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Collimation is about to make me dump both my Dobs

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#76 Nils Olof Carlin

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Posted 25 February 2012 - 02:39 PM

The astronomyshed video, and even better in this video, you see a potential problem of using the laser put into a Barlow lens, in such a way that the spot shadow is read *outside* the lens. The shadow is magnified by the Barlow, in such a way as to be geometrically distorted when falling on the 45 deg faceplate (it is offset, analogous to the offset of the secondary!). I suspect this just might introduce a significant error, it would take some calculation using realistic parameters to see what it does. Also, there is a strong reflection (despite coatings) from the lens. In this video, the reflection is just a tad smaller than the "shadow" of the spot's perforation, barely avoiding making it unreadable. I guess this is a matter of luck.
(With the blug and tublug, none of these problems arise).

Nils Olof

#77 calibos

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Posted 25 February 2012 - 02:46 PM

I see little need for that video camera. But it is quite doable to put a webcam on a Cheshire tool, even if you have to change the lens to one with longer focal length - this would let you twiddle the collimation knobs on the primary while watching the laptop...


Actually, this sounds like a terrific idea in need of practical implementation adapted to the Catseye Cheshire (aka Blackcat). The main design issue to solve is the mount adapter for the webcam: how to implement a mount that securely attaches to the Blackcat and centers the webcam lens over the peephole. The two other issues involved are choosing a webcam with a lens design/focal length appropriate for the application, and whether it's possible to design the mount so it accepts certain type(s) of off-the-shelf generic webcams, or whether it would be necessary to instead obtain the raw internal camera elements and build the mount around them.

It sure is handy whenever my scope is significantly out of collimation to begin the primary mirror alignment phase with the Glatter TuBlug/Laser and get the collimation close enough that what's left for the Catseye Blackcat and Autocollimators is the final tweaking to insure that alignment is dead spot-on and not merely close.


Hint hint ;) http://www.youtube.c...h?v=Vj12cx3tnsM

The problem is that reflections are at different virtual/real distances and it is hard to bring all to focus simultaneously.


Jason, I think you should post a link to that Vid everytime an acronym, formula and equation competition breaks out in a collimation thread between yourself, Vic and Nils Olof :D For the benefit of all those that get scared of collimation when they read all the math. Just to remind them that all those 20 pages of math basically amounts to a few twists of some adjustment knobs. :D

I used to be scared of the Cats Eye gear when reading all the formula and talk of offset pupils and PAE's and FAE's etc. After watching that vid a while back I realised that it only sounds complicated written down but after seeing the process in action its actually very simple. That vid is the best advertisement for Jims gear ever. I'll be getting the full set to join my Glatter gear as soon as funds allow. And do you know what, I'm not sure I even need it especially after the work you did on the templates etc for the Glatter gear but gosh darn it, I just want to play with the Cats Eye gear after watching those vids and knowing I have 'Perfect' collimation. :D

Your youtube video's have done more to remove the mystic or the fear of collimation than all the books put together. Of course there is a place for Vics excellent tomes and as many people as possible should of course learn the whys as well as the hows of collimation but for someone moving from other scope types to newts, just the fact that Vics collimation tutorials is an 80 page book rather than a pamplet is scary!! :D "Woah!! 80 pages, sounds complicated, think I'll stick to my 'fractor!!":D

#78 Starman1

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Posted 25 February 2012 - 04:53 PM

I tried to make mine as simple as I could, but it still ended up 19 pages:
http://www.cloudynig...hp?item_id=2677
But a lot of it is trying to make people understand what they're doing when they collimate.
The actual "instructions" aren't that long and could be distilled into on page if the pictures and illustrations were sacrificed.

#79 CatseyeMan

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Posted 26 February 2012 - 06:16 AM

I find it interesting that he invested money on Catseye tools as evidenced by the hotspot on his primary...

No tools - just the HotSpot.

#80 CatseyeMan

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Posted 26 February 2012 - 06:32 AM

I've watched that video before, and it's a very good one. Nevertheless, unless I overlooked something in one of the segments, nowhere does it show how you mounted the (web?)cam in position to view through the peep-hole of the cheshire or autocollimator...


The hardware used to generate the video is a special prototype setup I made for Jason in which the camera is secured squarely in the collimation tool to insure covergence of camera axis with focuser axis for both the cheshire and autocollimator. It is the genisis for a new CATSEYE "video" Cheshire on tap for introduction in the near future ... stay tuned ;-)

#81 CatseyeMan

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Posted 26 February 2012 - 06:56 AM

Watch This


The video technique potrayed prior to the Barlowed laser demo is equivalent in all respects to the functionality of a standard sight tube with perhaps some addional positional and axial error resolution provided by the camera/computer zoom display features. His demonstration of axial alignment correction via Secondary tilt/rotation adjustment after introducing Secondary rotational error does not indicate a laser "lie" but rather demonstrates the fact than there is an infinite number of Secondary tilt/rotation combination settings that will achive focuser axial alignment. This fact is the one pet peave of Vic Menard that he so ardently instucts folks to be cognisant of. Achieving a "circular" projection of the Secondary "ellipse" to the eye is a collimation factor often overlooked in the quest for axial alignments, and in severe deviation cases can effect image brightness and contrast.

#82 calibos

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Posted 26 February 2012 - 09:48 AM

I can't help but think of a quote from Bertrand Russel:

The trouble with the world is that the stupid are cocksure and the intelligent are full of doubt


Now I am not saying the guy in the Vid is stupid. I don't think he is, but I do wonder about the type of person that posts a tutorial youtube vid and speaks in an authoritive tone when he patently doesn't really understand what he is talking about and when a small amount of research would have shown him that he didn't really understand what he was talking about. I mean how can you not check your facts before proceeding with posting your vid. Then you have the corollory where Jason and Jim probably delayed the posting of their vid until they made sure they weren't making any mistakes, had all their facts straight and had polished the vid to perfection.

#83 Nils Olof Carlin

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Posted 26 February 2012 - 11:25 AM

I ventured:

The shadow is magnified by the Barlow, in such a way as to be geometrically distorted when falling on the 45 deg faceplate (it is offset, analogous to the offset of the secondary!). I suspect this just might introduce a significant error



I assume a "short" 2x Barlow with 70mm f.l. and the center of the target 85 mm from the (thin) lens. If you have a 6 mm perforation in the center marker to align, the edge of the shadow is offset by about 0.3 mm - perfect alignment would cause a miscollimation of the primary of 0.3 mm. This may not be obvious, but is not entirely negligible. But suppose the direct reflexion is too big and bright, and you have to align by the outer margin, of 10 mm diameter. Then the spot will be magnified to about 22 mm (barely fitting the target?), but the offset is about 0.8 mm - hardly negligible.
Caveat emptor

Nils Olof

#84 Starman1

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Posted 26 February 2012 - 12:05 PM

Several years ago I visited with the guys at Parks to find out how they collimated their scopes.
They did not center-mark their primary mirrors and I wanted to know how one did that.

I discovered that they spent an average of a day to a day and a half to collimate each scope! I kid you not. A day to a day and a half of manufacturing time spent collimating each newtonian telescope!

Mechanical tilts of the focuser and centering of the primary mirrors in the tube occupied quite a bit of time.
Primary alignment was done by shining a bright light into the tube without the secondary spider installed and getting the primary to reflect back at a transparent bullseye template over the front of the tube, aligning the optical axis of the primary to the centerline of the tube.

Then the spider was installed and the reflection of the primary was aligned with a transparent grid placed over the secondary mirror that had the proper amount of offset. You see, the secondaries were centered in the tube using templates. This process took several hours.

The result was a collimated telescope that did NOT have the reflected image of the primary centered in the secondary. It was offset toward the lower end of the secondary mirror.

Why this type of alignment?

Well, it was the way it was done in the '50s because of the long focal ratio telescopes of the time, and the illumination of the periphery of the field wasn't "on the radar". Some of the manufacturers of the time built an offset into the secondary holder so the spider could be centered but the secondary would be offset slightly bi-directionally (away from the focuser and toward the primary) so there would be uniform illumination at the edge of the field, but Parks did not do this.

The main problem, I pointed out, was that if the scope came out of collimation in transit, the person at home stood no chance of re-collimating the scope because all the popular collimation tools required a center marker of some sort.

The first thing I had to overcome in their thinking (remember, no computers or DSCs on their mounts) was that it wasn't essential to maintain the optical axis of the scope coincident with the mechanical axis of the tube. Once that was done, I got them to finally center mark their mirrors with triangles. Also, painstakingly centering the primary mirror in the tube to less than 1mm wasn't necessary because their mirror cells centered the mirrors just fine. And the focuser's alignment on the tube was close enough without shimming in order to make it accurate to less than a millimeter.

They had a set of the 3 Tectron collimation tools lying around, so I taught them the standard procedure used today to collimate scopes:

1) Center the secondary under the focuser using the front inside edge of the sight tube to do so. And rotate the secondary until it appears round.
2) Adjust the tilt of the secondary to point at the center of the primary using the sight tube's crosshairs.
3) Adjust the tilt of the primary, using the cheshire, until the center triangle is in the center of the central black area in the cheshire's reflection
4) Stack the triangles in the autocollimator using Vic Menard's Carefully Decollimated Primary (CDP) technique, doing the final alignment here using the locking screws on the primary mirror cell (their cells use locking screws because they use lightweight springs--that was something I couldn't get them to change).

Step 1 should take maybe 1 to five minutes depending on how far up or down the tube the secondary had to be moved.
Step 2 should take the same or less.
Step 3 should take the same time or less.
Step 4 maybe another ten minutes because of the monkeying around with locking screws.
Total time 10 to 25 minutes depending on how easily everything lined up. And that is because everything is being assembled from scratch and nothing will be even close to lined up at first. I emphasize that collimating a scope only slightly out of collimation is a 5 minute or less job.

I tried assembling one of the scopes, and the optical tube took 30 minutes to assemble from a pile of parts to a finished, collimated, optical tube assembly (the tube already had its holes drilled). And I didn't work that fast because I was talking to the guys as I assembled the scope.

The video techniques used by the referenced video maker reminds me a lot of the original Parks techniques. A lot of what's done is completely inessential to achieve collimation and is unnecessarily confusing to someone watching it. I won't argue that collimation is impossible using his techniques but I certainly will argue they are unnecessarily confusing and complicated. I've collimated hundreds of scopes, and his explanations were confusing to me.

There seems to be a subset of people who will never understand how to do something without watching a video of someone doing it. So I applaud any attempts by collimation experts like Jim Fly and Jason Khadder to create videos that show people how to do something that might sound confusing in a written explanation. I just wish that there weren't some commonly-linked videos so filled with misinformation and confusion.

#85 Jason D

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Posted 26 February 2012 - 02:00 PM

I ventured:

The shadow is magnified by the Barlow, in such a way as to be geometrically distorted when falling on the 45 deg faceplate (it is offset, analogous to the offset of the secondary!). I suspect this just might introduce a significant error



I assume a "short" 2x Barlow with 70mm f.l. and the center of the target 85 mm from the (thin) lens. If you have a 6 mm perforation in the center marker to align, the edge of the shadow is offset by about 0.3 mm - perfect alignment would cause a miscollimation of the primary of 0.3 mm. This may not be obvious, but is not entirely negligible. But suppose the direct reflexion is too big and bright, and you have to align by the outer margin, of 10 mm diameter. Then the spot will be magnified to about 22 mm (barely fitting the target?), but the offset is about 0.8 mm - hardly negligible.
Caveat emptor

Nils Olof


Very interesting observation, Nils Olof

For the benefit others, I put together the attached illustration.

The point is that no one accounts for the "offset" mentioned by Nils Olof and everyone attempts to center the shadow with the back 45 degree screen. Doing do will introduce an error described by Nils Olof.

Another important point. The error mentioned above mostly applies to a setup that uses a regular barlow with a regular laser collimator with a 45 dgree back window. It does NOT apply to Glatter’s TuBlug. The reason is that the distance between the barlow lens and the screen has the most impact on this error. For the TuBlug, the distance between the screen and the lens is virtually zero. Let me clarify further, the distance between the lens and screen is what will create the cone, hence, the offset mentioned by Nils Olof. For the TuBlug, there is really no cone, hence, no offset to worry about. OK, I will be picky. There is a cone because the laser virtual point is not at the focal plane but that cone has a very very large F-ratio which implies the offset will be virtually zero for the TuBlug.

Jason

Attached Thumbnails

  • 5092178-backwindow_barlow_error.png


#86 Jason D

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Posted 26 February 2012 - 02:05 PM

I discovered that they spent an average of a day to a day and a half to collimate each scope! I kid you not. A day to a day and a half of manufacturing time spent collimating each newtonian telescope!


:scared:

Very interesing post, Don

#87 Jason D

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Posted 26 February 2012 - 02:35 PM

but I do wonder about the type of person that posts a tutorial youtube vid and speaks in an authoritive tone when he patently doesn't really understand what he is talking about and when a small amount of research would have shown him that he didn't really understand what he was talking about. I mean how can you not check your facts before proceeding with posting your vid.


Keith, this guy has been making the "lasers lie" claims for several years. I did my best to correct him to no avail. Here is one example of many (TopHouse is the same guy):
http://stargazerslou...563-post20.html
But instead of engaging in a constructive discussion, here is what he had to say
http://stargazerslou...hotos-4.html#78

Jason

#88 Nils Olof Carlin

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Posted 26 February 2012 - 03:49 PM

I did my best to correct him to no avail.


Seems at the top posting on page 4, e quotes me - I may have posted on the forum, but if so, I have no memory whatsoever of doing so. Well, I'm an old man...
If someone tells you that you are not quite right (=wrong, but not nice to say outright) on some matter, the natural thing to do is write a rebuke, rather than considering the remote possibility that *you* may be wrong, more or less. This is what TopHouse did - suppressing that urge may be difficult, but as far as I recall, we both excelled in our early exchanges of ideas along time ago, so we know it is possible :grin: and on the whole, constructive.
Not that it has anything to do with anything, but I am so fascinated by the way he says "circles"...
Talking of horror movies, this one featuring the Men in Black is quite stunning - imagine you are a newbie stumbling on it :shocked:
But this one has one of the most fascinating laser spots I know :grin:

Nils Olof

BTW thanks for illustrating my offset ideas on the wrong placement of the Barlow - as I noted in my original posting:
this is *not* a problem with the Blug or Tublug!!!

#89 Jason D

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Posted 27 February 2012 - 12:41 AM

Nils Olof,
Check the attachment. The "offset" might not be as bad. Placing the laser virtual point above the focal plane will make the center spot image smaller at the barlow lens and will move the projection point farther away from the lens. The end result is a cone with higher F-ratio which implies smaller offset.
Jason

Attached Thumbnails

  • 5093232-backwindow_barlow_error2.png


#90 Vic Menard

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Posted 27 February 2012 - 11:32 AM

Very interesting observation, Nils Olof

For the benefit others, I put together the attached illustration.

The point is that no one accounts for the "offset" mentioned by Nils Olof and everyone attempts to center the shadow with the back 45 degree screen. Doing so will introduce an error described by Nils Olof.

Just to further clarify, what if the windowed laser is one like this. In this case, the target is not tilted, but the distance between lens and screen is still large.

(The images on page 8 describe the "multiple bounce dots" Mark Harry discussed earlier in this thread.)

#91 Nils Olof Carlin

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Posted 27 February 2012 - 12:55 PM

Correction:

If you have a 6 mm perforation in the center marker to align, the edge of the shadow is offset by about 0.3 mm - perfect alignment would cause a miscollimation of the primary of 0.3 mm. This may not be obvious, but is not entirely negligible. But suppose the direct reflexion is too big and bright, and you have to align by the outer margin, of 10 mm diameter. Then the spot will be magnified to about 22 mm (barely fitting the target?), but the offset is about 0.8 mm - hardly negligible.



I calculated the offset of the shadow. But the primary axis error at the focal plane is (as usually) only half this. The shorter the barlow, and the larger the circle on the shadow that you use, the more offset, but it seems likely that the collimation error will be less than my maximum of 0.4 mm, and the miscollimation will not be so large as to be obvious if you are not aware of the mechanism.

A perpendicular screen like the Baader collimator does not cause the offset of course, but you can't read it from below like you can with a tilted one.

#92 pacostiro

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Posted 27 February 2012 - 03:40 PM

Hi! I don't think the projection is correct. The distortion should also show a bigger magnification in the lower part of the shadow than on the upper part.
Could you do by drawing the projection of the center of the hotspot, I mean to draw the ray of light?

#93 Jason D

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Posted 27 February 2012 - 03:54 PM

Hi! I don't think the projection is correct. The distortion should also show a bigger magnification in the lower part of the shadow than on the upper part.
Could you do by drawing the projection of the center of the hotspot, I mean to draw the ray of light?


I understand what you are saying but that illustration is meant to clarify a point made by Nils Olof. It is not meant to be precise.

#94 pacostiro

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Posted 28 February 2012 - 01:15 AM

Ok, I understand. I think that trying to center the central hole rather than the edges could decrease a lot the error.
I exemplify with an easier to draw donut shape in this image how I think the projection should look like when the collimation is correct.
Posted Image
In some cases like mine where the marking is solid (a solid triangle), is more difficult to avoid it since the center is not projected in any way.
However, this remark is great and I never thought at it before seeing it here.


#95 Nils Olof Carlin

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Posted 28 February 2012 - 02:11 PM

Ok, I understand. I think that trying to center the central hole rather than the edges could decrease a lot the error.



Indeed. But the limitation here may be the direct reflection of the laser beam in the concave lens surface - in the astronomyshack video, I believe it is about as large as the reflection of the central hole. But if the latter reflection is smaller, it will be totally "blinded" and unreadable, so you have to use a larger circle, if any.. This depends on local geometry, I can't predict in a general case. Using a "longer" Barlow will also help much.
Actually, the thought hadn't occurred to me much before I wrote my first comment on the subject in this thread, and as it was late local time, I needed a night's sleep before doing some numerical estimates of the effect - arriving at the conclusion that the effect isn't clearly significant, and on the other hand, not clearly insignificant either. ;)
Your illustration looks quite good to me!

#96 Jason D

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Posted 28 February 2012 - 02:43 PM

In many cases you do not even see the center spot shadow and have to rely more on concentric circles that magically appear. See attachment. These circles are relatively large.
The 2X barlow will blur the shadow.

Attached Thumbnails

  • 5095929-laser_mate_barlowed.JPG


#97 Jason D

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Posted 28 February 2012 - 02:43 PM

Same scope but with Glatter and TuBlug

Attached Thumbnails

  • 5095933-tublug2.jpg



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