From: atm{at}misterg.fsnet.co.uk (Andy Gray)
To: atm{at}shore.net
Reply-To: atm{at}misterg.fsnet.co.uk (Andy Gray)


On Wed, 19 Feb 2003 01:09:25 GMT,  atm{at}misterg.fsnet.co.uk (Andy Gray) wrote:
>If someone would be good enough to look at the ronchi-graphs
>at:
>www.misterg.fsnet.co.uk/ronchi (just .jpg s, no html)
>and offer comment as to whether I need to get closer to a
>sphere before starting to parabolise, I would be most
>grateful!
>
>Mirror is ~F/6 7" (actually 180mm dia, 1100mm focal length)

I followed the advice, and continued polishing, getting to a much better
sphere with only another ~30 mins polishing, but still with a TUE. Went
straight into parabolising from there and after a 15 minute and a 5 minute
session ended up with a surface that TEX says is 1/9.2 P-V wavefront and
FigureXP (very professional software - thankyou!) tells me is 1/6.8 P-V
wavefront & 0.95 Strehl. Unfortunately, I let the lap
"topple" at the end of the last stroke, crunched the corner of
one of the pitch squares, and ended up with a 1/4" long scratch and a
couple of sleeks (a dent to the pride, but not much else).

Reasonably happy with this, I spent the next month building the rest of the
scope to try some star-testing. I'm only using a 30mm dia circular
secondary mirror at the moment (so the FOV isn't fully illuminated), but
I've still had some superb views of Jupiter and Saturn, an extraordinary
night viewing the moon when I managed to get good detail at 280x with a
cheap and nasty 4mm eyepiece from a 60mm refractor, and split a few doubles
(Algeiba in Leo, for example). It's been said before, but the feeling of
satisfaction is surprisingly strong. My attempts at star testing (such as
they are) haven't shown anything worrying, but I'm not sure that this is
valid with such a small secondary (it needs to be ~35mm) and a dodgy 4mm
eyepiece (10mm Plossl shows nothing).

Foucault (f_.jpg)& ronchi (r_.jpg) pictures of the mirror at this stage are at:

http://www.misterg.fsnet.co.uk/ronchi/afterfigure/

There's more contrast in these photos than is visible to my naked eyes. My
wife's comments are that the polish looks a bit rough (she makes optics for
a living, and is the only one who can see the sleeks mentioned above).
There's some signs of zoning on the Foucault photos, but this doesn't seem
visible viewing the test directly (but hey, this is the first time I've
done this!)

I'm not sure how I'll get on star-testing with a full sized, but un-coated
secondary, but will try this next. I still have a hankering to chase the
figure round a bit more, to polish the sleeks out, and to try and improve
the surface roughness a bit. (Anyone going to talk me out of it?)

I was using white Cerox 1650(?) from Beacon Hill Telescopes (UK). This is
supposed to be super stuff, but it drops out of suspension completely in
only a few minutes, which I find worrying. I had assumed that by using a
very light pressure (almost floating the lap) that I would improve the
surface finish, but this is what I was doing when I chipped the lap. I
think I'll thin the mix down a bit, instead. I also have some pink ceria
rouge, which I know will stay suspended for hours - maybe I'll try this,
too (I suppose a good test would be polishing the new secondary).

There's a photo of the mechanically completed scope at:
http://www.misterg.fsnet.co.uk/ronchi/afterfigure/thescope.jpg
(T shaped "hacksaw blade" spider, and SLR lens barrel focusser.)

Sorry to have rambled on so much.

Andy

Andy Gray, N.Wales, UK

--- BBBS/NT v4.00 MP