2.5-m Telescope Tracking Error Budget

Sloan Digital Sky Survey Telescope Technical Note 19970523

Charlie Hull and Walter Siegmund

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Contents

• Introduction
• Tracking error budget

Introduction

The specification of 70 milliarcseconds (mas) RMS and the goal of 28 mas RMS for telescope tracking performance was given in "Telescope Tracking Smoothness" (Sloan Digital Sky Survey Telescope Technical Note 19920608). A tracking error budget corresponding to the goal is given in that Technical Note. This budget was subsequently updated based on measurements in "2.5-m Telescope Drive Measurements IV: Acceptance Testing" (Sloan Digital Sky Survey Telescope Technical Note 19950608).

Tracking error budget

Table 1: Tracking error budget.

Subassembly	Component	Error	Effect	Notes
		(nm)	(mas RMS)
Az axis wobble				1
	Drive disk 2-D high freq. error	273	20.5	2
	Guide roller error	273	20.5	3
	Lower bearing 2-D high freq./nonrep. error	431	32.4	4
Alt axis wobble				5
	Bearing 1-D high freq./nonrep. error	47	10	6
Az encoding error				7
	Drive disk high freq error	216	3.1	8
	Encoder capstan error	150	17.2	9
	Encoder error		5.5	10
Az servo error			9.8	11
Alt encoding error				12
	Drive disk high freq error	92	1.3	13
	Encoder capstan error	150	17.2	14
	Encoder error		5.5	15
Alt servo error			9.8	16
Rotator bearing error		240	4	17
Rotator encoding error			0	18
	Drive disk error	1000	3.9	19
	Encoder capstan error	602	2.3	20
	Encoder error		0.2	21
Rotator servo error			0.2	22
Secondary motions				23
	Axial motion		20	24
	Transverse motion		5	25
Total			61.9	26

Notes:

1.  
2. Measured upper azimuth wobble with the lowest eight harmonics and 25, 50, 75 and 100 cycles/revolution of the azimuth axis removed (SDSS19950608). These are the lowest frequencies of the guide rollers.
3. Upper azimuth wobble at 25, 50, 75 and 100 cycles/revolution of the azimuth axis, i.e., guide rollers (SDSS19950608). Measurements of the assembled azimuth motion were used rather than the direct measurements of the guide roller runout since the former require less interpretation.
4. Measured runout after the half harmonic and lowest eight harmonic terms have been removed (SDSS19950608).
5.  
6. Measured runout after the lowest eight harmonic terms have been removed (SDSS19950608).
7.  
8.  
9.  
10. Heidenhein ROD 700 accuracy
11. ±2.5 arc sec
12.  
13.  
14.  
15.  
16.  
17. Large diameter bearing
18.  
19.  
20.  
21.  
22.  
23.  
24. This number is based on measurements of tilt due to the axial motions of the 3.5-m secondary mirror.
25. Assumes 1.59 µm basic step size, 3:1 lever reduction, triangle wave P-V/RMS = 3.46.
26. Assumes that all items are statistically independent and add in quadrature.

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Date created: 05/27/97
        Last modified: 09/30/97
        Walter A. Siegmund

siegmund@astro.washington.edu