J. M. Hill, Steward Observatory
Large Binocular Telescope Project
Technical Memo
UA-98-01 telspecs.tex 002s004g
July 6, 1998
This document summarizes the various input specifications to the final telescope design. Will it be finished before the telescope is built? Will it ever be finished? These specifications include: telescope drives and encoders, enclosure drives, instrument rotators and cable wraps, instrument and telescope utilities. Send corrections, questions and comments to J. Hill.
Removed CATV video distribution. Drastically reduced the number of coax cables. Removed Azimuth chilled water. Updated M2 and M3 dimensions for revised back focal distance. Added section on limits of telescope travel. Added section on telescope deflection tolerances. Removed UPS power from F/4 secondary utilities.
Added section on ``instrument envelopes'' and masses. Added chiller and heating water pipes to ``fixed enclosure utilities''. Updated requirements for ``mirror support air'' and ``mirror ventilation air''. Lowered total AC power estimates. Attempted to clarify limits of azimuth and elevation travel.
Added ``maximum observing windspeed'' of 80 km/hour to building rotation specifications. Added ``compressed air for mirror support'' and ``mirror ventilation air'' to elevation utilities. Reduced electrical service in azimuth utilities since telescope drives use 10 - 20 kVA not 80kVA. Split out listings for tertiary mirrors and laser guide star mirrors. Removed ``Bent'' from the focal station utilities list and replaced it with ``combined''.
Additional specs on the instrument rotators have been filled in. UPS power has been added to the on-telescope utilities. Elevation AC power was changed to 480 V assuming some on-board transformers. A list of utilities for the azimuth cable drape has been added. A list of utilities for the rotating section of the enclosure has been added.
The US Standard railroad gauge (distance between the rails) is 4 feet, 8.5 inches. That's an exceedingly odd number. Why was that gauge used?
Because that's the way they built them in England, and the US railroads were built by English expatriates.
Why did the English people build them like that? Because the first rail lines were built by the same people who built the pre-railroad tramways, and that's the gauge they used.
Why did "they" use that gauge then? Because the people who built the tramways used the same jigs and tools that they used for building wagons, which used that wheel spacing.
Okay! Why did the wagons use that odd wheel spacing? Well, if they tried to use any other spacing the wagons would break on some of the old, long distance roads, because that's the spacing of the old wheel ruts.
So who built these old rutted roads? The first long distance roads in Europe were built by Imperial Rome for the benefit of their legions. The roads have been used ever since. And the ruts? The initial ruts, which everyone else had to match for fear of destroying their wagons, were first made by Roman war chariots. Since the chariots were made for or by Imperial Rome they were all alike in the matter of wheel spacing.
Thus, we have the answer to the original questions. The United State standard railroad gauge of 4 feet, 8.5 inches derives from the original specification (Military Spec) for an Imperial Roman army war chariot. MilSpecs and bureaucracies live forever.
So, the next time you are handed a specification and wonder what horse's ass came up with it, you may be exactly right. Because the Imperial Roman chariots were made to be just wide enough to accommodate the back-ends of two war horses.
Author Unknown
A study is now underway to optimize the tradeoff between local seeing, pier height and wind-induced motion of the telescope and pier. A discussion on 13JUL94 produced the following error allocation for open-loop telescope tracking errors (tilt):
The following historical section was extracted from Hill (1990, SPIE 1236, 86).
In the spirit of building a telescope that matches the best atmosphere, the optics support structure is required to track open-loop with a smoothness to match the image motion caused by atmospheric turbulence. The tracking requirements of the telescope are set by two modes of operation. First, image motion should not significantly increase the point spread function during moderate length unguided exposures (minutes). Second, image motion should not degrade the diffraction pattern during rapid readout imaging (speckle, thermal IR imaging, interferometry). The telescope as a system must deal with five types of image motion on various timescales. The most basic motion is the diurnal motion of the sky. Given a stable clock and a good model of the atmosphere, this motion is quite predictable down to the level of a few hundredths of an arcsecond. The next set of motions are caused by flexure, hysteresis, and thermal drift of the telescope structure. Given a stiff steel structure, we can expect deformations of roughly one millimeter or equivalent pointing errors of tens of arcseconds. Systematic pointing variations can be measured as a function of position and temperature and removed from the pointing and tracking error with a lookup table down to the fraction of an arcsecond level. The following section discusses the image motion coming from tilts in the atmospheric wavefront. If the telescope is not in thermal equilibrium, temperature gradients could overwhelm the tabular calibration, but we must maintain equilibrium to preserve seeing. As we look at higher frequency errors we discover wind disturbance of the telescope position and internal torque disturbances. Our primary weapons against windshake are a short focal ratio to reduce wind torque, stiff drives, low wind cross-section and shielding by the dome. Finally we must design the telescope drives and supports to avoid high frequency drive errors and vibrations. These errors are extremely difficult to remove by guiding or other measurements of the focal plane images, and so contribute directly to increasing the image size. Vibrations are also very detrimental to interferometric measurements which require pathlength stability over the characteristic timescale of the atmosphere.
The RMS (1D) image motion, 
, induced by the atmosphere
is given by:
where D is the telescope diameter in meters, and 
is also expressed in meters. For an 8 meter telescope
in an 00 = 45 cm atmosphere, we expect 0.06 arcsec rms image motion.
For the gaussian case, 0.06 arcsec rms motion would provide a 0.14
arcsec FWHM long-exposure image. Since 0.06 arcsec rms motion is
derived by giving the entire error budget for wavefront tilt to
the mount, we clearly must reach some compromises to allow for
telescope - telescope alignment and collimation errors. Unlike image
size, image motion is constant with wavelength (neglecting
diffraction), because atmospheric phase errors are independent of
wavelength. Therefore, these tracking specifications should apply to
all wavelengths. In the thermal infrared where the telescope becomes
diffraction limited, we must consider the effects of image motion on
the diffraction pattern. The minimum image size occurs around 5
microns where 00 approaches the size of a single 8-meter primary. To
meet Marechal's criterion and preserve a Strehl ratio, S, of
0.8, again the whole error budget, image motion must remain smaller
than 0.031 arcsec rms.
This contributes a FWHM image size of 0.074 arcsec. To achieve
S=0.95 it would be necessary to halve these numbers.
In the absence of wind forces, telescope - telescope alignment is only affected by gravity and thermal effects, which are slow compared to atmospheric motion. These long term drifts are relatively easy to correct by monitoring the positions of the images in the focal plane. Within the linear range of the primary support mechanisms, it should be possible to steer the two primary mirrors into collimation by moving the three (six) hardpoints which locate the mirrors. This seems unusual, but no more so than tilting 2-meter secondaries which are as large as the primaries of today's telescopes.
After we have corrected pointing errors and drifts between the two telescopes, the next challenge for guiding is to remove the slowest parts of the atmospheric image motion to improve the long term image size. To estimate a timescale for the motion, we assume an outer turbulence scale of 100 meters and an upper atmosphere wind velocity of 20 meters/sec. This implies a timescale of up to 5 seconds. Image motions on timescales longer than one second should be correctable with normal guiding (moving the telescope) and tilting the secondaries. This allows us to loosen the tracking specification on longer timescales. The characteristic upper frequency of the atmospheric image motion is set by the pattern speed moving across the aperture. Using a 20 meter/sec wind and an 8 meter aperture we find a frequency of 2.5 Hz. Image motion and telescope motion on timescales longer than 0.1 second should be correctable with rapid guiding with a steering mirror if a sufficiently bright source is available within the isoplanatic region. A goal for rapid guiding would be to reduce net image motion below 0.01 arcsec rms -- actually improving on the atmosphere -- for frequencies below 10 Hz. Guiding at this level will certainly influence the design of the encoders and servo system, if not the telescope structure. Because the telescope aperture is so much larger than 00, the improvements in image size from guiding are not as great as they would be for a smaller telescope. In the best seeing, we can expect less than a factor of two improvement. For optical imaging and spectroscopy, where the field-of-view may be much larger than the isoplanatic region in the focal plane, image motion across the field may not be correlated. Differential image motion should not be a problem in the thermal infrared, where the isoplanatic region is roughly the size of the focal plane.
Adaptive optics correction of the wavefront is beyond the scope of this discussion.
The telescope should point to 0.3 arcsec rms at all times (night) with periodic recalibration of the open loop coefficients. The Multiple Mirror Telescope (MMT) already achieves this pointing performance. The telescope should track to 0.1 arcsec rms (1D) for periods up to 1000 seconds. The MMT currently tracks at 0.1 arcsec rms for shorter periods of time. This number also represents the blind offset specification for angular motion less than one degree. The telescope should track to 0.03 arcsec rms for periods up to 5 seconds. The value of 0.03 arcsec rms is somewhat larger than would be calculated from the 00 = 150 cm allowed in the wavefront error budget. This has been allowed because many of the other errors contribute less at large spatial scales. These tracking specifications should apply up to wind speeds of at least 6.7 m/sec (24 km/hour), and performance should degrade gracefully up to the maximum operating wind speed of 22 m/sec (80 km/hour) without exceeding three times the specification. The short timescale specifications imply an effective smoothness of a few microns for the drives and supports. Longer scale variations and temperature effects can presumably be taken out with encoders and look-up tables.
The fundamental encoder specifications are derived from the telescope tracking specifications listed above. We expect to have an encoder on the azimuth platform and one encoder on each of the two elevation C-rings. Additional encoders will be located on the instrument rotators etc.. Environmental parameters are listed consistent with the general telescope specifications.
We would like an angular resolution of
at least 27 bits (1 part in 
) with 28 bits as a desirable goal.
We have dropped the 0.01 arcsec tracking requirement discussed
previously, although we will still need 27 bit encoding to meet the
0.03 arcsec requirement. Some assumptions about the servo system have
been used to decide between 26 and 28 bits.
Using the encoder in a typical mountaintop environment, we have the following general environmental requirements. It would also be prudent if the encoder tape were tolerant of hydraulic oil from the nearby hydrostatic bearings of the telescope.
Assuming a strip encoder mounted on a 14 meter diameter cylinder we may derive the following subsidiary specifications. Each elevation C-ring would use half a circumference strip. From the angular encoder specs, we can derive some corresponding linear specs for the tape encoders. These linear specs are listed in the table below. The linearity requirements come from the desire to track smoothly for short periods of time without reference to the sky. Large scale non-linearities in the encoder are of little importance as they will be calibrated out along with telescope flexure and other similar effects. The maximum bit rate or maximum linear velocity indicates how fast the bits on the encoder need to pass by without getting lost. The minimum update rate indicates how often we want to feed position and velocity information to our control system.
The actual encoder mounting diameters are closer to 13 meters, but these tables have not been updated to reflect that change.
The actual encoders purchased in 1995 are Farrand Inductosyn tapes
with a physical pitch of 0.1 inches (2.54 mm). With nominal
14-bit interpolation, this is reduced to a linear resolution of
0.155 
. The diameter of the azimuth mounting surface is 13130 mm.
The equivalent diameter of the elevation mounting surfaces is 12960 mm.
The following description of the telescope azimuth and elevation limits is extracted from an email discussion in October 1996.
First, let us specify that the brakes of both the telescope and
the enclosure are sized so that they can stop from the full slew velocity
of 1.5 
in a distance of no more than 2 degrees AND no less than
1 degree. Note that stopping by brakes produces a deceleration which
is 2 - 4 times larger than that available from the motors; i.e. the brakes
are stronger than the motors.
Second, we will agree to move the bumpers back a small distance behind the final limit switch for elegance if not necessity.
Third, we will specify the position of a software limit which causes
the control system to reduce the allowed maximum velocity to a smaller
value (perhaps 0.3 ?). This software limit could have more
than a single step, but the principle is the same. In the case of the
enclosure with respect to the telescope, the software limit will refer
to relative velocity.
Fourth, as before we ask bumpers/dampers which stop the full slew
motion in a distance of 1 degree. If the bumpers need additional
distance for engineering reasons, we can engage them slightly sooner.
Because the 270 degree rotation of the enclosure has no bumpers, we've
added some extra angle for the emergency stopping distance. (In the table
below, 
277 degrees is the position by which the enclosure should certainly
have stopped after the final limit switch. Then there are 3 degrees more
before the cable chain is damaged.)
The revised table:
We have written the above assuming that the telescope and the enclosure are always able to move in their normal operating ranges without encountering any limit switches. Just beyond the normal operating range an initial proximity limit switch is encountered which serves as a warning to the control system that there is a problem. Upon sensing the initial limit switch, the control system immediately commands the telescope and/or enclosure to zero velocity under software or firmware control.
If the system does not heed the initial proximity limit, a second final limit switch disables the drives and applies the brakes in hardware just before the bumper is engaged. Even if the final limit switch fails, the bumpers are able to stop the telescope before reaching the ends of their travel.
If everything fails, damage to the cables and/or the steel structure will occur when a travel of 4 degrees occurs. Hopefully this condition is only hypothetical.
The instrument rotators/derotators and cable chains should have the approximately the same ranges as the enclosure absolute motion, but in reality they have slightly less travel than the enclosure absolute angles listed above.
In this scheme, pressing the emergency stop button cuts power to the drives, applies the brakes of both telescope and enclosure, and cuts power to the hydrostatic pads and other systems. The telescope should have stopped moving before the hydraulic accumulator drains down.
There is another debate about whether strong (as specified above) brakes are needed on the azimuth of the telescope. The MMT has disabled their original telescope azimuth brakes to avoid problems with the enclosure backdriving the telescope against its drives and brakes. I suggest that we need the telescope azimuth brakes for the pre-erection and we can decide whether to disable them later. Apparently the teeth of the gears are strong enough to withstand this backdrive force.
The previous calculation of the space necessary to stop the telescope at full slew velocity: Vmax=1.5 had:
If we consider to reduce the full slew velocity down to 
1 having the same maximum deceleration of 0.3 
we can stop in 1.5 - 2 degrees.
In any case considering the highest deceleration we have in the previous calculations we have on the mirror just a fraction of the 1.3g allowable load.
The rotation range of the front and back combined focus rotators is limited by the cable chains to a maximum of 250 degrees.
The telescope shall be able to achieve a maximum angular velocity in
each axis of 
. The maximum acceleration
shall be 
. These parameters and a 10 Hz
telescope will allow a 1 arcminute offset in 0.6 seconds, a 1 degree
offset in 4 seconds, and a 90 degree slew in 70 seconds. Motions less
than about 7.5 degrees are acceleration limited. It is desirable to
have a ``no-track'' cone no larger than 0.7 degree in diameter at the
zenith (peak angular velocity = 
). These
velocity specifications are not intended to substantially impact the
design or cost of a telescope which can meet the tracking goals.
The drive specifications for the co-rotating building enable it to follow the telescope and provide protection from foul weather. Maximum velocity and acceleration were revised July 1994. See also the limits of travel discussed in a previous section.
Shutter aperture width calculation comes from Tech Memo UA-91-02.
This section summarizes the cable wrap allotments for utilities to pass through the instrument rotators. This list came from discussions at the LBT Engineering meeting on June 3, 1993. The previous version of this list came from discussions held on November 20, 1990 (see UA-91-02). Cable wrap contents for the large Gregorian/Cassegrain instrument rotators are listed in rough order of priority. A subset of these cables will be used for the bent Cassegrain and interferometric focal stations. When a bore is specified, it refers to the section through the instrument rotator, not to long runs on or off the telescope. If cable wrap design becomes too difficult on the instrument rotators, then some of these cables might be derotated directly from the instrument. An important question still to be answered is: Where on the rotating part will all this stuff terminate?
The instrument ``cable wraps'' are more correctly called ``cable chains''.
Available 1 x 2 places on the telescope.
Available up to 3 x 2 places on the telescope.
Available up to 1 x 2 places on the telescope.
Available 1 x 2 places on the telescope.
Available 1 x 2 places on the telescope. (signals and power may be routed to electronics enclosure.)
Available 1 x 2 places on the telescope. (signals and power may be routed to electronics enclosure.)
Available 1 x 2 places on the telescope.
This is the first cut at the utility requirements for everything on-board the elevation structure. These utilities travel through the elevation cable drapes onto the telescope. This should be a superset of the above instrument and secondary utilities. This table represents the combination of the two elevation cable drapes on the left and right sides of the telescope.
30 kVA continuous/ 40 kVA surge, (480VAC, 60 Hz)
This is the first cut at the utility requirements for everything on-board the azimuth platform. These utilities travel across the azimuth cable bridge onto the telescope.
See the separate LBT Tech Memo UA-94-01 ``Error Budget and Wavefront Specifications for Primary and Secondary Mirrors'' for additional details.
These dimensions are from the January 1993 LBT Tech Memo UA-93-01 entitled ``1993 Baseline Telescope Description''. See the separate memo ``Dimensions for Large Borosilicate Honeycomb Mirrors'' from the Mirror Lab for additional details.
F/3.8 (naked)
1.25 m 2.8 m
10 arcminutes (vignetted at secondary)
These tolerances are intended to say what is required from the perspective of the optics and the astronomer. I have not worried about any special tolerances imposed by the steel structure, hydrostatic bearings and similar systems. (from a March 1996 email)
1 arcminute 1 arcminute 1 arcminute 0.5 arcminute (zenith) 1 arcminute (horizon) 0.5 arcminute (zenith) 1 arcminute (horizon) 0.5 mm across cell 1.0 mm 0.5 mm 2.0 mm 1.0 mm 1.0 mm
This is the first cut at the utilities required from outside the enclosure. These utilities come up the road through the utility trench from the utility building.
700 kVA continuous / 900 kVA surge,
This is the first cut at the utilities required from the fixed to the rotating enclosure. These are the utilities that go through the enclosure cable chain between the fixed and rotating sections of the enclosure.
400 kVA continuous/ 500 kVA surge, (4160 VAC, 60 Hz)
These instrument dimensions were previously specified in LBT Technical Memo UA-93-01, ``1993 Baseline Telescope Description''. I've now updated them to be consistent with the final design. These are nominal dimensions. Refer to the design drawings for specific details and precise dimensions.
Telescope Specifications
for the Large Binocular Telescope
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