter of the hole)
should equal the diameter of the cone of light at that particular
point inside the telescope tube.
Perhaps the best way to determine this is to draw a rectangle on
a sheet of paper that represents your telescope tube. It doesn't
have to be actual size, but does need to be to scale. At the
focuser end of the telescope, measure the diameter of the focuser
hole and mark that diameter on one end of your drawing. Then draw
a diagonal line from the top of the focuser opening (marked A on
the diagram below) to the top of the objective lens. At any
position along the length of the telescope tube, the baffle will
have a width equal to the distance between the top of the
telescope tube and that diagonal line.
an example to baffle my homemade telescope
Flocking a Newtonian reflector telescope
What
is flocking paper?
In
short: self-adhesive black velvet or felt that absorbs virtually
100% of light that strikes it and is so much more effective then
black flat paint.
Many telescopes suffer needlessly from low contrast images caused
by stray light reaching the focal plane. ProtoStar flocked paper
is a very black, self adhesive, material
(Note: if you wish, you can opt to leave the thin paper backing
on, and glue the material in place).
The flocked paper has a surface texture that is best described as
"peach fuzz," and the fibers don't shed like some
velvets and felts do. When looked at from low angles, it absorbs
virtually 100% of light that strikes it.
Applications include:
-Lining the inside of telescope tubes (especially the inside wall
opposite the focuser).
-Covering metal clips and hardware on the primary cell.
-Covering exposed edges of secondary mirrors.
-Lining the inside barrels of Barlow lenses and other eyepiece
accessories.
Darkening the telescope tube
We want to eliminate reflections as much
as possible, so the best thing we can do is to flock the
telescope tube. It's much more effective then even the
best ultra-flat black paint. Applying a flat black paint
to the inside of your telescope tube helps, but a
textured matt black surface is a much better light trap.
So we won't get into this as its definitely
counter-productive to what we are trying to accomplish:
the darkest tube possible.
You can clearly
see the difference
only a piece of
flocking paper on the front of the tube to demonstrate
the difference...much darker!
Let's use my own Skywatcher Pro
Newtonian reflector as the "guinea pig"
A beauty isn't she. Before we
continue, it isn't a very dificult modification to carry
out, but you should always be as careful as much as
possible when taking your telescope apart, so as not to damage it!
You'll also notice the plastic bowl. This is used to
cover the primary mirror and a secondary one to
cover...yes the secondary mirror.
I used an old bed cloth to wrap around my table. This
does not only protect my oak table, but needless to say
it also protects your telescope (scratches etc.). If you
are using non-adhesive flocking paper then you'll need
some plastic covering too as you will need to spray the
inside of the tube with glue. If the lateral, then please
cover the outside of your telescope too.
Before flocking, you will need to remove all optical
components, such as the objective lens (refractors),
primary and secondary mirror (reflectors), eyepieces,
focuser, finder, etc.
Please mark the mirror holder on the telescope (little
piece of tape) so you'll know exactly where to place the
primary mirror after flocking. I did the same thing with
the secondary mirror.
First remove the primary mirror, then the secondary
mirror (spider) , the finder base and finaly the focuser
(and of course the top ring of the tube).
Always holding the central assembly of the spider with
your hands (not touching the mirror) to avoiding dropping
it to the bottom of the tube and consequently damaging
the secondary mirror.
and put them away safely (is
also where the plastic bowls come in...protecting the
mirrors)
A "naked" Skywatcher
telescope tube ready to be flocked
Measure the length and width of
the flocking paper you will need. Cut the flocking paper
a bit larger then the length of the tube and a bit wider
then the perimeter of the scope tube. It is better to let
the flocking paper overlap a centimeter or two then
trying to fit it exactly together. You wont see the
overlapping when done anyway.
After flocking, cut the excesive flocking paper from the
edges and finish off the edges.
Let the flocking paper settle for an hour before putting
the telescope back together.
Now we need to cut out all of the holes. I
used an x-acto knife, which is easy on the larger holes
like those for the thumbscrews attatching to the spider,
and the hole for the focuser draw tube. However, for the
smaller holes this was a bit harder, and after making an
initial cut I had to enlarge the hole with a smaller
knife (and even a corkscrew helped). Make sure that you
don't leave any loose small pieces of adhesive, after
cutting out the holes, because if they fall on the
primary or secondary it's going to be a true pain to get
them off!
And finally, re-mount everything. First
start with the finder, then the focuser, then the
secondary mirror, then the top ring, and finally the
primary mirror (take care and mount the mirrors according
to the marker we placed earlier).
This is how it looks after flocking...
You will need to collimate the telescope
afterwards.
Remember
Number One Rule: "The best way to clean a lens is not to get
it dirty."
Number Two Rule: "Don't...unless you absolutely have
to."
Number Three Rule: "Blow off first and then determine if
cleaning is still necessary."
Cleaning reflector mirror
Brushing optics and carefully using compressed air to blow off particulates such as pollen and dust can usually get the mirror or optics back into top shape. Cleaning should only be done if there are stains or excessive spotting beginning to build up on the mirror's surface.
With commercial telescopes, cleaning the primary should be avoided at all costs....prevention is the best care you can give the optics of these telescopes: keep the back opening plugged at all times, even when briefly removing accessories... This keeps both dust and insects from floating in and it prevents humidity and damaging environmental pollutants from entering the inside of the telescope.
With some air pressure (produced by a compressor, a plastic bottle or a spray) most of the dust particles will disappear from the surface. Make sure that you do not though the mirrors! No brushes should be used for cleaning, because even the finest of them can produce tiny scratches.
If the removal of dust is not sufficient (their might be some spots and speckles on the surface) you should look for a small plastic bottle with a small hole at the top and fill it with distilled water. You should set the telescope frame some cm higher than the top of the table. Then you spray the water with some pressure entirely over the mirrors surface. The water will rinse down and the mirror will dry. Locate some clothes under the mirrors to remove the water.
Look at the mirrors surface after they had dried and if you see any color speckles which might be produced by fingerprints or some fat spots, you should repeat the cleaning procedure by using Alcohol or Propanol in a plastic bottle. Please do not use other chemicals because they might damage the coating.
You can also mix a cleaning solution composed of two parts of distilled water, one part of Propanol and some drops of a cleaning agent (not an agressive one).
The mirrors surfaces can be perfectly cleaned by this method, without touching the surface mechanically. You should always consider that the mirrors are the most valuable part of your telescope.
Do not remove the mirrors for cleaning purposes from their cells. A serious misalignment of the optics will result after reinstallation of the mirrors. A perfect alignment could only be managed by experienced amateurs or by an optical shop.
Some dust might also be present at the inner tube walls. This can be removed by using some wet cloth and by pressing it against the black rough coating.
You should not use oil in the interior of the tube, a thin film of Vaseline might protect screws from corrosion. Do not change the position of the adjustment screws and do not remove any screws for cleaning purposes.
Cleaning refractor lenses or optics
General tips
To try to obtain the best possible light transmission efficiency from our optics by cleaning them free of deposits, film and debris, do not ever forget that cleaning coated optical surfaces is the single-most damaging action that will be done to them, short of actual physical damage or breakage. No matter how careful, how delicate, nor what cleaning solution is used...every time cleaned will result in a microscopically-reduced optical performance than before cleaning. Note that the coatings themselves are nothing more than molecule-thick deposits of a very delicate film left on the optical surface from a vacuum process in which air is evacuated and the gases of the coating materials are gently and uniformly distributed across the glass surface after the vacuum container is void of air.
Never clean optical glass without gently dusting first!
To dust, use a square-cut very soft brush that is about 40mm wide with tapered bristles. Look for the very soft and flexible "touch up" and/or "delicate trim" brushes...most of these are short-handled and have the bristles as an angled radius cut. Make sure that the bristles are incredibly soft; I use the "cheek method" for testing softness: take the brush out of its package and push the tiny ends of the bristles hard against the cheek of your face...if they do not "prick" then they are fine for optical use. The more bristles on brushes the better and are usually the softer and better in quality.
Tips and tricks
The Lens Pen
Held like a regular pen, the Lens Pen uses an ingenious dry carbon-black cleaning compound on a small felt pad to clean lenses without liquids. Works great!
Eyepieces
Eyepieces must also be kept clean, because even a slight amount of oil or grease on the lens will smear and degrade the view. The outermost eye lens is especially susceptible to grease from eye lashes. To clean a lens, use lens tissue dampened with lens-cleaning solution. Lightly wipe the lens surface with the tissue to remove the grease. Lens pens are also good for this purpose, but to prevent scratching the lens's soft optical coatings, never rub with any pressure.
How to setup an equatorial mount
Read all about it here
Visit again soon for more tips and tricks.
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