The Telescope (pointing)•Pointing background•Current pointing errors.•Some problems with pointing.•Can we point during the painting project.

•For more info:

http://www.naic.edu/~phil

Positioning specs•BeamWidths: lband:200”, xband:30”

•A 1/8 inch offset at focus = 5” error.

•A 1 lambda motion in focus = 3 db gain loss

•5 deg F moves the platform 1 inch vertically

•Adding weight to the platform:

– 8/(24) kips lowers platform 1 inch

• tiedowns Retensioned/(Not retensioned) •Pointing model removes time invariant errors.

•Tiedowns, distomats used to remove time variable errors (from temperature changes).

The pointing model•13 term model in az & za with za lookup tbl.•Current model created May 2004 after:

–Alfa installation–Kevlar cables installed in dome–New compressors on dome–Dome side rollers tightened on 1 side

•Pointing errors immediately after the models:

•New pointing model needed after painting.

Date Az err asec Za err asec Total asec1998 3.82 2.37 4.49

1999 5.4 3.1 6.2

2000 5.68 3.45 6.65

2003 5.38 3.48 6.41

2004 4.20 2.41 4.84

AUSAC Meeting · January 16-17, 2007

Current pointing errors

• Pointing errors Jan06 to Sep06.– 1 azimuth term in errors

• probably coming from a tiedown at the wrong height

• Could also be caused by weight being rearranged on the platform

• Appeared close to mar05 when td12 jack was replaced.

– Structure in az errors add an az encoder table

AUSAC Meeting · January 16-17, 2007

Some problems we’ve had

• 2005 distomat failures solved after distomat windows replaced.

• Az encoder failures sept,nov06– Corrosion inside encoder– New encoders installed after fixing rfi– Re-installing encoder uses a mechanical index.

• A .1 inch error at 60 feet (index position) is a 5 asec error on the sky.

AUSAC Meeting · January 16-17, 2007

• Tiedowns can lose tension when the dome points at or away from a tiedown. – 27may04 calibratoin run.– 10Am, 84degF, za=16.5 az=155.

• Sept06 A2221 (galfa HI halo clouds).– sat at az=0 and drove between za 12 and 19 deg.– Took data 8:30 to 9:30 am. Temp=80 F.– Lost tension on at least 7 days.

Tiedowns losing tension

AUSAC Meeting · January 16-17, 2007

Tertiary drive to move between 2 positions

• New tertiary drive system to be installed during painting project. Move between standard and longer optical path (set and leave it).

• You lose some gain since out of focus.• We need to:

– figure out the new position and any pointing corrections it will entail.

– The online software needs to be able to control this.– We need hardware indices to verify that things are in

the correct position (in case of encoder failure).

AUSAC Meeting · January 16-17, 2007

How much torque to tilt Platform?(or can we paint and point..)

• Look at height of platform corners vs az sep06-dec06– 0 offset occurs when az perpendicular to tower.

• Fit line to points when azimuth points at a tower.

• Given dome, ch, and counterweight weight, compute the corner deflection vs torque. – cornerHghtIn=.908 - .0003312*torq(kip-ft).

– Since corners are 124 feet from the center:

platRotAngle(Amin)=2.1 - .00077*torque(kip-ft)

AUSAC Meeting · January 16-17, 2007

How much sky motion when platform rotates?

• Platform probably rotates about main cable connections.

• The sky angle gets larger as you move out in za (for a fixed platform rotation).

• You get about an 8 Asec pointing error for a 1000 kip-feet torque. This would be from placing 10 kips near the corner of the triangle. The result is linear in the torque.– The dome at 20 deg would give a 3 Amin error if the elevation

rails were not shimmed.

AUSAC Meeting · January 16-17, 2007

Sband tx horn

• New sband transmit horn installed may06– Wanted to over illuminate the tertiary.

• Increases the gain• Increases Tsys (we don’t care for tx horn)• Increases the kilowatts of power spilling over the tertiary.

– The gain increased by about 4% (.4 K/Jy)

• New sband receive dewar/horn to be installed soon.• …that’s it….