Optical Rangefinders

[dunmunro]

Accuracy of optical range-finders:

"The Services did not know the criteria of excellence of performance either desired or to be expected from trained personnel Both Services had a criterion of excellence of one Unit of Error, defined as an error of 12 seconds of parallactic angle at the eye. It was found that this level of performance could not be maintained by highly selected and trained test observers against a real target, even when flying accommodatingly planned crossing courses."

New Weapons for Air Warfare, Boyce (edit) p78.

CI Versus stereo:

"Since the Battle of Jutland in World War I, the British and American Services have argued which type of range finder should be used The British had adopted the coincidence type of instrument and the Americans the stereoscopic type. In order to test out the matter under semi-field conditions, the British sent a naval instrument and its crew to Fort Monroe to be matched against an American crew and their instrument. Both crews were specially trained on both types of instrument and each team was to alternate between the instruments during the tests. However, weather conditions delayed the completion of the comparative test, which had to be terminated when repairs were completed on the British ship and the instrument and its crew had to be returned. Hence the comparative test was inconclusive."

New Weapons for Air Warfare, Boyce (edit) p84.

The same volume states that the USN and Army determined that US stereo RFs were more accurate than US CI RFs when used with highly trained personnel, drawn from the 3% of the population with acceptable stereo vision, but the Barr and Stroud CI designs may have been better than than comparable US designs.

[dunmunro]

2nd battle of Sirte:

"...Our four cruisers, Cleopatra, Dido, Euryalus and Penelope,
with the eleven Fleet destroyers, turned at full speed
towards the enemy, while the convoy, supported by the
Carlisle, and six Hunt class destroyers, stood away to the
south. Our bows dipped into the heavy seas. At 2.42,
the enemy cruisers opened fire at long range, and I
sighted the faint flashes of their gunfire. The visibility
was poor due to low cloud accompanied by rain squalls
from the south-east. Wind and sea were increasing.
Though the enemy cruisers opened fire several times, all
their salvoes fell a long way short. The Euryalus rolled
quickly and butted into heavy waves at full speed. The
spray spattered over the bridge and, at times, either my
fellow rangetaker or myself had to climb precariously
out of the rangefinder compartment to clean the range-
finder windows with pieces of muslin. Salt spray caked
so easily on the glass. Astern, the convoy had dis-
appeared into the gathering storm.
At 2.56, the Euryalus opened fire with "A" turret
upon the nearest 8-inch cruiser, which for a few brief
minutes was clearly visible in a gap in the mist. I
obtained several clear range cuts of this cruiser in the
rangefinder eyepiece and I was dismayed to see that the
enemy's turrets were pointed towards Euryalus. The
Euryalus shook as salvoes left the two "A" turret guns
and once again the acrid smell of cordite filled the range-
finder compartment. The range was about 20,000 yards.
It was extremely difficult to hit a target at that range and
several of our spotting shots were not even sighted. In
the meantime, a number of 8-inch shells had dropped
close to Euryalus and we had to take avoiding action.
According to my range cuts, the range appeared to be
closing, and at 18,000 yards we opened fire with all the
main armament. I saw a straddle on the enemy cruiser..."

From Enemy Engaged; a Naval Rating with the Mediterranean Fleet, 1942-44, by Ronald Sired, p23

From the description, it would appear that the 15ft DCT RF was a duplex unit. This engagement was fought in a Force 8 gale, IIRC and the visibility was not so good.

[Bgile]

The Iowa class battleships had coincidence RFs in one turret ... I think it was turret 2, because that type was considered by the USN to be superior in night fighting.

I believe the US, Germany, and Japan all for the most part used stereoscopic rangefinders.

[Karl Heidenreich]

Germany and Japan used the higly superior sterescopic rangefinders, second to none during WWII, that´s right.

[dunmunro]

Germany and Japan used the higly superior sterescopic rangefinders, second to none during WWII, that´s right.

Unfortunately, the question of the superiority of Stereo versus Coincidence (CI) , was never answered satisfactorily during the war as I've pointed out above. The RN studied that question at great length after WW1 using both late war German and UK stereo RFs, and compared them to CI units and they ultimately decided that CI was better. Later the USN decided that stereo was better, but the UK firm Barr and Stroud had the most experience with naval RFs of any company and probably produced a better CI unit than anyone else. In the mid 1930s the RN experimented with stereo AA HF/RFs and decided to stay with CI. The quote that I provided above seems to imply that there was some comparative testing done, but not enough to decide which was better, and this in turn, seems to imply that the differences were not great.

Whether German Zeiss optics were inherently better than RN Barr and Stroud optics is another question. In WW1 they probably were (although the best RF at Jutland was probably the 15ft Barr and Stroud FT-24 on the QE class BBs), but it is not apparent that this was true during WW2.

[mike1880]

We've been round this one before; there are (relatively) primary RN documents from 1941 onwards that exactly state the RN's position on stereo rangefinders in WW2; they're quoted (but not adequately cited) in Friedman's "Naval Firepower". If someone had the time and inclination to go and find them this one could be put to bed for good.

Mike

[Brad Fischer]

The Iowa class battleships had coincidence RFs in one turret ... I think it was turret 2, because that type was considered by the USN to be superior in night fighting.

I believe the US, Germany, and Japan all for the most part used stereoscopic rangefinders.

Actually it’s Turret 1. All the USN’s fast battleship’s mounted a coincidence RF in turret 1; The Mark 47 in the North Carolina and South Dakota classes and the Mark 53 in the Iowa class. The USN assessed the accuracy to be identical between coincidence and stereo units for a given baselength.

Brad Fischer

[Bgile]

I stand corrected. :)

[dunmunro]

We've been round this one before; there are (relatively) primary RN documents from 1941 onwards that exactly state the RN's position on stereo rangefinders in WW2; they're quoted (but not adequately cited) in Friedman's "Naval Firepower". If someone had the time and inclination to go and find them this one could be put to bed for good.

Mike

Friedman references the 1943 version of Progress in Naval Gunnery to state that the RN agreed that Stereo was better in low light, (of course the corollary to this is that they might be worse with good lighting) and, in 1943 the RN adopted a policy of having a mix of stereo and CI units, although this was unlikely to have been carried out during the war. RN policy continued to favour CI on units with only a single RF, such as a DD, according to Friedman and by definition these were also AA RF/HFs, he also states that CI was retained for "close range" units. Friedman states that stereo was "clearly superior" for AA as aircraft "might present no feature on which a cut could be taken" but fails to note that RN CI RF/HFs used a strip field unlike RFs designed exclusively for LA ranging, and thus could range on indistinct objects. I've pointed out that the RN did compare stereo and CI for HA use before the war and did not change their policy in this regard for DDs all of which from the Tribal Class onward, had dual purpose RF/HFs.

I posted the info regarding the unresolved RN and USN trial of Stereo versus CI because it illustrates that this question was never really answered definitively during the war.

[Lutscha]

I wonder on what the often mentioned superiority of German and Japanese rangefinders is based on. Having read the navtec report on the Japanese ones some time ago, I did not find such assumption. I know it`s reported very often, but why?

I don`t dispute it but I always read it as a fact without any references.

[lwd]

The Japanese ones at least on the Yamato class had a longer baseline. German and Japanese optics were generally better than others but I'm not sure by how much.

[Byron Angel]

Go here -

http://www.admirals.org.uk/records/adm/ ... 86-259.pdf

- and scroll down to the section relating to the Fort Cumberland Rangefinder Trials. This post-WW1 RN test gives a good overview of the differences and advantages of stereoscopic versus coincidence range-finders. It is important to read and understand the full report itself, because the summary and conclusion are IMO somewhat at odds with the actual results developed. Also note that the 3m Zeiss unit was being tested against some late-war and post-war units of much greater base-length. It's my opinion that the particular capabilities of the stereoscopic range-finder, especially under hazy and low-light visibility conditions made it a better choice for use in combat.

The test also fails to account for the influence of the German "Mittlungs-Apparat" system in their range-finding system.


Byron

[dunmunro]

Go here -

http://www.admirals.org.uk/records/adm/ ... 86-259.pdf

- and scroll down to the section relating to the Fort Cumberland Rangefinder Trials. This post-WW1 RN test gives a good overview of the differences and advantages of stereoscopic versus coincidence range-finders. It is important to read and understand the full report itself, because the summary and conclusion are IMO somewhat at odds with the actual results developed. Also note that the 3m Zeiss unit was being tested against some late-war and post-war units of much greater base-length. It's my opinion that the particular capabilities of the stereoscopic range-finder, especially under hazy and low-light visibility conditions made it a better choice for use in combat.

The test also fails to account for the influence of the German "Mittlungs-Apparat" system in their range-finding system.


Byron

The report tries to separate out the mechanical advantages of the various RFs from their optical configuration, so while the German RFs seemed to perform better much of that performance was due to a superior mechanical design.

[dunmunro]

Earlier I had reported the following info:

CI Versus stereo:

"Since the Battle of Jutland in World War I, the British and American Services have argued which type of range finder should be used The British had adopted the coincidence type of instrument and the Americans the stereoscopic type. In order to test out the matter under semi-field conditions, the British sent a naval instrument and its crew to Fort Monroe to be matched against an American crew and their instrument. Both crews were specially trained on both types of instrument and each team was to alternate between the instruments during the tests. However, weather conditions delayed the completion of the comparative test, which had to be terminated when repairs were completed on the British ship and the instrument and its crew had to be returned. Hence the comparative test was inconclusive."
New Weapons for Air Warfare, Boyce (edit) p78.


I'm happy to report that I found a detailed account of the above trials and the results were not really inconclusive at all:

"COINCIDENCE AND STEREOSCOPIC RANGE FINDERS

The first of these reports is concerned with the comparative test of coincidence and stereoscopic range finders. (353) In these tests the America stereoscopic Height Finder Ml was operated against the British coincidence type Range Finders FQ 25 and UB 7, in ranging on fixed ground targets, moving naval targets and moving aerial targets. The coincidence and stereoscopic methods utilize the same basic principles of geometrical optics for the determination of the distance to a target. The two methods differ radically, however, in the nature of the criterion presented for human judgement. These British instruments were of the split field coincident type. American crews were being trained at Fort Monroe to operate the coincidence instruments but this plan was dropped when six British seamen, who were experienced range takers, were made available for the tests. Until recently the British Services had tended strongly to the coincidence type of instrument while the American Services had adopted the stereoscopic principle for long-base instruments a least. The decisions of both the British and American Services apparently grow out of different interpretations of the experience of the Battle of Jutland in World War I and are of no concern in this place

Tests were run in November and December 1941 using the British seamen on the British instrument and experienced American observers on the Standard M1. Bad weather conditions and various experimental difficulties and mishaps made it impossible to obtain a really satisfactory quantity of data before the tests hall to be terminated. Fixed target reading were made on targets from 2,700 to 14,500 yards. Only five aerial courses could be recorded and these were all level flight courses, at altitudes of 3,000 to 4000 yards and slant range between 4,000 and 12,000 yards Continuous contact was used. Nine courses were obtained on slow moving naval targets at ranges from 4,000 to 12,000 yards. In these latter course continuous and broken contact were used at different times.

It was found, throughout the tests, that the performances of the various instruments were more nearly alike when measured in external units (Reciprocal range) than when measured in terms of error at the observer's eye, in spite of marked differences in physical dimensions of the instruments. The American MI has a base length of 4.5 yards and used 12 power; FQ 25 with a 6 yard base used 28 power and UB 7, a portable instrument, has 25 power and 3-yard base. The coincidence instruments did not use internal adjusters but were calibrated on targets of known range. In other words. the net performance of the different instruments were essentially comparable although the instruments exhibited varying degrees of efficiency in performance relative to the size. On aerial courses precision errors of the four instruments were about alike when measured in reciprocal-range units. In UOE, the FQ 25 had comparatively poor precision, while the UB 7, for three of the five aerial courses, had very small precision errors. The number of aerial courses was too small to yield much information about consistency of observations from one course to the next.

For the naval target courses, one American instrument was not operating. Precision errors of the other three instruments there similar to those on aerial height courses. In reciprocal range units the three instruments had comparable precision. In UOE the FQ 25 was worse and the UB 7 was better than the American M1. Consistency error of the UB 7 was smaller than that of the M1, even when measured in reciprocal range units, while the FQ 25 was similar in consistency to the Ml, again in reciprocals units.

On ground targets the same general situation holds. Consistency errors of the four instruments over the 9-day period were the same when measured in reciprocal-range units. Again the UB 7 was better than the stereoscopic instruments in UOE and the FQ 25 was worse. Consistency over the 9 days was not perceptibly worse than daily consistency for any of the instruments. In other words, the readings over the 9 days did not scatter in total more than did readings for a typical day.

An analysis or these results leads to the following conclusions. (1) Performance of the coincidence and stereoscopic instruments was about the same when range errors were measured in yards (2) The UB 7 however, with a virtual base length smaller than that of the American stereoscopic instruments was more efficient than the stereoscopic height finders in terms of performance for its size, while the large coincidence instrument, the FQ 25. was less efficient in this sense. This situation held for all types of targets— fixed ground, naval, and aerial. (3) The UB 7 is somewhat better than the American instrument in consistency on naval targets, even when measured in external units.

This report is attached. as supporting data, to a Report to the Services issued by the Fire Control Division of NDRC (20) This points out that the tests indicate no important difference in the precision obtainable from the two types of instrument— coincidence and stereoscopic They do indicate, however, that the difference in performance between large and small instruments is by no means as great as would be anticipated from simple geometrical Optics The report concludes with the belief that stereoscopic and coincidence acuities are about equal. Under favourable conditions existing instruments of the two types perform about equally well, and the choice between them for any given purpose must be based on matters of convenience related to the particular conditions under which they are to be used ."

Rangefinders and Tracking
Summary Technical report of NDRC, Division 7 (Fire Control), volume 2, p17-18

The trial's conclusion was that there was no difference between the two type of RF...this is far from being inconclusive as reported by Boyce!

In fact, if there is no substantive difference, then the coincidence type is superior because it is far easier to operate and does not require the extensive training and stereovision selection required for the stereoscopic units. This isn't the first time that I've found NDRC (National Defence Research Committee) reports that seem to contradict the USN BuOrd conclusions.

[Byron Angel]

..... Very interesting material, Dunmunro. Thanks for posting.

Question: Did the document discuss "utility" factors separating stereoscopic and cincidence type range-finders? - for example, the ability of stereoscopic systems to range upon indistinct images which lacked a clear vertical element to be "cut" by coincidence type units.


Byron