Question about collimated display systems.

[Matt Olieman]

Mike (mikesblack), the first question that comes to mind, are we capable to form a cast for the mylar mirror? The answer is yes. Secondly can we vacuum form the mylar to the cast? The answer again is yes. So than where lies the problem, it's the imperfections in the mirror or the mold. First the mold has to be perfectly curved to act as lens and align with the curvature of the rear projection screen. Secondly foremost difficult part is the imperfections in the curve, a slightly flattened portion of the mold (mirror) and the whole mirror is out of whack.

Obviously, the mold can not be hand made, there is a tremendous amount of precision involved to make this work, that's the major short fall of a self made collimating mirror.

There is a video floating somewhere, at I saw within the past several years, where they showed to vacuum fit of the mylar to the form (mold). I was surprised how quickly it took place. The tiny holes in the mold has to be 10's of thousands and very small as not to have the slightest imperfection. The mylar is actually glued to the mold.

The development cost of a collimating mirror would be tremendous compared to buying a ready made mirror and screen. Then comes the high resolution projectors and then the special lenses, where talking about thousands of dollars for a single lens.

Mike Powell has presented amazing technical information regarding design of the collimating mirror for a cockpit, it certainly gives you a jump start. Even if you had the exact specifications, the above mentioned obstacles would still be there.

I'm not saying it can't be done, it's a matter of how much money you want to spend. I hope someone does come up with a solution to make an affordable design for a cockpit collimating mirror.

I hope I made some sense out of what I wrote, sometimes a just babble......

Matt Olieman

[mikesblack]

Matt,

Not at all. I appreciate your reply and If you don't mind, I have a few questions for you.

You mentioned many things. I'd love to find this video and if you know or find a link, I would be most grateful.

You mentioned here and I believe in another post on this topic that the Mylar is glued to the mold. Are you saying that the suction acts as a one-time process for which it enables the Mylar to be spread uniformly and finally joined permanently by glue? I was thinking that the vacuum was actually part of the set up that was used to keep the Mylar held to the mold and used continuously.

Also, do you have any idea what sort of result one would find using conventional projectors, assuming a good mirror, projection screen set up?

I have no idea the scope and scale of this project, so forgive my naiveté or simplistic thinking. If nothing else, I am always happy to learn new things.

Mike

[Neil Hewitt]

If you wanted to go for a simpler solution, it might be within a hobbyist's power to create a WAC (Wide-Angle Collimated) window display unit. Older sims used them, before large mirrors were practical, and plenty of sims still have them in today. You can still buy them new, too. Basically they use the same parabolic mirror as the cross-cockpit display but the mirror is sized for the window it's put in front of - the radius of the mirror being just a little larger than the width of the window. Put a half-silvered mirror directly in front of you at 45 degrees, then up top, put a CRT (I suppose you could use a very bright TFT) looking directly down onto the half-silvered mirror. The picture on the monitor is reflected onto the spherical mirror and then back at you, duly collimated. Get the focal length right, position the mirror and yourself in the right place, and bingo.

There are surprisingly few actual pictures of this in the internet, which is annoying... Matt Ford has a good section on his site about his Level D display system salvaged from an old Boeing sim which uses WACs:

http://web.me.com/mattford1/Site/Lev...al_System.html

The key is the mirror. You can get concave spherical acrylic mirrors from science and educational supply outfits, these are sometimes used by people creating solar stoves. I found one with a diameter of 600mm - can't find the link now, unfortunately - and it's possible they may go bigger. At 800mm - 1000mm they'd become useful for front-view windows on a smaller cockpit. Many are not a single mirror surface but made from strips of reflective material with joins, which is OK for a solar stove but not for sim use.

For a fighter pit or any kind of pit with small windows - maybe a GA pit? - this might be a goer. I doubt you'd be able to buy mirrors big enough for a heavy jet simpit without going to custom fabrication. You'd also have a lot of unneeded top and bottom to the mirror which you couldn't just cut off because the mirror would then deform. Probably have to cover the unused bits in black.

It's interesting that it seems very easy to get hold of reasonably large convex spherical mirrors - lots of companies sell big ones you put up in a corner of a warehouse so you can see all around you - but concave mirrors, not so much. Plus of course not just any old concave mirror will do.

I suppose at smaller sizes - up to a meter in diameter - you might get your local friendly plastics company to make you a few as a job lot.

But I think the full-on precision machined mirror you need for a cross-cockpit display is just beyond the capabilities of anyone without a proper manufacturing plant at his or her disposal. I suppose Matt Sheils might be able to manage it

[castle]

I went back to take another look at the "how it's made" video. Somethings to note.

The shape of the mirror section seems to indicate that the top edge starts at a latitude below the equator and and looks to be much closer to the "south pole" wherein the lower circumfernece or line of latitude has a much smaller radius than the upper latitude. Eyeballing it might be something like lat 20 degrees south to say around 50 degrees south.

The vertical curve along the top edge appears to have a rather large radius and is greater than the lower radius. If the person in the scene is 6 feet tall, my guess is that the radius of the sphere from which the mirror section is mapped is close to 15feet or more. Keep in mind the radius of the Artic Circle is much less than the radius of the Equator. Tough to tell as there is probably some distortion with the camera and depth of the image due to the lens. Note how the yellow beams of the holding frame are curved. I'll bet lunch that those beams are, in fact, straight.

The other item to note is the closeness of the projection screen to the mirror as it is visible when the scene shows the mirror moving into position.. Clearly, the projection screen is inside the focus of the mirror which produces the larger virtual image for the OTW. Also the view from inside the cockpit shows the mirror with wrinkles, so that would argue for an active vacuum system. Or did Matt really mean to say "glue" as in the sticky stuff.

Mike, you might want to rethink your calculations as to size, location, and perspective. The projection screen clearly overhangs the cockpit shell and can only be a few feet from the mirror.

Matt is right, It is a tough problem but I always like a challenge

JW

[Matt Olieman]

Concave spherical mirror. Thanks Neilh, I couldn't think of that term.

JW I'm with you, it's a tough problem and a fantastic challenge...... anybody..... GO FOR IT!!!!

Mike (mikesblack), The video..... I'll do my best to find it. Regarding the Mylar being glued. The vacuum holds it in place for several hours as the glue cures (yes, the sticky stuff ). They did not mention what the glue was, but I amagine something that would not cause imperfections.

Regarding the individual Concave spherical mirror, yes, Matt Ford uses it on his cockpit, you can find pics of it in the "March 2010" Builder of the month. Go HERE. Project Magenta's Flight sims use the individual concave spherical mirrors, matter of fact at one time they were selling them for around $4K each (NEW). I considered it. If you ever flew a sim with these type of mirrors it would not take long to be a bit frustrated, particularly if you want to be wrapped with scenery. If you're in the CPT seat, you can't see out of the F/O window, all though you can see out of the side windows. If you were able to see the F/O window, you would see the identical image as the CPT side (they are the same).

There is a GA Sim, I think a C172 that uses the individual Concave spherical mirror. I believe there are some posts here and pics of the sims progress, it was a school project in Europe (I think).

Regarding how close the mirror is to the overhead screen. The first time I walked between the cockpit and the mirror, I was amazed how close they were. First of all, from inside the cockpit, it gives you the impression the mirror has quite a bit of distance.... what an illusion!!! I was constantly reminded.... "DON'T TOUCH THE MIRROR!!!!"

This is an amazing topic and a fantastic challenge, I know...... I believe in our hobby we've conquered every aspect of building a Flight Sim Cockpit, except the concave spherical mirror, 150 to 180 degrees.

There was a time where builders believed, motion for the sim was not practical to make and was way beyond our budget. Someone in New Zeeland came up with an idea using Automobile axels, starter motors and other parts and made a full motion sim for around $3,000. We've developed further than that, and all sorts of contraptions are available in affordable form.

Our hobby still is not a cheap hobby, but compared to 8 to 9 years ago, you can build a sim for much less, AND it's plug and play. Oh well..... I can go on and on.... LOL.

I see hope for the 150 to 180 degree collimating mirror for the home build cockpit....

Matt Olieman

[geneb]

geneb,
My pit is 11.5 ft across at the floor and at the back. I have roughly 2 ft from side of pit to my wall, perhaps a little less on the other side. I am considering that my radius origin is located at the throttles in the center of the cockpit. The widest part of the circle and where I am measuring R is at the top of the Mylar screen, where the angle relative to the floor is 90 degrees.

That makes sense. I just wanted to make sure you were aware of the size requirements.

I'm considering the following.

Angle of arc from bottom to top of screen. Whatever I can do in order to keep the Mylar material wrinkle free. Width based on the arc based on 30 degrees and r= 6 feet is approx. 3 feet assuming my math is correct. ( 2Pi R) = approx. 36 ft diameter. So At 10 degrees of arc( 36/36) = 1 foot. I need to be sure that when I look out of the cockpit window I have enough viewing surface above and below cockpit window so there isn't an interruption to the image. If I bring these elements into consideration perhaps I might get away with 3 feet viewing surface. I need to experiment with curves of various radii and arc to see what will work.

Keep the mirror as close to the cab as you can without it shadowing the image and you'll be fine. There will be some experimenting for sure though.

g.

[castle]

Mike,

I came up with slightly higher numbers..

If you want a mirror that subtends 45 degrees of arc which seems to be the standard size, ( don't think 30 degrees will provide a sufficient FOV) for mirror with 6' radius

(45.0/57.2957) * 6.0 = 4.712 feet

you might be able to get by with a 5' radius mirror in which case you will need

(45.0/57.2957) * 5.0 = 3.92 feet

[Neil Hewitt]

I wonder what the effect would be if you took an acrylic mirror sheet and bent it into a parabolic curve just in one direction - horizontally - and then put a similarly-curved and appropriately scaled rear-projection screen at the focal point and projected onto that screen? Would you get partial collimation? What would that look like? I don't know enough optics to really think it through. I'm sure the reflected image would be distorted vertically at the corners, but if you removed that distortion using NTHUSIM what would the end result look like? Would your eye see it at a near focus or infinity or some kind of half-way house? Precisely bending acrylic mirror sheet into a simple curve is well within the capabilities of many people here.

Also - acrylic mirror sheet is basically clear acrylic with mylar on top / underneath. If the curve was gentle enough - say, using a large parabolic satellite dish, if you could get hold of one, as a mould - and you heated the acrylic up enough to be formable, would the mylar cope with the scretching and deforming without cracking or seaming? Could you even get the mylar film up to that temperature without it burning off? Anyone tried that?

I suppose you could maybe make several sectional moulds of an overall mirror shape using heavy-duty polystyrene blocks which are easy to carve and adjust, form acrylic to each one using the heat-it-in-the-oven technique, then look at mylar-covering each segment individually. That'd surely be easier to do? Yes, you'd get a less perfect display, but it'd be better than nothing by a long way.

Just throwing out ideas here...

[mikesblack]

I like those ideas neilh. I too wonder about the horizontal curve only. At least in terms of simulating paralax and proper runway allignment per Cpt or FO seat. This is something I will experiment with when I start to work with the mylar.

[ANDYSMITH]

Hi all,
I like the idea of the one dimentional curved mirror(cheap) and the problem I see is that it can not be mounted vertical, as in 90 degrees to your eyes. Because the "screen" must be over your head, you need to tip the mirror back at an angle to see the screen that is above and behind you. This will give your horizon a curve that can be at the right elevation directly ahead but will turn up at the outer edges like a smilley face. So my question for the day is would this "Immersive Display Lite" program that is on the main page be able to un smile the horizon and bend it back so it looks flat? It is only 40 bucks so I think I will buy it and take the mirror off by bathroom door and do some expirimenting. I already have a projector mounted to the cieling in the center of the room and a small mirror on the wall above the overhead so all I need to do is put a piece of white paper over the existing mirror and re focus the image on the paper then mount the large mirror in front of the sim.

Now that I am imagining this what about a curved screen, the one behind you, also bieng tipped at an angle might be able to curve the horizon the other way cancelling out the curved horizon on the big mirror?

My bathroom mirror is only about 18" wide by about 5 feet tall and is glass so I can't bend it much, but enough to test the concept. I will report my results tomorrow.

Andy Smith