Page 365 equipment and instruments 4515 



nor spray can possibly have access to the reflecting surface to cause deterioration. The mirrors are 

 circular in shape to facilitate the use of the special frame required to obtain the hermetic seal. 



b. SielUte mirrors. — Stellite is a trade name for an alloy of chromium, cobalt, and tungsten. 

 It takes a high polish, is very hard, and does not tarnish. Stellite mirrors, which have been used in 

 hydrographic sextants under the worst possible conditions for 10 years or more, show little or no 

 loss in reflectivity. 



Stellite mirrors have the advantage of a front reflecting surface, the error from nonparallelism 

 of the two surfaces, of course, being eliminated. The reflecting surface must be polished optically 

 flat throughout and be free from all scratches, cracks, blow holes, pin holes, or other defects. 



Stellite mirrors should be used in all hydrographic sextants used in launches and small boats 

 and in sextants used on board ship for hydrography. Occasionally the higher reflectivity of the 

 best glass mirrors may be advantageous for use in difficult observations. 



4515. Sextant Errors 



The principal errors which arise in angular measurements by sextant may be 

 divided into two general classes: (1) those due to defects of construction or adjustment 

 by the manufacturer, which cannot be remedied by the observer, and (2) those caused 

 by temporary maladjustments which can and should be corrected by the user. 



Errors of the first class result from one or more of the following causes: 



(a) The graduated arc or limb is not a plane, or its graduations or those of the vernier are not 

 accurately cut. 



(6) The pivot of rotation of the index arm is not coincident with the center of curvature of the 

 graduated arc, or not perpendicular to the plane of the instrument. 



(c) The surfaces of the mirrors are not within 2 seconds of parallelism, or the surfaces of the shade 

 glasses are not within 5 seconds of parallelism. 



(d) The line of sight, or the axis of the telescope, is not parallel to the plane of the graduated arc. 



Errors of the second class result from one or more of the following conditions: 



(e) The index mirror is not perpendicular to the plane of the instrument. 

 (/) The horizon mirror is not perpendicular to the plane of the instrument. 



(g) The horizon mirror is not parallel to the index mirror when the index arm is set at the zero 

 of the graduated arc. 



No correction can be made by the user to remedy the faults of inaccurate gradua- 

 tion or eccentricity, (a) and (b) above, although their amounts may be determined by 

 a careful calibration (see 4518), after which an allowance may be made for them. All 

 navigating sextants now acquirexi by the Coast and Geodetic Survey are tested for 

 these two errors at the United States Naval Observatory. Navigating sextants ac- 

 quired in the past may or may not have been so tested. If they have been tested 

 either by the manufacturer or at the Naval Observatory, a table of corrections will be 

 found pasted on the inside of the top of the wooden carrying case. 



Hydrographic sextants are not tested for these errors at the Naval Observatory. 



The prismatic error (c) cannot occur in stellite mirrors. Glass mirrors in which 

 this defect is found should be discarded from use and returned to the Washington 

 Office. The shade glasses on a navigating sextant to be used for astronomic sights 

 should be tested before use. Each shade glass should be tested separately by com- 

 paring an observation made without the shade glass with one made through it. If a 

 combination of shade glasses is likely to be used, each of these combinations should be 

 tested separately. No means of adjustment is provided and, when measurable errors 

 of this nature are discovered, the shade glass in error should be removed from the 

 sextant and returned to the Washington Office with a report of its defect. 



