1. Blanco Radial Supports Repair Procedures design review
Introduction, history & overview
Tim Abbott
Telescope scientist/manager

2. The Panel Derrick Salmon – Director of Engineering, CFHT
Tony Abraham – NOAO, Tucson
Doug Niell – NOAO, Tucson
Gabriel Perez – Gemini, La Serena

3. Schedule for the day Time Event Presenter 10:30am
Introduction, Description & History of the problem
Tim Abbott, Telescope scientist/manager
11:30am
Measurements and Optical Consequences
Roberto Tighe, Optical Engineer
12:30pm
Lunch
14:00
Setting the scene for the repair
Andres Montané, Mechanical Engineer
15:00
Coffee break
15:30
Details of the repair
Andres Montané
16:30
Break & executive session
17:30
Preliminary report of the panel
18:00
Close

4. Charge to the Panel
The panel is requested to address the following questions:
Is our understanding of the problem complete?
Is our proposed solution likely to succeed?
Is there anything we can add to our plans which would make us more likely to succeed?
Can the panel identify paths to failure?
Please structure the panel’s report with the sections “Findings”, “Conclusions”, and “Recommendations”.

5. Blanco radial support system
24 gravity driven actuators distribute the load around the circumference of the primary mirror.

6. Primary mirror cell P: active optics pad (3 marked of 33)
L: Load cell (3)
S: Seismic clip (4)
R: Radial support (3 marked of 24)
M: Mount point (4). M S S L P M P R R

7. A radial support M: Primary mirrorC B D E P G H A F IP
M: Primary mirror
H: “H-bar” attachment to primary (old style, also called “pad base”
IP: Invar “pad”
A: Counterweight Lever arm
C: Counterweight
D: “Pivot frame”
E: “Anchor pivot” - fits into socket (not shown) mounted on inside of telescope barrel
B: “Support Bracket” Point of attachment to mirror cell
F: Force vector radial support applies to primary mirror
G: Gravity vector
P: Pivot points.

8. Current status of primary radial supportsW E S 16 10 14 3 4 20 22 1 2 5 6 7 8 9 11 12 13 15 17 18 19 21 23 24
= broken
= new H-bar

9. Some history
Historically, the radial supports have failed by detaching from the primary mirror at the glue joint at the rate of ~1-2 per 1-2 years.
They fail preferentially in the SE quadrant (opposite the NW station)
They fail preferentially when under high, but not necessarily maximum, traction load.
When they fail they sometimes take glass plugs with them
2002: 4 repaired (#’s 12, 15, 16, 20), one failed immediately (#15)
Raised primary mirror 2.3mm
Installed passive break sensor system
2004: 2 repaired (#’s 15, 18), one failed immediately (#18) N W E S 16 10 14 3 4 20 22 1 2 5 6 7 8 9 11 12 13 15 17 18 19 21 23 24

10. Some history – the 2005 shutdown
3 supports repaired (#’s 5, 16, 18), 8 supports failed immediately, shutdown extended, one failed immediately (#15)
New techniques:
Weighed primary mirror - 34,700lb
This is 1,700lb more than the records state, but precision unclear
Installed 3 “new H-bars:”
Used glue injection & spacers for repair
Filled glass voids with glue/glass spherule mix
Installed micrometers (“Mitutoyos”) between primary & cell to measure radial displacement
Augmented counterweights
Second half:
Realized alignment pins necessary
Realigned as much of radial support system as possible
Radial supports first observed to bend (but significance not fully realized at the time).
Removed plenum
Mirror centered particularly carefully (recentered by ~1.5mm)
4th new H-bar installed.
Right now, we have 3 broken supports (#’s 14, 15, 21) 14 & 21 failed in 2007 N W E S 16 10 14 3 4 20 22 1 2 5 6 7 8 9 11 12 13 15 17 18 19 21 23 24

11. 4 unexpected discoveries
The primary mirror moves on its cell
Radial support attachment to the primary mirror is marginal
Radial supports bend
Misalignments have accumulated with each radial support repair.

12. 1. The primary mirror moves on its cell
Original telescope design had four radially constraining load cells between the central chimney and the primary mirror. Early tests showed that the chimney flexed with respect to the primary mirror sufficiently to break these load cells and they were removed. This left the primary mirror unrestrained against radial motions on its cell except for friction against the axial load cells and its balance with the radial support system.
Once a radial support fails, the balance in the system is lost and the mirror is free to move on the cell. In practice, the motions are limited by the lateral slack available in the radial support mechanisms. This amounts to about 1mm. [REWORD]
If that 1mm of free space is used up on any given radial support, it is in danger of bearing a significant portion of the weight of the mirror in a direction in which it is not designed to do so, and it may bind up. Both of these conditions may in turn induce strain beyond the designed limits on its epoxy joint with the primary mirror and cause it to fail.
Movement of the primary mirror on its cell and with respect to the primary mirror. The telescope was pointed at the south pole, then rotated 6 hours east followed by six hours west and then back to the meridian. Red and blue lines indicate movements of the primary mirror on its cell. Yellow and green lines indicate movements of the primary mirror with respect to the prime focus corrector. Measurements made Oct 2007.

13. 2. Radial support / primary mirror joints are marginal
Each radial support is attached to the primary mirror via four invar pads which are epoxied to the glass. An H-shaped invar bar (the “H-bar”) is bolted onto these four pads and the radial support applies traction or pressure to this bar at its center. Finite element analysis shows that the H-bar must bend; this redistributes the forces under each pad such that when it is at maximum tension, the peeling forces experienced by the epoxy are very close to its limit.

14. 3. Radial supports bend
When the telescope moves away from the zenith, each radial support bends elastically with gravity causing the counterweight on the end of the lever arm to displace with respect to the telescope superstructure.
If, as a result of that bending, the counterweights come into contact with the cell the radial support system will lose balance and the primary mirror will tend to move on the cell.
In around 1995 a plenum was installed around the radial support system to help direct cooling air. At high zenith angles, the radial supports will have come into contact with this plenum. The plenum was removed during the 2005 shutdown. During that shutdown, at least one radial support was observed to come into contact with the cell at high zenith angle.
Once a radial support has failed, the unbalanced mirror will move on the cell in the direction of the gravity vector, lifting the remaining radial supports as it does so, masking the effects of the bending in those supports.

15. Radial supports bend

16. Radial support FEA showing flexure

17. Radial support flexure
Observed displacement of radial support counterweights corrected for mirror movements.

18. 4. Misalignments have accumulated with each radial support repair.
Failed radial supports have been repaired over the years during re-aluminization shutdowns. Unfortunately, due to misunderstanding and miscommunication, inappropriate techniques have been used to align the repaired supports. As a result, misalignments have accumulated and the radial support system no longer has the intended lateral freedom of motion that was originally built in. Thus any motion of the mirror on its cell is more likely to drive the system into an over-stressed or bound condition. Indeed, in some directions there may be little to no freedom of motion and the radial supports already in a critical condition.

19. The plan for repair
Modify the system to prevent (or at least reduce) interference between the radial supports and the telescope structure.
Re-align the system to maximize the permissible motion without driving the system into an untenable state.
Strengthen the join between the radial supports and the primary mirror .
Improve the balance between mirror and radial supports.

20. What is “alignment”? - Assumptions
The following assumptions are generally made to simplify the radial support alignment process. We may want to consider the consequences if any of them are not true. This is not a complete list.
The geometric axis of the primary mirror is coincident with the optical axis of the primary mirror.
The density of the primary mirror is uniform so that the actual center of gravity is coincident with the geometric center of gravity.
The primary mirror is precisely circular.
The azimuthal plane1 of the primary mirror is parallel to the lower surface of the primary mirror.
Note 1) The “azimuthal plane” of the primary mirror is the plane which passes through the center of gravity of the primary mirror and is orthogonal to its optical axis.

21. Definition of “alignment”.
Proper radial support system alignment is defined by the following conditions when the system is fully assembled and relaxed, i.e. with the telescope pointed at zenith. Each requirement has associated tolerances, which are not stated here (mostly because they are not yet defined). I believe this definition to be complete, please correct me if I am mistaken.
The azimuthal plane of the primary mirror is horizontal, and parallel to the upper surface of the primary mirror cell.
The optical axis of the primary mirror is coincident with the axis of symmetry of its cell (i.e. the primary mirror is centered on its cell).
The radial support lever arms are parallel to the optical axis of the primary mirror and equidistant from its edge.
The radial support lever arms1 are perpendicular to the azimuthal plane of the primary mirror (i.e. they are vertical).
The radial support eyebolts2 all lie on the azimuthal plane of the primary mirror and point at its center of gravity.
The H-bars are at the center of their azimuthal travel.
The plugs on the tops of the horseshoe castings are centered in their ring girder sockets.
Notes:
1. “Radial support lever arms” here means the line which connects the fulcrum of the lever and the point at which it attaches to the eyebolt which is connected to the center of the H-bar.
2. The “eyebolts” are the bolts which transfer the radial lever arm forces to the H-bars.

22. Notes on alignment
This definition implies that the azimuthal positions of the Invar pads on the circumferential surface of the primary mirror are fundamentally determined by the positions of the radial support sockets in the telescope ring girder via the individual dimensions of the radial support mechanisms.
It may be sufficient to do a divide-by-24 making them equidistant around the circumference of the primary mirror. Question to panel: is it?
The radial positions of the radial support sockets in the telescope ring girder are determined by
deviations from circularity of the outer surface of the primary mirror,
the primary mirror’s location on its cell,
deviations from circularity of the outer edge of the cell,
the individual dimensions of the radial supports which determine the separation from the primary mirror of the radial support rotation axis while maintaining the radial support lever arm perpendicular to the azimuthal plane of the primary mirror.
deviations from circularity of the ring girder.