I have since ordered some Gugolz Pitch #55 from GotGrit.com suitable to my current working temperature of 16C.
Good move. Gugolz #55 is the one I use at working temps around 10C, (I add a small amount of Stockholm tar to it, to modify it to about equivalent to #50 [or slightly lower] for the lower temps I work at). Gugolz is nice to work with. I used resin (rosin?) moderated with pure turps and beeswax way back in the 1960's (loved the smell), it worked ok if one got the balance right with the ingredients for working temperatures.
Image record as promised from pervious post. I am attempting to polish an 8 inch mirror. I was using Tropical Resin Lap. The first attempts had way to many bubbles. The second attempt with the addition of more Castor Oil was much improved however only lasted one hour before it broke up. Obviously too hard and brittle. I have since ordered some Gugolz Pitch #55 from GotGrit.com suitable for my current working temperature of 16C.
I recently had a go at the 'tropical' lap myself. My problems were different to yours though Neil, went overboard with the castor oil and now have a lap which is easy to cut channels into, but which I fear is much too soft. Think I will give it a few more goes with the resin to see if I can get it right before I give in and go with pitch.
My lap pictured here, freshly poured (but prior to trimming).
Measuring progress polishing an 8 inch mirror. First picture has curved rough lines with scratches. The second picture is straightening up and looking even with some fine scratches. The target is to create a spherical mirror befor fine turning it into a parabolic mirror.
Looks like your moving in the right direction Neil. How many LPI is the Ronchi grating? You might want to consider reducing pic size a little. I use "paint" function to size my pics. My camera is a basic Canon A1200 (12.1 Mega Pixel). I do the 20% fix to give nice size pics for the forums.
Thanks Sopticals for the hint on pic sizes. I am in the figuring stage using 65 lines per inch. I also have 85 and 133 that I may get to. I am working on offsets in front of and behind ROC and comparing with computerised models. Attempting to asses under or over correction. The learning curve is using the correct strokes to improve the shape. Found even 5 or 10 minutes can make a difference at this stage. Have to be very patient.
Getting closer to a Parabolic 8" Mirror with the following comparison of the Ronchi calculator 85 grates per inch and a picture taken just using my iPhone. The first two compares inside ROC at -0.2 and the second two at Outside of ROC at 4.0. Pictures low quality
I set my offsets on the Ronchi calculator at -0.3, -0.2, -0.1, 0.0, 0.1, 0.2, 0.3, 0.4, 0.5. The minus offsets return results from zones closer to the centre of the mirror and the positive numbers results from the outside zones of the mirror.
Hi Neil
I believe Wayne Sainty of Saintech in Taree does most of the mirror coatings for this area. There was / is a price list in the archives here somewhere along with ph.number and address . Several forum members have used his coating services . Someone else may jump in with a quick link to the post.
Before and After pictures. It was purchased as an old mirror with chips in the rear structure. Once coated (silver) should the rear (back of mirror) chips effect the final performance?
hi Neil
Have been following your adventures as I have a few blanks sitting around that I would like to turn into mirrors when I get the time. As for chips on the rear edges, you wont even know they are there when the front surface is coated, they will have no effect on the image as you are only reflecting off the front surface. Suppose that goes without saying really.
I have a 10" blank after being pointed to Andrews.
I've started grinding but seem to be stuck at 3.18mm, I am looking to go to about 6mm which should be around F3. Full size glass tile tool in use which has gone from 8 mm to about 5.5 mm thick. Just pushing across the blank with about 60% cover.
Any suggestion about getting to 6mm appreciated.
Barry
I have a 10" blank after being pointed to Andrews.
I've started grinding but seem to be stuck at 3.18mm, I am looking to go to about 6mm which should be around F3. Full size glass tile tool in use which has gone from 8 mm to about 5.5 mm thick. Just pushing across the blank with about 60% cover.
Any suggestion about getting to 6mm appreciated.
Barry
If by pushing across the blank you mean tool on top then you should be mirror on top to deepen.
Location: '34 South' Young Hilltops LGA, Australia
Posts: 1,299
Barry,
Are you measuring the depth of the curve on the mirror or by measuring the edge tile thickness on the tool? If the latter, this is not a reliable method.
If you are using the mirror curve depth across the whole disk, I have written you an excel spreadsheet to do the mirror curve depth to focal length calculation. You have to input the diameter of the mirror in mm and the curve depth in mm, it will calculate the rest for you.
Personally, I prefer the sun method. As long as it is a sunny day, tack a piece of white card on the end of a long dowel rod. Longer than the focal length.
Slosh water on the ground mirror surface, point it and the dowel at the sun and mark where the sun comes to focus. Measure the distance to the white card and that is the focal length. You can also put distance marks on the dowel to simplify measurements. It's simple and bulletproof.
Gugolz #55
Gugolz #55 is the one I use at working temps around 10C, (I add a small amount of Stockholm tar to it, to modify it to about equivalent to #50 [or slightly lower] for the lower temps I work at). Gugolz is nice to work with. I used resin (rosin?) moderated with pure turps and beeswax way back in the 1960's (loved the smell), it worked ok if one got the balance right with the ingredients for working temperatures.
