Ok so I did the sharpie test at 25u and it is perfect contact (even dimming). I think it must have just been the channels, I dug new ones out with my fingernail.
F2.8 with a 20 mm eyepiece gives 21x. F3.2 with a 20 mm eyepiece gives 24x.
The field with a 100 degree eyepiece at 21x will be roughly 4.76 degrees. The field with same eyepiece at 24x will be about 4.16. So you lose 0.6 degrees by staying at F3.2. Personally I think a 4 degree field is huge so I would leave it at F3.2. It will be a little easier to figure too.
Polishing is going well, 2 hours done now but center has maybe 15% more to go and edge 40% as a rough guess? I am working mot. Also my channels are closing fast (soft acculap), which means channeling,which means pitch getting thinner, and now the pitch is id estimate 6 or 7mm thick? I think this lap will get me polished out but not into figuring. Unless i somehow pour new pitch onto the lap?
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
You'd also need to tell us what your Rochi LPI are. I assumed 4 per mm, ROC of 840mm. Using this you seem to have an over-parabolised edge. I'd return to a sphere using the full-size lap with a 1/3 D stroke, offset from centre. Likely you still need to polish out the mirror anyway so this shouldn't take too long.
Hi Scott,
Sorry for not replying earlier!
I now need to add parabolization to the 0.7 zone according to Bratislav, he says either full size scrubbing 0.7 down, or a subdiameter.
I'm waiting for my mentor to reply on cloudynights though.
So I had to return to sphere, and have been fixing a tde for the month. I only need about 1 more hour to fix it though! Woo!
Rod helped me make the beautiful wooden birch octagonal tube, it looks great! Now it needs a coat of polyurethane