TESTING my mirror HELP

[Daniel-K]

i need some help testing our 16inch mirror we have the tester but unsure how to get them to work or use them any help would be very great

Dan

[Merlin66]

Dan,

The slides look like a collection of Ronchi screens.

If one of these is placed across the light source on the tester and you look at the mirror through the rest of the screen aperture ( close to the centre of curvature = twice the focal length) You'll see a series of dark/ light lines across the mirror - the shape and regularity of these lines can be used to assess the mirror surface.

Google on "Ronchi testing"

[Daniel-K]

we cant get any sort of visual on the mirror even though we have polished it. how bright does the light source have to be

[Merlin66]

If it's polished completely and in need of final figuring, you should be able to get a good reflection from a bright LED when placed close to the centre of curvature....

What methods have you been using to test the mirror so far??

(If you have the tube/ supports you can look at the moon with a un-coated mirror - get a "feel" for the performance etc)

[billhinge]

you should be able to get a reflection with an old fashioned 1970's torch battery in a room with the curtains closed, you should be able to see something after fine grinding with a wet mirror surface

check your mirror and your tester are square to each other,and that your tester is at twice the focal length of your mirror (ie radius of curvature), think about it your lights reflection needs to come about a few mm's from its origin so the mirror needs to be stood up so that it is totally square on to you

who is we by the way?

[Daniel-K]

we have done 2 hours polishing, we can focus a light beam from skywatcher power tank on the wall

[Merlin66]

Hmmmm

two hours of polishing doesn't sound much on a 16" mirror...

OK you should be able to set up the Ronchi and see the lines....try this and let us know...

"How to make a telescope" by Jean Texereau highly recommended!

[Daniel-K]

Hmmmm

two hours of polishing doesn't sound much on a 16" mirror...

OK you should be able to set up the Ronchi and see the lines....try this and let us know...

"How to make a telescope" by Jean Texereau highly recommended!

i know i have read it may need upto 10 hours! but the surface of the mirror is starting to go clear. are we suppose to see the lines on the mirror and does the ronchi slide need to be in front of the light source?

thanks

Dan

[Merlin66]

Dan,

The mirror should not be "just going clear" but 110% clear all over - check with a powerfully magnifying lens that there are NO pits visible....

Yes the screen should go between the light source and the mirror - you look through it towards the mirror.

[billhinge]

ah, I see now, you are trying to see the ronchi pattern on your mirror , sorry my mistake

but as I said, I did my first ronchi test over 30 years ago using an old torch bulb and frosted glass so it doesn't need to be very bright - it is a very old test after all and would originally been undertaken with basic equipment

any chance you could supply the mirror details eg focal length?

[Daniel-K]

ah, I see now, you are trying to see the ronchi pattern on your mirror , sorry my mistake

but as I said, I did my first ronchi test over 30 years ago using an old torch bulb and frosted glass so it doesn't need to be very bright - it is a very old test after all and would originally been undertaken with basic equipment

any chance you could supply the mirror details eg focal length?

its a 16inch mirror with a sagitta of 5mm so its around f5ish

[billhinge]

so when your mirror is sufficiently polished you will be doing your test at a mirror distance of 4m and with your eye sighted just behind the light source and you can't see anything on the mirror surface?

Have you tried cutting the reflected beam with a 'knife edge' to see if it goes dark at ROC and you see moving shadow inside and outside of ROC? - personally I found this easier (it has been quite a while since I last did it though)

[Daniel-K]

the thing were struggling with is the light source. what kind of light source is needed? the one we i have is just a pin point and i dont think its bright enough to even reach the mirror at 4m

[JimD]

It's a long, long time since I made my mirror, but my light source was a normal 60 watt bulb in a metal box with a hole in it. Across the hole I had 2 razor blades making a slit (adjustable). I was mainly using a foucault test, but I did try a home-made ronchi screen (by opening out the slit on my light source and replacing it with the screen) and I got good vertical-line shadows on the mirror. And that was right from the fine grinding stage (wetted) right up to figuring.

[JimD]

Can't edit yet so to add - I forgot to say that the hole in the light box had a plastic diffusing screen over it, so the light was evenly distributed before going through the slit.

[Astrobits]

You must make sure that you have fully polished the mirror before trying to figure it. What happens if you find that you have a nice figure and still find unpolished pits? You might find that you think that a few small pits that are difficult to see will not matter. When it comes back from aluminising they will glare at you in all their "glory" if that's the right word .

I do not do any figure testing before I have removed all trace of pits. To check, make a light box about 12" square with a glass lid. Put a sheet of black paper with holes about 1" size on the glass and then put your mirror face up on top. This gives you a comfortable way to examine the surface with a 10x hand lens. The pits show up best near the edges of the clear holes. If you run your finger over the mirror surface under the lens any dust will move but pits will not.

The only test that you should be doing now is to check the evenness of the polish. Is the centre of the mirror more, or less, polished than the edge? If it is you must change the polishing strokes to even out the polishing.

Nigel

[Daniel-K]

Nigel and Jim thank you very much for the advice by the sounds of it we have to carry on polishing. this is our first mirror so were getting through it ok without too many hicups. for the polishing should we be doing 1/3 over center? this is what i have read anyway. how big should the slit be for the light to come out?

DAN

[Glasspusher]

Dan,

Have a look at my short article on Ronchi testing, it might help...http://www.nicholoptical.co.uk/pdf/The%20Ronchi%20Test.pdf

What size polishing lap are you using? You should be able to test the mirror at the radius of curvature after a couple of hours. Stand close to the mirror with a flash light and picjk up the reflection with your eye. Now move away from the mirror keeping the keeping the image of the light in your eye, it will begin to get smaller, when it reaches its smallest size you are at the radius of curvature. Now, place the ronchi tester at this point, you should be able to see the illuminated mirror with a series of vertical bands across it. To interpret these lines look at the article above. Hope this helps.

John

[Daniel-K]

its a 10inch lap wasnt sure on what size to do so went for this, its doing the job so far but im wondering if we will run into trouble when it comes to figuring it with it. should the verticle lines be clearly visible or are they hard to see?

[JimD]

.......... how big should the slit be for the light to come out?...

The narrower the better. Anything under 1mm should be OK.

[JimD]

Having viewed John's excellent link, I now realise that my memory is playing tricks on me. Obviously the grating does NOT go on the light source as I said in my earlier post. You keep the slit as it is and obseve the light reflected from the mirror through the grating. (Doh!)

[Astrobits]

Centre over centre with a sub-diameter lap on top will be working the centre of the mirror more than the edge and you will be heading for a nice hyperboloidal mirror if you continue. Is the edge more pitted than the centre? If so then use a w stroke to work the edge more and the centre less. Forget the figure testing until the mirror is FULLY polished.

Nigel

[Glasspusher]

With a 10 inch lap, as Nigel says, working thru the centre will produce an over corrected mirror, the curve will be deeper in the centre than the edge. You need to work with the stroke off centre to counter act this. An offset of about 3-4 inches from the centre should be ok. You need to test as soon as you can, if the mirror is overcorrected, shown by the ronchi bands bowing outwards just inside the radius of curvature, you need to increase the lap offset from the centre during polishing. The ronchi bands are easily seen if the mirror has a reasonable polish and your set up is ok. Type 'ronchi tester' into youtube.com for some helpful stuff. By the way the generally accepted sub diameter lap size, when working in this way, is about 75%D. For a 16 inch mirror your lap size would be about 12 inches. 10 inches is a little small, but still quite useable.

John

[Daniel-K]

this is very useful information, i will try the ronchi test on my next days off im back at work tomorrow so it will have to wait but im quite excite i have a better understanding about the test thanks john

Dan

[Astrobits]

I suggested the w stroke as the pics in your other post didn't seem to show that the mirror was on a turntable but just found the video In which case John's offset is the way to go ( and the way that I did it on my turntable ).

Nigel