13 replies to this topic

[emiddleb]

Has anyone else checked their center spot on the primary mirror?

I purchased a set of CatsEye tools because nothing I did seemed to come close on the collimation. Thought it was because I was using a 1-1/4 inch tool that was being thrown off by the adaptor, so I switched to 2" tools.

(I am still working on being adept at collimation... )

When using their spotting template for installing their red triangle, I found that the factory spot was off by about 1/8" of an inch or so.

Would this be enough to mess me up before? I was using a 1-1/4 Chesire and a laser that collimated (checked.)

Just curious... this was on my 12" LB purchased in November.

[Brian Carter]

I think that is pretty common, not just for the lightbridge, but all mass produced telescopes. It is a pretty easy fix, thank goodness, but yeah, it really hurts things early on.

[DeepSpaceTour]

The centre spot on my Antares Mirror was off centre by about 1/4 inch,I fixed it no problem,but still,it was quite a bothersome discovery!!

[KaStern]

Hello Ed,

the center spot should be located in the geometrical center of the
mirror`s front side.

But:
It could be possible that the optical center of the mirror is a little
bit away from the geometrical center(!)
That can happen if the mirror is not well centered during the grinding
procedure with a machine.
Normally this is not more than 1 to some millimeters.

If this is so, the center spot placement has to account for this to
achieve best results.

Regards, Karsten

[CatseyeMan]

I've been told there's no easy way (if any) to accurately locate the "optical" center; therefore, placement of the center spot at the geometric center is the consistent recommendation.

[Fireball]

Can I check my center spot without removing the primary ?
If yes, how ?

[Vic Menard]

Can I check my center spot without removing the primary ?
If yes, how ?

You can probably verify the placement of the primary mirror center spot with a precision holographic laser to an accuracy of a few hundredths of an inch (which really isn't that difficult to read!) At f/5, depending on your observing preferences and high magnification limitations, this may meet your collimation tolerance requirements.

[Vic Menard]

...It could be possible that the optical center of the mirror is a little bit away from the geometrical center(!) That can happen if the mirror is not well centered during the grinding procedure with a machine. Normally this is not more than 1 to some millimeters.

Generally speaking, the grinding procedure, machine or hand, delivers a sphere, where all points on the surface reflect back on themselves to the radius of curvature. The potential for offsetting the optical axis would then be attributed to parabolizing.

Carl Zambuto noted that the potential for this offset to occur with modern mirror making techniques, unintentionally, is very small, and the error, if any, would be less than a few hundredths of an inch (for those who prefer metrics, that's approximately 0.5mm.) For center spotting, this also assumes the paraboloid is a perfect figure of revolution. It's my opinion that the primary mirror edge is actually more important than the center with regard to image performance contribution, and a precisely placed primary mirror center spot accurately defines the edge!

If this is so, the center spot placement has to account for this to achieve best results.

I've read this comment from many sources and pondered it for many years. In my experience (and the experience of Nils Olof Carlin as well), I have yet to see well collimated (with precision tools and accurately placed primary mirror center spot) Newtonian optics (quality primary and secondary, thermally stabilized and properly supported, and excellent seeing) that did not pass "star" collimation.

[Nils Olof Carlin]

I've been told there's no easy way (if any) to accurately locate the "optical" center

Easy and easy - the way to do it would be by careful star collimation, then put in a Cheshire and see where the center spot ought to be. The key word here is careful - this calls for quite good seeing, watching for small asymmetries of the diffraction pattern near (or at) best focus, and perhaps repeating a few times to see if any apparent offset is consistent as to size and direction. The common or garden "star collimation" done far off focus is too insensitive to be of much use, I am sure. But if you have verified that the placement of the spot is correct, you need not expect the optical center to wander around much (of course, any asymmetry that causes coma deformation to the mirror will shift the apparent optical center some, but I wouldn't expect this to be a practical concern) and further collimation can be easier and more accurately done against the center mark.

As for checking the spot's placement, one could use a wooden stick a bit longer than the mirror dia., put two marks for the edge and one exactly midway between, then use the stick for checking in at least two directions. This may not work with a long, closed tube, though

Nils Olof

[emiddleb]

Fireball, it's not that big of a deal on the LBs. There are a half-dozen or so phillips screws around the bottom of the mirror tube assembly. Remove those and the tube easily lifts off, allowing access to the mirror. That's how I checked the center spot using the template that came with the CatsEye/Telecat tools. Highly recommend you give it a try... if you are "spot on" then that's one less thing to worry about, and if it's indeed off-center like mine, you are aware of it.

Thanks to everyone else who responded. Good info!

I spotted mine properly (I hope <g>) with the "red triangle" and am now just awaiting a final item to put it all back together and try collimating again (and of course, some clear skies, which seem to be mythical this fall... The Curse!)

Ed

[SeaDeep]

My centerspot on my LB was off by about 1/8" or more. Turns out that the center was actually on the center spot ring. While one day I will probably recenter it, my current solution is that I used a black sharpie and put a dot on the ring itself at the actual center point. I use my laser to hit that spot to verify secondary allignment. Then I use a barlowed laser technique to collimate the primary, noting the black mark which I can see in the laser return window.

Works well, for now.
SeaDeep

[gfogarty]

I've been thinking about this for a little while now, as I've just started builing an 8" f/5 dob, and this is worrying me a little.

Generally speaking, the grinding procedure, machine or hand, delivers a sphere, where all points on the surface reflect back on themselves to the radius of curvature. The potential for offsetting the optical axis would then be attributed to parabolizing.

Strictly speaking this isn't quite true; while the grinding does deliver a sphere, the (until now) unspoken assumption is that the center of that sphere lies on a line perpendicular to the plane of the rear surface of the mirror. This isn't guranteed; I can easily imagine some "wedge" being ground into the mirror so that, although the sphere is "perfect", its center is some distance off perpendicular from the plane of the mirror. The question is how often this might happen in practice. Has anyone here ever measured it?

GF

[Vic Menard]

...while the grinding does deliver a sphere, the (until now) unspoken assumption is that the center of that sphere lies on a line perpendicular to the plane of the rear surface of the mirror. This isn't guaranteed...

And it isn't really necessary either. Even if one edge of an 8-inch mirror were 1/10th-inch higher than the opposing edge, because the mirror is a first surface mirror, all you would need to do is to collimate the "wedge" out. If you're concerned that this would cause a lateral shift at the center (at the mirror surface), given the above circumstances, the shift is about 0.03-inch. Even the crudest mirror blanks I've worked with in my mirror making experience were well within this tolerance--perhaps 1 or 2 hundredths difference from one edge to the other, which would reduce the lateral shift to some thousandths of an inch. Again, it doesn't really matter, because mirror testing during parabolization establishes the center of the mirror surface, and that's normally going to be in the physical (measured) center of the mirror.

And welcome to Cloudy Nights forums!

[gfogarty]

Ah, now it seems obvious, thanks! I'll stop worrying.

Thanks for the welcome as well!

GF