Gun sight image size analysis

[RTHolmes]

really, I'm surprised I havent managed to dig this stuff out yet

[bustr]

The 100mph ring is a hold over term from the late 1930's biplane days. Because WW2 Barr & Stroud rings were adjustable to represent a wingspan of a known aricraft at a known distance. And air combat speeds were much faster, it is moot and an old common term to describe the circle on British gunsights. We use a fixed MkII ring. What wingspan at 300 or 200 yards do you want it to represent when you create your British M2 512x512 bitmap sight?

The radius of the graticule ring gave the deflection allowance for hitting a target crossing at 161 km/h (100 mph).

[Baumer]

Actually bustr as I understand it from Wolfs' book that describes the Mk.II operation, only the horizontal bars move in and out for the range selection the circle doesn't change. As for the bullet drop, the whole reticule image (bars and circle) would move up or down. Since AH accounts for the up down motion when you set the convergence this doesn't need to get modeled.

I have not be able to ascertain the ring dimension either, but if it's a 100 mph ring then it can be calculated. I'll work on that, and post up my calculations later this evening.

[Saxman]

What wingspan at 300 or 200 yards do you want it to represent when you create your British M2 512x512 bitmap sight?

Little hint on this one: if you want it to be a 30ft wingspan at 200yds set it to 50mil apart.

[RTHolmes]

only the horizontal bars move in and out for the range selection the circle doesn't change.

thats my understanding of it. couldnt find any reference to the range for the "100mph circle" either (I assume thats a 90deg snapshot at 100mph)

[bustr]

I found this at least giving the data for a 100mph ring at 200 yards. So at 200 yards the radius of the ring is 11.5 meters. You had an earlier formula which I think this number will compute the ring diameter at least to 200 yards in mil?

Typical parameters for a fixed gun were: own speed - 161 km/hr (100 mph), target crossing speed - 161 km/hr (100 mph 90 degrees across the pilot's line of sight), range 183 m (200 yds). In this situation the bullet would take 0.254 sec to reach the target, which would have travelled 11.5 m (37.7 ft).

From: http://forum.axishistory.com/viewtopic.php?f=114&t=17649

[RTHolmes]

yeah, I saw that, I suppose it may be the same although that is for 1916 ring and bead type sights. if it is the same, I make it about 126mil diameter. big if though.

[Baumer]

I suspect it's supposed to be rather larger as well (120+ Mil) but I haven't found any details to confirm it. Another point of reference according to Wolfs' book the Raf Mk.I and Mk.II should have the same ring size. So if anyone comes across details about the RAF Mk.I it would be helpful to know.

Having looked at many of the historical gun sight packs it seems pretty common that we made them to small, on average.

[bustr]

Middle of the page here:

http://www.spitfirespares.com/SpitfireSpares.com/Pages/gunsites.html

Some one has lite the bulb. If you click on the picture it becomes larger and you will notice the inner ends of the range bar. Has daigonal pointed ends like it has been swiped top left down to bottm right. Probably related to how the space is increased or shortend by the wingspan dial.

If you accept the picture as accurate, then its a matter of playing with it zoomed in photoshop. Zoom a screen capture from our spit16 so the two projector lenses are the same size. Then go from the dot and work diameters outward. that should give you the 100mph ring for the Barr & Stroud GM series sights.

[bustr]

The ring comes out aprox 150 pixels.

The reticle in the picture's proportions are the cross is a 10cm x 10cm cross. The inside diameter of the ring edge of light to edge of light is 5cm. Center of the lit ring side to side is 5.5cm. The glass on HTC's MkII reflector sight looks a different proportion than the ones from the web sight refrenced here.

[RTHolmes]

I like your thinking sherlock

I'm going to use that as a last resort, was thinking the Mk II youtube vid would be ideal because iirc its held at arms length so we know roughly the range to the sight for the real one. cockpit measurements give us the rough AH range so a combination of both tells us how big it should look in AH. the diameter in mils would be accurate and sooo much easier tho 

[Baumer]

Bustr that's a great web site, but I don't think it that's how the math works out for a Mk.II. One interesting point, on the second page is that the horizontal and vertical lines should extend all the way to the optics edge.

Just wish I had the money laying around to buy that stuff!

[RTHolmes]

yup thats what Im trying on my sights, just want to make the ring the accurate size

[bustr]

Based on the photograph, HiTech's reflector glass is not tall enough. By the way if you gauge the ring using 2pixels = a mil. The ring is 75mil and offline that's a Bf109 wingspan at 200 yards using a 512x512 mask and .mil file at 256. I set my space on the bar for that wingspan at 400 yards. I suppose I could also lower the whole reticule a few mil to adjust for bullet drop at 400 yards to be accurate....I used to make my old gunsights with a centred dot and one just below as a compensator....

Well I can extend the lines but I used the proportions from the picture. The distance one line extends out from the ring is a radius. I would need to enlarge my ring to about 80mil to proportionately lengthen the lines to fit completely across HiTech's reflector plate. But the vertical lines would over run the top and bottom edges due to the plate not being the same proportions as the one in the picure.

Updated: by bustr 12:10 pm pst 7-28-2010

I just expanded the Youtube video of the MkII to life size. At least his finger width matched mine.

Proportions:

A.) Reflector Glass width across center is 12cm. 

B.) Diameter of ring 5cm.

C.) Arms of the cross are 1.5 cm each.

I'm thinking there is a bit of variation in these rings and arms by who produced the hardware. And they are an aproximation of a 100mph ring at 200yards.

[bustr]

yup thats what Im trying on my sights, just want to make the ring the accurate size

OkyDokey,

Just finished a MkII reticle with an 80 mil/160 pixel ring. If you spawn, then zoom it in so that the glass is 12cm across and the ring is 5cm. I added so much blur and illumination to it, it now looks exactly and measures like the YouTube video you were refrencing.

512 x 512 bitmap black background.
Ring and line color Orange.
Ring 160 pixels.
Cross - Each arm is 100 pixels from ring edge out.
Horizontal bar center space - 76 pixels.
Lines and circles 2 pixels wide rough drawing.
Gussian Blur and Illumination effect until lines are 12 pixels wide high illumination orange.
Include a mil file with 256 in it.

I'm starting to see these blasted things in my sleep Holmesy. You want I should lift yur garage and rotate it 12 degrees to the left now..........