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Introduction

This describes my experience in creating my 1st 6" Newtonian RFT telescope. I have written this for my own use for making future mirrors. If it help you then good, if not tough.

The planned parameters were:

1. Mirror diameter; 150mm
2. Focal ratio; 4
3. Focal length; 600mm
4. centre of curvature; 1200mm
5. Sagitta; 2.34375mm

I discussed this project with Ron Kelton of Carlisle Astronomy Club, and read a variety of books some of which he kindly lent my. He also supplied me with a starter mirror kit with all the necessary blanks, grits etc. My brother, Walter who has an 8" Newtonian built by Ron, thinks I am mad to attempt this project.

Preparation

I started by building a type of tripod grinding/polishing stand after Texereau, see diagram. This took about 32 hours over 4 weeks, a lot of effort but I liked the trigonometry theory behind the design, it works quite well.

The blanks were Pyrex, the tool was previously used to grind/polished flats with a few chips on the edge. I chose the polishing surface as the tool. Starting of by grinding a 45-degree bevel around the edge using a "Stanley" double oil grinding stone, the rough side. The bevel was about 2mm wide.

The chosen mirror surface was already concave to about the correct depth. I decided not to grind it flat due to extra work, not a good idea, and the thickness of the blank was < 0.03"
.

I drilled small holes in a jam jar lid to make a sprinkler for the various sizes of grit and a used an insecticide sprayer thoroughly washed out. Return to top of page

Hollowing Out with 80 Grit

This is roughly grinding out a hollow in the mirror and forming the tool, basically removing unwanted material as quick as possible.

I set the stand up under the old apple tree and set off grinding using cords with the mirror centre about 1" in from the tool edge for about two hours mainly to get the tool formed to the mirror. This gave a 2" diameter hollow in the mirror plus removal of material next to the mirror edge due to its concave shape. The tool had about 1" wide ground band around its edge. I spent another hour grinding with the mirror centre along cords between 1" to 2" from the tool centre. At this point the Sagitta was nearly correct although there was still a small flat on the tool centre about 1" diameter.

Had a bash at testing, blacked the surface with 2B pencil to help reflect the light. Set up to do a knife-edge test but could not work out what to do. Made template with Pyrex with radius of 1264mm. This showed that the mirror edges were to low, i.e. the centre not ground out deep enough. This ties up with the fact that the centre of the tool is still flat.

Stroke	No Wets	Tool	e	Template	Comments
1/3 Cord	10	Flat at tool centre 2" dia.	0.10"	Rocked through mirror centre by about 0.005", either side	Pencil lines on tool abraded 1/4" wide starting 1/8" in from edge
1/3 Cord	10	Tool flat=1.5" dia.	0.11"	Template didn't rock with a central gap 2" wide @ >0.01" deep	As above
Diametrical, long strokes	15	Tool flat=1" dia.	0.13"	Gap width >3" dia. wide across centre	Pencil marks on tool abraded on edge to width of 1/8"
1/3 diametrical <=1/3	6	Tool flat=1" dia.	0.125"	As above	-
Diametrical <=1/3, tool on top	7	Tool flat=3/4" dia.	0.125"	Gap reduced to 0.01", 1" dia.	-
As above	10	Tool flat gone	0.12"	Gap =>0.01", 3.3" dia.	Pencil lines on tool abrade at edge 1/4" wide, some marks in centre of mirror
As above	6	-	0.12"	Gap dia. to 2.8"	More marks in mirror centre from pencil marks on tool
As above	6	-	=<0.12	Gap reduced <0.01", and 2.5 dia."	As above
As above	12	-	0.115"	Gap to <0.05" width 1.25" to 2.75" dia. depending on which way the template is placed on the mirror	Chamfer on mirror now ground out
1/3 diametrical	20	-	As above	As above	Ground new chamfer since slight chipping off tool edges
Diametrical, long then short 1/3 strokes	5 & 5	-	0.11" to 0.115"	Gap reduced to <=0.01"	-
As above	8 & 4	-	As above	As above	-
Long Diametrical	10	-	0.095" which is about right	Template fits	Bubbles appear event

3 wets using 400 grit using 1/3 diametrical with mirror on top. Real problem with near ringing of mirror to tool making grinding very difficult. Due to not enough grit.

6 wets @ 400 grit with tool on top, 1/3 diametrical. Added more grit so easier. 1.5" wide band from edge inwards ground smooth centre less so, therefore not spherical. Hopefully can sort this during next grinding stage. Smooth enough to test for centre of curvature (CoC), measured at about 1320mm +-50mm, using torch next to eye along centre line of mirror which was wetted with water to make it reflective.Return to top of page

Grinding @ 120 Grit

8 wets 2 lots of 4 wets, two wets with mirror on top and two with tool on top, result: as above, e measured with feeler gauges=0.915". Big pits <0.2mm 10 in all.

4 wets, two with mirror on top and two with tool on top. Result: 10 pits about 0.2mm diameter (8 thou) found by scanning surface with a x10 eye piece, CoC measured as above at 1325mm, and e=0.0915" measured by feeler gauges.Return to top of page

Grinding @ 240 Grit

2 wets with mirror then tool on top, each wet lasting 10 minutes. To check for pits marked out 1/2" squares on mirror with a soft pencil, and used a x10 eyepiece to find and measure pit sizes and number (should have done this earlier). Result: 33 pits, most <=0.1mm some about 0.2mm diameter.

2 wets as above. Result: large pits gone, 2 bubbles in glass @ 0.2mm dia, 3 bubbles in glass @ 0.1mm dia, 5 pits between 1.5 and 1 mm dia, and 5 pits between 0.1 and 0,05mm dia., e=0.0895", r=3.095", CoC=1292mm using touch next to eye, probably should have used Foucault tester. Decided to go to 400 grit. This was a mistake since it took a long time to grid out the above pits and others I suspect I didn't see! Return to top of page

Grinding @ 400 Grit

During this stage the mirror and tool were rotated during grinding, firstly mirror on top then tool on top, using a "W" stoke at a 1/3 D unless stated otherwise. Alternating the tool and mirror on top during grinding maintains the mirror shape obtained in the previous grit. Used a good squirt of washing up liquid in the water sprayed into the top at the start and during each wet, this stopped the sticking of the mirror to the tool. This has not happened to my yet, but I think it would cause deep sorrow at this stage.

Grinding Time is the actual1 total grinding time, so does not include the time taken for washing, testing, etc

No of Wets	Grinding Time (min.)	Comments
2	16	4 bubbles =0.2mm dia., some pits =0.1mm dia., many =>0.05mm and <0.1mm dia.
2	16	Ignoring bubbles in the glass there were about 78 pits, some =0.1mm dia. and most were <=0.05mm dia.
1	8	Swapped mirror & tool half way through wet. Many pits <=0.05mm dia.
2	16	Many pits <=0.05mm dia.
2	16	Large pits reduced in number because of reducing the pressure of the mirror/tool on top during grinding. Many pits=0.05mm dia. around the central area of the mirror.
2	16	57 pits <=0.1mm dia. CoC=1132mm measured using my Foucault tester, e=0,0905".

Decided to got to the next grit.Return to top of page

Grinding @ 600 Grit

Decided to got to the next grit.Return to top of page

Grinding @ 800 Grit

Note difficult to find exact CoC using Foucault tester, and difficult to measure distance between the mirror centre, the knife edge, and the slit of the light source.Return to top of page

Summary of My Grinding Process