deepening sagitta

[GrahamL]

I started grinding my 6 " today ,, good fun

I measured the sagitta as being close to f 8 quite quickly it seemed

after a few wets at 1/3 COC I checked it again and found it it was .75 mm (target 1.15).

I believe I maybe bought the chordal strokes in to quickly and when
changeing strokes to a more spherical action lost the high area around the outside of the blank loseing quite bit of my depth. ?

as this is my first try its all new .

Right now I've been sticking with 1/3 COC MOT and the sagitta
does seem to be getting deeper (slowly)

any quicker way to proceed ?


input most welcome

[sopticals (Stephen)]

Stay with the current stroke.Dont try to hurry.Have patience, because you will have to find lots of patience, and forbearance, when you get to the polishing stage, and greatly more so during figuring.Take your time and ENJOY the journey.

[GrahamL]

Thanks Stephan

Yes I'm in no hurry , Just wasn't sure if there was another way to get back on track .

I'm still grinding MOT 1/3 COC 80 grit and today measured the depth at 1.00 mm

Theres a very small air bubble half from centre I may have to get past yet but next couple of days should get me close to target

Should I start alternateing to TOT when I get close ?

regards graham

[sopticals (Stephen)]

Hi Graham, stay with mirror on top while using the #80 grit.What is your target sagitta? What progression of grits are you using? I used the following for my larger (18" to 25") mirrors: #46,#80,#120,#220,#400,#800 Silicon carbide (for all). With smaller mirrors I wouldnt use #46 (but when having target sags of 6-8+mm the courser grit saves time in the initial hogging out).

[GrahamL]

Hi Stephan

1.2 mm (F8) would give or take a little would be ok , the scopes focal length isn't really fixed and will adjust down or up to suit the mirror but somewhere around the F 8 mark would be ideal I think.

I havn't decided on the progression from here but I have the following
carbo #120 #220 #280
aloxite #400 #600 #1000
and one that is smudged that I think reads cerox #1650
I also have a big chunk of hard as a rock pitch

what would you suggest to move on to next ?

I just checked the surface for good contact as it mentions on the stellafane site ( drawing lines ) and everything seems pretty good.

graham

[sopticals (Stephen)]

Hi Graham,

Others may disagree, but if I were you, looking to maintain a sagitta of around 1.2mm, then skip the #120 grit and go with the #220 next. 1 hour to 90mins should be all you need to remove all pits left from #80.

I would use a little TOT during this time to:
a) remove more quickly pits (#80) from the edge regions of the mirror, and
b) to control the depth of the sagitta.

The smaller the number of grades of grits used the less likely large grit contamination is to occur as one goes through the fine grinding process.

I would suggest the following progression of grits for your 6" mirror:

#80, #220, #400 (1 -2 hours last two), #600, #1000 (1 hour each).

Use a little TOT with each grit to get a uniform across the surface grind and to manage sagitta. (though sagitta will be less sensitive to change as the grit sizes get smaller).