Quote Stanley’s 'A' Catalogue
FEATURES OF SPECIAL INTEREST
Internal Focussing Telescope
INTERNAL FOCUSSING TELESCOPES are common to all our theodolites and levels,
whereby focussing is effected by means of an auxiliary lens within the telescope. The
advantages of this telescope are :—-
1. The overall length of the telescope remains constant and the balance of the instru
ment is preserved.
2. The telescope is dust and damp proof.
3. Collimation errors are practically eliminated.
4. The telescope is anallatic, i.e., requires no constant for stadia work “ distance
measurement.”
5. Compared with the external focussing telescope, the length of the telescope is
reduced without loss of magnification.
THE RESOLVING POWER OF A TELESCOPE.
The distance at which the -01 ft. graduations on a Levelling Staff can just be detected
is generally considered to be a measure of the resolving power of the telescope. The
greater the distance, the better the resolution.
In so far as this criterion is affected by inherent optical characteristics, our telescopes
give the maximum possible resolving distance due to the care taken both in optical design
and in manufacture and assembly of the lenses.
High magnification is of no benefit if obtained at the expense of image brightness.
Allowance has to be made for good luminosity in unfavourable light conditions. The
following table gives the value for our surveying instruments under ideal conditions and
with an average observer :—
Instrument Cat. No.
Resolving Distance
A. Optical Theodolite
950 feet
A. 21, 25, 26
1,100 feet
A. 68
800 feet
A. 352, 368, 510, 520, 530
1,000 feet
A. 500
750 feet
It should be pointed out that these are the distances at which the black and white
divisions just merge into a uniform grey pattern. The distances at which the staff
divisions may be read comfortably, may be taken as just less than half the tabulated values.