Pitch lap supplies in Oz

Gregted (Greg Tedford) · #1 19-01-2016, 11:00 AM

G'day all,
I made some telescopes about 15 years ago and I think I used resin from an equestrian supply store for the pitch.
I have just read somewhere that some have used beeswax as a lap also.
This may be wrong but I will be making another telescope soon ( 12 incher ) and I was wondering where you get your pitch/beeswax from in Oz and what have you had success with.
I am also interested in getting in touch with anyone in the Gladstone, Qld area who may have made their own telescope.
Apologies if this has been asked but I did a search and couldn't find any mention of resin as a pitch lap.

chardie (Leslie) · #2 19-01-2016, 08:54 PM

an apiarist will have beeswax or try the local bee club in the white /yellow pages . if you go to some sunday markets that sell honey you could most likely get it from them. failing that perhaps melt down a non scented non coloured candle. i think the bees wax is only used to weaken or make the pitch just a little more malleable correct me if im wrong
edit even a surf shop may have plain beeswax blocks as i think surfers use it on their boards now i am out of ideas lol

robbo4 (Rob) · #3 20-01-2016, 06:51 AM

I do not think Beeswax or resin will do at all.
You will need pitch.. I get mine from Gotgrit in
the U.S.A. He is good to deal with.
I am doing a 12 incher at the moment but my
problem is with the Foucault test as I have had
Cataract surgery and am also 77 and it is hard
to interpret shadows.

Gregted (Greg Tedford) · #4 20-01-2016, 11:56 AM

Thanks Rob and Leslie for your responses.
I am pretty sure I did use resin 15 years ago from the local saddlery and the 6 and 8 incher I made polished out very well.
The resin was a combination of powder and rock and melted with castor oil to an acceptable hardness with the Thumb nail test.
Resin is also used for rock climbing and gymnastics.
I'll melt some soon and let you know how it turns out.
Rob, what focal length\sagitta are you going for on your 12 incher?
I was thinking of something faster than I have done before. Maybe f\4.
You are doing well pushing glass at 77. Hope it turns out well for you.

Satchmo · #5 20-01-2016, 12:32 PM

Greg

Polishing pitch could cost $60 or more per kilo to land these days.

Gum Resin or Rosin can certainly be used for laps so long as you add some fine wood flour which will give it properties more similar to traditional pitch .

A 'tropical' lap mixture was sold by Astro Optical for many years to mirror makers - it was supposed to work better for our warmer conditions . I certainly used variants of it to make hundreds of mirrors when I was there .Eric Witcombe told me he thought the pitch in the mixture was just to give the lap a familiar black colour and probably makes no difference .

My old copy of the AOS publication Building a Low Cost telescope lists the mixture for an 8" mirror at 20 dgerees C as :

1lb Gum Rosin
3 oz of wood flour
55ml Castor Oil
1 oz pitch

Adjust Ratios of Rosin to oil for hardening and softening .

Mixing the wood flour into the rosin brings back many old memories

I don't know where you will get wood flour these days but perhaps a wood working shop where they do finish sanding ?

You would need a very soft lap for figuring F4 mirrors .

Hope this helps

robbo4 (Rob) · #6 20-01-2016, 03:03 PM

I remember that mixture as it was, I believe , created by some Professor
in one of our Universities, I did buy some but never used it.
You can buy pitch for U.S.39.75 per Kilo from GG but post is dear from U.S.
You can make your own sawdust with a belt sander but would have to make sure there is no grit in it.

robbo4 (Rob) · #7 20-01-2016, 03:15 PM

P.S.
The focal length of my mirror is f5.7.
It has a slight T.D.E. and a small hill in the middle.
I have tried Photographing to give me an idea of how
it is shaping but have found it very difficult to place the reflection.
I shall next try to use a small scope to look at the Mirror and if that fails
I shall have a go with making a Bath Interferometer

pdalek (Patrick) · #8 20-01-2016, 04:14 PM

Check
http://www.telescopes-astronomy.com....ccessories.htm
A resin lap kit is available at $15Lap resin kit Lap resin kit

Rac (Raymond) · #9 20-01-2016, 04:24 PM

I know how hard it will be viewing a Foucault image after cataract surgery as my left eye needed that after damaging its lens 3 years ago. My right eye is still good though.

I get my pitch from a friend at the NZ refining company. I'm not sure if you could see if you can buy some from a similar company in Australia, If you could just ask for uncut pitch with nothing added to it. Straight out of the tap it is about the same as Gugolz Pitch #55 and it is exactly the same as Gugolz and don't let anyone tell you otherwise. They only use it for burning to make steam for making vacuum for other uses. I have had thoughts of seeing if I can buy heaps and package it up for selling at realistic prices unlike Gugolz.

dave brock · #10 20-01-2016, 04:54 PM

I've made all my mirrors with a mixture of Rosin and Stockholm Tar (about 7-1 ratio).

Dave

Satchmo · #11 20-01-2016, 05:04 PM

Yes it was Dr Con Tenukest ( sp ? ) from University of NSW in the 1960's

I actually own his old polishing machine , which used to be a Draper configuration - now heavily modified . I can figure mirrors up to 24" on it although rough polishing of 24" is done on a 1 meter capacity machine

Quote:

Originally Posted by robbo4

I remember that mixture as it was, I believe , created by some Professor
in one of our Universities, I did buy some but never used it.
You can buy pitch for U.S.39.75 per Kilo from GG but post is dear from U.S.
You can make your own sawdust with a belt sander but would have to make sure there is no grit in it.

Satchmo · #12 20-01-2016, 05:15 PM

Quote:

Originally Posted by robbo4

P.S.
The focal length of my mirror is f5.7.
It has a slight T.D.E. and a small hill in the middle.
I have tried Photographing to give me an idea of how
it is shaping but have found it very difficult to place the reflection.
I shall next try to use a small scope to look at the Mirror and if that fails
I shall have a go with making a Bath Interferometer

Rob
Can I recommend to you as your mirror is reasonable 'slow' - a simple null test will be much easier to use . If you can use a ronchi grating on a star it will make interpretation very simple- you are just aiming for straight lines . John Dobson used to use trap door style back ends to his telescopes do he could quickly take mirror in and out for testing. This can be good for final testing / tweaking and the comparative ronchi test ( Google Mel Bartels ) good for rough figuring . Knife edge testing can be notoriously difficult for the beginner . I guess it really depends if you are after a usable mirror soon or alternatively `the journey is the goal ' ..

robbo4 (Rob) · #13 21-01-2016, 09:00 AM

I have made a number of mirrors over about 50
years and have had no trouble with the Foucault
Test until now. It is my eyes that are the problem
I guess, but I shall keep trying. I have read Mel
Bartels stuff and have another blank rough ground
to just under F4. Being optimistic I guess.

Satchmo · #14 21-01-2016, 10:15 AM

No worries Rob - just in case you were new to it and struggling with testing . When I used to work at AOS they also sold mirror kits and I was a bit dismayed at how few mirrors that came for aluminising were actually any good ( perhaps 1 in 30 ? ) - I always thought that a simple ronchi screen on a star would have been far more effective for beginners .

Quote:

Originally Posted by robbo4

I have made a number of mirrors over about 50
years and have had no trouble with the Foucault
Test until now. It is my eyes that are the problem
I guess, but I shall keep trying. I have read Mel
Bartels stuff and have another blank rough ground
to just under F4. Being optimistic I guess.