Category

Requirement

Reasons and Discussion

*Telescope Sky Coverage

The telescope will point and track above 30 degrees altitude when it's operating.

DIMM will be used to examine the seeing properties with respect to azimuth so all sky coverage is required.

*Mounting

The telescope will be mounted in alt-az fashion. Sufficient space will be allowed in the "closed" mode for the telescope to track Polaris continuously whether or not it is operating.

The alt-az mount allows continuously tracking Polaris during the daytime, which speeds evening setup.

*2.5 m Building Position

The DIMM will operate regardless of the position of the 2.5 m enclosure on its rails. A clear view of Polaris will be available at all times.

We would like to be able to work on and test DIMM without having to move the 2.5 m building. An unobstructed view of Polaris is desirable.

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. An override switch to allow "observing" mode while the telescope is closed will be available.

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.

The override mode in STANDBY is to allow tracking while the system is closed. It is anticipated that leaving the telescope pointed and tracking on Polaris during the daytime will speed nighttime setup.

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. The only electrical conductors entering the box will be for AC power, and these shall be equipped with sufficient lightning protection to prevent damage to other on-site instruments if the DIMM takes a direct lightning hit.

DIMM may take a direct lightning hit and we want to make sure that this does not damage the SDSS telescopes or their instrumentation.

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

For setup, the observer opens a non-removable manual cover. If necessary, this action applies power to the telescope drives. The observer will visually (or with a television aid) sight Polaris and another star at large angular distance from Polaris (60 degrees or more - Armin?) to obtain a pointing model. The observer starts the software if necessary. To shutdown, the observer will set the telescope on Polaris and manually close the cover.

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.

*Eyepiece access

Setup requires visual acquisition of two stars at large angular separation to determine a pointing model. This may be done with either an attached finder telescope or through a device that switches between the CCD and an eyepiece (for example, a flip mirror) in the telescope focal plane. If the latter, this system will have a positive detent for positioning and will not require a setscrew or other device that might come loose during the night.

Pointing precision required is better than one arc minute (Armin?).

While an eyepiece is preferred, access may limited enough to warrant using a television or other visual aid.

The alt-az mounting requires a two-star pointing model to enable telescope tracking. The observer needs to center two widely separated stars in the telescope. This might be reduced to a single star if we can maintain tracking on Polaris during the daytime (but this might not be possible if the DIMM needs to be stowed quickly).

Using a 90 degree pickoff mirror ("star diagonal") we imagine that visually sighting Polaris and some other bright star in the east or west will be possible with an eyepiece.

*Accessibility

Trained observers ranging in height from 58 inches to 77 inches will be able to perform the daily/nightly DIMM tasks without stepstools, ladders, etc. Permanent platforms, steps, etc., may be installed if needed. These will need to be approved by APO site engineering staff since they will almost certainly be placed inside the platform railing.

Ladders and stepstools in the proposed location will almost certainly be destroyed by the 2.5 m building if left in the wrong place. Damage to the building is also possible.

*New Filterwheel

A new filter wheel or slide will be permanently installed. There will be at least three positions: (1) Clear (really a BK7 or similar clear glass to maintain focus), (2) on-band H2O (what's the spec?), and (3) off-band H2O (what's the spec?). This may be a custom part or an appropriate high quality commercial device.

Another option is to replace the existing ST-5 with an ST-237, which can be had with an internal filter wheel that I assume (but do not know) can be controlled from the SBIG software package (possibly saving some development costs).

Control software will be written or modified by Armin Rest (UW) so consultation between JHU and UW is needed to get this right.

The filter installation affects the eyepiece or television field acquisition system since it might occupy similar space in the focal plane (unless a finder telescope is used for pointing).

New hardware (ST237, filterwheel/slide, filters) will be purchased by JHU.

*No Robotic Operation

DIMM will operate without human interaction during the night but there is no anticipation that DIMM will become a robotic system, that is, automatically start and stop, detect rain, etc. Human intervention will be needed for setup and shutdown (see above). No design compromises will be made to accommodate future robotic operation.

Anticipating future robotic upgrades requires all the work except final parts procurement for success. We state here that humans will always be available for setup and shutdown and the design will not anticipate future robotic modifications.

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. No more than 10 watts will be allowed to escape the enclosure at any time, whether or not DIMM is operating. The excess 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.

Possible heat dumps include venting hot air well away from the telescope or using the in-ground glycol circulation system at APO.

*Self-induced seeing

The enclosure will not degrade DIMM seeing measurements by more than 5% above open-air conditions.

The desire to operate in high winds (which requires wind shielding) conflicts with avoiding locally induced convection and wind flow patterns.

*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.

Experiments currently underway at APO will determine the nature of wind shielding required to keep the telescope from shaking too much.

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.

*Software

New software will be developed by Armin Rest, University of Washington, under ARC agreement with UW (that is, JHU does not pay software development costs). JHU developers should keep in close contact with UW personnel, especially regarding the impact of new hardware or changes to existing hardware.

DIMM has not been used often in all-sky mode and some software modifications are needed to make this better.

DIMM has never controlled a filter wheel and new software will be needed for this function.

*Spares

No spares beyond what already exist, will be provided. JHU will note items it believes may see end-of-life before 2005 or have exceptionally long acquisition lead times (longer than three months).

Spares are expensive. Do we get to call the "second DIMM" spare parts?