Re: [Utah-astronomy] Green laser backlash
Some of the members of OAS recently picked up some of the green laser pointers. We did a bulk order of 10 and got them for $102 with shipping. They gave us a price break and only one shipping charge. I think they got them from greenlaserpointer.com. Kay or Lee might be able to comfirm that. Dave
In my experience I have found lasers worse than useless for collimating Newtonians. See the June 2002 issue of Sky and Telescope for an explanation of the problems. As a test I set up a Newtonian telescope with the diagonal offset almost an inch in the wrong direction and by tilting the diagonal and adjusting the main mirror the laser converged as it is supposed to do; but the star images were terrible. A sight tube, Chesire and autocollimator are far superior for collimation. Several times I have seen laser collimated telescopes that were dramatically out of collimation with shadow of the secondary far off to one side. I have not tried the new holographic lasers that project a pattern so I can't comment on them. Clear Skies Don Colton
This thread was started merely to gather information on the green laser as a star-pointer, Don, but I agree with you completely. Manual collimation, with a star-test for tweaking is the best way to go, but many folks either can't get the knack or don't want to take the time. Some just like gadgets. The collimators that project a grid are a step in the right direction. Rich and I used one on my 10" last year, and while it was close, a star-test later revealed that collimation was in fact off by about 1/10" at the image plane. While this may be satisfactory for visual use at low to medium powers, it is totally unacceptable for a "fast" Dob, imaging of any kind, or high-magnification viewing. Chuck --- "Don J. Colton" <djcolton@piol.com> wrote:
In my experience I have found lasers worse than useless for collimating Newtonians. See the June 2002 issue of Sky and Telescope for an explanation of the problems. As a test I set up a Newtonian telescope with the diagonal offset almost an inch in the wrong direction and by tilting the diagonal and adjusting the main mirror the laser converged as it is supposed to do; but the star images were terrible. A sight tube, Chesire and autocollimator are far superior for collimation.
Several times I have seen laser collimated telescopes that were dramatically out of collimation with shadow of the secondary far off to one side.
I have not tried the new holographic lasers that project a pattern so I can't comment on them.
Clear Skies Don Colton
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The (holographic) laser collimator I use (Laser Max) gets my 16-inch close enough for my taste, only takes a minute to do, and has saved me from having to center-dot my mirror. The times that I've done a star test afterwards has shown a good pattern. Is the star test the best confirmation of collimation, or does the cheshire do a better job? It would be interesting some time to line up a few different scopes of various focal lengths, try different lasers, and compare collimation results with traditional collimation tools and star testing (along with one or more of you collimation pros!). Maybe I've been kidding myself all this time... -Rich --- Chuck Hards <chuckhards@yahoo.com> wrote:
This thread was started merely to gather information on the green laser as a star-pointer, Don, but I agree with you completely. Manual collimation, with a star-test for tweaking is the best way to go, but many folks either can't get the knack or don't want to take the time. Some just like gadgets.
The collimators that project a grid are a step in the right direction. Rich and I used one on my 10" last year, and while it was close, a star-test later revealed that collimation was in fact off by about 1/10" at the image plane. While this may be satisfactory for visual use at low to medium powers, it is totally unacceptable for a "fast" Dob, imaging of any kind, or high-magnification viewing.
Chuck
--- "Don J. Colton" <djcolton@piol.com> wrote:
In my experience I have found lasers worse than useless for collimating Newtonians. See the June 2002 issue of Sky and Telescope for an explanation of the problems. As a test I set up a Newtonian telescope with the diagonal offset almost an inch in the wrong direction and by tilting the diagonal and adjusting the main mirror the laser converged as it is supposed to do; but the star images were terrible. A sight tube, Chesire and autocollimator are far superior for collimation.
Several times I have seen laser collimated telescopes that were dramatically out of collimation with shadow of the secondary far off to one side.
I have not tried the new holographic lasers that project a pattern so I can't comment on them.
Clear Skies Don Colton
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Rich, it's not the Cheshire or sight tube that does the collimation, it's the person using them! Different people can end up with different results using the same equipment. Patience & experience is the key no matter what system you use. The star-test is a reality check. Many folks don't use a high-enough power to clearly see the diffraction pattern, or try it in less than ideal conditions. The star test, using image artifacts as it does, is subject to bad seeing. Even using an artificial star doesn't eliminate bad seeing if the air is turbulent or inhomogenous along your line-of-sight. And if your optics are not top-notch, you won't get a good diffraction pattern no matter how good the seeing is. What's good enough is largely dependant on f/ratio. The faster the scope, the more critical collimation is. Most Dobs are f/6 & under, and as you'd expect are the worst offenders. F/4 systems must be dead-on to get diffraction-limited performance at the center of the field. For most visual use, a laser is fine. Don cited the recent S&T article by Nils Olof Carlin, and this would be a good starting point for laser users. Nils advocates using a Barlow to increase the sensitivity of the laser, and this can go a long way to eliminate the pitfalls. Chuck --- Richard Tenney <retenney@yahoo.com> wrote:
The (holographic) laser collimator I use (Laser Max) gets my 16-inch close enough for my taste, only takes a minute to do, and has saved me from having to center-dot my mirror. The times that I've done a star test afterwards has shown a good pattern. Is the star test the best confirmation of collimation, or does the cheshire do a better job?
It would be interesting some time to line up a few different scopes of various focal lengths, try different lasers, and compare collimation results with traditional collimation tools and star testing (along with one or more of you collimation pros!). Maybe I've been kidding myself all this time...
-Rich
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I read the S&T article when it came out but never fully understood it -- call me dense (I usually deserve it), if the equiptment is accurately machined and you have a good, sturdy focuser, I still can't comprehend why it wouldn't render a very accurate picture of the optical path. I also found that using a barlow renders the holographic pattern on my laser useless! I suspect that technique is intended for the simpler single beam designs. Oh well. I suppose I ought to revisit the article and give it a more careful reading... Rich --- Chuck Hards <chuckhards@yahoo.com> wrote:
Rich, it's not the Cheshire or sight tube that does the collimation, it's the person using them! Different people can end up with different results using the same equipment. Patience & experience is the key no matter what system you use.
The star-test is a reality check. Many folks don't use a high-enough power to clearly see the diffraction pattern, or try it in less than ideal conditions. The star test, using image artifacts as it does, is subject to bad seeing. Even using an artificial star doesn't eliminate bad seeing if the air is turbulent or inhomogenous along your line-of-sight. And if your optics are not top-notch, you won't get a good diffraction pattern no matter how good the seeing is.
What's good enough is largely dependant on f/ratio. The faster the scope, the more critical collimation is. Most Dobs are f/6 & under, and as you'd expect are the worst offenders. F/4 systems must be dead-on to get diffraction-limited performance at the center of the field.
For most visual use, a laser is fine. Don cited the recent S&T article by Nils Olof Carlin, and this would be a good starting point for laser users. Nils advocates using a Barlow to increase the sensitivity of the laser, and this can go a long way to eliminate the pitfalls.
Chuck
--- Richard Tenney <retenney@yahoo.com> wrote:
The (holographic) laser collimator I use (Laser Max) gets my 16-inch close enough for my taste, only takes a minute to do, and has saved me from having to center-dot my mirror. The times that I've done a star test afterwards has shown a good pattern. Is the star test the best confirmation of collimation, or does the cheshire do a better job?
It would be interesting some time to line up a few different scopes of various focal lengths, try different lasers, and compare collimation results with traditional collimation tools and star testing (along with one or more of you collimation pros!). Maybe I've been kidding myself all this time...
-Rich
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--- Richard Tenney <retenney@yahoo.com> wrote:
I also found that using a barlow renders the holographic pattern on my laser useless! I suspect that technique is intended for the simpler single beam designs.
Yep, you're absolutely correct. Works only with the dot-type collimator. I recently came up with a design for using one of those $5 cheapie lasers as a collimator. Might have a prototype working one day when things slow down here. C. __________________________________ Do you Yahoo!? Yahoo! SiteBuilder - Free, easy-to-use web site design software http://sitebuilder.yahoo.com
Chuck wrote: Might have a prototype working one day when things slow down here. Yeah right! Chuck - would that be before or after the filter class, the eyepiece abuilding class and the dozen other things you keep teazing us with. The only way this is going to happen is if we all take up a collection to pay your rent and buy your food for a month so you can take a vacation. Your wife gets you the first week and we get you for the next 3 weeks. I am not sure 3 weeks would do it. Jim --------------------------------- Do you Yahoo!? Yahoo! SiteBuilder - Free, easy-to-use web site design software
Chuck I was just teasing! Come Back! Oh! I forgot to mention the Bino class too. Jim Gibson <xajax99@yahoo.com> wrote: Chuck wrote: Might have a prototype working one day when things slow down here. Yeah right! Chuck - would that be before or after the filter class, the eyepiece abuilding class and the dozen other things you keep teazing us with. The only way this is going to happen is if we all take up a collection to pay your rent and buy your food for a month so you can take a vacation. Your wife gets you the first week and we get you for the next 3 weeks. I am not sure 3 weeks would do it. Jim --------------------------------- Do you Yahoo!? Yahoo! SiteBuilder - Free, easy-to-use web site design software_______________________________________________ Utah-Astronomy mailing list Utah-Astronomy@mailman.xmission.com http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy --------------------------------- Do you Yahoo!? Yahoo! SiteBuilder - Free, easy-to-use web site design software
Rich It would have to be machined accurately to very high tolerances; much higher than practical for any commercial or home built telescope. As the article mentions even if the laser beam coming out of the eyepiece misses the "exact center" (i.e. the precise diagonal offset center) of the diagonal by only 1mm you have introduced a 2mm error which will not allow good images in a fast scope. For your f/6 scope the tolerances are much more forgiving. As implied above if you were to precisely offset the diagonal and mark a dot on the diagonal using Dave Kriege's template to get an accurate offset center dot and then centered doted the mirror; you would probably be OK with a laser if it precisely hit that center dot on the diagonal and then hit the center dot on the primary and returned by the same path. In order to get this to happen you would probably have to adjust the focuser, truss tubes etc. in order to get everything precisely square. Its much easier and just as effective to use the sight tube, chesire, and autocollimator since as explained in the article the telescope does not have to be precisely square with these tools. To use the laser the telescope must be precisely square to a very high tolerance. Clear Skies Don Colton -----Original Message----- From: utah-astronomy-bounces@mailman.xmission.com [mailto:utah-astronomy-bounces@mailman.xmission.com]On Behalf Of Richard Tenney Sent: Tuesday, August 12, 2003 1:02 PM To: Visit http://www.utahastronomy.com for the photo gallery. Subject: Re: [Utah-astronomy] Lasers and collimation questions I read the S&T article when it came out but never fully understood it -- call me dense (I usually deserve it), if the equiptment is accurately machined and you have a good, sturdy focuser, I still can't comprehend why it wouldn't render a very accurate picture of the optical path. I also found that using a barlow renders the holographic pattern on my laser useless! I suspect that technique is intended for the simpler single beam designs. Oh well. I suppose I ought to revisit the article and give it a more careful reading... Rich --- Chuck Hards <chuckhards@yahoo.com> wrote:
Rich, it's not the Cheshire or sight tube that does the collimation, it's the person using them! Different people can end up with different results using the same equipment. Patience & experience is the key no matter what system you use.
The star-test is a reality check. Many folks don't use a high-enough power to clearly see the diffraction pattern, or try it in less than ideal conditions. The star test, using image artifacts as it does, is subject to bad seeing. Even using an artificial star doesn't eliminate bad seeing if the air is turbulent or inhomogenous along your line-of-sight. And if your optics are not top-notch, you won't get a good diffraction pattern no matter how good the seeing is.
What's good enough is largely dependant on f/ratio. The faster the scope, the more critical collimation is. Most Dobs are f/6 & under, and as you'd expect are the worst offenders. F/4 systems must be dead-on to get diffraction-limited performance at the center of the field.
For most visual use, a laser is fine. Don cited the recent S&T article by Nils Olof Carlin, and this would be a good starting point for laser users. Nils advocates using a Barlow to increase the sensitivity of the laser, and this can go a long way to eliminate the pitfalls.
Chuck
--- Richard Tenney <retenney@yahoo.com> wrote:
The (holographic) laser collimator I use (Laser Max) gets my 16-inch close enough for my taste, only takes a minute to do, and has saved me from having to center-dot my mirror. The times that I've done a star test afterwards has shown a good pattern. Is the star test the best confirmation of collimation, or does the cheshire do a better job?
It would be interesting some time to line up a few different scopes of various focal lengths, try different lasers, and compare collimation results with traditional collimation tools and star testing (along with one or more of you collimation pros!). Maybe I've been kidding myself all this time...
-Rich
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Don You mentioned: Dave Kriege's template How do I get such a template? Jim --------------------------------- Do you Yahoo!? Yahoo! SiteBuilder - Free, easy-to-use web site design software
It's in his book The Dobsonian Telescope: A Practical Manual for Building Large Aperture Telescopes by David Kriege and Richard Berry, available from Willmann-Bell. He has a page that you can xerox that has a master template good for several sizes of diagonals. There is also a table that gives the proper offset. Make sure you offset in the right direction, it isn't obvious. Clear Skies Don Colton -----Original Message----- From: utah-astronomy-bounces@mailman.xmission.com [mailto:utah-astronomy-bounces@mailman.xmission.com]On Behalf Of Jim Gibson Sent: Tuesday, August 12, 2003 1:52 PM To: Visit http://www.utahastronomy.com for the photo gallery. Subject: RE: [Utah-astronomy] Lasers and collimation questions Don You mentioned: Dave Kriege's template How do I get such a template? Jim ---------------------------------------------------------------------------- -- Do you Yahoo!? Yahoo! SiteBuilder - Free, easy-to-use web site design software
The star test is always the best if done right. If you get a good star test at 50x per inch you are in great shape, but you have to use high power. At lower powers unless the star is precisely centered you may not get accurate results. Be sure to examine a slightly defocused star on both sides of focus for symmetry and concentricity. The image of the diagonal, if applicable, should be centered. Clear Skies Don Colton -----Original Message----- From: utah-astronomy-bounces@mailman.xmission.com [mailto:utah-astronomy-bounces@mailman.xmission.com]On Behalf Of Richard Tenney Sent: Tuesday, August 12, 2003 11:35 AM To: Visit http://www.utahastronomy.com for the photo gallery. Subject: [Utah-astronomy] Lasers and collimation questions The (holographic) laser collimator I use (Laser Max) gets my 16-inch close enough for my taste, only takes a minute to do, and has saved me from having to center-dot my mirror. The times that I've done a star test afterwards has shown a good pattern. Is the star test the best confirmation of collimation, or does the cheshire do a better job? It would be interesting some time to line up a few different scopes of various focal lengths, try different lasers, and compare collimation results with traditional collimation tools and star testing (along with one or more of you collimation pros!). Maybe I've been kidding myself all this time... -Rich --- Chuck Hards <chuckhards@yahoo.com> wrote:
This thread was started merely to gather information on the green laser as a star-pointer, Don, but I agree with you completely. Manual collimation, with a star-test for tweaking is the best way to go, but many folks either can't get the knack or don't want to take the time. Some just like gadgets.
The collimators that project a grid are a step in the right direction. Rich and I used one on my 10" last year, and while it was close, a star-test later revealed that collimation was in fact off by about 1/10" at the image plane. While this may be satisfactory for visual use at low to medium powers, it is totally unacceptable for a "fast" Dob, imaging of any kind, or high-magnification viewing.
Chuck
--- "Don J. Colton" <djcolton@piol.com> wrote:
In my experience I have found lasers worse than useless for collimating Newtonians. See the June 2002 issue of Sky and Telescope for an explanation of the problems. As a test I set up a Newtonian telescope with the diagonal offset almost an inch in the wrong direction and by tilting the diagonal and adjusting the main mirror the laser converged as it is supposed to do; but the star images were terrible. A sight tube, Chesire and autocollimator are far superior for collimation.
Several times I have seen laser collimated telescopes that were dramatically out of collimation with shadow of the secondary far off to one side.
I have not tried the new holographic lasers that project a pattern so I can't comment on them.
Clear Skies Don Colton
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Wow, 800x is a lot of magnification! Guess I've been using way too little power after all (more like in the 300x range). Thanks, Rich --- "Don J. Colton" <djcolton@piol.com> wrote:
The star test is always the best if done right. If you get a good star test at 50x per inch you are in great shape, but you have to use high power. At lower powers unless the star is precisely centered you may not get accurate results. Be sure to examine a slightly defocused star on both sides of focus for symmetry and concentricity. The image of the diagonal, if applicable, should be centered. Clear Skies Don Colton
-----Original Message----- From: utah-astronomy-bounces@mailman.xmission.com
[mailto:utah-astronomy-bounces@mailman.xmission.com]On
Behalf Of Richard Tenney Sent: Tuesday, August 12, 2003 11:35 AM To: Visit http://www.utahastronomy.com for the photo gallery. Subject: [Utah-astronomy] Lasers and collimation questions
The (holographic) laser collimator I use (Laser Max) gets my 16-inch close enough for my taste, only takes a minute to do, and has saved me from having to center-dot my mirror. The times that I've done a star test afterwards has shown a good pattern. Is the star test the best confirmation of collimation, or does the cheshire do a better job?
It would be interesting some time to line up a few different scopes of various focal lengths, try different lasers, and compare collimation results with traditional collimation tools and star testing (along with one or more of you collimation pros!). Maybe I've been kidding myself all this time...
-Rich
--- Chuck Hards <chuckhards@yahoo.com> wrote:
This thread was started merely to gather information on the green laser as a star-pointer, Don, but I agree with you completely. Manual collimation, with a star-test for tweaking is the best way to go, but many folks either can't get the knack or don't want to take the time. Some just like gadgets.
The collimators that project a grid are a step in the right direction. Rich and I used one on my 10" last year, and while it was close, a star-test later revealed that collimation was in fact off by about 1/10" at the image plane. While this may be satisfactory for visual use at low to medium powers, it is totally unacceptable for a "fast" Dob, imaging of any kind, or high-magnification viewing.
Chuck
--- "Don J. Colton" <djcolton@piol.com> wrote:
In my experience I have found lasers worse than useless for collimating Newtonians. See the June 2002 issue of Sky and Telescope for an explanation of the problems. As a test I set up a Newtonian telescope with the diagonal offset almost an inch in the wrong direction and by tilting the diagonal and adjusting the main mirror the laser converged as it is supposed to do; but the star images were terrible. A sight tube, Chesire and autocollimator are far superior for collimation.
Several times I have seen laser collimated telescopes that were dramatically out of collimation with shadow of the secondary far off to one side.
I have not tried the new holographic lasers that project a pattern so I can't comment on them.
Clear Skies Don Colton
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participants (5)
-
Chuck Hards -
Don J. Colton -
DunnDave＠aol.com -
Jim Gibson -
Richard Tenney