advice please on greenlasers used for rangefinding

[ian333]

Hi.
I'm new to forum use but basically I want to use a laser as a range finding aid on my air rifle. The basic idea is that you mount a laser pointer above the telescopic sight. The crosshairs of the scope and the dot of the laser are both zeroed at a chosen range (say 35 yards). If the laser dot aligns with the crosshair you know the target is at 35 yards. If the laser dot is above or below the crosshair you can get a good idea of the yardage to target by seeing how far (using the mil dots on the crosshair) above or below the dot is.
To get to the point. Some laser pointers are available commercially for this purpose but they are usually red. They don't have a very good dot brightness so cannot be seen easily in daylight. generally they do not have a very focused dot (most quote sizes of around 4 inches dot size at 100 yards). A Greenlaser is available from a company called Beamshot and this has a dot that can be seen easily in daylight and the dot size is 1.75 inches at 100 yards. Unfortunately it is prohibitively expensive!
Does anyone know of a laser pointer or module that gives a dot that can be seen in daylight, that is fairly small in size (i.e.dot around 1.5 inches at 100 yards), that can be placed inside a tube so I can fix adjusting screws to zero it vertically and horizontally?
Thanks,
Ian

 

[bootleg2go]

Hi ian,
A 1.5 inch dot at 100yards would mean a laser with a 0.416mrad divergence, if you find a laser with such a small divergence as this it will probably be very expensive as most laser specs have a 1.2mrad divergence and that's pretty standard. When shopping and asking about this, don't go by what the sales person tells you. You need to go by the spec. as that is what your really going to get. Just look to see what the divergence is in "mrad" 1.2mrad is equal to .0012 and then multiply that number by the distance to the target and the result will be the beam diameter... in the same units you used for the distance. So .0012x100yards= .12 yard beam diameter. convert to inches by multiplying by 36 (number of inches in a yard) and you get 4.32 inches.

Good luck
Jack

 

[ian333]

Hi jack,
Thanks for your explanation. I have seen the mrad specs you are talking about. From article I have read in the airgunning press it seems that some of the red lasers will give a smaller dot than the 4.32 inches you say would be normal for this type of laser. Is it possible that the laser itself gives a divergence of 1.2 mrad but the beam is then focused with a seperate lens? If so perhaps the lens type is more what I should be looking at rather than just the laser specs.

Cheers,
Ian

 

[comozo]

Surefire.com shows a green laser sight but, I could not find info regarding it, I'd call them and ask.
Another option is buy a green pointer module and put it in a custom housing, I made one from Brass nipples, plumbing supplies. You can also change the optics to decrease the beams divergence. A Plano-concave[PCV] or Double-concave[DCV] short focal length lens and a Plano-convex lens with a long focal length. Lenses should be Anti-reflective coated. PCV and DCV can be bought from Melles Griot and www.optosigma.com -5mm and -4mm respectively are the shortest focal lenses I have found so far. PCX lenses are easy to find at any focal length. Try www.edmundoptics.com. You want to construct a Galilaen beam expander. You could also buy a beam expander. If you like, I can send you instructions on how to use brass nipples for a custom housing just PM me.
These are some options. Here is a useful program for calculating beam divergence. http://www.lightmachinery.com/gausbeam.html
Focal lengths for the expander lens can be negative or positive. Use negative focal lengths. The longer the Rayleigh length the better. You'll also want a short length as possible waist diameter compared to the Rayleigh length.
Input these numbers
beam size 1mm
Divergence 5mm
Separation length 10mm
First lens -5mm
Separation 49.0090364
Second lens 54mm
Beam diameter at surface indicate the beams diameter at the second lens.
54 / 5 = 10.8 x beam expansion.
Now start changing one input at a time to see how it affects divergence, don't change the focal lengths at first.
Rayleigh length: Sams Laser FAQ
"Another way of characterizing divergence is to calculate how far the beam can travel before it expands significantly. There is a maximum distance that a beam of light can be kept collimated. Usually it is called the 'Rayleigh length' or 'Rayleigh range' as shown in the diagram: Divergence, Beam Waist, Rayleigh Length <rayleigh.gif>. (Note that the divergence in the diagram is GREATLY exaggerated). The Rayleigh length depends on the wavelength and the minimum diameter 'waist' of the beam (as well as the beam cross-section but that is for the advanced course!)."

 

[ian333]

Wow!
Thanks comozo.
There is enough here to keep me busy for a while!
I'll follow up the website addresses you suggested and try to make sense of it all. Thank you for the invite to PM you. I will do this and let you know how I get on but don't hold your breath 'cos I have to access the Net when my son lets me use his computer so time online is limited!
Ian