From: "Thomas Janstrom" 
To: "Andy Gray" 
Cc: "atm" 
Reply-To: "Thomas Janstrom" 


Andy,

First off, hi and greetings from Oz.

> 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.

I found that too, the pics of my 8" DK primary being tested on my web
site do show much more roughness than seen with the naked eye.

> 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.

OK, my opinion, your lap is getting too thin, the thiner it gets the harder
it acts. I have just finished polishing the secondary for my DK and the
channels were closing in about 30-45 mins depending on weather I was
working at night or during the day. So your lap is not "too
soft", but this is really temperature dependant, what is the temp of
your workspace?

> 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.

Sounds about right, I would try to avoid waltzing the tool at the ends of
the strokes as this can lead to a turned edge.

> 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?

Doesn't sound normal, my CeOx will stay in suspension for hours and the
water is always pink in coulour even after months of not being used. BTW I
use a glass jar and a soft cammel's hair brush to add the polishing agent,
and for polishing "less is more" for best results I use the
nearly clear "water" at the top of the water column.

Let me know if you need some of the pink CeOx as I can send you some no
worries, just reply of list in that case.

Clear skies, Thomas
http://www.tjanstrom.com
"Don't make me set the laser printer to stun"

--- BBBS/NT v4.00 MP