Stephen,
The best thing to do at this point is schedule a two-hour window some afternoon
or evening when you can collimate your Newtonian from scratch.
When was the last time you cleaned the optics? If it's been more than a year,
include this in your agenda. Think of it as giving your scope the spa
treatment. When you're all done, you'll have an instrument ready to perform at
its best.
And keep in mind, collimation isn't a mystical art. It's a process.. At some
point along the path to aligning your optics, a step was not successfully
completed. If you take your time, start at the beginning and evaluate each step
as it's completed, you will find where the problem lies. Whatever that problem
is, it can be corrected and you will have a collimated instrument.
In addition to your collimation tools, you'll need a bubble level, preferrably
one that can check both a horizontal and a vertical surface for level. Also,
check the center mark on your primary mirror.
A simple way to check the position of the center marker is to use a thin piece
of packing tissue paper. Outline a square on the tissue, each side as long as
the diameter of your primary mirror. Double check this outline. If the sides
are all the same length, you've drawn a square. Cut the square out out and fold
neatly into quarters. The center of the square is where the folds intersect.
With the tissue folded, cut off that corner to make a hole just big enough to
accomodate one of those binder paper hole reinforcers.
Then, unfold the paper and lay it carefully across the face of the primary
mirror. Use tape of fix the paper along the side of the mirror--not the face,
obviously--in at least two places. Does your marker coincide with the hole in
the center of the tissue? If not, remove the marker and replace with a new one,
using the hole as a guiding device.
Once this is accomplished, you're ready to go.
Step 1: Rough Collimation-Squaring the Focuser
Begin by checking the focuser. Lay the OTA on pillows or some other support
material that can be adjusted until the OTA is level along the horizontal line.
Check the focuser by using the bubble level to check its vertcal (along the
side of the focus tube) and horizontal (across the focus tube opening)
surfaces. If both indicate level, then the focus tube should be squared to the
light path.
Step 2: Rough Collimation-Positioning the Secondary
Next, check the orientation of the secondary mirror. Use your sight tube for
this. Take your time and carefully position the secondary so, when viewing
through the focus tube, it appears *both* perfectly circular in shape and to
have equal space, all around, between the inside edge of the focus tube and the
outside edge of the secondary. Ignore the appearance of the primary mirror.
The keys to this step are making sure that the secondary has a circular outline
and that the border space between it and the inside edge of the focus tube is
equal.
Step 3: Rough Collimation-Aligning the Secondary
Using the adjustment screws on the back of the secondary mirror mount, adjust
the tilt of the secondary until the outline of the primary mirror is centered
within the secondary. During this steop, ignore the position of the center
marker on your primary mirror.
After completing this step, check the position and shape of the secondary
mirror. If it looks elliptical or if the border space around the outside of the
secondary is uneven, then you're gonig to go back to step 2. What you want is
for the secondary to still look circular and to still have equal amounts of
border space after Step 3 is completed.
If you need to go back to step 2, move the secondary either one full turn
toward or one full turn away from the primary. Then, lock it in place and
repeat Step 3. You shouldn't have to repeat Steps 2 and 3 more than a few times
until the secondary will still have that circular shape after the completion of
Step 3.
Step 4: Rough Collimation-Aligning the Primary
Using the adjustment screws on the back of the pimary, adjust the tilt of the
primary until the center marker corresponds with the intersecting vanes of your
secondary spider. When this step is complete, you're ready to laser collimate.
Step 5: Laser Collimation
Perform the barlowed-laser collimation procedure. This procedure elliminates
the potential problems a miscollimated laser can introduce.
You need to fit a mask at the bottom of the Barlow. I used an old broadband
light pollution filter for this. I cut a circular piece of index card to the
diameter of the filter opening. Then, I used a handheld hole punch to make a
hole for the laser to pass through. Finally, I threaded the filter onto the
Barlow.
In this procedure, you use the primary mirror adjustment screws to adjust the
tilt of the primary such that the dark donut of the center marker outlines the
hole through which the laser exits the Barlow. When you achieve this result,
laser collimation is complete and you're ready to use the star test to make any
final collimation adjustments.
Step 6: The Star Test
Center a star at high power (1-mm exit pupil or smaller) and slightly defocus
both "in" (toward the optical tube) and "out" (away from the optical tube). If
the diffraction pattern shows concentric rings of light and dark centered
around a central point of light, then your collimation is finished. If not,
adjust *only* the primary mirror tilt until the diffraction patterns on either
side of focus are identical.
Step 7: Observe
Use Jupiter or Saturn to check the final collimation. They should snap to focus
and show lots of detail in your telescope. If they do, take as much time as you
want enjoying the best planetary views you've ever had with the scope. Then,
move on to another showpiece. Have fun.
Regards,
Bill Ferris
"Cosmic Voyage: The Online Resource for Amateur Astronomers"
URL:
http://www.cosmic-voyage.net
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