Figure 1: A difference of two data cubes taken of the AO fiber point source in the Kn3 filter and 100 mas scale. Between the two datasets, the SPEC scale was changed from 100 to 20 and back to 100 mas.
OSIRIS was serviced in February 2009. We think that we have repaired a thermal short, but we were unable to determine a cause of the image motion with changing plate scale. It's possible that the thermal cycle will change the performance of the SPEC image motion. We expect to return OSIRIS to full capability on March 31, 2009.
Starting in late 2008, we noticed images shifting on the OSIRIS SPEC detector when we would change the OSIRIS plate scale. There is an offset among the 4 OSIRIS plate scales, but this is known and accounted for by software. The movement was closely aligned with the x-direction on the AO bench so initial investigation focused on the AO bench. Eventually, the AO bench was was not the cause of the motion, but evidence pointed to something inside OSIRIS.
Figure 1: A difference of two data cubes taken of the AO fiber point source in the Kn3 filter and 100 mas scale. Between the two datasets, the SPEC scale was changed from 100 to 20 and back to 100 mas.
Also in late 2008, the thermal performance of OSIRIS became unstable. The dewar temperatures would change rapidly and by large amounts (see Figure 2). Initially, there was some correlation between warmer temperatures and OSIRIS being inside AO; however, the temperatures were erratic both inside and outside of AO. The main issue was that we could no longer control the OSIRIS SPEC detector at its operating temperature of 68 K (Figure 3). Measurements show that at 72 K, the dark current of the SPEC detector is a factor of 6 greater than the dark current at 68 K. Furthermore, while we could maintain the IMAG detector at 70 K, its control circuit seemed to cause temperature oscillations. As we had not changed the control parameters, we posited that there was a different heat load on the instrument for which our control parameters were incorrect.
Figure 2: Erratic dewar temperatures in OSIRIS from late 2008, early 2009
Figure 3: Erratic detector temperatures in OSIRIS in early 2009
In January 2009, Support Technicians reported trouble connecting the helium supply lines to the OSIRIS closed-cycle refrigerator (CCR) cold head. Upon inspection, Instrument Technician Mike Wagner found that in the process of disconnecting or reconnecting the helium lines, the cold head was being twisted and tilted in its mount. He noted that the connection/reconnection practice was different that that originally specified for OSIRIS. Instead of connecting the helium lines through an interconnect panel, Support Technicians were connecting directly to the OSIRIS cold head. It is thought that the change was made such that the procedure was more similar to that for NIRSPEC when it goes into AO; however, NIRSPEC has two cold heads so its helium line connection on the instrument is via a built-in interconnect panel, not a separate panel as is the case for OSIRIS. Furthermore, the OSIRIS cold head rides on a flexible bellows to reduce vibrations. After discussion with Jim Lyke, Mike Wagner changed the helium line connection procedure back to the original form that uses the interconnect panel.
A service mission for OSIRIS was planned for February and March 2009.
A service mission for OSIRIS was planned for February and March 2009. The OSIRIS CCR compressor was turned off on the morning of February 4, and the instrument was allowed to warm. The dewar was opened on February 17 by Jim Lyke, Randy Campbell, Dwight Chan, Mike Wagner, and Grant Hill.
Upon opening, a thorough inspection of the dewar was made. We found a "smoking gun" that can explain the temperature performance. Inside the dewar, the cold head is connected to a copper assembly (Figure 4). There is about 1/2 inch of clearance between the copper assembly and the inner radiation sheld. It appeared that any rotation or tilt applied to the cold head via the helium line connection could cause contact between the assembly and the radiation shield. Indeed, we see evidence of rubbing both on the radiation shield (Figure 5) and on the top horizontal plate of the copper assembly (Figure 6).
Figure 4: Copper assembly connected to the OSIRIS cold head
Figure 5: Copper residue on the inner radiation shield that is evidence of intermittent thermal touch-off.
Figure 6: The top horizontal plate of the copper assembly shows discoloration that is evidence of thermal touch-off
In addition to the changes to the reconfiguration procedures, we also modified the copper assembly to prevent future touch-offs even if the cold head is inadvertantly rotated or tilted. Figure 7 shows where we literally cut corners in OSIRIS to improve performance.
Figure 7: The front corners of the top horizontal plate of the copper assembly were cut to prevent future thermal shorts.
As a part of normal dewar maintenance, we replaced the getter material with brand new, vacuum-baked Zeolite immediately before closing OSIRIS.
Looking at optics, we found the locking rings for the lens barrels on SPEC scale turret #1 (SPEC wheel #1) loose. In the case of the 100 mas scale locking ring, it had nearly backed off the threads all the way (perhaps 7-8 threads). We attribute this to the vibration of the stage as it moves from scale to scale. Figure 8 shows the correct location of the locking rings. When loose, the lens barrels can move to adjust focus. It is interesting to note that the 4 locking nuts on SPEC scale turret #2 were tight.
Figure 8: The proper location of the locking ring on the lens barrel.
We proposed adding set screws to the locking rings to give an additional source of friction to keep the locking rings in place. We discussed this idea via email with PI James Larkin and the designer of the wheel, John Canfield, both at UCLA. Larkin thought the idea was fine and also suggested alternatives. After discussions with our Instrument Engineer, Dwight Chan, Mike Wagner, and several Support Astronomers, we decided to go ahead with our set screw idea.
Machinist Neil Felton drilled and tapped holes in the locking rings for use with special soft-tipped set screws that Dwight Chan had. We needed soft-tipped set screws so that the threads of the lens barrels would not be damaged. Neil used no oil in his work and the set screws were vacuum-baked before installation. Jim Lyke installed the modified locking rings with the set screws. One such ring is shown installed on the 100 mas plate scale lens barrel in Figure 9. The set screws are 2-56 and require a 0.035-inch hex key.
Figure 9: The modified locking ring with set screw installed on lens barrel
Because the image offset issue affected all scales in the same way, we expected to find some alignment issue with one of the 2 SPEC scale wheels. Both wheels were noticeably more loose than the SPEC filter wheel (very similar design), but the performance of all stages was good. No stage faulted during testing and each stage seemed to settle into the correct positions. We note that the performance of a stage while warm and at atmospheric pressure may be different than while cold and under vacuum.
We set up an alignment laser on the AO bench and attempted to demonstrate and image offset. We were unable to measure such an offset. We do not know the cause of the image shift with changing plate scale.
During the last two service missions, we have found debris inside the dewar. This debris is located below the OSIRIS imager. It is green and bears the marks of threads. It appears to be Loctite that has flaked off. We removed the debris before closing the dewar.
We spent some time wiping many surfaces and hardware with wipes wet with alcohol in an attempt to clean the dewar. We note that there are many surfaces that we cannot reach safely that show signs of grit. Our intention is to clean the dewar in order to be able to warm pump OSIRIS to a lower pressure.
Figure 10: Green dewar debris in place
Figure 11: Close up of the dewar debris showing the indentions due to threads.
OSIRIS was serviced in February 2009. We think that we have repaired a thermal short, but we were unable to determine a cause of the image motion with changing plate scale. It's possible that the thermal cycle will change the performance of the SPEC image motion. We expect to return OSIRIS to full capability on March 31, 2009.