Author Topic: Sight picture (Read 1489 times)

Saxman · « **on:** February 11, 2007, 10:15:21 PM »

I had a question about the visible gun sights.

On some aircraft the gun sight that appears is at times almost so tiny as to be practically unusable. Some sights, as a result, almost can't be used on certain aircraft at all. While this wouldn't necessarily be a problem, it DOES get in the way in some cases of using historical gun sights with the craft that used them (which some people like to do).

An example that I come across most often (for obvious reasons if you check my stats) is the F4Us (specifically, the 1A and later).

Trying to use the full Mk. 8 sight image (with the extended main crosshair) gives a gun sight that is too small to be practical. However, even using the version with the crosshair shortened to fit the outer concentric ring is still bordering on too small: sometimes to the point of cluttering the sight picture unless using a high transparency setting.

The reason I question the size of the sight picture is because of the size of the sight reflector in comparison:

On the left is the Mk.8 with the reduced crosshair. Useable, but still bordering on too small, often to the point where even with transparency reduced to JUST visible a target gets completely lost in the sight The right is the full Mk.8 sight image, which even at this level of transparency generates WAY too much clutter to really be practical.

Now, note the size of the reflector. Even accounting for the fact that the projected sight wouldn't occupy the ENTIRETY of the lense, shouldn't the projected image be LARGER? Especially considering the design of the F4U's sight glass? As it is, it almost looks like the sight is being projected on a point BEYOND the glass.

Modified image. I'm not ENTIRELY certain of the mechanics of a reflector sight, other than it that being light shining through the cutouts of a solid plate/insert, and then projected on angled glass. However considering the distance between the projected image and the actual sight glass, wouldn't the sight image be magnified somewhat (which is why it appears LARGER than the actual reflector in this image)?

This isn't limited to just the Corsairs. While it's one *I* notice most because it's my preferred ride, I've seen it in other aircraft, such as to a limited extent on the FM-2 and F6F (which are far less troubled by it because the gunsight is placed so close to the head position).

Is it possible to review the gun sights and verify the size of the sight pictures in the game?

Charge · « **Reply #1 on:** February 12, 2007, 07:08:01 AM »

Keep in mind that the sight is not projected on the oblique glass panel but it enables you to see the reflected image of the sight pattern magnified by the lense. Thus the sight pattern can not be bigger than the lense of the sight and it is usually quite a bit smaller than that.

-C+

Saxman · « **Reply #2 on:** February 12, 2007, 10:53:15 AM »

Three aircraft that all used the Mk.8 gunsight, from left to right; F6F-5, FM-2, F4U-1A, set to use the same full Mk.8 image.

Compare the distance from the edge of the lens in the F6F/FM-2 to the edge of the actual sight, to the same distance in the F4U.

In the FM-2, the width of the sight is roughly 70% the width of the lens (estimated, as it's more difficult to determine here, as the glass is just textured on). In the F4U, the sight is 55% the width of the lens. This is a pretty SIGNIFICANT difference.

F4U with the sight picture corrected to the same proportions as in the FM-2.

While still small, there's a pretty notable difference between the size of the gun sight in the first and second picture, the latter of which is somewhat less cluttered. This is also a very rough estimate as I'm trying to use my graphics program for scaling, so it could be off by a couple or ten pixels (I may still be a little small, since the edge of the lens I would need to measure from is obscured by the modelling).

It could also be that the sight image on the F6F/FM-2 is too LARGE. However either way, if three aircraft have the same sight, why would one of them have such a proportionately smaller reflected image?

Mace2004 · « **Reply #3 on:** February 12, 2007, 11:04:10 AM »

I've noticed a significant variation in the size of various gunsight reticles also. I'd guess that this is a result of people replicating actual reticles without keeping them the proper proportion but don't know for sure if HT accurately models them in the default sights, they probably do.

The F-4U sight you're talking about should be pretty easy to check because, unlike many others, it's labeled with both a 100 and 50 mil circle. It also has a 2mil pipper (the center dot) and 20 mil graduations between the circles. The only thing we need to come up with is a good target to measure and appropriate mil size (50mils) at a good range.

"Mil" is short for milliradian which is an angular measurement. Rule of thumb is that 1 mil subtends 1yd at 1,000yds. 50 mils at 200yds would then equal 30ft so find an airplane with a 30ft wingspan and fly 200yds behind it. If the reticle is correctly sized then each wingtip should touch the 50mil circle. At 100yds the wingtips should touch the 100mil circle.

Edit: just a quick note but the size of the reticle relative to the objective lens is irrelevant. The systems are designed to give an ability to accurately measure distance if the size is known or size if the distance in known.

Mace2004 · « **Reply #4 on:** February 12, 2007, 11:11:58 AM »

Quote

Originally posted by Saxman
However either way, if three aircraft have the same sight, why would one of them have such a proportionately smaller reflected image?

They should all present identically sized reticles assuming your head position is correct in each image. If they're different something's wrong.

Bronk · « **Reply #5 on:** February 12, 2007, 11:17:16 AM »

OT

Any one have data on how to properly use this type of sight.

I use the brit big circle with a pipper for everything.
While I have gotten better at useing this sight for all purposes.

I'd like to know what all those lines are for and how to use them on the above gun sight.

Bronk

Krusty · « **Reply #6 on:** February 12, 2007, 11:25:32 AM »

Also, I think head position changes the size of it. Same gunsight from plane to plane could be different sizes because of default head distance and/or position. Try moving in.

Saxman · « **Reply #7 on:** February 12, 2007, 11:38:26 AM »

All three head positions are the default forward view, which should be roughly centered on the aircraft's actual seat position (and if it's not, I think it's something that should be adjusted). It should also be noted that the default view in the F4U has always been set further from the sight than for the F6F (even though from images I've seen, it doesn't appear that the sight in the F4U is any further from the pilot than it is in the F6F).

Mace2004 · « **Reply #8 on:** February 12, 2007, 11:41:04 AM »

Quote

Originally posted by Bronk
OT

Any one have data on how to properly use this type of sight.

I use the brit big circle with a pipper for everything.
While I have gotten better at useing this sight for all purposes.

I'd like to know what all those lines are for and how to use them on the above gun sight.

Bronk

The reticle is combination sight designed for both Air to Air and Air to Ground work. The larger circles are designed to allow a pilot to more accurately determine range by the relative size of an aircraft's wingspan to the 50 and 100 mil circles. The horizontal, vertical and 45deg lines are reference lines. The horizontal lines makes it easier to determine if you're level with the horizon or a target aircraft's wings. The vertical line would be used to determine your lift vector and, when you've set up an ideal tracking shot is an approximation of the bullet flight path. This isn't very accurate except at short ranges because bullets are also affected by gravity but it's meant just as a reference. Say you're in a left hand turn and pulling lead for a shot, the target needs to be below the center of the pipper (center dot) and to the left of the vertical line, how much to the left depends on the geometry. The 45deg lines are probably helpful in setting up high angle snapshots. Again, as a fixed sight this reticle is merely a reference and assists the pilot's sight picture.

The vertical hash marks are designed to give a vertical reference for accurate air to ground work. Unless you're in a 90deg dive the pipper (center dot) will not provide an accurate reference because of the ballistics of the bombs/rockets. During pre-flight, a pilot would determine how he intends to deliver his ordinance (dive angle, speed, release altitude and type of ordinance) and determine the appropriate mils to use as his aiming point. The shallower the dive, slower the speed and farther the release point all affects the bomb impact point. Say a guy has gone to the books and determined that with a 45deg dive, 400mph dive and 3,000ft release point the bomb will impact 80mils below the center of the reticle. Just count down 4 of the hash marks (each is 20mils) from the pipper and that's your aimpoint so put that on the target and, assuming the rest of your delivery profile is accurate release at 3,000ft. In practice, you become familiar enough with difference deliveries that you can adjust on the fly. Also, I'm not sure about this particular sight but many have an adjustible pipper. Instead of counting down the number of hash marks you'd set the pipper at minus 80mils and then the pipper is your aimpoint.

Saxman · « **Reply #9 on:** February 12, 2007, 12:22:05 PM »

Ok, did some testing on the F4U-1A.

For the test, I used the FW-190D, with a wingspan of 34ft, so probably about as close on this test I'll get. Also it's one of the offline drones, so I'd rather use that than try and do this in a furball.

Anyway, first I tried to keep the Dora in the sights. The screen was taken from the default head position. Zoom was used to make it easier to line up on the target.

Here, the range counter indicates the 190 is 200yds out. According to Mace, at 200yds distance the 190's wingtips should be roughly touching the edge of the inner ring. However here, the wings are touching the OUTER circle, Roughly double the size it SHOULD be.

In this image, the counter shows the Dora at 400yds. The wingtips are touching the inner circle, and once again according to Mace's data this is approximately TWICE the distance it should be.

Realizing the range counter doesn't give the exact range, I next used the target to try and get a more accurate picture.

In this image, I set the target to a distance of 10yds, about as close as I could usefully make it, then took a screen from the exterior to get a comparison. I'm slightly zoomed in on the aircraft here to get ground clutter out of the way (figured I'd do this on the ground to make it easier to line up on the center of the target).

Roughly, the Dora's wingspan is equivalent to the diameter of the second ring.

As best I could, I then lined the F4U's sight on the target set at 200yds distance. Even though I don't have a PERFECT measure of the Dora's wingspan in relation to the target, this image still falls almost EXACTLY on the test using the range counter and Dora illustrated above.

The gun sight in the F4U-1A, 1C, 1D and 4 (since they all use the same reflector setup) appears just around HALF the size it should be (accounting for any distortion due to the target being set 10yds ahead of the Dora, and that the Dora has an additional 2ft on either wingtip).

Here is the F4U-1A's sight roughly adjusted, based on the data from this test.

Mace2004 · « **Reply #10 on:** February 12, 2007, 04:35:09 PM »

Nice test saxman. I did some checking myself and the reticle does change in size according to the head position but I'm curious if this happened in your typical WWII fighter?

Optics can be quite interesting, for instance the behavior if the sights in AH replicates at least most of the typical reflector gunsights I've seen in that you could move your head around and as long as you're looking through the combining glass (the angled glass that reflects the reticle) the reticle is properly aligned. This, I believe, is a result of the gunsight being focused near infinity. I can't recall if the size of the reticle changed when leaning forward though, as that's not typically what you do but it sure seems to me it didn't.

A good example to illustrate the ability to move about some with a reflector sight compare the P40B where there is only one position that the iron sight is accurate while the reflector sight allows some movement. This is part of the reason for the reflector system as opposed to just a glass with an etched symbol on it.

Does anyone know more about the optical properties of the WWII sights? Not talking about Gyroscopic or LCOS, just WWII reflector sights in general.

Saxman · « **Reply #11 on:** February 12, 2007, 04:43:42 PM »

Too late to edit the last post, but I redid the Dora superimposed over the target at a range of 1yd to make it a more exact comparison (the new image is visible in my last post). This now shows that, using the full Mk.8 sight, the F4U-1A and later have a sight image almost RIGHT at 50% the size it should be. The question I suppose that Mace puts forward, is it an error in the sight ITSELF, or in the default head position? To test this, I went back in and moved the head position forward to approximately where the it would have to be to make the sight match Mace's data.

The below image is at normal zoom with adjusted head position. The target is set for 200yds and the image was resized for the benefit of viewers without 1600x1200 resolution monitors.

Given how far forward the head position would need to be moved for an accurately-sized sight, I'm inclined to believe the error is in the sight itself (trying to fly with the head in this position proved EXTREMELY difficult).

As a point of comparison, this is the same version of the Mk.8 sight as it appears on the FM-2, (default head position, zoomed in) with the Dora superimposed over the target (actual range 200yds). Although still slightly undersized, it much more closely matches the historical sight. The sight on the F6F is roughly the same.

F4UDOA · « **Reply #12 on:** February 12, 2007, 08:09:42 PM »

Skillfully done Saxman

Saxman · « **Reply #13 on:** February 12, 2007, 08:19:39 PM »

Woo! I identified an error!

So how 'bout it HTC, can we get the F4U's sight image fixed? It seems this would be a relatively simple fix by enlarging the area occupied by the sight image by ~double.

Charge · « **Reply #14 on:** February 13, 2007, 03:13:15 AM »

AFAIK and what I have seen the reticle seems to be float over the target and the distance between your eyes and sight does not really affect the size of the reticle. So it should not grow bigger as it does now when you move your head closer to sight as if it was projected on the oblique glass panel.

To get it to work correctly in optical sense HTC would need to move the reticle picture to some distance in front of cockpit (somewhere around engine cowling) and only visible through sight glass. That would make the reticle immune to distance changes when you move you viewpoint inside the cockpit and it would look like it floats in the air just over the target as it does look like IRL. When I first time looked though a reflector sight (Revi) I didn't see the reticle at all because I was looking at the glass! When I realized that I would need to focus my eyes in the distance where the target would be the reticle suddenly materialized over the target. It actually looks pretty cool.

You can actually build one by yourself and try it.

You need two pieces of plexi glass or regular glass and short piece of cardboard pipe and a magnifying glass or a camera objective. Paint the other piece of glass very thinly black and use a pin to scrape a reticle picture in the paint. The size of the needed reticle depends of the optical qualities of the used magnifier so you need to experiment to get it right. Cut the cardboard pipe to proper length and attach the painted glass plate with a reticle to the bottom of it and the lenseto the top. Attach the other piece of glass obliguely over the lense. When you apply light to the bottom of the constrution you should be able to see the reticle in the distance. You may need to adjust the distaces of pieces a bit to get it right.

This exaple was build from camera objective:

http://www.virtualpilots.fi/img/mm03/day2/slides/112_1279.html
http://www.virtualpilots.fi/img/mm03/day2/index.html

Here is BlauK wondering where the heck is the reticle at:

http://www.virtualpilots.fi/img/mm03/day2/slides/112_1259.html

If you build your own sight you will also realize how well your head needs to be aligned to see the reticle at all! That means that shooting while maneuvering would mostly happen without a proper sight solution and with tracer guidance only.

-C+