SYSTEM DESIGN NOTE
SDN0002.02 - Environmental Cover/Entrance Window Requirements
SYSTEM DESIGN NOTE
SDN0002.02 - Environmental Cover/Entrance Window Requirements
| Prepared by | Date | Approved by | Date | Rev. | Rev Date |
| Gary Muller | 3/31/99 | N. Gaughan | 4/1/99 |
1. Introduction
The environmental cover is intended to protect the entrance window when the instrument is not actually in use for observations. It will therefore normally be kept closed when the instrument is off the telescope, or when the instrument is on the telescope but not being used for observations. The latter situation would occur during the day or during bad weather, or when another instrument is being used and GNIRS is on stand-by. The cover would not be closed during telescope slews or similar brief intervals between observations.
The cover is a warm external mechanism, and therefore does not act as a true “dark slide” or “shutter”.
2. General Requirements
1. O-ring seal to keep window free of dirt, dust, and moisture during shipping, handling and periods of non-use of instrument. No purge air is available during these times.
2. Manual actuation capability for times when power to instrument is not available.
3. Light seal between ISS and instrument. This light seal is for optical wavelengths and is not as stringent as IR light sealing requirements. Solder bead not necessary for light seal. Blade to be enclosed in light-tight box. The light seal must not degrade under environmental conditions. It must be flexible or otherwise designed (e.g. labyrinth) so that it doesn't exert forces on the dewar/ISS.
4. The available space is 25 mm. between the ISS and the front of the window.
5. One minute maximum actuation time (for motor powered operation only, does not apply to manual actuation).
6. Uniformly purge window with dry air during observations to prevent condensation of moisture or accumulation of dust on window. (Gemini to provide air, NOAO responsible for drying air). The flow must be non-turbulent and at near-ambient temperature. Need to allow for (a) pressure reduction, (b) manual adjustment of flow rate and manual readout [computer control not required]. One issue we did not discuss is whether any remote monitoring of the flow is required. It would be useful to know whether air is flowing, or whether there is a blockage/disconnect; exact flow rate is less important.
7. Window to be purged during periods when blade is exposing window. Window not to be purged during periods when blade is covering window. The air shut-off must be automatic (i.e. mechanical or electrical actuation rather than manual actuation). A manual valve of shut-off is permitted for use when the instrument is removed from the telescope and disconnected from the Gemini air supply.
8. Blade must support pressure differentials in both directions. Range of (external pressure - internal pressure) is from +15 psi to -10 psi. The -10 value might be reduced if a pop-off value or something similar was provided. On the other hand, if you can pressurize to 10 psi differential, you could do this on the summit and have the instrument arrive in Hilo with still some pressure difference left; this would tend to prevent diffusion of moist air into the window cavity. That is, a 10 psi pop-off is preferable to <8 psi.
9. Window to install from outside of dewar.
10. Use stepper motor for actuating blade to conform to existing electronics configuration. As this is a warm motor, unlike those driving other mechanisms, there will be some differences in its preparation. The overall drive system should be compatible with the cryogenic motor drives as much as is reasonable. Motors and other components which are subtly different should be clearly labeled.
11. Use micro switches to sense in and out positions.
12. Safety override feature to prevent injury of fingers, etc during operation.
13. Environmental Cover Assembly to be modular in design to allow easy
access to window. Separate modules for window assembly and blade
actuation assembly are desirable.
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