I need a concave mirror with a focal length of 150 feet

[gary350]

I need to cut down a 3 year old dead tree from top down so tree causes no damage with small pieces falling. I need a mirror with a focal length of 150 ft. 12" diameter to 36" diameter will work good but I can't think of any easy way to build it. Nothing like this for sale on Ebay.

I have a 30" Fresnel lens that I use to burn stumps it works great.

Tree service wants $2000.

 

[Baluncore]

I need a mirror with a focal length of 150 ft.

Your mirror will be close to flat, so it does not really matter if it is a parabola, a sphere or an ellipse.

You could use many smaller flat mirrors, mounted Fresnel like, on a flat surface, maybe plywood with a pyramid back frame to prevent twisting. Calculate the spacers needed on the plywood face, to tilt the mirrors at the correct angles.

 

[berkeman]

I have a 30" Fresnel lens that I use to burn stumps it works great.

Sorry, but I guess I'm a little dense today. You want to cut down a tree piece by piece with a mirror/lens from 150' away? That does not seem very practical to me. And even if you could, is it less dangerous to have flaming pieces of wood falling into your neighbors' yards (and onto their roofs)?

It would seem more practical just to rent a truck/van with a bucket arm to lift you and your chainsaw and rope up far enough to cut down the trees piece by piece while lowering each piece with the rope:

Or even simpler would be to use a bow and arrow type setup to launch a rope up high on each tree to get a rope attachment high enough to pull the tree so it leans down to the right into the open area and just cut it down with a chainsaw at ground level.

 

[Baluncore]

Or even simpler would be to use a bow and arrow type setup to launch a rope up high on each tree to get a rope attachment high enough to pull the tree down to the right into the open area and just cut it down with a chainsaw at ground level.

When you attach a single rope to a tree, in an attempt to pull it down in a preferred direction, it will usually fall left or right, at 90° to the rope. That is because one side of the hinge will always break before the other. You would do better placing the rope to prevent it falling in the worst way possible, or using two or three ropes to control it all the way down.

Those trees would be a trivial job for an experienced tree feller. For the minimum cost, you could hire a local feller for an hour to drop them. Then you clean up the remains on the ground.

If I was closer, and it could be done in less than an hour, I would offer to drop them for free, as a community service.

 

[berkeman]

When you attach a single rope to a tree, in an attempt to pull it down in a preferred direction, it will usually fall left or right, at 90° to the rope. That is because one side of the hinge will always break before the other.

I'm definitely not an experienced tree feller, and for larger trees I would never attempt what I suggested. But for skinny little things like shown in the OP's picture, I have used a rope and my Jeep to bias them in one direction, and my son-in-law used a front notch and a back cut to bring them down in the direction I was pulling them.

 

[DaveE]

Just hire a tree guy, dude.
Your concave mirror isn't an antigravity device, pieces will still fall, but in a much less controlled fashion.
Before you can effectively think outside of the box, you need to know what's in the box. We don't do trees, but others do. I bet they know more than you or any of us.

Anyway, you could watch this for entertainment. I did and I don't have a tree to remove (but apparently I have some time to waste)...

 

[Baluncore]

... I have used a rope and my Jeep to bias them in one direction, ...

You were lucky that time. You must always expect it to fall sideways, because maybe next time it will. I am sorry, but I have this vision, as the tree falls backwards, of a Jeep being launched from a trebuchet.

 

[sophiecentaur]

I have a 30" Fresnel lens that I use to burn stumps it works great.

As you say, a reflector would be needed and not a lens, which can work on tree stumps (low down).

Burning a section of a tree would need an image of appropriate size to be formed on the tree in order to get a high enough temperature. A lens with an area of, say 0.5m2 which would capture around 500W. That 500W would need to be focused on a small area of the tree if you want the temperature to be raised high enough (say 300C) to char the wood. Would the optics actually allow this? It wouldn't be good enough for the 500W to be spread over the full width of the stem. For a lens with focal length of 50m an image of the Sun would be size of 0.5 degrees at 50m, which is a disc of about 4cm (a check would be good). I question whether that would produce significant burning of a relatively large piece of wood ( even given infinite time).

Short focal lenses produce much smaller spots with appropriately higher temperatures. A larger area of reflector could do it but you'd need good optics for the image size not spill out more.

Better to consider felling in the normal way. 50ft is not high, plus you say it's dead. Two ropes pulling in a V, attached as high up as you can get (and a tether near the cut to stop the base from leaping about when it goes ), would protect neighbours. 2K is an awful lot to pay. It could be a lot cheaper if you shopped around

PS is the tree really as fat as your sketch suggests?

 

[Rive]

I have a 30" Fresnel lens that I use to burn stumps it works great.

The likely outcome is one (three, actually) tree-big torch lighted near that building, with flaming pieces falling all around.
Feels very safe.

Tree service wants $2000.

By the picture they look too old to be climbable, so that may be a reasonable price.

Renting a truck/van with a bucket arm (and a fellow who knows how to use it) looks like a good idea.

 

[sophiecentaur]

with flaming pieces falling all around.

. . . and no way to get water up to put it out.

Bottom line is that it is unlikely to work and, even if it did, it could be a disaster.
$2k seems excessive. I wonder if that estimate was based on a visit or accurate photo?

I used to drop a 10m mast which weighed about 80kg. Only scary if I considered the possible damage if we lost control. Those nearby trees would limit the amount of damage in the absence of fire.

 

[Rive]

$2k seems excessive. I wonder if that estimate was based on a visit or accurate photo?

I don't know the local prices, but when the tree is too old or damaged to be trivially climbable the price usually goes up.
Especially if it is a damage sensitive job (close to a building).

 

[Baluncore]

Bottom line is that it is unlikely to work and, even if it did, it could be a disaster.

Spoil sport.

You could quietly use an industrial IR laser to start the fire during an electrical storm, then as a good Samaritan, you could cut the tree down in the emergency, with the buildings still covered by their insurance. Take lots of pictures, you should gather plenty of evidence for the coroner.

I once felled a 50' hollow tree that was on fire, and roaring like a Roman candle. It was threatening the area and the firefighters wanted it lying down neatly. It surprised me how quickly it could be done precisely, when standing under the shower of sparks, with no time to stop and think.

Maybe the $2000 is all to cover the cost of planning the event, to maintain professional insurance and the workplace safety requirements.

 

[gmax137]

Get a couple more quotes/estimates.

 

[Averagesupernova]

When you attach a single rope to a tree, in an attempt to pull it down in a preferred direction, it will usually fall left or right, at 90° to the rope.

I have never heard of this. A tree that is evenly loaded branch-wise generally will fall in the direction of the rope/cable if the proper notch and back cut are made. I fell one this way last week. Two man job. One guy on the saw another on a tractor to pull. Once the tree starts to tip it's pretty much decided that's the direction it will go. It is important to make the notch large enough so the hinge does not break off too soon. Also, cut above the notch several inches when cutting from the back. Like this:


Cutting even with the notch is dangerous. It is more likely for the trunk to split and the back of the trunk above where the cut is being made will snap outwards in an instant. I know of people seriously hurt or killed from this. Also, if a tree does fall to the side of the direction desired the reason is likely because there is severe side load that should have been dealt with prior, or the back cut was made too low.
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As I mentioned, you don't want the notch to break too soon. I've seen teeny tiny notches cut that don't allow the tree to fall far enough before the notch closes up. Two things can happen here. One is that the tree hasn't fallen far enough to put enough strain on the hinge to break it. It hangs up. It is desirable for the saw to be away from the trunk when the hinge breaks. How is this possible at this point? Also the tree is hanging with no good way to determine if/when it will snap. The other thing that can happen is if the hinge does snap the tree is too close to vertical yet and there is a greater chance of the trunk rolling a significant amount before it hits the ground. May or may not be important.
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The one I fell last week had a small window of error for the direction. It was right next to a building and other obstacles were around it. It went exactly where we wanted it. It started to pinch the saw on the back cut so we put a little tension on the cable. At that point there was no turning around. It fell perfectly.

 

[Baluncore]

It started to pinch the saw on the back cut so we put a little tension on the cable. At that point there was no turning around. It fell perfectly.

That was a well-balanced tree with a flexible hinge, so you could have used a wedge in the back cut, rather than a rope. The problem comes when people use a rope to pull the tree in a way that it does not want to go. Why else would a beginner have used a rope.

The other problem comes when people use a crane to remove and lower entire limbs from the tree as they are cut free. It is hard to guess the weight of a limb. The limbs removed become progressively bigger, approaching some limit, until the crane cannot support them, and you don't know that until the limb is hanging on the hook. Also, the boom starts elevated close to the crane, then the boom is lowered away from the tree. That increases the moment on the vehicle, again trapping the operator in a crane destroying situation.

 

[Averagesupernova]

That was a well-balanced tree with a flexible hinge, so you could have used a wedge in the back cut, rather than a rope.

It's balance was a little better than I thought it would be. The problem with a wedge in the back is there was no room to swing a hammer against the wedge. Too close to the building. And I wasn't going to take a chance because there was significant loss of limbs on the cable side. I had assumed it would have taken more than a wedge anyway. I was surprised when the cable was pulled slightly and the tree kept moving in the correct direction. We just kept ahead of it with tension until it was well into the fall. Took place in about a second or two.

 

[gary350]

This is suppose to be a fun, build a concave mirror project, not a chain saw project. I own a chain saw, 100 ft of chain, come along, ladder, I could cut these trees down if I wanted to. I want to build a concave mirror. I can do the math for the tilt of each circle of mirrors but getting the tilt correct so sun reflection is exactly correct 1800" away is not easy. Tilting the mirror .001" creates a lot of movement 1800" away. At the moment I am considering using 2" square mirrors. 4 mirrors in the first donut circle, 9 mirrors in the 2nd circle but I only drew 8 mirrors in the drawing. Each circle gets larger diameter it has more mirrors. When finished it will look like a Fresnel lens with several circles of 2" mirrors. If focal length was only 3 ft this would be an easy project.

 

[Rive]

I think you should check up on the relevant episodes of the MythBuster series first.

Ps.: they had some adequately serious budget and adequately (safety-) serious attitude. Remember: the Sun burns.

 

[Baluncore]

When finished it will look like a Fresnel lens with several circles of 2" mirrors.

Why circles? If the mirrors are rectangular, lay them out in a rectangular grid. You only need to place accurately calculated shim spacers on the flat backing plate, to aim each of them in the correct direction.

It is best to use front silvered mirrors. The cheap alternative would be to use an aluminium foil. Gold leaf is very thin and low cost on eBay, probably cheaper than glass mirrors. You could make your own flat glass plates, each with a gold leaf front surface.

 

[Averagesupernova]

I once used a 12 gauge shotgun to chip away at a branch that was mostly broken off but still hanging on. Damage from a storm. It was nothing I wanted to get very close to. I caught a bit of heat over it when I mentioned it on this forum. Unconventional for sure but not like burning something off over a period of hours in a location it is difficult to reach if something goes wrong. You best hope if you get it started that you don't have a wind storm come up and spread it around. I have an idea this won't be a ten minute per cut job. It takes a long time for a six inch chunk of wood in a wood furnace/stove to break up. It will take longer in your case. Having a brush pile on the ground on a calm day with the ability to put it out with a suitable supply of water is one thing. The inability to get a fire put out 40 feet in the air is quite another. You could always ask the fire dept to be on standby. They'll get a good laugh.

 

[berkeman]

You could always ask the fire dept to be on standby. They'll get a good laugh.

Or borrow your neighbor's cat and show it a fake bird you have pulled up the tree with a string. After the cat gets about half-way up, call the fire department to come rescue it. Once they get the ladder truck positioned and the ladder up, you could just mention "Oh by the way, while you're up there..."

 

[sophiecentaur]

If focal length was only 3 ft this would be an easy project.

And feasible because of the size of the spot ( Sun's image). You are limited by the Solar Constant and the size of the spot at 50m.

Any low cost replacement (fresnel etc) would limit the resolution of the spot. It's no good just warming up the wood. It has to be maintain a temperature of 300+C.

The same consideration needs to be applied for photographic imaging. The f number of the lens determines the exposure time, for a given illumination and that's with good optics.

 

[gmax137]

The same consideration needs to be applied for photographic imaging. The f number of the lens determines the exposure time, for a given illumination and that's with good optics.

Maybe it is an ISO 3200 tree.

 

[sophiecentaur]

Maybe it is an ISO 3200 tree.

Lol. More likely, an ISO 0.1 tree

Well, Physics is a pretty universal field of Science. Parallels can be useful.

 

[gary350]

This works.

 

[sophiecentaur]

This works.

Did you actually make it work? What was the size and focal lengths of the added elements and how big was the image of the Sun at the final focus?

It seems to me that the arrangement in the diagram is the equivalent of a single mirror with a focal length of the original 150 ft.

 

[Baluncore]

This works.

I think you should mount the lens, fixed and aimed at the target, then track the Sun with a flat-mirror heliostat, mounted on a clock driven axis, parallel to the Earth's axis. The only moving part then is the flat heliostat mirror, and that can be balanced. That is the opposite light path to the one you show.

 

[sophiecentaur]

I think you should mount the lens, fixed and aimed at the target, then track the Sun with a flat-mirror heliostat, mounted on a clock driven axis, parallel to the Earth's axis. The only moving part then is the flat heliostat mirror, and that can be balanced. That is the opposite light path to the one you show.

If this is to be used for several hours, how does the angular range work?

But what about the image size at the focus on the tree? You have even less than 1kW of incident power and an image size of say 4cm diameter. What sort of temperature will that produce? Fine for boiling water but what about "Fahrenheit 451". Tell me if I have got the basic optics wrong. If not, it won't work at that distance.

 

[DaveE]

This works.View attachment 366123

Great! What happened to the tree then?

 

[sophiecentaur]

Great! What happened to the tree then?

I reckon that, on a humid day, the leaves / twigs could be steaming (if you look closely).

There are many sites that describe the OP's idea but the only one I found that 'really worked' was this one which used 127 one foot square mirrors with a wooden target at unspecified distance.

This is an Engineering Project and the actual numbers count. The OP does not describe a solar furnace because his dimensions are not appropriate. Great fun to do anything like that but best to start off with realistic aims.