Page 2 of 3 FirstFirst 123 LastLast
Results 11 to 20 of 27

Thread: How to achieve the finest laser line possible with the least spread in the opposite..

  1. #11
    Join Date
    Feb 2011
    Location
    New Hampshire
    Posts
    3,513

    Default

    After doing a few back of the envelope calculations, I am confident that this is relatively easy. A few watts over .1mrad will be easily visible. In Alaska, you are entering the more difficult time of the year to do this, but the astronauts are almost sure to have access to solar blind filters that pass the very common 532nm wavelength. This wavelength has been the workhorse for lidar and ranging experiments on satellites as well as on aircraft for decades. The advantage of the line vs the spot for detection is that it will decrease the time that the same energy deposits on any detector (including the eye) as the ISS passes through the beam. This is probably not that important. I believe that spreading out the laser to any extent larger than the tracking error is counter productive.

    With a big budget, feed the 4W 532 through a beam splitter cube and use the full aperture of a telescope to image/track the station and simultaneously paint it with the laser. A blocking filter for 532nm in front of the tracking camera would add more protection against scattered light from the laser.

    With less equipment, co-align an imaging scope with a beam expansion scope on a terrestrial target and then visually center the station in the scope as it passes over head. This is a little more iffy, but I think you could do this.

    Regarding safety, I come from the astronomical end of things rather than the entertainment end and once you have acquired the permission (others on this forum can help you there) please get a spotter when you are doing this. Don't do this alone. Another set of eyes can keep you out of trouble when you get focused on one problem or another.

  2. #12
    Join Date
    Aug 2014
    Location
    Doha, Qatar
    Posts
    379

    Default

    Planters, thank you for your thoughts and suggestions on this. The project is a long term one I've been dreaming about for three years now, so trying to pull it together quickly, or even this year, isn't a concern for me, just need to figure out how to best do this and great to have others insights on the problems.

    Edit: I'm looking at your suggestion, I thought I understood earlier but now I'm not sure I understand the purpose for the beam splitter cube.
    Last edited by Laser57; 04-04-2015 at 17:30.

  3. #13
    Join Date
    Feb 2011
    Location
    New Hampshire
    Posts
    3,513

    Default

    The big budget system uses the full aperture of the telescope to image the station using ambient light over the whole spectrum (minus the narrow band at 532nm due to the narrow band filter in front of the camera). Almost all DPSS lasers in the several watt power range are based on vanadate and are polarized. I should have said the beam splitter cube would be a polarizing beam splitter cube. This allows the laser to enter the beam path at right angles, strike the cube and be reflected 90 degrees to be sent out of the telescope. In theory you could use the narrow band filter to accomplish this, but these tend to have poor wave front quality. The cube will inevitably scatter some of the light inside of the telescope (it's not perfect) and this scattered light will be of mixed polarization and so it could pass through the cube unhindered by the polarizer layer and so enter the camera. Hence, the narrow band (high OD) filter.

  4. #14
    Join Date
    Aug 2014
    Location
    Doha, Qatar
    Posts
    379

    Default

    Perhaps I can use the 160X 150mm beam expander the same way, as both a beam expander and a telescope with camera?

    Click image for larger version. 

Name:	160x_zpsrskhrx9d.jpg 
Views:	5 
Size:	69.5 KB 
ID:	46745 Click image for larger version. 

Name:	160xx_zpsekhuwnvg.jpg 
Views:	4 
Size:	62.7 KB 
ID:	46746

  5. #15
    Join Date
    Feb 2011
    Location
    New Hampshire
    Posts
    3,513

    Default

    You might be able to, but this telescope is afocal and so you will have to fabricate an imaging adapter so as to feed the camera at a focus rather than a parallel beam. You will also need a mount and the 90 degree input adapter for the laser. You might consider using this as a stand alone laser launcher piggybacked on the telescope/mount like in the lower cost version I described above. The tracking vs eyeball targeting for this setup depends on what else you've got.

  6. #16
    Join Date
    Aug 2014
    Location
    Doha, Qatar
    Posts
    379

    Default

    Stand alone telescope paired with the beam expander does appeal from a simplicity standpoint, and I've been wanting a telescope for a few years now but if they are not perfectly aligned to one another to the nats back side, it won't track enough to be able to spot the ISS. Although I still want to work out a way to make an ultra low divergence line too, in addition to this for another project. Best I can figure is using a low divergence laser and having a moving mirror for that. I've been looking at some 10 X 10 inch stage mirrors for lasers trying to figure a way to make one work, a fast sweep won't happen with one of those. Thanks.
    Last edited by Laser57; 04-05-2015 at 08:14.

  7. #17
    Join Date
    Feb 2011
    Location
    New Hampshire
    Posts
    3,513

    Default

    You only have to align these two scopes to the divergence limit. One watt =approx 1x10^20 photons/sec. The eye,can barely detect about 100 photons/sec and a good CCD camera can detect around one/sec when placed onto a few pixels. Assuming the same aperture (very roughly 1cm), your 4W laser should be detectable by the lower sensitivity eye when the laser has spread to a circle nearly 1000kM in diameter!
    With a divergence of 2,500 mrad the laser should be detectable. You do not need extremely low divergence to send enough photons to the station and super low divergence will just make pointing harder.

  8. #18
    Join Date
    Aug 2014
    Location
    Doha, Qatar
    Posts
    379

    Default

    Just for the sake of discussion if I were to produce a line which is 45 degrees wide with 4 watts of 532nm, any idea how well that might be seen with the eye by the ISS?

  9. #19
    Join Date
    Feb 2011
    Location
    New Hampshire
    Posts
    3,513

    Default

    It would be seen well.

  10. #20
    Join Date
    Aug 2014
    Location
    Doha, Qatar
    Posts
    379

    Default

    I have an idea, if I can find long enough cylinder lenses, I can have a mount machined for them to fit over the output of the 150mm beam expander, lets say, covering about 25% of the output of the lens, spreading them out equally across the face, leaving slots for the normal output of the lens. If I am correct, which I'm asking here, wouldn't that produce a line with a reduced divergence the same as the 160X expander telescope has, but only use 25% of the output power? If you are an optics man, or anyone here is, which is better to use for this, half cylinder lenses or full round ones?

    Thank you again, hard to get the answer for some of this googling.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •