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


On Sun, 30 Mar 2003 23:33:06 -0600,  "Dan Chaffee"
 wrote:

>Firstly, don't bother with star testing an uncoated primary
>with an uncoated diagonal. The image is too dim for any
>meaningful analysis whatsoever.

One blind alley avoided, thanks Dan.
>
>Second, from the looks of the images you may have full
>correction in the outer 12mm or so; maybe too much.

This is backed up by my measurements - I've gone slightly too far, plus the
centre is a bit deep.

>The mirror really has too much dog biscuit to hope  further
>paraboloizing will clean it up.

The dog-biscuit is much more apparent in the digital pictures than to the
naked eye, but I agree it's there. I have no qualms about backing off to a
sphere - I was amazed at how quickly the figure altered.

Lap is ~7/8" pitch squares stuck to glass tool (as Texereau). It is
now ~1/8" thick (from 5/16"), and I've has to trim it 3 times so
far to keep the channels open (about every 1-2 hrs polishing). My
impression is that it's a little soft, if anything, but this is from a
position of ignorance(!) I may have to re-make it, anyway, as it's not been
used for over a month.

I *think* I am getting good contact, as the resistance is quite high (but
doesn't feel "oily" - if you get what I mean), even without
deliberate pressure - enough to make my arms ache after a few minutes. I
certainly started noticing "tight" spots (uneven resistance) as I
continued a parabolising stroke, which weren't there when polishing
normally. It certainly doesn't stick and grab during the stroke.

I'm taking about 3 seconds for each complete stroke (i.e. 1-2 s away, 1-2s
back) but because of the resistance, the lap sometimes stops at the end of
the stroke (origin of the dog biscuit??). To avoid this, I've been trying
to round over the ends of the "V" to keep the lap moving. Each
stroke is accompanied by a faint "singing" (from the glass!!!) -
not squeals and squeaks, but these can be induced if I push the wet. Adding
more water doesn't really make much difference.

As mentioned, I'm using "Cerox" from Beacon Hill Telescopes. This
is a white powder that drops out of suspension really quickly (5 minutes),
leaving clear water above - I have to shake the squeezy bottle every wet -
Is this normal? I'm used to seeing pink-ish cerium oxide that takes hours
and hours to settle out, and still leaves a "muddy" water after a
day or so. I'm using just a bit more water than is needed to cover the
powder for the mix, and adding about half a teaspoon full every wet - just
enough to spread over the surface with my fingers. I'm TOT, rotating the
tool slightly during every stroke, and making a small side-step in the
opposite direction ever stroke, too.

I associate roughness with too rapid polishing, so given the above, the
only thing I can think to try is to thin down the CeO2 mixture and
concentrate on a smooth stroke, or to try the pink rouge (industrial
polishing powder).

The more I think about it, the more I think I should try the pink CeO2,
even if it just allows a smoother polishing stroke.

Thanks for the thoughts so far, and for the opportunity to think out loud.
I'm not struggling for answers (yet!), just curious. Any suggestions are,
however most welcome.

Off to get the glass out of the 'scope, and dig up the pink CeO2....

Andy.

Andy Gray, N. Wales, UK.

--- BBBS/NT v4.00 MP