In outline, the PT collimation procedure is:
- Make the primary mirror flat in its cell by adjusting its tilt. We assume the primary's optical axis is now parallel to the telescope's mechanical axis.
- Translate the secondary mirror so its vertex is on the telescope's mechanical axis. This is done by moving the secondary until its vertex mark is centered in a collimating telescope mounted near the Cassegrain focus. The collimating telescope is assumed to be mounted so its optical axis coincides with the mechanical axis of the telescope.
- Tilt the secondary mirror so its optical axis is parallel to the telescope mechanical axis. This is done with the collimating telescope in autocollimation mode. A bright target sent out of the collimating telescope is reflected by the secondary mirror. A beamsplitter on the collimating telescope allows viewing of both the original and reflected images. By tilting the secondary mirror, the two images can be made to coincide indicating that the secondary mirror axis coincides with that of the collimating telescope.
- Center the primary mirror in its cell. This is done by watching for symmetry in the out-of-focus image of a bright star in the eyepiece while translating the mirror.
- Center the second corrector. The second corrector is built into the CCD dewar, which is clamped to the filter box. The dewar should be centered on the optical axis of the telescope.
- Center and de-tilt the CCD. You're unlikely to do anything about this but for the record the CCD is well centered on the optical axis (the axis of the second corrector) but is tilted a bit with a roughly N-S gradient. Since the CCD mounting hardware inside the dewar was made to high precision, the tilt arises either in the CCD carrier (the CCD might not have been glued in exactly flat) or is on the filter box (the bottom plate of the filter box isn't perpendicular to the optical axis of the telescope).
Step-by-step (not tested yet):
| Primary Mirror should be Level | During installation the primary mirror is made level in its cell. Confirm that this was done by checking the log and asking the person who most recently reassembled the telescope. Three tip-tilt screws, one under each of the mirror support whiffles, are used to make the mirror's edge a fixed distance from the top of the mirror cell. These screws are never to be adjusted "on the sky" (photo). |
| Remove Instruments | If the instruments are not already off the telescope, follow the instructions for removing the primary mirror up to and including the step "Remove corrector lens." This is a normal state during reassembly after aluminization. NB: At this point, the telescope should be pinned in both axes and the Dec axle tension should be OFF. The HALT MOTORS button on the control panel should be IN. |
| Add trim weights |
We want the telescope to be close to
balanced.
You do not need to be in perfect balance
but the telescope should not be too heavy
to move safely.
I can imagine in the usual scheme of things the sliding black counterweights will be out of reach at this point. If they're not, bring them down and you might not have to fool with the trim weights. Attach trim weights to the primary cell until the telescope feels balanced or until your run out of room for weights. Trim weights the arc shaped lead weights that fit on the outer bolt circle of the primary cell. You'll find them in the bottom drawer of the toolbox. Be sure to use the screws in the bottle labeled "Trim Weights." The telescope may still be top heavy after adding all the trim weights. It will certainly want to "fall" north. |
| Install Collimating Telescope | Locate the collimating telescope (it's the same one used on the 2.5 m) and its PT mounting fixture. Mount the telescope on the 20-inch telescope. NB: The screw holes in the back of the primary cell are not tapped through to prevent accidentally contacting the primary mirror. You need screws of exactly the correct length to mount the collimating telescope. |
| Pin Telescope at Service Position | You may want to roll the tool boxes out of the way for this. Position the ladder north of the telescope. With someone holding the telescope tube, pull the pin on the DEC axle and move the top of the telescope north, towards the horizon. Pin the DEC axle near the horizon. I'm not certain if there's a position exactly at the horizon; if not, it's about six degrees below the horizon. |
| Adjust Sliding Counterweights | Move both of the black sliding counterweights to the lowest position, near the telescope center section or to a "balance when horizontal" situation. |
| Remove Light Shield |
You probably want to check the secondary
mirror centering before removing the light
shield, since if it's OK, you don't need
to remove the shield.
Remove the light shield around the secondary mirror by unscrewing the four screws around its top edge. It's not possible to remove it from the telescope tube so tie it up around one of the secondary supports. Be careful not to touch the secondary mirror. |
| Center Secondary |
With direct viewing through the collimating
telescope focus on the center mark on the
mirror (you might need to open the dome
to get enough light to see the mark).
The mark is on a piece of Kapton tape
stuck to the mirror's center.
Use the four nylon screws on the side of the secondary mirror cell to center it in the collimating telescope. This is much easier with two people. Be careful not to tighten the screws too tight; they should only be snug enough to keep the mirror from slipping around. |
| Replace Light Shield | When you're satisfied that the centering is OK, attach the light shield. |
| Adjust Secondary Tilt | Turn on the reticle light and focus so you can see the outgoing and return targets clearly. Adjust the screws on the plate holding the secondary cell to center the return image in the collimating telescope, right over the outgoing image. These screws pull the mirror cell up against a spherical washer so you need to loosen some before tightening others. Don't tighten them too much or you'll warp the secondary cell and mirror. Use only enough pressure to keep the mirror in place without straining the cell. |
| Double-check Secondary Centering | Turn off the reticle and refocus to see the centering target directly. It should not have moved but if it did, remove the light shield, re-center, and repeat the tilt adjustment. |
| Return to Zenith | Unpin the telescope, push it to zenith, and replace the pin. |
| Remove Collimating Telescope | Unscrew the collimating telescope from the primary cell. Store the mounting unit in the bottom drawer of the tool box and return the telescope to wherever it's normally stored. |
| Remove Trim Weights | Remove the trim weights from the back of the primary cell, making sure the original weights are in their proper locations. Return the trim weights to the bottom drawer of the toolbox. |
| Install Corrector Lens | After checking that the primary baffle is in place (install it if it isn't), install the top (first) corrector lens. |
| Install Filter Box | Use the pneumatic lift to install the filter box. |
| Install Eyepiece | Clamp the eyepiece holder into the back of the filter box after removing the cover plate ("moth blocker"). |
| Balance the Telescope | See Wendell Jordan. Generally speaking, if you've got the trim weights in the right place, balancing requires only that the two black counterweights be positioned correctly. DEC Balance should be checked at zenith and at far south positions. Wendell Jordan is currently the only on-site person trained to balance the PT. |
| Tension DEC drive | See Wendell Jordan. This procedure should be done only by trained personnel to avoid damaging the drive disks. Using the spring scale, tighten the DEC drive tension screw until breakaway occurs at about 10 lbs. Maybe a bit more. |
| Check Balance | Use the drive motor current meters as a sanity check on the balance. Check balance near the zenith and at high zenith distance. |
| Initialize Telescope | Check the telescope coordinate setup and reinitialize at the zenith if necessary. |
| Bright Star Exam |
Point to a really bright star and examine
the image with an eyepiece.
You'll need to open the shutter (toggle switch
on the shutter control box) and select the
g-band filter (from MOP).
You'll need to take the telescope relativly
far from its usual CCD focus value to find
the eyepiece focus position.
Using a high power (15 mm or shorter) eyepiece,
study the image.
Just out of focus you'll notice some astigmatism.
Farther from focus, you'll see a donut of light.
Notice the difference inside and outside of focus.
Unless the telescope was incorrectly disassembled and reassembled, the donut of light should be fairly symmetric. If not, you'll need to go to the next step and adjust the primary mirror translation. |
| Adjust Primary Translation |
Use the four centering screws around the base
of the primary mirror cell to make the donut
of light perfectly symmetric.
You'll need to loosen one of the screws before
tightening its opposite.
When you're finished, make sure the screws are
snug but not tight.
You don't want to pinch the mirror, but you also
don't want it sliding around.
Generally speaking, the primary will be well centered if the telescope was disassembled and reassembled according to instructions. You will often not be able to improve on this initial centering but most people cannot resist. |
| Remove Eyepiece | Remove the eyepiece holder and install the filterbox cover (moth blocker). |
| Install CCD | See Jon Brinkmann.
Install the CCD dewar on the filter box.
Before doing this, you should look closely
at the clamp system after removing the cover.
Notice the tiny pins poking out of the clamps
pointing to the optical axis.
These pins center the CCD dewar on the optical
axis and are adjusted by setscrews (well, they
are the setscrews).
Besides clamping the dewar, you'll also need to connect the cables, dress the CryoTiger hoses, and power up the system. |
| Adjust CCD Centering |
Take a few short r-band images near the zenith
on a night of good seeing.
The stars should look similar across the field,
right into the corners of the CCD.
Any asymmetries should be radially symmetric,
that is, things should point to the center,
and more or less similar in size at similar radii.
Image elongation in a large area of the field that cannot be attributed to tracking errors may indicate that the dewar is not centered on the optical axis (the dewar carries the second corrector lens, which needs to be centered on the optical axis to perform well). Consider sliding the CCD dewar along that direction to remove the elongation. This is done by unclamping the dewar (make sure the latches are in the "safety" position!) and adjusting the centering pins. It's worthwhile doing the trigonometry ahead of time (or take your calculator to the dome) to turn the three screws the right amount to get the direction correct. You want to move the CCD in the direction that makes things symmetric. Typical motions to start with are around 0.5 mm translation. You can figure out the direction but it's just as fast to try one and see what happens. This test should be done late in the night, well after the telescope has thermally relaxed since the far field images change character quickly and oddly until the telescope has had a chance to stabilize. |
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