From: atm{at}misterg.fsnet.co.uk (Andy Gray)
To: atm{at}shore.net
Reply-To: atm{at}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 ~œ30 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 œ30 - 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 œ30 isn't too bad for a diagonal - one quick go at rescuing the
situation, and if I get nowhere, I'll spring the œ30, 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 œ30 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?

œ30 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!}

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