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Curious Foucault Readings After Star Lap

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#1 msheald

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Posted 30 November 2020 - 07:53 AM

Hello! I've started to use a star lap on my 10" f3 mirror. I've not used a star lap before, and the readings seem strange.

 

    I generated the star lap using Mel Bartel's program for a 7-petal lap with Mirror on Top and 1/3 CoC strokes.

 

    I had thought that the center would deepen, so when I use my 9-zone Coude' mask and zero on zone 1 then the outer zones should be relatively longer in ROC when compared to zone 1. The opposite seems to  be occurring.

 

    I started with a sub-diameter square tool about 2-1/2" x 2-1/2" using CoC strokes in order to get initial figuring.

 

    With the star lap, after the last three sessions (each session is 60 minutes long with an initial cold press and then 10-minute press after each 20 minutes of polishing), the readings went from

 

Zone1   0      0     0      0

Zone2   29   21    0     -4

Zone3   39   32    0     -8

Zone4   38   28   -8     -12

Zone5   38   26   -10   -13

Zone6   33   21   -11   -14

Zone7   21   17   -16   -16

Zone8   22   17   -15   -16

Zone9   35   40   -5     -10

 

    I use  1/4" x 20 carriage bolt and dial divided into 50 divisions, so a "1" represents a movement of 0.001" in ROC.

 

    This almost looks like the star lap is acting like a full sized lap and taking the mirror back to a sphere. Part of that is good - when I started, zones 6 to 9 were way too short. There was/is a TDE, which is much better. Still, the readings are opposite what I was anticipating, and I am not sure how to use the star lap in the future.

 

    Thank you for your help and best regards.

 

Mike


Edited by msheald, 30 November 2020 - 08:27 AM.


#2 kfrederick

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Posted 30 November 2020 - 07:56 AM

Mind me asking why F3 ? on a 10 inch .Is this your first mirror ?   



#3 TOMDEY

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Posted 30 November 2020 - 08:11 AM

The scope will be for imagery only... right?    Tom



#4 msheald

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Posted 30 November 2020 - 08:35 AM

Hello! It is for imaging/photometry with appropriate coma corrector for a full sized sensor.

 

    I've made some mirrors in the past - a several 4-1/2" and 12" for classical Cassegrain (which I was not happy with and had refigured, so I would not call it a success). I thought a 10" f3 photometry scope would be a great COVID-19 lockdown project which would complement my 8" LX200 that I use for photometry. A 10" f3 mirror sure is a different beast than my other projects, though.

 

    I also mis-spoke. Zones 6 to 9 had too short ROC - essentially turned up zones rather the TUE compared to zone 1. There was a TDE edge though. I used the sub-diameter lap for initial control of the outer zones. From the Foucault zonal readings, the turned up zones were corrected faster  much better than when using the sub-diameter lap. Also, by Ronchi, the TDE is much better after using the star lap for these sessions since the sub-diameter lap did not touch the TDE.

 

    Also, previously I got it to 1/5 wave by excluding zone9 according Figure XP, but I was having trouble with the edge and Zone9. I had to decide whether to mask the outer 1/4" or keep working it and try to correct the edge and zone9. Unfortunately, when I continued to to to correct the zone9 and the edge, I had several scratches develop. So, I reground the bevel and repolished the mirror. Now, I'm back to figuring, and I thought I would try using the star lap this time instead of sub-diameter lap. Best regards.

 

Mike


Edited by msheald, 30 November 2020 - 08:59 AM.

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#5 kfrederick

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Posted 30 November 2020 - 08:48 AM

Great  bet it work good on Night Vision too 



#6 davidc135

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Posted 30 November 2020 - 09:47 AM

This doesn't help with your question but:

 

A 10'' F/3 sphere needs 8.35 waves correction. It so happens that the corrector from a 4'' F/10 sct e.g Meade 2045 corrects a max of 8.68 waves (or less) if placed in the converging beam. This gives an on-axis solution. I'm not sure how it looks off-axis with coma and vignetting but it might be worth investigating depending on how the figuring goes. 

 

BKSo started a thread 'Aplanatic reflecting Schmidt telescopes' here on 26 Oct 20.

 

David


Edited by davidc135, 30 November 2020 - 10:03 AM.

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#7 Pinbout

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Posted 30 November 2020 - 09:56 PM

Show a ronchi along with your zonal readings



#8 msheald

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Posted 05 December 2020 - 07:49 AM

Sorry! I had gone on to subdiameter lap again and did not get Ronchigrams along with the zonal readings.

 

    I'm about to try it again after getting close to what I want with the subdiameter lap. I'll keep you posted and shoot some Rocnhigrams as needed. Best regards.

 

Mike



#9 ccaissie

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Posted 05 December 2020 - 01:14 PM


 I thought I would try using the star lap this time instead of sub-diameter lap. 

 

 

 

In my experience, a star lap IS practically a sub diameter lap with a softer action out near the edge.  When I make a sub lap, I scallop the edge so it has that same "fuzzy" action on the edge....




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