ATM Diagonal learning curve (long!)

[atm@misterg.fsnet.co.uk (Andy Gray)]

Hello all,

Just thought I'd share my experiences trying to make a 1.5"
diagonal mirror for my 7" Dob. (My guiding light in this the
2nd edition of Texereau's book)

I started off with a scrounged piece of 10mm thick float
glass that was just big enough to trepan an oversize blank
for the mirror (~3" diameter) using an old boot polish tin
as the tool - just like the "Every boy can make..." books!
There was enough spare to trim to a ~4" diameter piece to
cast the lap on.

(a previous post deals with the effects of casting a lap on
painted MDF. Not Good)

As mentioned previously, I have a ~3.5" optical flat marked
[lambda]/8, and I checked the flatness of the blank against
this using a small fluorescent lamp as a light source.
Although I could see the fringes with the blank laid on top
of the flat, the brightness & contrast was much better if I
held a "page magnifier" (large fresnel lens) over the glass
parts and both illuminated & observed them through it - this
is similar to the plano/convex lens in the book.

Stages:
1) Blank ~2 fringes 'cave
2) ~15 mins TOT --> 7-8 fringes 'vex!
3) Many hours MOT --> 2 fringes 'vex
4) More hours MOT --> 1.5 fringes 'vex [aaargh!]
5) 15 mins lengthened stroke (~0.7d) --> <1 fringe 'vex
[Wahey!]
6) More messing around in 10-15 minute bursts MOT, various
length strokes --> As near to perfect as I can tell
[honest!]
7) Careful inspection prior to cutting up --> 3 short
scratches that I can't cut around [ :-((( ]

{I've done it once, so I'll do it again - after all it'll
cost me ~£30 to buy, and it would be quite nice to have made
it myself....}

8) Repeat steps 3-6 and reach an acceptable flat (~1/4
fringe + ~1 fringe roll off at edges, which will be cut
off). 

{I decide this is a reasonable result. I've been put off
trying to trepan the blank into a "sausage slice" elliptical
diagonal & have decided to cut it to an octagon with
bevelled edges top & bottom (minimal extra obstruction over
elapse ~ 7%). I'm irresistibly drawn to a diamond tile saw
in B&Q for £30 - bought on the justification that I would
have spent that on a ready made diagonal.}

9) Test cuts on scraps of the 10mm glass. Saw is a bit
vicious, but otherwise works a treat. Checked cut pieces for
flatness --> no problem!! [:-)]

10) Stuck ~1.5mm "picture frame" glass over polished blank
with bees wax / paper and cut precious blank to an octagon
EXTREMELY carefully. Bevelled 3 edges top & bottom, cleaned
up saw cuts and bevel with cheap diamond whetstone (works
great) and gently removed protective glass, bevelled edges
on polished side with trembling hands --> Looks the
"bees-knees" [Nothing to this...]

11) After much careful cleaning, checked flatness --> 2-3
fringes roll-off on bevelled edges.

{intermission, during which domestic animals, spouse,
fixtures, fittings and offspring see how sulky a 37 year old
man can be}

12) Try to polish cut mirror --> Massive roll-off at edges
after 5 mins polishing [Stupid hunk of glass...]

{maybe £30 isn't too bad for a diagonal - one quick go at
rescuing the situation, and if I get nowhere, I'll spring
the £30, and put it down to experience...}

13) Surround the diagonal with scraps glass and manage to
get them all contacted to the optical flat at the same time.
Stick a glass disk covered in hot pitch to the back of them
& let it cool --> A "blocked up" mirror like Texereau's
first method.

14) Repeat steps 1-5 without smoothing (concerned about
loading the pitch with grit) --> Noticeably astigmatic flat,
but without roll-off at edges [getting there...]
15) A few more hours --> Not too bad a flat (~1/2 fringe,
but "saddle" shaped [probably OK, but just try a little bit
longer]
16) Scratch it. [withdraw from social interaction for
several days]

{There's no way I'm paying £30 for a diagonal having come
this far..}

17) Repeat steps 1-5 --> repeat step 16

{I don't care if diagonals are being given away free with
corn flakes, I'm going to make one...}

18) Can see glass flaking away at edges of blank. Try to
bevel edges 'in-situ' with diamond whetstone. It slips.--> a
proper scratch, glass dust and all.

{I don't care if diagonals are being given away free with
corn flakes, four at a time I'm going to make *this piece of
glass* into one...}

19) Cheat - take blocked up pieces to work and get it
smoothed & polished on a polyurethane lap --> 50+ fringes
'vex, but smooth, shiny & no scratches.

{I switch to a small neon light as a source at this stage,
to get away from the multi-coloured fringes from the
fluorescent lamp}

20) Repeat steps 1-5 --> some fine sleeks visible on
inspection.

21) Repeat step 20 many times. Heavy pressure on the blank
helps stop sleeks --> reasonable flat [:-)]

22) Freeze the whole thing & remove the finished diagonal,
apply bevel to polished face & test --> 
http://www.misterg.fsnet.co.uk/ronchi/diag.jpg

Maybe 1/2 fringe of orange neon light if I'm being generous
to myself.

What's the verdict?  Worth coating?

£30 would have been a bargain.

Andy.

Andy Gray, N. Wales, UK.

{I'm not sure why I've written all of this - hopefully you
will understand!}