Telescope Optical Systems

Mirror Mounting / Removal

Removing any mirror requires at least two people and in some cases three. For help please consult Mark Klaene at Apache Point Observatory.

Tertiary mirror

The tertiary is mounted on a rotating table to allow access to both Nasmyth ports. The rotation mechanism is mounted from behind the telescope. Extreme care must be taken to remove the bolts that mount the tertiary to the table, and not to remove the bolts that mount the rotating mechanism to the telescope, or else the latter will fall out of the back end of the telescope!! The rotating mechanism mounts to a plate that is mounted to the telescope stovepipe from the primary mirror cell. See later section on the tertiary.

When we removed the tertiary May 2013, we removed the bolts on the inner circle, and lifted the tertiary mount off of the table; it has a pin, and needs to be lifted straight up. We then left the entire rotary table mechanism in the cell. We did this at a vertical orientation, but this did then require the we lift the tertiary out over the primary; doing it at an angle might be easier. Note, of course, we took the baffle off first.

OBSOLETE: removing the OLD tertiary:

Old reinstallation:

  1. Attach tertiary tripod to tertiary

  2. Attach tertiary support to tertiary tripod

  3. For old system, feed threaded rods through tripod and adjust


OLD Tertiary adjustment (OBSOLETE):

To adjust the tertiary, remove the cylindrical baffle around the mirror. The tertiary is adjusted using three nuts on threaded rod behind the mirror itself. The three threaded rods are mounted symmetrically around a ball-pivot. Adjusting the nuts causes the mirror to pivot around it's geometric center. It is possible to piston the mirror by making sure the central bolt is loose, then adjusting the nuts on all three of the threaded rods, iteratively; it make take a little while to move in piston, and the mirror will need to be readjusted in tilt, but it is possible to do in a moderate amount of time.

Primary mirror

We removed the primary in February 2000 for realuminization. We removed the mirrors for repolishing in 2002. We removed the primary again in May 2013 for realuminzation/overcoating; this was done at ATOC in Albuquerque where they put down aluminum plus 0.5wave (680nm) SiO2.

See photo documentation at There is some older photo-documentation of this process in the Photo Album in the control room. On the day of removal, make sure that arrangements have been made for a suitable crane to hoist the primary mirror cell. Make sure there are sufficient personnel to operate the crane, and handle the mirror this takes a minimum of three people. You will need straps to attach the mirror cell to the crane. Mark Klaene at Apache Point Observatory is experienced with handling this mirror and should be consulted regarding questions. Also, Mark has the straps for handling the mirror cell. Aluminization is a process that takes between 3-5 days. The typical schedule should be as follows: On day 1, remove the mirror cell and transport to Sunspot. Strip the old coating that day if possible and prepare for aluminizing. Day 2, aluminize. Day 3, check the coating and return to the telescope. Extra days will be required if the first coating turns out poor.

Notes on reinstalling optics from June 2002/October 2003:

Secondary mirror mounting and actuators

The top end of the telescope is mounted to the telescope using an eight strut system, with turnbuckles on each of the eight struts.

The secondary mirror is mounted to a tripod structure. A shaft attaches to the tripod and goes through a linear bearing on the telescope top end. The three legs of the tripod are attached to three secondary actuators using flex couplers; the actuators allow for focus and tilt adjustment of the secondary mirror.

In Feb 2009, something happened which led the flex couplers to all fail, causing the mirror to tilt and become unfunctional. At that time, Tom Harrison, Charlie Park (Engineering), and Yuping (MTEC) investigated the situation and found several issues: a destroyed linear bearing, some rusting on the shaft, some cracks in the turnbuckles, and a poor design of the struts used to attach the secondary cage to the top end. At that time, they remade a new set of struts (putting in a 90 degree rotation between the holes at each end), got new turnbuckles, got a new shaft, and got a new linear bearing. They also improved the safety system, putting a better stop at the end of the main shaft, and installing some straps on the tripod that will catch the mirror if the couplers fail.

They found that the ideal amount to insert the actuators into the couplers was not to insert them as much as they had been before (this allows more tilt). As a result of the new mounting, however, the previous focal position placed the acuator shafts very close to the limit switches. As a result, the limits switches needed to be moved back a bit.

The bearing that holds the secondary shaft is fragile and may be destroyed if the shaft is removed without putting something in its place. Charlie designed a tool to allow the shaft to be safely removed. This should always be used if the secondary is removed.

Periodically, the secondary actuators should be checked to make sure they are not binding. The primary cause of binding is if one actuator locks up for some reason and stops moving, causing the shaft to exceed its tilt limit. The shafts should be periodically cleaned of dirt and dust that builds up in the grooves and seems to cause the binding. The actuators should be mildly lubricated.

The actuators are connected to the secondary with three flex couplers. Before decoupling a actuator or coupler from the secondary, the secondary should be tied to prevent it from ``falling''; this is especially essential for the top actuator. One should take care when decoupling the actuators from the secondary: if all three couplers are removed, the secondary will lose all support except for the central attachment, and so care must be taken to let it down gently. If all three are removed, it becomes much more difficult to attach them again, becuase the secondary must be held up while the couplers are connected. It is recommended to just remove one at a time if possible. Care must be taken to make sure the couplers are securely attached to the secondary!

There should be spare secondary actuators; we do occasionally have failures. Additional replacements may be ordered from Eastern Air Devices. The part number is: LA23GCKJ-A213.

The secondary actuators are controlled by the PC38 in the TOCC computer, as the T, U, and V axes of that controller. When looking at the primary mirror, the U actuator is the one on top, the T actuator is at the lower left, and the V actuator is at the lower right. The T actuator is hooked up to cable P113B, the U to P114B, and the V to P115B. There is a separate cable that connects to the limits switches that must also be connected.

Measuring motion of the actuators gives .625 inches per 10000 steps, or .001587 mm/step. The actuators are located at a radius of 6.385 inches (162.19 mm) from the central shaft .

Secondary mirror reinstallation


Aluminizing the mirror must be done by a qualified person. Currently, Mark Klaene and Jon Davis are both qualified to help with this procedure. There is a custom-made band to hold the mirror in the Sunspot aluminizing chamber. This band is located in the 1m crate, which is usually kept in the base of the 3.5-meter enclosure.

Aluminization at Sunspot Feb 2000.

Aluminization after repolishing, June 2002?

Secondary and tertiary aluminized and overcoated by ATOC (fomerly Optical Surface Technology) in Albuqueque, 2007? Primary?

All mirrors removed May 2013. All coated and overcoated at ATOC: secondary and tertiary with Al + SiO2 (half wave at 500nm), primary Al + SiO2 (half wave at 680nm); the different overcoat thicknesses were to try to provide overall near-constant throughput as a f(wavelength) and to maintain decent UV throughput.


Secondary Adjusment

Put spot on secondary using print overlay of mirror, kept in secondary mirror crate. Collimating telescope in tertiary mount. Center and tilt secondary.

Tertiary Adjusment

Collimating telescope in Nasmyth mount. Adjust tertiary.

There is fixture for mounting the APO collimating telescope at the Nasmyth mount on the back of the filter wheel. Using this, one wants to adjust the tertiary such that the collimating telescope can be focussed on the center of the secondary, and also that it can be focussed back on itself. Both things are changed by either tilt or piston of the tertiary, so it is an iterative process to get both things to focus; at a given piston, adjust the tilt such that the retroreflection is aligned, then see if the center of the secondary is centered; if not, repiston the tertiary and try again. Hopefully, you will see if the change in piston made the agreement better or worse, and you can converge on a proper alignment of the tertiary.

Another method we have used for the tertiary is to map out the image quality in the focal plane (using the guider on its stage), and determine the location of the best images, and use this as the optimal center of the field. To get this location onto the science camera, you tilt the tertiary. The method for doing this has been to use a laser pointer mounted at the top of the telescope. Adjust the laser so that it is pointed at the guider pickoff at the location of the best images. Then adjust the tertiary until the laser spot is pointed at the science camera.

Three components are needed to set up the laser for adjustment. The first is a flexible flashlight-holder. The holder has a clip on one end that may be attached to the secondary struts. The second component is the laser pointer itself. Finally, you need two small clamps. One small clamp may be used both to hold the laser pointer in the holder and keep the laser pointer powered on. The second clamp may be used to help hold the flashlight holder to the secondary mounts. CAUTION: When the laser pointer is powered on, take great care not to look into the beam.

Primary Adjusment

On sky, adjust primary to minimize coma.

Tertiary Adjusment

On sky, minimize astigmatism with tertiary rotation?

Puntino Shack Hartman - final adjustment

In 2011, we purchased a Puntino Shack-Hartmann sensor with associated software from Spot Optics. To reduce the cost, we provided an existing SBIG camera (ST8) for use with the system. The SBIG ST8 is controlled by a special version of the software that Spot Optics provided us with.

The Puntino needs a reference frame to compare the star frames with. This is obtained by screwing in the small LED that came with Puntino (the instrument must be off the telescope to do this, and the aluminum mounting plate taken off - mark the orientation!). One then takes a reference frame with the software.

The Puntino mounts at NA1 to an aluminum plate that is clamped to the filter wheel. To try to match the back focal distance of the CCD camera (and as a result allow the guider to be in focus for acquisition and guiding), some extra spacers (not currently known how much) should be inserted. If mounted in a repeatable orientation (TOP towards CCD controller) then one can use repeatable commanding for offsetting the telescope and tilting the secondary to produce repeatable results on the SH.

Before using, the filter wheel should be moved to an OPEN positions!

See Appendix E for details of different Puntino runs.

Tertiary and tertiary control

In July 2008, we obtained a new tertiary mount from ACE. The new tertiary is on a rotating table to allow use of both Nasmyth ports. In addition, the new tertiary mount was designed to allow easier adjustment of the mirror alignment, although this is still manual, not motorized adjustment.

The rotation mechanism must be inserted from the back end of the telescope. It mounts to a plate that is bolted on from the front side of the telescope to the flange on the stovepipe that extends up through the primary. Installing or removing the rotation mechanism requires two people, because the screws thread in from the front, but the mechanism is inserted from the back. The tertiary itself is bolted to the rotation table from the front. Extra care must be taken when removing the tertiary to remove the correct bolts: if the bolts to the rotation mechanism were removed, the mechanism would fall out of the back end of the telescope! (I think it must not be possible, or at least very undesireable, to pull the entire thing out from the top.)

The tertiary rotation is controlled by a stepper motor which drives a worm gear that is coupled to the rotation table. To obtain stable positioning of the tertiary, there is also a brake that clamps the table in place. The brake is controlled by a 24V DC motor. The brake MUST be released before the table is rotated, and should be activated while motor power is still being applied after a move. There are limit switches that are triggered when the brake is in the open or closed position.

There are two home switches near the position of the two ports. We have obtained reasonably repeatable positioning by moving to a home switch, then moving past the home switch in the same direction. Then reverse direction until home is hit, and reset the position counter. Then the tertiary can be moved, ideally in a single commanded move, to the desired position, as determined by the optical alignment procedure.

Cable connection from tertiary: (but note, we have two cables coming out! maybe the motor power one was originally in the big connector, and we had to break it out for some reason???)

Cable connections inside of ACE box

Cable connection from ACE box to IO38 box


Alt-az telescope can present challenges for proper baffling. The 1m has two baffles located in the Nasmyth tube, a baffle around the tertiary which has components both around the tertiary and up towards the secondary, and a baffle around the secondary, which has both a vertical and horizontal component.

As of July 2004, the Nasmyth baffles are located 2 inches and 7 3/4 inches from the rotator mounting surface (in the direction of the tertiary). These have a 4.5 inch diameter central hole.

Mirror Cleaning

The original notes are in The Blue Notebook under the "Optics" section. For questions, contact Mark Klaene at Apache Point Observatory. To obtain CO2 cylinders, talk to Mark or Norman Blythe. CO2 cylinders are obtained from Valley Welders in Alamogordo. One cylinder typically lasts for about five cleanings. A cleaning session for the primary, secondary and tertiary typically takes about 30 minutes.

To clean the mirror with CO2:

To wash the mirrors (old procedure?):