kroum
sage
Reged: 08/28/08
Posts: 203
Loc: Bay Area, CA
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A couple of years ago I centered my primary mirror. I used the paper-circle-with-hole-in-it method, but I don't think I did a very good job at it. I want to redo it.
My question is this: How did you guys remove the sticker before redoing it?
And another thing, Is there an easy way that doesn't require any expensive gear to do it more accurately? I traced out the mirror on a piece of paper, then I cut it out, but I can't cut it out perfectly, nor perfectly center it on the mirror afterword.
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mazzefr
I'll take door number three...
Reged: 02/12/07
Posts: 1763
Loc: Behind the Wheel in PA
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I use alcohol and toothpicks to remove the existing center spot
fixed linky
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Franco
Orion XT12i w/ MoonLite
13T6, 9T6, Pan24
Scopos 35mm
Lumicon UHC, Baader M&S
DIY- Denver Chair, Light shields
"All that is necessary for the triumph of evil is that good men do nothing." (Edmund Burke)
Lehigh Valley, PA
Edited by mazzefr (10/27/08 05:42 PM)
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tigerroach
sage
Reged: 08/13/08
Posts: 364
Loc: Houston, TX
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As far as placing the new one, the Catseye people sell templates.
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Brian
TeleVue TV-102, Gibralter alt-az mount
Webster 14.5" f/4.3 truss dob *under construction*
Canon 10x30 & 15x50 IS binocs
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jimmoscheck
Pooh-Bah
Reged: 01/08/07
Posts: 1094
Loc: Under the SE Michigan lightdom...
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Yeah, the Cateye template made it real easy to mark the center. You'll also get a Catseye triangle with the template. Highly recommended.
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Jim Moscheck
14.5" f4.3 home built Kriege stlye truss
Stevens primary
"A mind is like a parachute. It doesnt work if it's not open." - Frank Zappa
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kroum
sage
Reged: 08/28/08
Posts: 203
Loc: Bay Area, CA
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Thanks for the replies. I was hoping to do this without having to order stuff online, but I might end up doing it for the template... Again though, how do you center the template and is there a hole in the middle so you can put the sticker on? Do they give you just one triangle sticker with the template?
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jimmoscheck
Pooh-Bah
Reged: 01/08/07
Posts: 1094
Loc: Under the SE Michigan lightdom...
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IIRC they give you one with the template but you can buy another. The template is clear plastic with circles scribed on it for different mirror sizes. The center has a small hole that you place a piece of scotch tape over. The tape holds the triangle(with the sticky side down) to the underside of the template, you center it on the mirror and simply press down with your finger to stick the triangle exactly in the center of the mirror. It's actually much faster than it just took me to describe.
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Jim Moscheck
14.5" f4.3 home built Kriege stlye truss
Stevens primary
"A mind is like a parachute. It doesnt work if it's not open." - Frank Zappa
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KerryR
sage
Reged: 12/05/07
Posts: 424
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I recently did this on my 16". Be bery carefull removing the old one. I pulled a some of the aluminum off of my mirror, even after soaking the ring with water, alcohol and soap (cleaning solution). Make sure you really soak it, and perhaps use pure alcohol which might disolve the sticky stuff better than the cleaning mix. Luckily, the pulled off aluminum is only in the center, and most of it is under the new ring, but it's irritating because it shows when I collimate with a barlowed laser, and makes centering the image a little more difficult.
I centered the new ring this way, using a self-sticky ring: Cut out a square containing the ring. Remove a small section of the paper backing to expose a small crescent of sticky stuff. Gently place the ring on the mirror, making sure the sticky crescent doesn't stick. Using a tape mesure that either shows 1/32s or mm, pull measurements from many sides, constantly adjusting the ring (remember, it only has a small crescent of sticky exposed, and you must make sure you don't accidentaly push it into contact while measuring/moving) until all measurements are as close to the same as possible. Once you are satisfied with the measurements, push the sticky crescent down, very carfully. Be sure you didn't move the ring when you did this by re-checking your measurements. If everything is still OK, bend the paper back upward (the crescent portion of the ring stays put because it's stuck down) and remove the remaining backing, and finish sticking the ring down. This is tedious, but quite accurate if you do enough itterations...
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Kerry
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mazzefr
I'll take door number three...
Reged: 02/12/07
Posts: 1763
Loc: Behind the Wheel in PA
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Quote:
I use alcohol and toothpicks to remove the existing center spot
fixed linky
Fixed link to a thread with a couple of ideas but I think you did them already. I know there are others floating around here somewhere. 
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Franco
Orion XT12i w/ MoonLite
13T6, 9T6, Pan24
Scopos 35mm
Lumicon UHC, Baader M&S
DIY- Denver Chair, Light shields
"All that is necessary for the triumph of evil is that good men do nothing." (Edmund Burke)
Lehigh Valley, PA
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hudson_yak
sage
Reged: 11/15/07
Posts: 498
Loc: Hyde Park, NY, USA
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I did something sort of like this, but the measuring was becoming tedious. Since my dot was a 1/4 inch round (using a notebook hole punch to make) I cut four equal-length paper strips that, when placed in a cross pattern with ends lined up at the mirror perimeter, left an open area in the middle to place the dot.
The strips were about a 1/4 wide, and to make sure they were all the same length I first cut a 1-inch strip of the proper length for my mirror radius (with accurately square ends) and then cut that lengthwise into the narrow strips.
After some partial sticking/adjusting, as Kerry described, and making sure the strips hadn't moved, I wound up with the dot quite well-centered, checking with a ruler afterward.
This method wouldn't work very well with a triangle mark though.
Mike
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Meade 8" LX10
Orion ED100 + SVP
Meade 12" LightBridge
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Buck
sage
Reged: 04/29/05
Posts: 475
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This is easier. Get a piece of wrapping tissue, the kind that is wrapped around a shirt or robe. It can be had where wrapping paper is sold. Lay it out flat and set the mirror on it and carefully trace around the edge of the mirror. Now fold the circle in half, then fold it in half again so you have a 1/4 piece of pie so to speak. Now nip a very small piece of the tip off, then open the paper. You should now have a tiny hole in the exact center of the circle. Lay the paper over the mirror aligning the circle to the mirror edge and use a marker to place a dot, through the hole, onto the center of the mirror. Your mirror is now center spotted. Now place a donut or triangle on the mirror with the dot centered in the hole. All done.
Buck
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hudson_yak
sage
Reged: 11/15/07
Posts: 498
Loc: Hyde Park, NY, USA
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Not so sure it is easier. For one thing, you have to unmount the mirror. For another, it's not too hard to imagine getting the tracing and/or folding and/or dotting and/or marker sticking off a millimeter or even more with this method.
Mike
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Meade 8" LX10
Orion ED100 + SVP
Meade 12" LightBridge
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kroum
sage
Reged: 08/28/08
Posts: 203
Loc: Bay Area, CA
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Quote:
Not so sure it is easier. For one thing, you have to unmount the mirror. For another, it's not too hard to imagine getting the tracing and/or folding and/or dotting and/or marker sticking off a millimeter or even more with this method.
Mike
That is precisely what I think I did last time, so I'm doing it over.
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Buck
sage
Reged: 04/29/05
Posts: 475
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I guess it would depend on the mirror mounts. All the ones I've done were easily removed. Several solid tube and 2 truss types and all were held in with retainer clips so a few screws and the mirror is out, and 12" was the largest. I checked them on a horizonal turret table and spun the mirror till it was concentric and the dot was spot on in all but one which I had to do over. You do have to be careful but it is quite easy. However the Catseye product is the best and most accurate.
Buck
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Starkler
professor emeritus
Reged: 11/04/05
Posts: 698
Loc: Australia, Melbourne
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Be aware that not all mirrors have an even bevel all the way around. I discovered this when i found that my catseye template just wouldnt line up with the mirror and i suspected that the template was not perfectly circular. The template later proved to be circular and my bevel did not.
Any of these template solutions have this limitation using the bevelled edge as the visible reference.
One solution I have heard of is butting the mirror edge against a block of wood and using a divider holding a fine tip pen set at slightly > mirror radius, mark a small arc at the centre, rotate mirror 120 degrees, mark again, then once more.
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Geoff
15" SDM truss dob | Vixen r130sf | GSO 10" dob
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hudson_yak
sage
Reged: 11/15/07
Posts: 498
Loc: Hyde Park, NY, USA
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Quote:
using the bevelled edge
A good point. I lined up my little paper strip ends with the vertical ground surface to avoid that issue, ignoring the bevel.
Mike
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Meade 8" LX10
Orion ED100 + SVP
Meade 12" LightBridge
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Buck
sage
Reged: 04/29/05
Posts: 475
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I always use the vertical edge or OD of the mirror.
Buck
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kroum
sage
Reged: 08/28/08
Posts: 203
Loc: Bay Area, CA
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Ok, I'm trying to understand this but what do you mean Mike, by the "vertical ground surface"? Also Geoff: by divider, do you mean a protractor with a felt tip pen?
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hudson_yak
sage
Reged: 11/15/07
Posts: 498
Loc: Hyde Park, NY, USA
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I meant the extreme outer edge of the mirror, the part that's machined (ground) into a cylindrical profile. I knew it would be confusing the way I phrased it but nothing better came immediately to mind.
The assumption usually made is that the mirror is formed, both the parabolic surface and the outer edge, while being turned on the same machine, such that the optical center (where you want to mark it) should be measured from there, not from the inner edge of the bevel, which may not be as precisely machined.
Mike
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Meade 8" LX10
Orion ED100 + SVP
Meade 12" LightBridge
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Albie
Post Laureate
Reged: 02/22/05
Posts: 3548
Loc: Alberta,Canada
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Quote:
how do you center the template and is there a hole in the middle so you can put the sticker on?
Here is a picture of my 10" mirror being spotted.The template has a hole in the middle.The triangle is under the template with a piece of tape on top of the hole.The tape keeps the triangle in place.You peel the backing off the triangle and then center the template on the mirror.Then you press down on the triangle and presto it's stuck in the center of your mirror.The glue on the triangle is stronger than the glue on the tape so there's no chance of peeling the spot off when you remove the template.
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Skywatcher 10" f/4.7 Dob with Moonlight CR1
Televue 2x barlow and 5T6,7T6,9T6,16T5 and 26T5 Televue Naglers.
SkyMaster 15x70,Regal LX 8x42 and Noble 10x28
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Albie
Post Laureate
Reged: 02/22/05
Posts: 3548
Loc: Alberta,Canada
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Template gone spot remains
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Skywatcher 10" f/4.7 Dob with Moonlight CR1
Televue 2x barlow and 5T6,7T6,9T6,16T5 and 26T5 Televue Naglers.
SkyMaster 15x70,Regal LX 8x42 and Noble 10x28
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Starkler
professor emeritus
Reged: 11/04/05
Posts: 698
Loc: Australia, Melbourne
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Quote:
Also Geoff: by divider, do you mean a protractor with a felt tip pen?
I mean something similar to this http://www.carpettool.net/380.jpg
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Geoff
15" SDM truss dob | Vixen r130sf | GSO 10" dob
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proud uncle
Pooh-Bah
Reged: 01/22/07
Posts: 1357
Loc: Central Texas
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Quote:
Yeah, the Cateye template made it real easy to mark the center. You'll also get a Catseye triangle with the template. Highly recommended.
I have three related questions:
Can the template be saved and re-used on other mirrors? Of course, it would be necessary to get additional triangle spots.
Jim Fly has both red and white reflective triangles. What are the pros and cons of one vs. the other?
He also offers triangles with or without the central hole punched out. He states, as I recall, the central hole is mostly for use of laser collimators. Is there a preference for those planning to use sight tube and Cheshire?
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Kenneth
Zhumell 10" Dobsonian (f/4.9)
2" 32mm WA eyepiece
9mm, 12.5mm, and 20mm Plossls
6mm TMB/BO Planetary
2" 2x ED Barlow
Nikon 10x50 binocular (6.5 deg FOV)
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jimmoscheck
Pooh-Bah
Reged: 01/08/07
Posts: 1094
Loc: Under the SE Michigan lightdom...
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Quote:
Can the template be saved and re-used on other mirrors? Of course, it would be necessary to get additional triangle spots.
Yes.
Quote:
Jim Fly has both red and white reflective triangles. What are the pros and cons of one vs. the other?
He only had red when I ordered, but I would have preferred white. I use a laser so I have the one with the hole. I also use an auto-collimator to do the final tweak of the secondary and white would certainly be easier to see than the red triangle.
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Jim Moscheck
14.5" f4.3 home built Kriege stlye truss
Stevens primary
"A mind is like a parachute. It doesnt work if it's not open." - Frank Zappa
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kroum
sage
Reged: 08/28/08
Posts: 203
Loc: Bay Area, CA
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Thanks for the photos Albie, that was very informative. Actually seeing it done, I no longer have to imagine how it works.
Geoff: We were thinking the same thing, and using different words. 
I believe I have enough information to do this now. I just have to decide whether to drop money for the very nice looking template, or use a protractor which also sounds promising.
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redsquash
super member
Reged: 08/05/08
Posts: 105
Loc: JAPAN
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Quote:
This is easier. Get a piece of wrapping tissue, the kind that is wrapped around a shirt or robe. It can be had where wrapping paper is sold. Lay it out flat and set the mirror on it and carefully trace around the edge of the mirror. Now fold the circle in half, then fold it in half again so you have a 1/4 piece of pie so to speak. Now nip a very small piece of the tip off, then open the paper. You should now have a tiny hole in the exact center of the circle. Lay the paper over the mirror aligning the circle to the mirror edge and use a marker to place a dot, through the hole, onto the center of the mirror. Your mirror is now center spotted. Now place a donut or triangle on the mirror with the dot centered in the hole. All done.
Buck
No need to put a dot is there? Just put the small circle you cut out, back into its original place, with some adhesive, with the template, which is on the mirror
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Vixen R130 f/5
powermate 2.5
vixen LVW 5mm ,13mm
Vixen LV 8-24mm zoom
Whats the next lens to complete this set up.please send me a PM if you like
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redsquash
super member
Reged: 08/05/08
Posts: 105
Loc: JAPAN
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What margin of error is acceptable when marking the centre of a primary mirror?
Its impossible to mark dead centre if you use small enough units.
Is there a formula which tells you what is an acceptable error?
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Vixen R130 f/5
powermate 2.5
vixen LVW 5mm ,13mm
Vixen LV 8-24mm zoom
Whats the next lens to complete this set up.please send me a PM if you like
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Buck
sage
Reged: 04/29/05
Posts: 475
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Never assume that the previous donut or triangle was in the correct position. The dot itself makes a good reference point to aim a laser while adjusting the secondary, provided the laser is also collimated.
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Buck
sage
Reged: 04/29/05
Posts: 475
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If there is a formula, I do not know what it is. As far the acceptible amount of error, is 1mm or less is the number I hear most. The faster the mirror the closer it must be.
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Jason D
Carpal Tunnel
Reged: 10/21/06
Posts: 2067
Loc: California
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Quote:
Is there a formula which tells you what is an acceptable error?
There is. It is "d=0.034D" where "d" is the error and "D" is the Diameter of your primary (aperture). The formula was provided by Nils Olof and Vic Menard. If you are using a Paracorr then the error margin drops by 6X and the formula becomes "d=(0.034D)/6".
Aside from formulas, if you mis-center the spot then you will introduce focuser axial misalignment which will translate to the tilting the primary's focal in relation to the EP focal plane. Think of it as holding a magnifier against a tilted page.
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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hudson_yak
sage
Reged: 11/15/07
Posts: 498
Loc: Hyde Park, NY, USA
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Do the math on that and you would get 10mm for a 12-inch mirror. That is the error allowed for focuser axis (tilt) collimation. The dot needs to be centered far more precisely than that to get the primary collimated well enough. There's a different formula for that one, somewhere, but to generalize you want to get it as close as you can, inside a 1 mm error would be a good target.
Edit: This page talks about tolerances for both types of errors. The 1A error is the important one. It's dependent on F-ratio, not diameter. Basically, as I said you want it less than 1mm off to feel good about it.
Mike
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Meade 8" LX10
Orion ED100 + SVP
Meade 12" LightBridge
Edited by hudson_yak (11/01/08 04:59 PM)
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CatseyeMan
Vendor (Cats Eye Collimation)
Reged: 12/16/04
Posts: 207
Loc: Huntsville, AL USA
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Quote:
Quote:
Is there a formula which tells you what is an acceptable error?
There is. It is "d=0.034D" where "d" is the error and "D" is the Diameter of your primary (aperture). The formula was provided by Nils Olof and Vic Menard. If you are using a Paracorr then the error margin drops by 6X and the formula becomes "d=(0.034D)/6".
As Mike stated, the formulations you quoted are for "Focuser" axis error tolerance.
From Vic's "New Perspectives on Newtonian Collimation" 5th edition (if you don't have your copy yet Jason, you will soon), pa. 41, the Primary Mirror axial mm tolerance is .005 x fno^3. At f5, it's 0.63mm; for f4, it's .32mm.
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Jim Fly - Manufacturer
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Jason D
Carpal Tunnel
Reged: 10/21/06
Posts: 2067
Loc: California
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Quote:
(if you don't have your copy yet Jason, you will soon)
Jim, I am looking forward to receive my copy 
Vic, I need your help here. Refer to the attachment
Figure “A” (Nirvana – no errors): I included it for reference. The coma-“free” area (shown in purple) is centered and perpendicular to the focuser axis
Figure “B” (Tilt error) shows the 0.034D calculation at the primary end assuming no lateral error at the focuser end. That is, the coma-“free” area is still centered but tilted. The 0.034D calculates the maximum allowable tilt at which no part of the coma-“free” edge exceeds ¼ PV error. (Note: Let us leave the paracorr effect out of this discussion for now)
Figure “C” (Lateral error) shows the 0.005F^3 calculation at the focuser end assuming no tilt error. That is the maximum allowable shift of the focal plane before the PV error at the center of the FOV exceeds ¼ PV.
Now, let us assume someone accidentally placed the |
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