View Single Post
  #57  
Old 12-11-2018, 07:09 PM
sfarndell (Scott)
Registered User

sfarndell is offline
 
Join Date: Nov 2007
Location: Melbourne
Posts: 5
Hi Logan

My experience is more with larger mirrors (18" f4), where you work with sub-diameter laps, so I'll share what I know:

More information is needed:
1. Have you polished enough? 4 hours is relatively short if polishing by hand.

2. Are you inside / outside focus? If you're inside focus, then you have some issues (use short strokes with small overhang slightly off-centre to lower the centre)

3. If you're outside focus, then it's not so much as a kink, but a spherical centre and some parabolisation towards from the 70% zone and you don't seem to have a TDE, which is good news! F2.8 is a pretty aggressive / short focal ratio. Check mel-bartels' website for the matching ronchigram to determine if you've over-parabolised the edge. If not, all you need to do is use a subdiameter lap (4") and parabolise the centre (short chordal strokes with minimal overhang close to COC), then smooth with full size lap.

Last edited by sfarndell; 12-11-2018 at 07:22 PM. Reason: Inside / outside focus advice was muddled
Reply With Quote