Review Comments on the new GNIRS Preliminary Design
by
Tom O’Brien, Ohio State University, April 14, 1999

The NOAO GNIRS Project asked Tom O’Brien, Ohio State University, to participate in a “Virtual Review” of the GNIRS Preliminary Design on Wednesday, April 14, 1999.  The purpose of this review was to take advantage of Tom’s experience and to go over any of his comments/ideas/guidelines that would help us in our design effort.
Tom graciously accepted our request, was able to view the material on the GNIRS web site, and comment on the design.  Tom made comments in the following areas:

•  Service access and alignment
•  External structure, thermal and flexure
•  Mounting of electronics
•  Use of Minco warm up heaters
•  Cryo-cooler location
•  Cool down time
•  Thermal background
•  Baffles
•  Vacuum performance
•  Mechanism drives and encoders
•  Alignment
•  Cold motors

1. Service access and alignment. Tom had concerns about access to mechanisms (this was a big criticism of his on the original GNIRS design). He cautioned that we keep in mind the accessibility to various parts and mechanisms of the instrument while developing the design.  A goal should be the ability to remove individual mechanisms without disturbing the rest of the instrument, that modular design is a big deal.  He stated that our going to cold motors was a good choice.

2. External structure and flexure.  Tom asked if our "brute-force" structural approach was sufficient to achieve our flexure goals without a compensation system, that we might want to consider some sort of active flexure compensation, such as transverse motion of the detector.   He noted that OSU's LBT optical spectrograph and some other optical spectrometers do have active flexure compensation.

Our response was that the OIWFS will compensate for flexure in the first order. [Larry Goble]

Further comment: The OIWFS only keeps the object on the slit, it doesn't compensate for internal instrument flexure between the OIWFS pickoff mirror and the detector. (TPO)

3. Mounting of electronics. Tom asked why our electronics boxes were supported on main truss rather than a separate truss, which would isolate their moments from the instrument.

Our response was that we did it this way to save weight, and that our analysis suggests that the weight of the additional truss could go into the instrument support truss and provide the requisite stiffness. [Larry Goble, Gary Muller]

4. Use of Minco warm-up heaters. Tom commented that for warm-up of the instrument, the heaters OSU likes to use are MINCO Kapton substrate type "pads" instead of resistors.  They use the biggest MINCO sheets made (5" x 15" with 400W capacity) which they epoxy to the aluminum structure.  This method avoids hot spots, which are prone to resistor heating, and is very economical. OSU has had good experience with these.

Our response was that we certainly will look into these. [Andy Rudeen]

5. Cryo-cooler location. Tom asked if the cryo-cooler placement is close to the heat sources now.  He remembered that the original GNIRS had a very long thermal path to cool the instrument.

Our response was that the coolers are mounted on the main bulkhead now and will be closely coupled to the load. [Larry Goble]

6. Cool down time. Tom noted that, at present, there is no pre-cool system included in the GNIRS design.  He suggested we consider LN2 pre-cooling, at least during the test phase, where time is at a premium.  A small (5 l) flask (with an autofill system) would be a sufficient reservoir for instrument checkout and could be removed before commissioning.  He recommended that we put it in our design layouts, and not use it unless needed.

Tom asked if we were planning to use superinsulation, stating that he was not a real proponent of this if radiation loss limits can be achieved by shield(s).

Our response was that we had talked about using it, but that it was too early in the design phase to make that decision. [Larry Goble]

Tom mentioned that the cool-down rates of optics follows the square of the lens diameter, and that our lens cool-down time should be fine given the long instrument cooldown time, and that we (NOAO) had a pretty good understanding of these.

7. Thermal background.  Tom asked if the detector would be "redblocked" (5.4-5.7um) for reduced thermal background.

Our response was that we have no plans for redblock filter. [Jay Elias]

8. Baffles.  Tom asked for an explanation of our baffling approach, if we were using baffle tubes or a cold cavity approach [Comment by JHE – this was not 100% clear in the telecon, but I think that what Tom was trying to say was that a fully sealed enclosure – cold cavity – was to be preferred to a complex system of baffle tubes, which was certainly something he did not like in the old design]. He asked how we would baffle around the OIWFS and pick-off mirror, mentioning that the primary mirror of the Offner would be a good place to put a baffle, as it would reject off-axis scattered light and background.  He suggested a one-pass labyrinth between the window and the instrument front plate and suggested that we may want to put extra black treatment in the fore-optics portion, and asked what kind of treatment on the inside of the cold cavity would be used.  It may be a good idea to have a labyrinth between the camera turret and the detector to take full advantage of the reduced solid angle that the detector can see through the relatively slow cameras.

Our response was that the structure now makes up the baffling, along with radiation shields.  Wiring is source of radiation now that cold motors are used. We are using cold cavity structure and cold shields to do the baffling, and not using tubes around the optical path as done before.  We plan to use hard black anodize on the inside of the cold cavity. We will be using a cold baffle immediately behind the window. [Larry Goble]

9. Vacuum Performance. Tom stated that cryo-pumping water with the detector during warm-up is a concern, and that we might consider using a sacrificial cold element as a condenser.  He stated that OSU uses LN2 cooled charcoal instead of mechanical pumping to achieve really good vacuum.

Tom asked how we were going to control water in system/condensation.  He said that he uses activated charcoal instead of molecular sieve for getter, but that he had no strong opinion on this.  OSU uses activated charcoal sieve, which doesn't require baking, and also has no desiccating properties.

Tom said that he has considered using a lump of material that acts as a thermal lagging surface to keep water off the camera lenses.  This is just a mass that gets cold fast during cool-down and stays cold longer than the lenses during warm-up.

Tom recommends not using a turbo-pump on the dewar and starting cooling as soon as rough pumping is done.  He stated that a 50mm vacuum port should be fine as cooling creates a high vacuum, especially if you have an LN2 pre-cool with some charcoal on it that could cool quickly and achieve high vacuum long before the entire instrument is cold.

Tom also said that OSU has been putting ionization gauges on their instruments to get a pressure reading.

Our response to this was that we have a difference of opinion with Tom in this area. There will be a getter on the coldest part of the cryo-head.  We plan to pump to achieve a good vacuum before starting cooling, and are considering putting a turbo-pump directly on the dewar, which will take over after roughing.  This pump is lighter than the equivalent valve that would have been used to go to an external turbo- pump to achieve the same result.  We are considering using a cold mass to keep condensation off the camera lens and the detector. We are going to use ionization pressure gauges .Our cryo-pumping medium is molecular sieve.  [Larry Goble, Dick Joyce, Jay Elias]

10. Mechanisms. Tom asked how are we doing mechanism drives now, would we be using worm gears and ratchets.

Tom said that in TIFCAM they mill a binary absolute encoder with stack of micro-switches a "low tech" absolute encoding method.  Their linear mechanisms have no encoding, but use high and low hardware limit-switches, which don't depend on software.  Their zero point reference algorithm uses 2 micro-switches: one for rough home, then back off and read a cam-operated second switch within 1 motor step of the rough switch.  This also gives diagnostic capability, but is exercised every motor revolution (concerns about lifetime).

Our response was that we plan to use highly geared mechanisms with dead reckoning motor stepping.  Filter wheel may have spur gear cut on edge of wheel with spur gear coupled to stepper motor.  Slit wheel drive may a linear slide instead of wheel; all slits on a mask.  We count steps and use precise flexure-mounted micro-switches for homing. [Larry Goble]

Further comment: I still feel that using high reduction gearing is not the way to go.  It puts more stringent requirements on motor life, adds to motor running time and hence heating, and adds a complex geartrain with attendant lubrication, life, and reliability concerns, and still doesn't achieve the accuracy or repeatability of flexure mounted zero-backlash detents.  (TPO)

Tom stated that they cool filter wheels through bearings only, with no thermal strap. He gave a general description of mechanism encoding used with OSU instruments, using micro-switch encoding and detents to back-drive motors for positioning.  GNIRS positioning requirements for some mechanisms may require more precision than possible, but suggested that the camera turret, having the tightest accuracy and repeatability specs, would be a good candidate for a detent or other locking mechanism.

Our response was that our camera turret just counts steps with no detent. [Larry Goble]
Gemini has relaxed camera (and other turrets) repeatability specifications. [Jay Elias]

Tom suggested the use of a micro-switch on the mechanism and a cam switch on the motor shaft to achieve positioning within 1 motor step.  Lifetime of motor cam switch is an issue.  He said why not use a detent and back-drive the mechanism?  He was nervous about un-detented camera turret repeatability as it puts huge requirements on the gearing accuracy.

Our response was that we only need repeatability and not accuracy. [Larry Goble]

Tom stated that he approves the use of Vespel. SP3 is a superb polyamide material - they are going to use it for motor bearing ball spacers instead of teflon (self-lubricating). They use Universal Grinding leadscrews and bronze nuts in actual instruments with a spray on moly-di-sulfide lubricant.  The lubrication is marginal and Tom would suggest SP3 as a far better choice for a leadnut material.  He is using an SP3 lead nut in a new instrument and its performance is extremely encouraging, especially compared to metal to metal contact.  However, the design must accommodate the huge difference in CTE between metals and SP3.

Our response was that we will use Vespel SP3 for screws and maybe nuts. [Larry Goble]

11. Alignment.  Tom asked if we would have a pupil-viewing lens.

Our response was yes, but it isn't focusable, we do focus detector though (~ 1mm motion detector focus). [Jay Elias]

12. Cold motors.  Tom stated that they do not thermally strap their cold motors, but also have no background concerns in the K-band.  They measure electrical winding resistance of Portescap cold motors when ON.  They only flange mount their motors, and that no bind-up problems have ever been observed at ~ 80K operating temperature.  They use full-step motor control and do not rely on motor steps for accurate positioning.

What is the lifetime of in-house cryo-conditioned Portescap motor bearings? [Larry Goble]

Tom's answer: Motors they previously used with standard metal ball bearing spacers had moderate lifetimes; with the OSU teflon spacers that replenish the dry-lube they expect very long lifetimes.  They are going to make the spacers out of Vespel SP3 next (NOAO is trying this on their initial efforts to modify a Portescap motor).  They don't alter the Portescap motor shim stack and bearing preload from factory presets.  They've run ~50 modified Portescap motors with only 1 armature failure (immediate), no bearing failures, and 2 stators burned up because of operator error (overdriving them electrically).  Their motors are used at ~ 80-85K and tested in LN2 baths occasionally.

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