Category

Requirement

Reasons and Discussion

Telescope Sky Coverage

Two possibilities:

(1) An unobstructed view of Polaris is needed at all times. Polaris is about 1 degree from the celestial pole. Maintenance operations (e.g., polar alignment) will require wider sky coverage but we allow this to be a special, rather than operational, situation.

(2) Some people argue that wider, if not complete, sky coverage is essential.

Almost everything is easier if we restrict required sky coverage to Polaris.

Sky coverage affects almost all aspects of the DIMM enclosure design including its location, moving mechanisms, and operations so until this is decided we cannot proceed with details. (To be specific in this document, I've assumed the operational mode is Polaris-only, option (1) - au.)

2.5 m Building Position

The DIMM will operate regardless of the position of the 2.5 m enclosure on its rails.

It's been noted that this might be too strict a requirement: "if the 2.5 m is not observing, we don't need DIMM." AU argues that we would like to be able to work on and test DIMM without having to move the 2.5 m building. DIMM may also be useful for other APO telescopes even if the 2.5 m is not operating.

2.5 m Building Movement

DIMM need not operate or produce good data while the 2.5 m building is in motion.

The proposed mounting location, on the 2.5 m platform, will vibrate while the building is being moved. Since moving the building is not done often and is of short duration, we will accept lost data during that time.

Location

Outboard the railing near the West end of the North railing of the 2.5 platform with no incursion inside the work area defined by the railing. Note this is along the North railing, not the West railing as has been discussed in the past.

This spot is accessible without moving the building (although raising the west door would be needed to approach the instrument) and will have a clear view of the celestial pole regardless of 2.5 m pointing or building position. In particular, the DIMM can operate if the 2.5 m building is closed. We want to keep the area inside the railing free of obstructions to avoid interfering with usual and maintenance operations.

Authorized Access (security)

No locks (key or combination) will be needed for full access to the instrument.

Locks impede startup procedures and inconvenience the occasional visitor who might need access. Users will need to enter the 2.5 m rolling building to get near the permanently mounted DIMM; no further security is needed or desired.

Height

DIMM will not extend more than 24 inches above the top-of-railing plane.

2.5 m telescope interference is the minimum requirement; French Leger reports that 24 inches satisfies this but should reconfirm after the location is decided. We believe there is no other height restriction.

Other Envelope Restrictions

There are no restrictions on the physical envelope besides those listed above.

APO does not use or have plans for this space.

Weight, Component

Individual components will each weigh less than 50 pounds.

Parts must be lifted over the railing by no more than two people during installation and maintenance. A third person may be needed to tighten fasteners.

Weight, Total

150 pounds.

Don't want to overload the support structure or make the enclosure too difficult to install. We can probably handle much more than this but less is better here. This is a loose requirement.

Installation Procedure

Installation procedures will be reasonably simple and safe, with dangerous or complex situations called out in the documentation.

Installation and dismantling is unlikely to occur more than a few times during the life of the instrument but we can lose equipment by dropping it over the rail.

Power Modes

OFF: All power inside the enclosure is removed by throwing a switch inside the enclosure.

STANDBY: All components powered up except the telescope drive. The telescope cover is closed. This is the usual daytime mode. Question: Is it OK to run the CCD camera all the time with cooling enabled?

OBSERVING: The telescope cover is open and the telescope is tracking.

Switching between "OBSERVING" and "STANDBY" is done by the single action of opening the telescope cover (go to "OBSERVING" when the cover is opened).

Each component has its own power switch (computer, CCD, telescope, etc.) there is no need for more finely determined power control.

Weatherproofing, Snow

In "standby mode" the enclosure will shed snow away from the 2.5 m platform and protect its contents from snow accumulation. Melted snow will not enter the enclosure or drip onto the 2.5 m platform. Ice locks on moving parts should be anticipated and avoided if possible. In "observing mode" the telescope should be protected from wind, including direct wind blast from below the enclosure. There should be easy access to sweep out snow accumulation. Melted snow should drain out. In all power modes the electronics should be completely protected from snow.

Extended unattended snowy periods should not require special attention. Occasional snow incursion while observing should not be a problem; it's OK to require some cleanup by humans in severe weather. Wind from below the platform can bring dust, snow, and water into the enclosure if it isn't properly protected.

Weatherproofing, Rain

In "standby mode" the enclosure will be weathertight, protecting its contents. In "observing mode" the telescope should be protected as well as possible. Sufficient drainage to prevent water accumulation should be provided. In all modes the electronics should be completely protected from rain.

Rainfall can be heavy and/or wind driven. Heavy rainfall should not affect the system in standby mode. The instrument will not be operated when it's raining but sudden storms may find it exposed for a few minutes. Occasional water incursion while observing should not be a problem.

Electric Power

APO will provide a dedicated 15 amp (minimum) 115VAC UPS circuit to a location specified by JHU. Conduit will enter the instrument and hookup will be to a JHU-provided circuit breaker box inside the enclosure.

This arrangement allows the designers to lay out the power distribution inside the enclosure and avoids outdoor electrical sockets.

The UPS power is needed to keep the computer running. Placing a UPS device inside the enclosure is rejected because of the additional heat generated and likelihood of UPS failure.

Data Port

APO will deliver in conduit at least two pairs of fast Ethernet fiber optics terminated in ST connectors at a location specified by JHU. One pair will have live ethernet, the other pair acts as spare fiber. JHU will mount a CAT-5 to FO repeater (specified by APO) inside the enclosure.

This gives JHU designers freedom to lay out data lines within the enclosure.

Lightning Protection

The enclosure should provide lightning protection for its contents by following usual procedures for grounding, AC line protection, etc.

This box will be near lightning strike points.

Polar Alignment

JHU will do the initial polar alignment to the refracted pole and provide documentation for the procedure.

It is likely that existing documentation for this task will need to be customized for the new enclosure.

Cleaning Access

No-tools access should be provided to make cleaning with a vacuum cleaner nozzle a reasonable task.

Insects, dust will need to be cleaned out occasionally. Making this job easy can save time and increase the frequency with which it's done.

Setup and Shutdown

To start the system, the observer opens the non-removable manual cover. This action applies power to the telescope clock drive. The observer adjusts the telescope hour angle to the appropriate value and centers the pole star. The observer starts the software if necessary. To shutdown, manually closing the cover will be sufficient.

The enclosure will be designed for maximum convenience for the setup and shutdown observer.

We believe the observer overhead on a carefully designed system will be acceptably small. Manual operation and setup reduces equipment and maintenance cost.

No Robotic Operation

There is no anticipation that DIMM will become a robotic system. No design compromises will be made to accommodate future automated operation.

Anticipating a successful future robotic upgrades requires all the work except final parts procurement for success. If robotic operation is desired, this decision should be made now.

Documentation

JHU will provide electronic dxf drawings and HTML pages organized in a single hierarchical directory. The HTML will be viewable with simple web browsers on the unix, linux, Windows, and Mac systems in common use by APO and SDSS staff and users.

This scheme is easily transported, updated, and viewed by the widest number of different computer systems.

Lights

The system will have no exposed light sources (IR or visible) while in operation. No pilot lights in the open and covers to prevent computer screen glow from being seen.

We want it dark to avoid interfering with 2.5 m operations.

Heat/power

The entire system will consume less than 100 watts of power at any time. This heat will be dumped in a way that does not affect 2.5 m imaging.

We want not to disturb 2.5 m seeing.

Heat from the CCD cooler will be difficult to control but it might be small enough not to be a problem.

Wind

The system will operate in sustained winds up to 35 mph with gusts to 45 mph from any direction. Effective operation is defined as usable data on 90% of measurement attempts. It will survive (closed) wind gusts up to 90 mph.

The APO telescope closure condition is gusts at 35 mph so this guarantees DIMM can operate whenever the big telescopes are running.

Operating Temperature

The system will operate normally when the ambient air temperature is between 0 and 90 degrees F and relative humidity is between 5% and 95% noncondensing. The system will survive extended excursions to -20 F and 110 F.

The temperature extremes at APO seems to be 0-90 F.

Telescope Compartment

An isolated area for the telescope. This area should be at ambient air temperature during operations. It should provide complete protection for the telescope when it's not observing. Sufficient ventilation to maintain the thermoelectric CCD cooler should be provided. Perhaps a small cover over a hole that the telescope looks through or perhaps a rollaway box.

This protects the telescope from the wind and the elements both while observing and in standby mode.

Electronics Compartment

An area separate from the telescope compartment will house the computer, CCD controller, network equipment, and UPS. Convenient access to the computer keyboard and screen while standing and reasonable access to the CCD controller and network controllers is needed during setup and maintenance. It should keep the enclosed components at proper operating temperature except during setup and maintenance.

A separate area for these components is needed to control heat.

Mounting Fixtures

The system will be mounted onto the steel structure of the 2.5 m building platform. After consultation with APO engineering staff, JHU will provide specifications for mounting points and structures to be welded or otherwise attached to the platform. APO will pay for, design, and manage the construction of parts that are permanently attached (e.g., welded) to the building platform.

A permanent mounting solution for DIMM requires solid mounting points.

Vibration

Someone (Armin?) should produce a requirement here or we should decide that previous tests indicate the simple solution of attaching it to the platform is acceptable.

The enclosure should be designed so that wind does not induce vibrations that affect the data. For example, loose panels that might catch the wind should be avoided.

The enclosure will be mounted solidly to the platform, which transfers platform vibration directly to the telescope. This is OK since people are usually not walking around out there while observing.

Vibration is likely a secondary concern as long as bad data due to vibration can be detected and rejected (Armin?). However, continuous vibration on the platform will likely cause too much data to be rejected so we need to find out if there are any motors, fans, etc., that couple to the platform that would require more vibration resistant mounting schemes.