Author Topic: 10" f/3.45 hyerbolic astrograph project  (Read 130 times)

Jason Bryant

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10" f/3.45 hyerbolic astrograph project
« on: December 24, 2017, 04:20:02 PM »
Hi all,

I present for you a ronchi test I just made from my 10 inch mirror.
This was sitting on a shelve for a couple of years because of much work and real-life stuff interfering with my spare time.

It had been nearly polished out when it got mothballed about the shelve.
The mirror will be a quick hyperbolic astrograph mated with a Baader MPCC.
The focal length has become a bit shorter that planned but I'm gont move with it as it's currently, circa fl = 877 mm.

When polishing I had difficulty with the border, it had been lacking behind.
I kept on with COC 1/6d strokes, shifting MOT & TOT.  Did that for quite a while.
Foucault revealed a high zone on the border and approximately 1 cm in.  (not sure I still possess the foucaultgrams)
Then attempted some W strokes also.  Dug a hole.
Subsequently I blamed the soft lap and left a new with a little harder pitch (Swedish).  It got better I believe.
Subsequently the job got mothballed.

As soon as I started the job again, I went with a more competitive approach.
I'd TOT and circa 60% overhang to both sides with strokes only over the border.
3 or 4 sessions of approximately 5 min each (two times around).

Think it did good.  Edge much longer in par with the rest of the surface.  But clearly TDE now.
Still not satisfied with the polish of the final half cm inside the border.  Laser shows much more pits, center has almost none.
So I need a better polish on the border before attempting to start figuring.

I require a bit of advice of which approach to take now.
If I do much more of the competitive stuff I will earn a serious TDE, so I've done enough of this for today, I think.

Oh btw, I use a full-size tool, thick dental plaster, rather heavy.
I've got good contact, cold-press frequently.  I am able to make the mirror chime LOUDLY by turning the tool slowly!

Excited to hear what you believe

EDIT: Oh btw, the mirror appears filthy, it is dried cerox on the surface.  I use a blue LED in my tester and cerox seemingly looks quite black in blue lighting...