How is the secondary mirror attached to its housing?
On most Cassegrain-style telescopes (Schmidt-Cassegrain,
Maksutov-Cassegrain, Ritchey-Chrétien, Takahashi Mewlon), the three collimation screws
also
hold the secondary mirror and its mounting plate to the secondary housing.
An example is the Meade 10" f/10
three-screw. This is why it's so important
not to remove more than one collimation screw at a
time when installing Bob's Knobs. The secondary mirror mounting plate on
these telescopes
pivots on a tang or spring at its center, and the collimation screws, which
are threaded into the plate, adjust mirror tilt.
Other telescopes have a fourth screw in the center of the secondary
housing. An example is a version of the
Celestron C14. This central screw usually holds the secondary mirror assembly
to its housing. The collimation screws, which are typically headless
set screws, press against the mirror
mounting plate rather than thread into it. When installing Bob's Knobs on
these scopes, it's still important to remove only one collimation screw at
a time to reduce the risk of rotating the secondary mirror and disturbing
its alignment with the primary mirror.
Several Celestron telescope models with the Fastar
option have a fixed central knob mounted on the secondary housing for
the user to hold while unscrewing the outer ring. The three collimation
screws on this telescope thread into the secondary mirror mounting plate
and hold it in place.
Some Meade models, such as the newer
8" f/10 six-screw, have three coil springs
between the inside of the secondary housing and the mirror mounting plate,
instead of a single central spring or tang. These telescopes are identified by six
screws on their secondary housing instead of three. The outer
three screws hold the secondary assembly together, and the inner three
screws adjust collimation. Each collimation screw passes through a coil
spring and holds it in place. Before fitting Bob's Knobs, first tighten
each collimation screw until the coil spring around it is fully
compressed. The screw is sufficiently tight when it becomes slightly
harder to turn. Do not force the screw beyond this point. When installing knobs, do not loosen more
than one screw at a time or the coil springs could slip out of position.
If this happens, you must disassemble the secondary to reposition the
springs. More information on the Meade six-screw secondary can be found
here.
The larger Takahashi Mewlon telescopes (the Mewlon 300
is an example) have a
"push-pull" collimation arrangement. Three screws thread into the
secondary mirror mounting plate and hold it to the housing, and three
additional headless set screws press against the mounting plate and lock
it into place. This system eliminates the central pivot to
provide room for the electric secondary focuser.
Each of the above designs can also be used to secure the
secondary mirror on a Newtonian-style telescope. The most common method
has a central securing screw with three collimation screws around
the perimeter of the secondary housing. An example is the
Meade 10" Schmidt-Newtonian.
On most telescopes, the secondary mirror is attached to its
backing plate with double-stick tape. It is extremely important to use
Bob's Knobs with the correct screw length on your telescope. If the screws
are too short, you run the risk of dropping your entire secondary mirror
assembly. If the screws are too long, they can push against the back of
the secondary mirror and pop it off of the backing plate, with equally
unfortunate results. Always check screw diameter, length, and thread pitch
before installing knobs. The screw length of our knobs should be within
about 1.5 mm (1/16 inch) of the factory screw length. If you have any
doubt about whether your Bob's Knobs have the correct screws, please
contact us before installation
Is there any danger of dropping my secondary mirror during installation
of Bob's Knobs?
Not if you follow our instructions carefully and use the correct knobs
for your telescope. By removing only one
factory collimation screw, replacing it with a knob, and repeating this
process for the other two collimation screws one at a time, there's no
danger of dropping the secondary mirror. Our instructions also recommend
pointing the optical tube assembly (OTA) slightly downward during the
installation of our knobs for an added safety measure.
On most telescopes, the secondary mirror is attached to its backing
plate with double-stick tape. It is extremely important to use Bob's Knobs
with the correct screw length on your telescope. If the screws are too
short, you run the risk of dropping your entire secondary mirror assembly. If the screws are too long, they can push against the back
of the secondary mirror and pop it off of the backing plate, with equally
unfortunate results. Always check screw diameter, length, and thread pitch
before installing knobs. The screw length of our knobs should be within about
1.5 mm (1/16 inch) of the factory screw length. If you have any doubt
about whether your Bob's Knobs have the correct screws, please
contact us before installation.
Does each telescope model require a different set of knobs?
Yes, each model of telescope generally requires a different set of
knobs because secondary designs vary. As aperture increases, secondary
assemblies also become larger, and longer screws are needed for
collimation adjustment. Screw diameter and thread pitch can vary as well.
We tailor knob diameter to the size of the secondary to make collimation
adjustment as precise as possible. Therefore, always use knobs that are
designed specifically for your telescope make and model.
Since telescope
manufacturers often re-design their hardware during production, check that
the knob threads match the factory screw threads in diameter, length, and
thread pitch before attempting to install the knobs.
My telescope isn't in your product list. How can I get knobs?
If your telescope isn't in our product list, we may still be
able to provide knobs for it. First, check our
custom knobs page for a large variety
of knobs and screws that we have in stock. You will also find directions
on how to measure the size of screw in your application. If you find what
you need there, you can order by clicking on the appropriate Add to Cart
button. Otherwise you can contact us directly.
First, take a photograph of your factory screw in your scope so we can
determine the amount of room available for a knob. We will also need the
screw size, or a clear photograph of the screw next to a ruler, so we can
match the knob to the correct screw. If a design looks feasible, we will
quote you a price. As always, your satisfaction is guaranteed, so if our
design doesn't work for you, just return the knobs for a full refund.
Will the factory dust cover hit the knobs when installed?
The factory dust cover will fit normally on most telescopes with Bob's
Knobs installed. Also, our knobs will usually clear the Kendrick Kwik-Focus
and various full-aperture solar filters. On a few models, the inside of the factory dust cover sits
very close to the secondary housing, and it may contact the knobs. This is an issue with Meade 8 inch (20 cm) SCTs
with 3-screw secondary housing in both
f/10 and f/6.3 (except the LX90), Meade 10 inch (25 cm) SCT in
f/6.3 only, and some versions of the Celestron 6 inch. On these models, we include with your knobs a set of special bumpers that
you can apply to the inside of the dust cover (Meade) or on the plastic
corrector plate ring (Celestron) so that the dust cover sits slightly
away from the knobs when installed. Also, the dust cover on affected Meade scopes is thin aluminum,
so you can manually press it from the
inside center to contour it for additional clearance if
needed.
Some variants of the Celestron 8 inch (20 cm) SCT with
standard-thread factory collimation screws have a plastic
"twist-on" dust cover that cannot accept either bumpers or
hand-contouring. If you install your C8 knobs without the enclosed white
spacers, they will clear the inside of this dust cover. The spacers are
used for the C8 with the Fastar option to raise the knobs slightly so that
they will not contact the inside lip of the secondary housing. Spacers
should always be used when installing C8 SCT knobs with metric threads.
If you find that your dust cover hits the knobs when
installed, please
contact us
and we will send you a free set of dust cover bumpers.
My knobs have plastic spacers. Will these cause collimation to
drift?
Some of our knobs are equipped with white nylon spacers to
allow the knob to stand higher for clearance and to adjust screw length.
Along with the other materials used in telescopes, nylon changes its
dimensions slightly with temperature. However, this will not affect
collimation adjustment because all three spacers are affected in the same
way.
The most common reason for an SCT to lose collimation is from
primary mirror shift during focusing. You can see this effect by
defocusing a star with a clockwise turn of the focus knob, collimating,
then defocusing using a counterclockwise turn of the focus knob. You will
usually see a slight mis-collimation due to primary mirror shift. This
effect can be minimized by approaching focus by turning the focus knob in
the same direction it was last turned before collimating. That is, if you
defocus a star with a clockwise turn of the focus knob for collimation,
then make your last turn of the focus knob clockwise when focusing an
object for observing. (Primary mirror shift while focusing an SCT can be
eliminated by locking the primary mirror and installing an external
Crayford-style focuser.)
Primary mirror shift can also occur while pivoting an SCT on
its mount as it is aimed at a different part of the sky, making a quick
collimation touch-up necessary. Finally, collimation almost always changes
slightly during transport and from the stresses that occur as the OTA is
being secured to its mount.
I removed a factory collimation screw and now the knob threads
won't reach. What now?
This can be an issue on certain Meade and Celestron SCT
models, especially if our installation instructions aren't followed
carefully. Before proceeding, check the knob thread length -- it should be
within one or two thread turns of the factory collimation screw thread
length. If not, please contact us and check that
you have the correct knobs.
Meade Cassegrain-style models with four or six factory secondary screws
have a set of springs between the secondary housing and mirror
mounting plate to provide the correct tension for collimation precision.
When a factory screw is removed, the springs may push the secondary
mirror assembly away from the hole in the housing such that the knob
thread may not reach.
If your knob has a spacer, remove the spacer. If the threads
are now long enough, install the knob and tighten until resistance is
felt. Do not force it. Tighten the other two factory screws in the same
way. This procedure will compress the springs and move the secondary mounting plate closer to the
housing. Remove the knob, install the spacer, and replace the knob.
Exchange the remaining two factory collimation screws with knobs, one at a
time.
If the knob won't reach even without its spacer, or if it doesn't
have a spacer, tighten the remaining two factory screws until resistance
is felt. Do not force the screws. This may draw the secondary mirror
mounting plate close enough for the knob threads to reach. Install the
knob, and exchange the remaining two factory collimation screws with
knobs, one at a time.
If the knob thread still won't reach, obtain a longer screw
with the same thread type as your factory collimation screw. We can
provide such a screw if you contact us, or you
can obtain one at your local hardware store. Thread the long screw two
turns into the secondary mirror mounting plate and pull it outward while
tightening the remaining two collimation screws until resistance is felt.
Do not force the factory screws. Remove the long screw and install a knob.
Exchange the remaining two factory collimation screws with knobs, one at a
time.
More information on the Meade six-screw secondary can be
found here.
Once the knobs are successfully installed, they are probably
too tight now. Check the next section, "How tight should the knobs be,"
for guidance.
Meade and Celestron Cassegrain-style models with three
Phillips or button-head Allen secondary
screws don't use any springs. Instead,
the secondary mirror assembly pivots at its center on a projection
protruding from the back
of the secondary housing. When the collimation screws are tightened, the
plastic secondary housing flexes slightly and provides tension. Removing a
factory collimation screw may release tension and allow the secondary mirror
assembly to move away from the secondary housing.
If a knob thread won't reach after removing a factory
collimation screw, point the OTA slightly downward so nothing can fall onto the primary
mirror. Loosen each remaining factory collimation screw a maximum of one
turn. Do not remove either screw. Both screws should now be somewhat
loose. You should be able to manipulate these screws to move the third
hole in the secondary mirror mounting plate closer to the secondary
housing so the knob can reach it. Install the knob two turns into the
secondary, and repeat the installation of the other two knobs, one at a
time.
If the knob thread still won't reach, obtain a longer screw
with the same thread type as your factory collimation screw. We can
provide such a screw if you contact us, or you
can obtain one at your local hardware store. Thread the long screw two
turns into the secondary mirror mounting plate and pull it outward while
loosening the remaining two collimation screws until the long screw moves
sufficiently close to the secondary housing that the knob thread will
reach.
If you accidentally remove a collimation screw, do not
loosen any of the others until this screw (or a knob) is (re)installed into the
secondary mirror mounting plate. Exchange the remaining factory screws, and the long screw, with knobs,
one at a time.
Once the knobs are successfully installed, they are probably
too loose now. Check the next section, "How tight should the knobs be,"
for guidance.
More information on the Celestron secondary can be found
here.
How tight should the knobs be?
After adjusting collimation, the knobs should be neither "too tight"
nor "too loose." There is often less than one knob turn between these
two extremes, especially on smaller telescopes. (However, see our note
below for the Meade six-screw secondary.)
A knob is too tight when there's a marked increase in effort to turn
it. Also, the extra tension on the secondary mirror mounting plate may cause
optical aberrations to appear temporarily. Forcing a knob even tighter may damage either the knob or the
secondary mirror assembly. A knob is too loose when the secondary mirror
assembly rattles when a knob is wiggled. A telescope will not hold its
collimation when the knobs are loose.
After installing Bob's Knobs, you can easily determine the range
between "too tight" and "too loose" by experimenting with them prior to
performing the coarse collimation. Once all three knobs have the correct
tension, collimation is achieved by tightening and loosening different
knobs while keeping all three within the correct tension range.
The Meade six-screw secondary behaves differently from the
above examples due to the coil springs surrounding each collimation screw.
If our installation instructions are carefully followed, the secondary
springs will be fully compressed and the knobs will be too tight after
installation. Each knob should be loosened one or two full turns before
performing rough collimation. This will put the adjustment near its
mid-range position. The secondary mirror assembly will not rattle if a
knob is too loose; instead, the knob will come completely out of the
secondary. To prevent this, do not loosen any knob more than three turns
from its fully tightened position. More information on the Meade six-screw
secondary can be found here.
How do I perform coarse collimation after installing Bob's Knobs?
When Bob's Knobs are first installed, the telescope will usually be out of
collimation. It's often easier to do a coarse collimation in daylight
first, and then finish with a star at night.
Cassegrain-style telescopes:
Before you install Bob’s Knobs, swing your
optical tube assembly (OTA) on its mount so you can look directly into the
front from 5 to 10 mirror diameters away. You’ll notice that the various
primary and secondary mirror reflections form a set of concentric circles
in a properly collimated scope. This is what you want to see again after
installing the knobs. (Hint: cut a small hole in the center of a piece of
white paper or cardboard and look through it to eliminate extraneous
mirror reflections.)
After installing Bob's Knobs,
tighten and loosen appropriate knobs until the mirrors and their
reflections are once again concentric, and the secondary mirror is held
securely. If a knob becomes harder to turn when tightening, loosen the
opposite knobs slightly instead. Check your owner’s manual for additional
guidance for your particular scope.
The coarse collimation procedure is also given in our instructions,
along with some illustrations. Instructions
are available as pdf files in English,
French,
German,
Italian,
Japanese, and
Spanish.
Newtonian-style telescopes:
These telescopes can usually be collimated accurately using
an optical aid or laser collimator. Follow the collimation instructions in
your owner's manual.
How do I collimate using a star?
Star collimation using Bob’s Knobs is very
simple. First, let your telescope cool to ambient temperature
before attempting collimation. Find a moderately bright star and defocus the image so
three or four
diffraction rings are prominent as seen through the eyepiece. These rings
will be concentric in a collimated scope or
skewed if collimation is off.
(If your telescope is warmer than the ambient air, air currents will cause the rings
to
have a pinwheel appearance with rough edges. Wait a while longer before
adjusting collimation.)
To achieve collimation, reach around and
carefully adjust the knobs while looking through the eyepiece until the
diffraction rings are concentric. With experience you can use the shadow
of your hand on the star pattern to determine which knobs to adjust.
Remember to loosen and tighten different knobs as needed so the secondary
mirror assembly remains secure without over-tightening, which is indicated
by a marked increase in the effort needed to turn a knob. Use your scope’s
slow motion controls to re-center the star in the eyepiece during the
collimation process. By using higher magnification you can increase
collimation precision. Check your telescope owner’s manual for more
information on collimation specifics.
The star collimation procedure is also given
in our instructions, along with some illustrations. Instructions are
available as pdf files in English,
French,
German,
Italian,
Japanese, and
Spanish.
How can I use the shadow of my hand to aid collimation?
On most Cassegrain-style telescopes smaller than 12 inches (30 cm) in aperture, you can reach
around and adjust Bob's Knobs while looking into the eyepiece for immediate
feedback. As your hand moves into the light path, you'll notice a
shadow on the star's diffraction ring pattern. Slightly tighten the knob
nearest the place where your hand's shadow crosses the narrowest ring
spacing. If the knobs are tight enough already, then slightly loosen the knob
nearest the place where your hand's shadow crosses the widest ring
spacing. Re-center the star in the eyepiece and repeat the process until collimation is correct. With practice, you can have your scope collimated in less than a
minute.
Should I collimate with or without a star diagonal?
The general rule is to collimate your telescope with the same
configuration that you intend to use. If you plan to do visual observing,
collimate with the star diagonal in place. For most camera work, collimate
without the star diagonal. Bob's Knobs make collimation so easy that you
can re-collimate, if necessary, any time your configuration changes.
What about using a laser collimator?
Laser collimators generally work well on Newtonian-style
telescopes with a flat secondary mirror. On Cassegrain-style telescopes,
which have a curved secondary mirror, a laser collimator will
work properly only if the secondary mirror is perfectly centered above the
primary mirror. Unfortunately, telescope manufacturing tolerances are such that
slight secondary mirror offset is inevitable. Under these conditions, the
laser collimator may give a false indication of collimation. Using a star
is much more accurate. Note, however, that a laser collimator can be used
to speed the process of rough collimation after Bob's Knobs are first
installed.
