Lens surface accuracy question; 1/2, 1/4 or 1/10 wavelength?

[steve001]

Here's a lens with a very long focal length
150MM DIAMETER F/17 PCX LENS 2600MM FL - Surplus Shed

Even longer.
http://www.surplusshed.com/pages/item/l3919d.html

 

[Alaskan]

I came across that one searching Google too, but its focal length is too long for what I use, that and it is 6" diameter, I have lots of lenses that diameter and larger. I was hoping to find like 18-24 inch diameter PCX lens I could afford, but the only ones I can afford are at 2 lambda surface accuracy, while that will work, the tolerance is a bit loose, that and heavy as hell.

I've started looking at Dobsonian mirrors, they are far less expensive, lighter, and from what Planters recently told me the tolerances are much better. I'm thinking the cheapest way to achieve ultra low divergence is to buy a used Dobsonian telescope and shoot a laser into its eye piece, might need something changed at the eye piece, but other than that they appear to be plug and play for this use, if there is enough range of focus. I still need to research more on this.

I think it would be a hoot to have a 16 inch Dobsonian telescope to put a spot on a cloud with. If using a low power PL520 diode the beam would probably be difficult to see, but the spot so concentrated on a 5,000 foot high cloud base it would get some folk scratching their heads. That and I've always wanted a good telescope, as long as I don't damage it with a laser it will be a win-win to get one.

 

[steve001]

I came across that one searching Google too, but its focal length is too long for what I use, that and it is 6" diameter, I have lots of lenses that diameter and larger. I was hoping to find like 18-24 inch diameter PCX lens I could afford, but the only ones I can afford are at 2 lambda surface accuracy, while that will work, the tolerance is a bit loose, that and heavy as hell.

I've started looking at Dobsonian mirrors, they are far less expensive, lighter, and from what Planters recently told me the tolerances are much better. I'm thinking the cheapest way to achieve ultra low divergence is to buy a used Dobsonian telescope and shoot a laser into its eye piece, might need something changed at the eye piece, but other than that they appear to be plug and play for this use, if there is enough range of focus. I still need to research more on this.

I think it would be a hoot to have a 16 inch Dobsonian telescope to put a spot on a cloud with. If using a low power PL520 diode the beam would probably be difficult to see, but the spot so concentrated on a 5,000 foot high cloud base it would get some folk scratching their heads. That and I've always wanted a good telescope, as long as I don't damage it with a laser it will be a win-win to get one.

Using the second lens link this is what I got https://lightmachinery.com/optical-...-beam-propagation/?key=IefemV-0IUKN9KD7m14kjw This 160,000,000.0mm converted to miles is 99.4 with a spot size of 40cm or 1.5748 at that distance

 

[Alaskan]

Something isn't right in the setup of that online graphical calculator, if I see what I think I see.

 

[steve001]

Something isn't right in the setup of that online graphical calculator, if I see what I think I see.

Maybe. Try the archived app.. gausian beam rev 2.0.xls