I am having a conniption trying to place a dot as precisely as possible on my 22" primary. Does anyone have any advice on this - I just recently had the mirror recoated and so it's time to place the dot!
Hang it up, throw darts and when someone shouts "Bullseye"........
I'll go away now......
OK, random thoughts/guesses:-
Simple plastic ruler measurements from the edge? Being concave, the ruler will sit above the surface when rested on the edges.
Put it on a flat and horizontal (check) surface, add a small drop of water with a little detergent to break the surface tension to the approximate centre. A few vigorous taps and the drop should settle at the lowest point?
I used a folded paper template made from the tissue paper Issac from C Quin wrapped the mirror in
Got fingerprints etc on the mirror but had to clean up every thing anyway after mounting it in the cell
I believe the coatings are tough enough
Here's a pic of us in the bath!
(baby not me!)
1. Back of mirror is not perfectly parallel to the front surface? I guess could place a spirit level across the front of the mirror (just touching the edges) and adjust until level in several directions.
2. Damn surface tension means the water droplet "sticks" and fights gravity - hence the add detergent suggestion and tapping - not sure how hard you'd have to tap a 22" lump of glass to transmit a "shock" to the droplet - maybe a big sledgehammer!
Will have to do a similar thing shortly with my 16" primary. It currently has a cross marked in the centre with a permanent marker but I want to accurately place a stick on doe nut.
The method suggested with my collimating tool instructions, which sounds reasonable, is to find a sheet of reasonably stiff paper, place the mirror on it and mark out a disk by running a pencil around the edge of the mirror. If you cannot source paper large enough then tape a couple of sheets together.
Cut out the disk, fold it in half then half again to form a sort of acute triangle then snip off the point, unfold the paper and you have a disk with a square hole in the centre, lay the paper over the mirror, secure it with tape to the mirrors edge in a few places so it cannot move sideways, (obviously any pieces of tape would be adhered to the paper prior to placing it over the mirror so no pressure would be exerted on the mirror surface). You now have accurately defined the centre of the mirror and could use the cutout to place a doe nut or mark a spot.
It goes without saying that if this was attempted the surface and whole adjacent area that the mirror would be placed on should be wiped down with a damp cloth so there is no dust or loose crap in the region.
1. Back of mirror is not perfectly parallel to the front surface? I guess could place a spirit level across the front of the mirror (just touching the edges) and adjust until level in several directions.
2. Damn surface tension means the water droplet "sticks" and fights gravity - hence the add detergent suggestion and tapping - not sure how hard you'd have to tap a 22" lump of glass to transmit a "shock" to the droplet - maybe a big sledgehammer!
OK, maybe not such a good idea after all
No no you use your collimating mallet , rubber of course
Let me throw up a question I have thought about before, that maybe the mirror makers can answer. Presuming the mirror is a pretty good circle, is the bottom or lowest point (for want of the right word) of the parabola absolutely going to be in the physical centre of the mirror. In theory, it should be and that's where the paper template method and the ruler method expects it to be, but what is the chance (say in these machine manufactured mirrors) that it is actually a bit/lot off centre from the physical centre of the circle?
Let me throw up a question I have thought about before, that maybe the mirror makers can answer. Presuming the mirror is a pretty good circle, is the bottom or lowest point (for want of the right word) of the parabola absolutely going to be in the physical centre of the mirror. In theory, it should be and that's where the paper template method and the ruler method expects it to be, but what is the chance (say in these machine manufactured mirrors) that it is actually a bit/lot off centre from the physical centre of the circle?
It should be center or it would show an Astigmatic figure.
the radius of the circle placed on the edge and done 3 iterations approximately 120 degrees. it does not matter that the mirror dips down, the center will always be the center. a parabola will have a point where it = zero and that will be the center of the circle. easy done. utilize paper tissue and draw a outline of your mirror and away you go.
OK, here is what I am on about - but we'd better start from the premise that I don't know nothin!
I often see mention of poor results if a laser collimator is hitting the primary even a small amount (less than a mm) off mirror centre (not barlowed laser). I have seen mention of finding that the spotted centre was not the centre on some mirrors. Recently I was washing a primary and wondered what to do if the centre "doughnut" came off in the process (It didn't).
First image (sorry - scanner to server to mailbox has died? so back to trusty mobile phone!) is meant to be the centre cross section of a super accurate mirror - axis of parabola = physical axis of mirror. Second image is a wonky mirror - axis of parabola is offset compared to the physical axis of the mirror - but the parabolic surface is all present. Third image is a wonky mirror where the parobola is deeper so there is no top flat surface and the LHS height is less than the RHS height - so part of the parabolic surface is missing.
All spoken by someone who clearly knows nothing about grinding mirrors! Feel free to set me right.
Now the images are a gross exaggeration and that may the only issue - such exaggeration is just unreal - the axis of the parabola will always be close enough to the axis of the mirror.
I guess both wonky versions would produce readily seen gross problems optically?
So is worrying whether the axis of the parabola is coincident with the axis of the physical mirror an issue? Can we happily use the brown paper folded into quadrants to locate the centre spot?
I know what your getting at Eric. A mirror test, teste the whole surface.
So if it was a perfect parabola just off center, the gross error from the off set side would translate into an astigmatism.
Here you see some Ronchi screens, just imagine what they would look like if the patterns were all to one side
Buy a large piece of clear plastic or acetate, scribe a circle that's the same outline as your mirror, punch a hole in the centre of the circle and use the outline to centre it over the mirror.
or... buy one of these masks already made from the catseyecollimation guy...
As bird just said, but i would use thin cardboard.
Use a compass or similar LARGE doode wacker type instrument, or just sit the mirror on and draw around it.
Cut it out, and make a small hole in the middle where the compass or similar thingy wacker already made its centre.
Place over the mirror, and dot the center with liquid paper, etc.
Wow!!! Thanks everyone. I attempted the outline on paper thing but we had "difficulties". I'll have a look at MrFry and perhaps draw a firmer template. When it comes down to fractions of a mm it starts to get very finnicky!:than x:
I know what your getting at Eric. A mirror test, teste the whole surface.
So if it was a perfect parabola just off center, the gross error from the off set side would translate into an astigmatism.
Here you see some Ronchi screens, just imagine what they would look like if the patterns were all to one side
But if I wasn't going to do a Ronchi test of, say, my humble little GSO 12 mirror, would star testing tell me quickly if I had one of these wonky mirror problems?
But if I wasn't going to do a Ronchi test of, say, my humble little GSO 12 mirror, would star testing tell me quickly if I had one of these wonky mirror problems?
absolutely positively centred primary dot
