Centre Spotting a Newtonian Mirror

iceman (Mike) · #1 21-02-2006, 08:30 AM

Hi all.

Steve H (janoskiss) has written a great how-to on the quick, easy and precise way of centre-spotting your mirror..

You can read the Article by navigating to the IceInSpace How-To page, or directly by clicking on the link below:

Centre Spotting a Newtonian Mirror

Thanks to Steve for writing the article!

If you'd like to contribute a review, how-to or other article, please contact me.

iceman (Mike) · #2 21-02-2006, 09:23 AM

Article submitted

rmcpb (Rob) · #3 21-02-2006, 11:06 AM

Steve,

Great and simple but I would suggest that people should at least measure the diameter of their mirror and make the arcs just over the radius otherwise someone will make their arcs too big and actually draw on their mirror in the area that catches the light and wreck their mirror.

It has to be the easiest way to do the job I've seen though.

Cheers

janoskiss (Steve H) · #4 22-02-2006, 09:05 AM

Quote:

Originally Posted by rmcpb

Great and simple but I would suggest that people should at least measure the diameter of their mirror and make the arcs just over the radius otherwise someone will make their arcs too big and actually draw on their mirror in the area that catches the light and wreck their mirror.

Yes, that is a good tip if you don't trust your eye. I did the mirror in the photos without any measurements, but the secondary obstruction was quite large for this f5, so there was no real risk of marking the mirror in the wrong place.

Mr Bob · #5 22-02-2006, 04:12 PM

Handy to know when my scope finally arrives. Thanx.

AstroJunk (Jonathan) · #6 22-02-2006, 08:17 PM

Wouldn't be able to use a barlowed laser with this technique though. I would go for the paper-scissors route myself!

janoskiss (Steve H) · #7 22-02-2006, 08:33 PM

Why not? What does paper-scissors (no rocks!

) have that is lacking with the compass method?

AstroJunk (Jonathan) · #8 22-02-2006, 09:20 PM

The barlowed laser method relies on the shadow of the central paper ring does it not? (I've only done it once, but it seemed a nice technique).

What could be more easy than to fold a circle of paper in four and snip the center? A five min job surely. Mine was. I wouldn't let a set of compasses near my primary!

janoskiss (Steve H) · #9 22-02-2006, 09:53 PM

Affixing the paper ring is the last step, and comes after marking the centre, which is done just the the same whether you use a compass or a paper template.

AstroJunk, your remarks about not letting a compass anywhere near your mirror parallel my sentiments about not using a paper template and potentially damaging the surface of my primary. The felt-tip marker in the compass only needs to touch the centre of the primary in the shadow of the secondary, but a paper circle needs to cover the entire mirror. So compass is safer IMHO.

wavelandscott (Scott) · #10 22-02-2006, 10:07 PM

A nice solution and one I had not considered...first instinct was to go to the folded paper route...The pictures and graphics in the article are very helpful...interesting and one to remember!

AstroJunk (Jonathan) · #11 23-02-2006, 12:38 AM

An optical surface has next to nothing to fear from pressure applied to it tangentially from a porous non greasy medium such as paper. The self same optical surface has plenty to be worried about by an errant compass point striking it or a rough felt tip point accidentally scoring its surface past the shadowed zone. You're happy with those risks and that's just dandy. I personally feel that the method has inherent dangers sufficient that and I would not be willing to recommend it as a How-To to a general audience; but that's the icemans problem and not mine thankfully.

iceman (Mike) · #12 23-02-2006, 06:33 AM

Quote:

Originally Posted by AstroJunk

I personally feel that the method has inherent dangers sufficient that and I would not be willing to recommend it as a How-To to a general audience; but that's the icemans problem and not mine thankfully.

I think it's up to the individual to decide if it's too dangerous for them. Should we also not recommend washing mirrors at all, just because someone could damage them? Of course it's a risk - but it doesn't mean you can't have a how-to article detailing how to do it.

Even with folded paper, someone could slip and put a greasy hand all over it. There are risks in everything you do.

Like anything involving your optics, the best advice is caution - make sure you're prepared, take your time and understand what it is you're about to do.

matt · #13 23-02-2006, 07:00 AM

Gotta say I agree with Ice on this issue.

The 'How To's" can only show us .... how to.

The common sense? Well, that's entirely up to you!!!

I like the How To's but surely it goes without saying that many of these come with some kind of cautionary caveat????

barees63 · #14 23-02-2006, 08:59 AM

The best center-spotting technique I found (and I looked at them all believe me) was the catseye spotting template, the (precision - as opposed to dodgy paper ring) triangular center-spot is attached precisely to the center of an acetate template which marked for your mirror size, you line it up then press in the center and you're done.

ChubbyStarstuff (Pete) · #15 03-06-2011, 09:10 PM

Pardon me commenting so late. I'm not sure if this has any point (pardon the pun), but I noted that Steve H mentioned he'd never seen this method used before...

I have! (This fact astonishes me, as I'm shockingly inept at all kinds of mathematics!)

It's in the first set of Euclid's books (I can't remember which book, but I'm pretty sure it's books 1-3 (out of 13)). It's actually described in the proof of making an equilateral triangle using only some string and a pencil/goosequill/whatever (instead of a compass), but this extrapolation (finding the centre of any circle using only string/setsquare/pencil) appears in the next few lemmas and proofs.

The reason I know about this at all is because I bought the set of Euclid's books as a way of teaching myself the basics of maths again. Unfortunately the greek used is, well, greek (i.e. not English) - so I bought an English translation.

The problem there is that English translations of Euclid fall into 2 categories - a) 18th century with archaic lingo but no mathematical assumptions (with the focus on the original greek language used, not the mathematics involved), or b) 21st century with clear language but assuming the reader is not actually learning mathematic proofs from the books, but instead using them as "recreation" while sitting in the Advanced Mathematics chair in King's College!

If anyone can recommend a _readable_ version of Euclid that's still available, it'd be much appreciated - almost as much as I appreciated Steve's article! (Yes, there *was* a point to this meandering!)

GTB_an_Owl (Geoff) · #16 04-06-2011, 12:11 AM

well i hate to shock you all

but my mirror was centered by this method with one exception

NO FELT PEN

done with a pointy compass - and yes the mark is permanent

it was done by a person with many years in the business and is the way he does all his mirrors

done lightly - it simply will not affect the mirror

when i wash my mirror and the center spot comes off - i simply put another over the marks

geoff

gmbfilter (Geoff) · #17 04-06-2011, 10:16 AM

Quote:

Originally Posted by GTB_an_Owl

well i hate to shock you all

but my mirror was centered by this method with one exception

NO FELT PEN

done with a pointy compass - and yes the mark is permanent

it was done by a person with many years in the business and is the way he does all his mirrors

done lightly - it simply will not affect the mirror

when i wash my mirror and the center spot comes off - i simply put another over the marks

geoff

Sounds like a great Idea

multiweb (Marc) · #18 04-06-2011, 11:13 AM

That's a hell of a lot of drawing on the mirror to find the center IMHO.

The simplest way to do this is put the mirror upside down, draw the perimeter, find the center of the circle with the method indicated in the tutorial on the paper or even easier fold the paper template in 4, then drop the center spot on the mirror by having it stuck on the paper while matching the edge of the mirror. Catseye has made such a template. No brainer.

David Fitz-Henr · #19 23-06-2011, 02:51 PM

One thing worth noting in the quest for perfection is that the centre of the paraboloid is unlikely to be the exact geometric centre of the mirror. When I picked up my 12.5" conical mirror from Mark last year he mentioned that the centre of the paraboloid will "wander" around by a few millimetres in the final stages of figuring. I would suppose that is why final collimation on a star is recommended (although seeing conditions usually make it difficult to achieve this!).

AstroJunk (Jonathan) · #20 23-06-2011, 11:18 PM

'Fess up - who's been selectively pruning!