Thread: The Comparator In Use

The other day I received an email from Ray advising me that he had volunteered my services on PM. A bloke there had a Reilang oil can and was chasing the dimensions of some of its missing springs. Ray knew I had a similar oil can - http://www.woodworkforums.com/showth...61#post1620361

Upon dismantling the can I realised that whilst I could jam the jaws of my dial caliper between the coils to measure the wire, the reading varied depending on the pressure exerted on the wire. More importantly, I didn't want to damage or bend the springs. The answer was the Mitutoyo PJ250 Optical Comparator I had bought as an appetiser from fellow forum member Peter Fou. Appetiser because I bought it just before acquiring his wonderful tool and cutter grinder.

I have 0.005mm micrometer heads fitted ( thanks to Alan "C-47" ), the comparator came fitted with .0001" mics, so the measurements obtained would be a 10X improvement on the dial caliper. The micrometer heads have adjustable barrels allowing zeroing. So to measure the spring wire diameter it was nothing more difficult than bringing the wire into focus, aligning the cross hairs on one side, zeroing the mic and then winding the micrometer to move the cross hairs to the opposite side and reading the micrometer. The 0.29mm I had squeezed out of the caliper was actually 0.3mm.

I used the 20X lens, for more precise measurement with the tenth mics fitted, there is a 50X lens and for general close up sightseeing there is also a 10X lens. Sadly the latter lens' half reflection mirror has lost most of its mirroring. If my memory serves me correctly, the titanium (?) mirroring was/is applied by some vapour deposition process, hardly a back yard thing.

With a dial indicator of suitable sensitivity fitted to the, unfortunately not fitted, vertical indicator mount, accurate depth readings are also obtainable. Depth readings that would be pretty difficult to obtain otherwise.

The comparator is a neat bit of gear. Like centring scopes and auto collimators, not everyone's cup of tea but for those of us who have them, they add another dimension to our back shed explorations.

Bob.

I used the 20X lens, for more precise measurement with the tenth mics fitted, there is a 50X lens and for general close up sightseeing there is also a 10X lens. Sadly the latter lens' half reflection mirror has lost most of its mirroring. If my memory serves me correctly, the titanium (?) mirroring was/is applied by some vapour deposition process, hardly a back yard thing.

I have seen a chromium plating arrangement, using a vacuum chamber. The chromium was in a wire form twisted together with resistance wire. The resistance wire had a current passed thru it to melt the chromium. Most mirrors were silvered. I have not heard of titanium being used, but that does not mean much. This device was at Adelaide Uni and the chamber was made of glass to make the process visible.

Dean

Every home should have at least one

Large telescopes use Aluminium coated surface mirrors. I cant say why they don't use silver though? maybe its easier to get off when its time for re-coating.

I wonder if you could replace the surface light and half mirrors with a ring of LEDS much like the Isoma. The light wouldn't be spot of vertical but as you have a ring of lights I'm not sure that would matter.. not sure that it wouldn't though lol

Nice pictures as always.

Stuart

Originally Posted by Stustoys

Every home should have at least one

Large telescopes use Aluminium coated surface mirrors. I cant say why they don't use silver though? maybe its easier to get off when its time for re-coating.

I wonder if you could replace the surface light and half mirrors with a ring of LEDS much like the Isoma. The light wouldn't be spot of vertical but as you have a ring of lights I'm not sure that would matter.. not sure that it wouldn't though lol

Nice pictures as always.

Stuart

It is my understanding that it is matter of toxicity. Although the aluminium coating has slightly lower reflectiveness, it is much safer to manufacture. I may be wrong tho. I guess the cost would be a factor.

Dean

I seem to vaguely remember a chemistry experiment in high school that silvered a test tube (mix silver somethingate together with something else and silver precipitated out). Al probably replaced it as it was cheaper. If I had the space, PVD would be something I'd try setting up though as it leads to all sorts of fun combinations.
I replaced the light source in my comparator with a LED bulb and it does not seem to work as well. As good as LED are getting I think for a comparator the brighter the better.

Michael

Silver has a reflectivity of up to 99% compared to aluminium, which is around 90%. Gold gives the best reflectivity but produces a gold tint.

Here is a YouTube video of making glass mirrors with silver nitrate. I have not watched it. Just a comment to say maybe it is possible.

Dean

Originally Posted by Oldneweng

I have seen a chromium plating arrangement, using a vacuum chamber. The chromium was in a wire form twisted together with resistance wire. The resistance wire had a current passed thru it to melt the chromium. Most mirrors were silvered. I have not heard of titanium being used, but that does not mean much. This device was at Adelaide Uni and the chamber was made of glass to make the process visible.

Dean

I don't know where the titanium came from Dean. Could be one of those getting old things. I had a bit of a dig on Google and mention was made of a thin silver coating allowing both light transmission and reflectance.

Mitutoyo offered a full reflection mirror which doesn't look too complicated. Stu had been experimenting with hard drive discs as mirrors for his prototype centring scope adaptors. I scrounged a hard drive from work and pulled it apart, mirrors abound!

Might be one of those afternoon gadget projects that runs into couple of weeks.

Bob

052 (Large).jpg

I doubt cost has much to do with it when we are talking about 8m telescope mirrors. If I recall correctly it takes 6g of Aluminium to coat the 8.2m VLT mirror.

Micheal are you talking about the profile light? I'd think that's more tricky than the surface light.

P.s. does Silver still beat Aluminium in infrared?

A place to start if you are looking at re-silvering. a.g. Thompson P/L. Tel. 1800 888466 I'm sure that they did vaccuum deposition in the 70's company has had owner changes since then I think. Hope this helps.
tinkera

Hi Bob,

The thing that could be a problem with the side mirror(as your scan shows) it if the object isnt flat, you could end up with shadows. But if you have LEDS all the way around, that should improve things. More so on the 10x than the 20X as they will be further from the work. making the "off vertical" angle less. I dont have a 20x or 10x so I cant test it for you. Of course when Michael reports back about his LED I might be full of it

Stuart

Originally Posted by Stustoys

I doubt cost has much to do with it when we are talking about 8m telescope mirrors. If I recall correctly it takes 6g of Aluminium to coat the 8.2m VLT mirror.

Micheal are you talking about the profile light? I'd think that's more tricky than the surface light.

P.s. does Silver still beat Aluminium in infrared?

Cost does not have a lot to do with the direct coatings of large telescope, mirrors but if you consider the huge number of smaller telescopes etc. around the world and then factor in the extra costs of safety handling and disposal of wastes it would make a difference.


According to Wikipedia, silver is effective right into the far infrared spectrum, but it does drop off considerably at about 550nm.

I had an idea to make a telescope, including lens at one stage. I was thinking around 8in to 12in. Nothing came of it. A mate at work has one which was home made by someone else. It is 16inches I believe. I think the lens was bought tho.

Bob,

Mitutoyo offered a full reflection mirror which doesn't look too complicated. Stu had been experimenting with hard drive discs as mirrors for his prototype centring scope adaptors. I scrounged a hard drive from work and pulled it apart, mirrors abound!

Might be one of those afternoon gadget projects that runs into couple of weeks.

You have just reminded me that I have a couple of those disks in the shed.

Just as long as the afternoon project only occurs in the afternoon!

Dean

Originally Posted by Stustoys

Micheal are you talking about the profile light? I'd think that's more tricky than the surface light.

Can I call a friend?
I have had this for several years but I'm yet to get it functional - I need to get a hood made and a stage for things to sit on. I think once that is done I may dispense with the light reflecting from the side and try illuminating directly from underneath.
Anyway - brightness -
P1020118 (Large).JPG
With the lights out at 9:30pm this is as bright as it gets (the exposure was 1/2.5 seconds from memory)

With the the lights on, it looks like this
P1020120 (Large).JPG


Michael

Originally Posted by Oldneweng

Cost does not have a lot to do with the direct coatings of large telescope,

Hi Dean,
I might have confused things, I meant that the cost of using silver wouldn't be a factor, so there must be some other good reason to use Aluminium. Maybe recoating is a pain or getting an even coating thickness?......I thought small scopes where silvered?

Google!

http://articles.adsabs.harvard.edu//...00018.000.html


Hi Michael,

That's a surface light.
Profile light comes from below, works much better but of course you can only see the edge/profile/silhouette.
I think the lens on my surface light is broken. I'll have a play tomorrow if I get a minute.

Stuart

Originally Posted by Michael G

Can I call a friend?
I have had this for several years but I'm yet to get it functional - I need to get a hood made and a stage for things to sit on. I think once that is done I may dispense with the light reflecting from the side and try illuminating directly from underneath.
Anyway - brightness -
P1020118 (Large).JPG
With the lights out at 9:30pm this is as bright as it gets (the exposure was 1/2.5 seconds from memory)

With the the lights on, it looks like this
P1020120 (Large).JPG


Michael

I reckon the LEDs need to go MG. Mitutoyo at 8.15. Lights out. 1/6 at f3.2 No idea what the ISO setting is. For 79 bucks, who cares.

053 (Large).JPG

Originally Posted by Anorak Bob

The other day I received an email from Ray advising me that he had volunteered my services on PM. A bloke there had a Reilang oil can and was chasing the dimensions of some of its missing springs. Ray knew I had a similar oil can -

I knew you were the goto guy, who else has a Reilang Oil Can fetish AND an optical comparator...

That comparator is a nice bit of gear,

In similar vein we were discussing the other day, how I could get a micrometer stage under the Elmo video presenter.. maybe one day...

Ray