|
| IMACSEngineeringRunSep04 |
UserPreferences |
| Magellan Telescopes | FrontPage | RecentChanges | TitleIndex | WordIndex | SiteNavigation | HelpContents | moin.sf.net |
Experience and observations point to IMACS not being aligned on the telescope axis and perhaps wandering with instrument rotation. Investigation has exposed loose parts and other problems that probably contributed to the misalignment but the full details not understood.
Other instrument flexure might also contribute to the symptoms (guider motion might mimic axial misalignment, for example) and could be explored as well.
The primary purpose is to measure, and if possible adjust, the IMACS alignment on the telescope. A procedure for checking the alignment should also be developed. Other problems such as focal plane tilt, focus, and server problems will be dealt with at another time.
Numbers (X.XXX) should be filled in and the list checked for completeness (and items added, if necessary)
This list is open to changes and revisions.
Instrument Mechanical Alignment
Other Tasks
What's the procedure to check that the tolerances mentioned above are within specification without using the direct measuring equipment? Write it down here. This is useful for quick checks and also to catch other items (flexure within the instrument). I can imagine a set of exposures on the pinhole mask with the rotator as well as the same on a star field and guider images. But we need to write down the details.
For my purposes (Steve Gunnels) ordinary dial indicators are adequate (actually, preferred).
* Investigated concerns of problems (suspected operating failure and noise) with the f/2 shutter as noted in several night reports. A short M3 cap screw was found lodged in the (south pointing) delrin blade guideway. The blade was damaged by the screw (bent a tab up on the tailing edge, preventing proper motion) but repairable. The damaged blade was repaired by flattening and filing and should be replaced when possible. The shutter was tested and shown to be in working order after reassembly. Possible source of the screw includes hardware associated with the filter frames.
* Finished design and drawing of the PTFE (or similar) slides for the platform bench slit mask storage unit. A copy of the drawing was left with F. Perez, as requested.
* A redesign for a new f/2 direct imaging baffle was started. The new design is a 'cleaner' assembly and is to include a provision for a removable Hartmann mask with the ability to 'snap' the mask into one of four positions.
* Investigated new plumming options for the dewar glycol cooling system with F. Perez. Redesign considerations include:
* Modified f/2 filter server to address reported operating problems. The 'top' delrin rail block shims were plained 0.5 mm to reduce alignment tolerences, allowing for smoother motion on the insert/retract operation. The system was checked and tested after reassembly as shown to the working as expected in the four cardinal orientations wrt gravity. Key/keyway and handle/arm clearances were checked. Little force was needed to manually insert test filters.
* Disassembled off-axis Shack-Hartmann lenslette array barrel to confirm optics mounting. The optics were found to be rigidly held in their mounts and solidly affixed in the barrel. This was confirmed by A. Uomoto and D. Osip. The camera was reassembled, tested and demonstrated to be operating as expected. Also, the stiffness, flexure, and mechanical assembly (loose screws, etc.) of the camera was investigated in an effort to discover a possible cause for the off-axis Shack-Hartmann focus issues. The drive assembly, specifically the drive chain slack) of both Shack-Hartmann and Principal Guiders were checked and found to be as expected.
* Carried out IMACS alignment tasks. See full report to be presented by Steve Gunnels.
* Continued work with D. Osip and S. Schaller on off-axis Shack-Hartmann issue. After alignment and guide camera work, a new LED spot template was obtained. Next, the center of rotation was located and found to be in the exact same location as the pre-alignment center. Continued testing with Center Field Shack-Hartmann active showed no improvement in the defocus as seen in the off-axis guider as compared to prealignment work.
Important Issues for Discussion
Further discussions with S. Gunnels and D. Osip brought to light that the telescope mechanics and alignment procedures currently employed may deliver a properly collimated and aligned beam to instruments mounted to the NIR only.
The following points are presented for consideration:
* When the telescope is pointed at Zenith there is a tilt experienced by the altitude disks. This tilt is a result of the bending moments on the lower OSS which is a reaction to mass of the upper OSS and the fixed, stationary master lateral hydrostatic pad.
* This tilt seen in the altitude disks are also seen by the NIR.
* It is understood that during a normal telescope collimating run, after the primary and secondary are aligned, a collimating telescope is mounted to the NIR and used to align the tertiary. As the telescope is moved in elevation, lookup tables are created to correct for the motion of the secondary/tertiary as a function of elevation angle, in an effort to deliver the optimal focus at the NIR.
* Also consider that as the telescope rotates in elevation towards to horizon the tilt in the altitude disks reduces (they become more parallel) with the changing loading conditions. Also the bending moment seen in the lower OSS reduces and the tertiary moves towards the secondary.
For an instrument mounted to the NIR disk these motions are accounted by, and corrected for, with the alignment procedures and lookup tables as described above. However, this is not believed to be the case for an instrument mounted to the Naysmyth platform, as IMACS is. In this situation the beam that is delivered to the instrument by the telescope is not expected to be aligned with the instrument rotation axis at any elevation angle other than when the telescope points to Zenith (because IMACS is aligned and centered to the rotator with the telescope at this angle). At all other angles the optical axes may not be collinear. In other words, although the center of the focal surface is delivered where it is expected, the focal surface delivered by the telescope may be tilted to what is expected.
Other issues that require further investigation and discussion:
Work still to be done includes:
* Investigate the reported problem of the 200 l/mm grism not readily returning to the wheel when released by the clamp.
---
