Interfacing Visitor Instruments to the Keck Telescopes

Interfacing Visitor Instruments to the Keck Telescopes

This guide is meant to convey the essential information needed by an instrument builder to judge the compatibility of the instrument with the Keck II telescope and to estimate the effort required to observe with it. It is not intended that this guide provide all the information needed before an instrument is brought to Hawaii; many more details will have to be settled before that happens, through exchange of information between the instrument and telescope engineers.

The primary mirror comprises 36 hexagonal segments each 1.8 m across the corners. The maximum diameter of the primary is 10.95 m and its focal length, 17.5 m. The telescope aperture area is equivalent to that with a circular aperture 9.96 m in diameter.

The mirror segments are maintained in mutual alignment both in angle and in piston, using measurements of relative axial positions of neighboring segments, with capacitive sensors accurate to about 5 nm.

The f/15 secondary forms a Ritchey Chr�tien system in conjunction with the primary, giving zero coma. It may be used with a (flat) tertiary mirror mounted on the elevation axis, when the beam can be sent to either of two Nasmyth platforms (left and right) or to any of four bent Cassegrain focal stations, designated 1 through 4, with 1 nearest the left Nasmyth and 4 nearest the right Nasmyth platform. Position 3 at the right bent Cassegrain focus of Keck II is available for mounting visitor insturments. This position includes a rotator on which to mount the visitor instrument, an off-axis guide camera, as well as some electrical and cooling interfaces. This position has not been commissioned at f/15, but could be if a proposed visitor instrument requires it.

Keck II is provided with an infrared chopping capability on the f/40 focus, but it differs from the Keck I f/25 system in having a simpler secondary mirror drive with a more restricted angular chop range. The Keck II f/40 secondary is referred to as the Infrared Fast Steering Mirror (IFSM).

The IFSM provides performance as listed in Table 1.

Table 1. Features of infrared secondary mirrors

Note: The maximum amplitudes shown in Table 1 are conservative, allowing for the center of the chop being considerably offset in angle, e.g., to minimize aberrations with an off-axis detector. For larger amplitudes than shown, details of the requirement would have to be studied.

For Keck II, the chopping f/40 secondary mirror has its final focus parfocal with the f/15 system; i.e., it feeds the conventional Cassegrain, bent Cassegrain, and Nasmyth foci, rather than forward Cassegrain focus used by the Keck I f/25 system.

The f/40 secondary mirror is undersized and has a central hole so that it reflects no radiation to the detector from beyond the periphery of the primary mirror or from the primary mirror�s central obstruction, for chop angles up to +/- 150 arcsec on the sky. The secondary mirror chopping mechanism is contained within an annulus behind the secondary mirror so that the detector sees direct skylight through the mirror�s central hole and outside its external profile, apart from spider vanes supporting the chopping secondary assembly within the main secondary module.

Synchronization of the IFSM to the instrument is via the "chop trig" and "chop synch" hardware signals. The instrument can trigger a chop by changing the state of "chop trig." "Chop Synch" is set true by the IFSM at the beginning of a chop and set false upon completion and is available, if necessary, to the instrument.

Both signals are differential using 26LS31/32 driver receiver pairs.

Table 2 provides the main optical details of the alternative foci. The aberrations are expressed as rms diameter, meaning 2 x rms radius of ray deviations from the image centroid at the test plane in which this rms is minimized.

Table 2. Main optical features of foci

If an instrument is displaced axially from the position at which the optical performance is optimized and the secondary mirror is moved to recover best focus, some spherical aberration results. Table 3 shows the magnitude of the effect. In each case, the secondary axial displacement has been chosen so as to give an rms image diameter of 0.10 arcsec on axis. The aberration is linearly related to the axial displacement. The effect on off-axis images is also shown.

Table 3. Optical effects of axially displaced focus

The primary mirror segments, the f/15 secondary mirror and the tertiary flat are optically polished Zerodur with aluminum coatings. The f/40 secondary mirror is diamond-turned electroless nickel on beryllium; the f/40 mirror has gold coating. Studies are under way to establish the practicality of coating primary mirror segments, f/15 secondary, and tertiary for Keck II with silver.

The Keck II bent Cassegrain visitor port guide camera has only been commissioned at the f/40 focus. It could be commissioned at f/15 if a proposed visitor instrument so requires.

Figure 1 shows the major mechanical features of the bent Cassegrain focus. Those proposing to mount instruments at this position, either directly to the fixed flanges or attached to existing rotators, should request more detailed drawings with layouts of threaded holes, etc.

Figure 1. (a) Relationship of focal surfaces to bent Cassegrain fixed flanges, (b) Relationship of focal surfaces to bent Cassegrain rotators.

A separate document with details for interfacing to the f/40 bent Cassegrain port rotator and guider is available on request.

General purpose instrumentation cables are terminated at the instrument interconnect panels located at the bent Cassegrain focus location on one end and at the Instrument Interconnect Rack (IIR) at the other end. The IIR is located in the computer room, next to the control room. Each telescope has a separate computer room and IIR. The following types of instrumentation cables are provided:

� Coax

� RG58

� Twisted Pairs with an overall shield (TPOS)

� Twisted Pairs with individual shields (TPIS)

� Optical Fibers: 62.5/125 micron multimode

Table 4. Number of cables per location

Single Phase, 115 Volt, 60Hz power is available at the visitor port. Clean and conditioned power is available. Connection is via standard duplex outlets and/or an MS style connector, P/N PT02SE2296S.

Visitor instruments interface with the Keck computer systems via ethernet. In this section, we detail the ethernet software protocols. Section 5 details ethernet hardware interfaces.

A visitor instrument may require a computer interface to the telescope, the chopper, and/or the guider. Each of these systems has defined levels of interaction. In each case, "level zero" refers to the simplest method of interaction and references only currently existing software.

We also discuss higher levels of interaction that provide tighter coupling and broader control. These higher levels may require more programming effort.

Level zero interaction assumes that the visitor�s instrument computer comes equipped with ethernet and a method for using ethernet to execute commands remotely on a foreign host (for example, unix/rsh, multinet/rsh, pctcp/wrsh, etc.). In this section we refer to this mechanism simply as "rsh."

6.1.1 The telescope

As with most Keck subsystems, a user controls and monitors the telescope by writing and reading keywords. Keywords are written using the "modify" command. For example, to set the telescope focus to 14.5 mm, we type the command

The "dcs" acronym on the command line refers to "Drive and Control System," the official name of the telescope control software at Keck.

Similarly, to read back the current focus, we type

(For a complete description of DCS keywords and their function, see KSD_46.)

Using rsh, the visitor can focus, offset, or make other commands interactively from the instrument computer command line. In addition, the visitor can write scripts that intersperse instrument commands with telescope commands to, for example, take a series of images in a box pattern.

If the home institution of the visiting team has ethernet access to a Sun workstation running either SunOS or Solaris, Keck can provide a telescope simulator for testing scripts before the visit.

6.1.2 The chopper Keck II f/40

The level zero interaction with the chopper is nearly identical to that for the telescope. For example, to set the chopper throw to 40 arcseconds, the visitor instrument uses rsh to execute the command

(For a complete description of chopper keywords and their function, see KSD_78.)

No chopper simulator currently exists.

6.1.3 The guider

Again, the level zero interaction for guiding occurs by remote execution of show and modify commands via rsh. The visiting instrument can correct the telescope�s tracking by modifying collimation keywords. For example, to correct elevation by 0.3 arcseconds, the visitor�s computer would use rsh to

The instrument�s autoguider software must keep track of the rotator position (by reading the keyword "rotpposn") to generate corrections in the az/el frame. (KSD_40 describes Keck coordinate systems.)

The autoguider system must also avoid oscillations by limiting the update rate to < 1Hz, with a maximum gain of 0.4.

Rather than generate software to correct telescope tracking directly, the visiting instrument team can choose to provide guide camera images to the Keck guider system and thereby take advantage of the guider software that is already in place at Keck. The Keck guider system provides a user interface and control software for acquisition, guiding, offsetting while guiding, compensation for field rotation, dispersion correction, and automatic centering of the telescope. The protocol for transmitting guider images from the instrument guide camera to the facility guider software system is described in KSD_13 and KSD_19.

Even if the proposed visitor instrument includes its own guider, it is strongly recommended that the guider provided at the right bent Cassegrain visitor port be used instead.

Level one interaction assumes that the visiting instrument team has chosen a control computer that supports Keck resident software. As of this writing, Keck resident software runs on Sun/ SunOS, Sun/Solaris, DEC/Alpha, and PC/Linux computers.

The method of interaction for level one is very similar to that for level zero. The only difference is that show and modify commands are run locally on the instrument computer instead of remotely via rsh on Keck computers.

The primary advantage of level one over level zero is avoiding some of the difficulties of rsh such as deferred evaluation and redirecting standard output. A second advantage is the ability to run the telescope simulator locally during development.

Level two interaction assumes that the visiting instrument team wishes to write C programs that interact with the Keck facility, has chosen a control computer that supports TCP/IP direct socket communication, and does not want to install Keck resident software.

As with level zero and level one, the instrument controls and monitors the telescope by writing and reading keywords. In the level two approach, the user program makes calls to the standard Unix subroutines socket(), connect(), send(), and recv().

The primary advantage of level two over level three is that it provides a C language API on a wide variety of platforms and does not require installation of Keck-specific software on the instrument host.

Level three interaction assumes that the visiting instrument team wishes to write C programs that interact with the Keck facility, has chosen a control computer that supports Keck resident software (see Section 6.2), and is willing to install Keck resident software.

As with levels zero, one, and two, the instrument controls and monitors the telescope by writing and reading keywords. In the level three approach, the user program makes calls to subroutines ktl_read() and ktl_write().

Level three is used by the home institutions that provide Keck�s facility instruments.

Keck currently uses the STB data archive mechanism (see "NOAO Save the Bits Archive Manager�s Guide") for its facility instruments. Visitor instruments are responsible for taping and archiving their own data.

Coolant is piped in insulated tubing to all the various instrument locations and to the socket for the secondary mirrors (where it is used with the chopping secondary).

The coolant is a mixture of 70% water and 30% ethylene glycol. Its freezing point is minus 15�C. The mixture has a specific heat of 3611 J/kg�K (0.8625 Btu/lb�R) and the density is 1093 kg/m³ (68.3 lb/ft³).

Typical operating pressure at the Nasmyth deck level is 345 to 414 kPa (50 to 60 psig). Maximum system pressure at the Cassegrain instrument connect panel with the telescope at the zenith is 552 kPa (80 psig). The instrument plumbing should be able to withstand a pressure of 620 kPa (90 psig). There is a differential pressure regulator between the supply and return connectors at each panel. The differential pressure is adjustable, and is typically set at 275 kPa (40 psi).

The design flow rate from the right bent Cassegrain instrument connect panel is 4.9 lpm (1.3 gpm). Coolant flow is continuous, except in the case of equipment failure or power outage.

The coolant temperature is 3�C below the dome ambient air temperature. Typical range of the coolant temperature is -10�C to 0�C. The temperature rise of coolant through the instrument should be not more than 3�C. The heat load from instruments at the right bent Cassegrain positions should not exceed 1000 watts.

Fittings on the instrument connect panels are 1/2-inch Parker FS-500 series quick connect couplers. The instrument�s coupler on the supply side should be male (Parker FS-502-8FP), and the return coupler should be female (Parker FS-501-8FP).

We recommend the instrument have the following provisions:

� Heat exchanger(s) and air circulation as required.

� Sufficient insulation to limit heat loss into the dome to less than 100 watts to minimize seeing degradation.

� An adjustable flow control valve to establish the correct temperature rise in the coolant.

� Temperature sensors at critical points with provision to shut down equipment in the event of overheating.

� A flow sensor in the coolant supply line to shut down critical equipment if flow is lost.

There is no personnel access to any foci during observing. For the bent Cassegrain foci, there is free space around and behind the instrument, but access is possible only with the telescope pointed at the zenith which places the instrument axis about 1.5 meters above the access walkway. Any but the smallest instrument would have to be handled to and from the bent Cassegrain focus with an overhead hoist.