Go Back   IceInSpace > Equipment > ATM and DIY Projects

Reply
 
Thread Tools Rate Thread
  #1  
Old 15-04-2018, 08:40 PM
Lognic04's Avatar
Lognic04 (Logan)
Registered User

Lognic04 is offline
 
Join Date: Mar 2016
Location: Melbourne, VIC
Posts: 889
Making a 10" Dob

Hi all,
I have finally decided to take the plunge into mirror making! Going old school!
I am planning to grind my own 10 inch (250mm) f/6 (1500mm FL) Mirror to go into a 10 inch dobsonian tube that has been sitting unused.
Picking up the materials on Saturday, and am going to get help from the ASV all the way
This is my first time mrror making! :O
Going to regularly update this thread on the progress!

Last edited by Lognic04; 16-04-2018 at 07:00 AM.
Reply With Quote
  #2  
Old 15-04-2018, 09:59 PM
Atmos's Avatar
Atmos (Colin)
Ultimate Noob

Atmos is offline
 
Join Date: Aug 2011
Location: Melbourne
Posts: 6,980
Awesome!
This is one of the things to do on my bucket list.
Reply With Quote
  #3  
Old 16-04-2018, 06:59 AM
Lognic04's Avatar
Lognic04 (Logan)
Registered User

Lognic04 is offline
 
Join Date: Mar 2016
Location: Melbourne, VIC
Posts: 889
Quote:
Originally Posted by Atmos View Post
Awesome!
This is one of the things to do on my bucket list.
Reply With Quote
  #4  
Old 16-04-2018, 07:01 PM
Lognic04's Avatar
Lognic04 (Logan)
Registered User

Lognic04 is offline
 
Join Date: Mar 2016
Location: Melbourne, VIC
Posts: 889
Few questions:
Is there a way to measure my curvature relatively accurately without a spherometer?
I am seeing different methods of testing the mirror, is a ronchi/focault combination fine? Or is it better to map it with a PC?
Thanks!
Reply With Quote
  #5  
Old 16-04-2018, 09:43 PM
OzEclipse's Avatar
OzEclipse (Joe Cali)
Registered User

OzEclipse is offline
 
Join Date: Jun 2012
Location: Young Hilltops LGA, Australia
Posts: 1,177
Hi Logan,

Good on you. I made my own mirror for my first scope when I was 15.

While you are grinding the easy way is to swish some water over the ground surface and focus the sun. I had a long wooden stick 2m long. I attached a piece of white card to one end and marked distance at the other end. You can use 100mm markers for the longer lengths then drop down to 10mm markers near your target focal length. Take the mirror outside on a sunny day, slosh water over the surface and focus the sun on the white card, read the focal length off your distance markers.

A 10" f6 has a 2.6mm deep sagitta or curve. If you don't have the sun, you can lay a straight edge across the whole diameter of the mirror. Then compare the centre depth to a depth marker like a 2.5mm drill.

You only use these methods when you are rough grinding. Get the focal length a couple of inches longer than you want by the end of the 80 grit grind. The fine grind will take the rest off. By the time you get to Ronchi or Foucault test (not Focault) you are polishing and the focal length won't change much during polishing. You can however use these tests to determine the focal length accurately.

Have fun mate.

Joe

Last edited by OzEclipse; 16-04-2018 at 10:11 PM.
Reply With Quote
  #6  
Old 17-04-2018, 05:56 AM
Baza (Barry)
Registered User

Baza is offline
 
Join Date: Mar 2014
Location: Heathcote NSW
Posts: 82
Logan
there is a mirror making thread with lots of info, also have a look at https://stellafane.org/misc/links.html#Equatorial and http://www.bbastrodesigns.com/JoyOfMirrorMaking/ both have heaps of info burt you might have to dig a little.
http://www.mirror-maker.com/
awaiting the progress reports. Barry

Last edited by Baza; 17-04-2018 at 05:57 AM. Reason: another link
Reply With Quote
  #7  
Old 17-04-2018, 07:05 AM
Lognic04's Avatar
Lognic04 (Logan)
Registered User

Lognic04 is offline
 
Join Date: Mar 2016
Location: Melbourne, VIC
Posts: 889
Quote:
Originally Posted by OzEclipse View Post
Hi Logan,

Good on you. I made my own mirror for my first scope when I was 15.

While you are grinding the easy way is to swish some water over the ground surface and focus the sun. I had a long wooden stick 2m long. I attached a piece of white card to one end and marked distance at the other end. You can use 100mm markers for the longer lengths then drop down to 10mm markers near your target focal length. Take the mirror outside on a sunny day, slosh water over the surface and focus the sun on the white card, read the focal length off your distance markers.

A 10" f6 has a 2.6mm deep sagitta or curve. If you don't have the sun, you can lay a straight edge across the whole diameter of the mirror. Then compare the centre depth to a depth marker like a 2.5mm drill.

You only use these methods when you are rough grinding. Get the focal length a couple of inches longer than you want by the end of the 80 grit grind. The fine grind will take the rest off. By the time you get to Ronchi or Foucault test (not Focault) you are polishing and the focal length won't change much during polishing. You can however use these tests to determine the focal length accurately.

Have fun mate.

Joe
Thanks for that Joe! I will definitely be using these techniques!

Quote:
Originally Posted by Baza View Post
Logan
there is a mirror making thread with lots of info, also have a look at https://stellafane.org/misc/links.html#Equatorial and http://www.bbastrodesigns.com/JoyOfMirrorMaking/ both have heaps of info burt you might have to dig a little.
http://www.mirror-maker.com/
awaiting the progress reports. Barry
Thanks Barry!

Reply With Quote
  #8  
Old 17-04-2018, 09:55 PM
Lognic04's Avatar
Lognic04 (Logan)
Registered User

Lognic04 is offline
 
Join Date: Mar 2016
Location: Melbourne, VIC
Posts: 889
Another newbie question, what should I make my tile tool with? Does it need to be curved?
Thanks!

Last edited by Lognic04; 18-04-2018 at 06:08 AM.
Reply With Quote
  #9  
Old 20-04-2018, 05:33 AM
Baza (Barry)
Registered User

Baza is offline
 
Join Date: Mar 2014
Location: Heathcote NSW
Posts: 82
double post
Reply With Quote
  #10  
Old 20-04-2018, 05:34 AM
Baza (Barry)
Registered User

Baza is offline
 
Join Date: Mar 2014
Location: Heathcote NSW
Posts: 82
you need tiles of the same hardness all the way through. non glazed exterior tiles seem to work. they can be stuck to dental plaster. you will need to waterproof plaster of paris if you use that. details and photos on stellafane & numerous other sites.
It will curve with correct use.
Reply With Quote
  #11  
Old 21-04-2018, 08:06 PM
Lognic04's Avatar
Lognic04 (Logan)
Registered User

Lognic04 is offline
 
Join Date: Mar 2016
Location: Melbourne, VIC
Posts: 889
DAY 1! 2 hours work - flattening the back of the blank with 80 grit, almost done. Lot of fun so far!
Reply With Quote
  #12  
Old 22-04-2018, 09:13 AM
xelasnave's Avatar
xelasnave
Gravity does not Suck

xelasnave is offline
 
Join Date: Mar 2005
Location: Tabulam
Posts: 16,866
Good on you Logan should be good experience before you tackle a 20 inch f4
Good luck ( remember the harder you work the more good luck you get) and post often.
Alex
Reply With Quote
  #13  
Old 22-04-2018, 09:15 AM
xelasnave's Avatar
xelasnave
Gravity does not Suck

xelasnave is offline
 
Join Date: Mar 2005
Location: Tabulam
Posts: 16,866
And you know we are all expecting a full length movie.
Alex
Reply With Quote
  #14  
Old 22-04-2018, 01:55 PM
Troy's Avatar
Troy
Registered User

Troy is offline
 
Join Date: Feb 2008
Location: Hunter Valley
Posts: 946
Are you hand grinding?
Reply With Quote
  #15  
Old 25-04-2018, 10:03 AM
Lognic04's Avatar
Lognic04 (Logan)
Registered User

Lognic04 is offline
 
Join Date: Mar 2016
Location: Melbourne, VIC
Posts: 889
Hehe thanks Alex. I am hand grinding.
Look forward to that video!
Reply With Quote
  #16  
Old 28-04-2018, 01:48 PM
Lognic04's Avatar
Lognic04 (Logan)
Registered User

Lognic04 is offline
 
Join Date: Mar 2016
Location: Melbourne, VIC
Posts: 889
I am back to grinding, just wondering if i should regularly flip the mirrors? I am only flattening the back.
Reply With Quote
  #17  
Old 28-04-2018, 05:47 PM
OzEclipse's Avatar
OzEclipse (Joe Cali)
Registered User

OzEclipse is offline
 
Join Date: Jun 2012
Location: Young Hilltops LGA, Australia
Posts: 1,177
Hi Logan

To make a perfectly flat (optically flat) surface, you need three blanks and they have to be regularly flipped top to bottom and swapped in and out of the pair.

I am understanding that you mean you are grinding the back of the mirror flat? Unless it is really uneven, it's not necessary to flatten the back.

However if you want to do it, then yes, do flip the 2 mirrors otherwise the top one will become concave. Flipping two mirrors, will make them roughly flat which is all you need.

Cheers

Joe

Last edited by OzEclipse; 28-04-2018 at 06:13 PM.
Reply With Quote
  #18  
Old 28-04-2018, 06:43 PM
Lognic04's Avatar
Lognic04 (Logan)
Registered User

Lognic04 is offline
 
Join Date: Mar 2016
Location: Melbourne, VIC
Posts: 889
Quote:
Originally Posted by OzEclipse View Post
Hi Logan

To make a perfectly flat (optically flat) surface, you need three blanks and they have to be regularly flipped top to bottom and swapped in and out of the pair.

I am understanding that you mean you are grinding the back of the mirror flat? Unless it is really uneven, it's not necessary to flatten the back.

However if you want to do it, then yes, do flip the 2 mirrors otherwise the top one will become concave. Flipping two mirrors, will make them roughly flat which is all you need.

Cheers

Joe
Thanks Joe. It is mostly done now anyway, so soon I can go onto rough grinding! Hopefully I don't use all my 80 grit on this!
Reply With Quote
  #19  
Old 28-04-2018, 06:45 PM
Lognic04's Avatar
Lognic04 (Logan)
Registered User

Lognic04 is offline
 
Join Date: Mar 2016
Location: Melbourne, VIC
Posts: 889
Mirror right now. That is the back
Attached Thumbnails
Click for full-size image (20180428_175120.jpg)
199.2 KB172 views
Reply With Quote
  #20  
Old 29-04-2018, 07:56 PM
Lognic04's Avatar
Lognic04 (Logan)
Registered User

Lognic04 is offline
 
Join Date: Mar 2016
Location: Melbourne, VIC
Posts: 889
Phew! The back is flat! I have started on the rings, they really were nasty at first but they are almost gone now - 4 or 5 more wets to go. Another question! How long should a wet last? Usually i let it go until it is a very light grey, but i feel like this is a huge waste of time, in the time i spend stretching out a wet i feel like i could have done 2 "normal?" wets in between.
Reply With Quote
Reply

Bookmarks

Thread Tools
Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT +10. The time is now 08:12 AM.

Powered by vBulletin Version 3.8.7 | Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
Advertisement
Bintel
Advertisement
Testar
Advertisement