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Nathan F
sage
Reged: 10/10/08
Posts: 223
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I'm in the process of building myself a tuBlug-like barlowed laser collimator for my dob out of acrylic tube, rod, and a $15 adjustable-focus laser module.
For a quick proof of concept, I just stuck everything together for a hand held test, and to my surprise I found that movement of the diffused laser causes the center spot shadow to move quite noticeably. Here's a video of the shadow dancing around: http://vimeo.com/7410420
And here's a pic of the whole thing in the light.
The same movement of the shadow was also observed by sticking my Antares 1.6x barlow in the focuser with a holed cap (traditional barlowed laser method) and moving my GLP around while pointed at the primary through the barlow.
I was under the impression that the entire point of barlowed laser collimation was that the spot's shadow's reflection's location (holy possessive nouns, batman!) on the cap / blug / tuBlug was not dependent on a perfectly aligned light source. IE, it was my understanding that the shadow was supposed to be consistent relative to the alignment of the primary, regardless of focuser slop or poorly collimated laser collimators that may have a crooked beam relative to the focuser axis.
Anyone care to explain? Thanks.
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12 inch f/5 Deep Space Observer Dob
31mm Nagler, 13mm Ethos, 1.6x Antares 2 inch Barlow
Celestron SkyMaster 15x70 Binoculars
Homemade Maple Parallelogram Binocular Mount
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sixela
Postmaster
Reged: 12/23/04
Posts: 10878
Loc: Boechout, Belgium
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The shadow only doesn't move when the focal plane is just in between the virtual point source and the screen. You're very far from the focal plane there...
With a classical barlowed laser the barlow makes a virtual point source that is further from the tube than the focal plane, but the screen is at the bottom close to the secondary.
Also, there's only little dependence between the orientation of the laser and the results if the distance between barlow and laser isn't too large (that's one of the things that makes the TuBLUG work).
Why did you make the tube so long, by the way? That's only making things more difficult.
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400mm f/4.46 self made Dobsonian on Tom Osypowski equatorial platform
Orion Starblast (114mm f/4 reflector, Alt/Az)
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Nathan F
sage
Reged: 10/10/08
Posts: 223
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Quote:
The shadow only doesn't move when the focal plane is just in between the virtual point source and the screen. You're very far from the focal plane there...
With a classical barlowed laser the barlow makes a virtual point source that is further from the tube than the focal plane, but the screen is at the bottom close to the secondary.
Also, there's only little dependence between the orientation of the laser and the results if the distance between barlow and laser isn't too large (that's one of the things that makes the TuBLUG work).
Why did you make the tube so long, by the way? That's only making things more difficult.
I'm afraid I'm not quite following what you're saying completely. Are you saying that the shadow won't move if and only if the focal plane (the focal length of the mirror?) resides precisely between the output of laser emitter/lens and the 45 degree window?
If that is the case, why did I also notice the exact same movement when doing the traditional "paper cap on the barlow" method while holding a GLP in more or less the same place my regular laser collimator would fit into the barlow. Was I still just outside or inside of the focal plane?
With regards to your statement about the distance between the barlow and the laser not being large, I should point out that the lens on this adjustable laser module is probably 10mm from the emitter at best - hard to get closer than that.
As for the length of the tube and placement of the 45 degree window, this was just a quick test of the laser module. The plan for this when finished is to look more like the tublug, with the 45 degree window flush with the top of the focuser. The extra length of the tube will hold the batteries, on/off switch, etc.. I just did it this way for now so I could tape the 45 degree window in place easily.
I'd really like to get this thing working eventually, so your help is much appreciated.
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12 inch f/5 Deep Space Observer Dob
31mm Nagler, 13mm Ethos, 1.6x Antares 2 inch Barlow
Celestron SkyMaster 15x70 Binoculars
Homemade Maple Parallelogram Binocular Mount
Edited by Nathan F (11/03/09 04:42 AM)
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sixela
Postmaster
Reged: 12/23/04
Posts: 10878
Loc: Boechout, Belgium
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Quote:
I'm afraid I'm not quite following what you're saying completely. Are you saying that the shadow won't move if and only if the focal plane (the focal length of the mirror?) resides precisely between the output of laser emitter/lens and the 45 degree window?
No. The virtual point source is one (absolute value of the) barlow focal length away from the barlow out the tube, in the barlow's axis if the light strikes the barlow perpendicularly (no matter where exactly the laser beam strikes it). The screen is, well, where your screen is.
And the reading is only insensitive of focuser axis collimation if the focal plane lies in between.
Tilting the laser --with respect to a barlow that stays well aligned along the focuser axis -- is different: it does move the virtual point source, though usually not by much (for a properly clamped laser collimator, but if you move it around on purpose, things are different). Again, if the focal plane is in between the virtual point source and the screen you are less sensitive to these errors.
Quote:
If that is the case, why did I also notice the exact same movement when doing the traditional "paper cap on the barlow" method while holding a GLP in more or less the same place my regular laser collimator would fit into the barlow. Was I still just outside or inside of the focal plane?
Chances are the screen was inside and the vitual point source outside of the focal plane. Also, your barlow was still nicely along the focuser axis.
Yes, a TuBLUG usually also gives you an unbalanced reading, which makes it important to rack the focuser in, make it short, put the laser close to the barlow, and have small mechanical tolerances.
Quote:
I should point out that the lens on this adjustable laser module is probably 10mm from the emitter at best - hard to get closer than that.
If the barlow is, in fact, attached to the laser then all bets are off when you move it. The one feature that makes a barlowed laser work so well is that no matter what happens,, the *barlow's* axis always stays nicely along the focuser axis. The TuBLUG only works because the barlow is in the TuBLUG (not the laser) and because it's built in a way that centres the virtual point source well.
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400mm f/4.46 self made Dobsonian on Tom Osypowski equatorial platform
Orion Starblast (114mm f/4 reflector, Alt/Az)
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Nathan F
sage
Reged: 10/10/08
Posts: 223
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I'm still not following so well, but for a moment, let's forget about my half-baked home made tuBlug, and my laser module with the built in lens.
As another test, I just put a paper screen with a center hole on the end of my focuser draw tube and inserted my Antares 1.6x barlow into focuser. (Basically, the classic barlowed laser setup.)
I then tested the movement of the shadow on the screen by hand holding a red laser pointer inside the barlow and wiggling it around slightly. I did this at five different rack positions, from racked all the way in to racked all the way out.
And at each rack position, I held the laser at 3 locations, from almost touching the barlow lens, to about half way to the top of the barlow, and all the way at the top where an eyepiece would seat.
In each and every instance, I observed what appeared to be an *identical* amount of shadow movement, both within all the different configurations in this experiment, as well as to my original test run with my DIY tublug and adjustable lens laser module way out at the end of the tube.
This empirical observation makes me think that placement of the screen, lens, and laser is more or less irrelevant, but I don't want to jump to any conclusions.
Regardless, the more I look at it, the more I realize that the movement of the shadow is only great then the laser is pointed quite a bit off to the side. (IE, the diffuse spot on the primary is way out of alignment with the center dot and is only catching the side of it.) That being the case, I am inclined to think that in most instances where a laser collimator is pretty close to perfect, accuracy will be good. Still, I think it's a misnomer to say that barlowed lasers are totally immune to any kind of laser alignment / focuser slop issues.
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12 inch f/5 Deep Space Observer Dob
31mm Nagler, 13mm Ethos, 1.6x Antares 2 inch Barlow
Celestron SkyMaster 15x70 Binoculars
Homemade Maple Parallelogram Binocular Mount
Edited by Nathan F (11/03/09 05:29 AM)
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hudson_yak
Pooh-Bah
Reged: 11/15/07
Posts: 1106
Loc: Hyde Park, NY, USA
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Nice long topic covering these issues here.
To reiterate/expound on what Sixela has already said, the Glatter tuBlug concept does have a potential accuracy issue with it, that you have to pay for the convenience it provides. Both the laser point source and target lie outside the focal plane. This makes issues of tool tilt, and accurate focuser axis collimation, more critical than they are for a cheshire or indeed the barlowed laser as it was originally conceived.
You can verify the accuracy you are getting by comparing the results to what you get with a cheshire. Also, you can do other verifications such as ensuring the reading does not change when racking the focuser in and out, and also when rotating the tool in the focuser, and the laser in the barlow.
I've done all these sorts of tests with my Glatter tuBlug, several times, and am a happy user.
Mike
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Meade 8" LX10
Orion ED100 + Losmandy GM-8 + Berlebach UNI 24
Meade 12" LightBridge
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Jason D
Post Laureate
Reged: 10/21/06
Posts: 3311
Loc: California
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Here is my take:
When you insert a laser inside a barlow then attach a perforated screen at the bottom of the barlow, sensitivity to wobble will be minimum – hardly noticeable. The reason is because in this setup, the virtual pivot point of the wobble is close to the center between the virtual laser emitter source point and the center of the screen. That is, when the whole thing tilts (wobble), the screen center and the virtual laser emitter source point will move in opposite directions -- almost by the same amount -- cancelling each other’s movement. The result is an almost stationary center spot shadow with respect to the screen.
In your case, you have a fixed location for the screen. Now when you wobble the laser+barlow, the virtual emitter point will move; however, the screen will not. Therefore, there is no motion cancellation and the effect will be readily noticeable as shown in your video.
Of course, there is NO wobble when you insert a Glatter into a tuBlug which means the above phenomenon will not be observed.
Jason
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XT10 classic with premium optics
Tri-knob CR2 with compression rings
Round Table Platform
4.5" StarBlast
6" StarBlast6
TV EPs
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Nathan F
sage
Reged: 10/10/08
Posts: 223
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Quote:
Here is my take:
When you insert a laser inside a barlow then attach a perforated screen at the bottom of the barlow, sensitivity to wobble will be minimum – hardly noticeable. The reason is because in this setup, the virtual pivot point of the wobble is close to the center between the virtual laser emitter source point and the center of the screen. That is, when the whole thing tilts (wobble), the screen center and the virtual laser emitter source point will move in opposite directions -- almost by the same amount -- cancelling each other’s movement. The result is an almost stationary center spot shadow with respect to the screen.
In your case, you have a fixed location for the screen. Now when you wobble the laser+barlow, the virtual emitter point will move; however, the screen will not. Therefore, there is no motion cancellation and the effect will be readily noticeable as shown in your video.
Of course, there is NO wobble when you insert a Glatter into a tuBlug which means the above phenomenon will not be observed.
Jason
That makes sense to me.
As for wobble being at a minimum in the classic barlowed laser configuration, that's sort of the conclusion that I reached, myself: a reasonably collimated laser collimator used in a decent barlow will result in good accuracy - in my test, the hand held beam probably ventured off from center by an order of magnitude more than even the most out of whack laser collimator would ever be.
The unfortunate consequence of all of this is that my little homemade tuBlug just got a whole heck of a lot harder to build, since I essentially need to make the laser/lens collimatable now, rather than just sticking it in so that the diffused light covers the spot adequately and calling it good. But, if I'm not mistaken, once I get to a point where I can rotate the whole unit in the focuser and have the diffused light blob rotate directly around the exact center of the primary, that should be good enough.
I have a sneaking suspicion though that this project may end up going nowhere fast if I can't get this thing mechanically sturdy enough that a) I can collimate it accurately, and b) that those adjustments stick after tossing the thing around a bit. I see now why people pay Glatter the big bucks for this stuff. 
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12 inch f/5 Deep Space Observer Dob
31mm Nagler, 13mm Ethos, 1.6x Antares 2 inch Barlow
Celestron SkyMaster 15x70 Binoculars
Homemade Maple Parallelogram Binocular Mount
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