Polishing with sub diameter lap

[JRM]

Hi All,

    

 Anyone have a link or information on the use of sub diameter laps?

I need a refresher course.

it has been nine months since last I pushed glass but the wrist is all healed

and life hopefully has stopped throwing me some bad curves and the time has 

come to get back in the saddle,

I know mirror on top 1/3 center over center stroke makes the curve deeper 

and the exact opposite with lap on top but this is with full size laps.

If I want to deepen the central area of the glass with sub lap on top what length

of a stroke should be used,hell I feel stupid asking but I am not use to sub

laps and prefer not to go full size.

oh well, maybe I can have a chuckle over this tommorrow.

Thanks

Rick

[mapstar]

Hi Rick,

Good to see you getting back into it. My struggle continues but I'm getting there and learning a lot although on the subject of sub diameter laps I'm definitely no expert as I keep finding pitfalls in everything I seem to do to the glass!

Cheers  

Damian

[Glasspusher]

Rick,

I think you are talking about a parabolising stroke using a sub-diameter lap on top of the mirror. I would use a 50% diameter lap with minimal edge overhang. Stroking through the centre will have a quicker action than a tight 'w' stroke, another option is an elliptical stroke. It all depends on the size and speed of the mirror, larger faster mirror may need a lap smaller than 50% D, maybe stroking centre thru centre. A smaller longer focal length mirror would be OK with a 50% lap with a 'w' stroke across the whole mirror. Hope this helps.

John

[RAC]

I take it it's 20", what f ratio is it?

[JRM]

I take it it's 20", what f ratio is it?

it is an F-5

[Astrobits]

You will find it impossible to parabolise a 20" F/5 mirror with a full size lap so using sub-diameter laps is the order of the day. Follow Glasspusher John's advice.

Remember that the sphere that you start with needs glass taken away from all over EXCEPT at the edge, so don't have any stroke overlapping the edge.

Nigel