Page 2 of 4 FirstFirst 1234 LastLast
Results 11 to 20 of 37
Like Tree2Likes

Thread: Mirror Grinding/Telescope Making Package

  1. #11
    tinkerer's Avatar
    tinkerer is offline Main Sequence
    Points: 5,067, Level: 48
    Level completed: 59%, Points required for next Level: 83
    Overall activity: 0%
    Join Date
    Jun 2010
    Location
    minneapolis
    Posts
    108
    Points
    5,067
    Level
    48
    Thanks
    14
    Thanked 57x 45 Posts

    Default



    Corey

    I bought (2) 4.25 inch diameter blanks. The second one in case I really screwed up. They are secondaries for a gregorain scope I am working on. One of them is through the fine grinding stage and ready to be lapped and figured. The 3rd blank is 12.5 inch dia. If you take a look at the grinding away thread (somewhere around page 6 I think) there are pictures of the three of them.

    Al Powers

  2. #12
    Art Bianconi's Avatar
    Art Bianconi is offline Banned
    Points: 7,670, Level: 61
    Level completed: 40%, Points required for next Level: 180
    Overall activity: 0%
    Achievements:
    First 1000 Experience Points200+ Posts Achievement!400+ Posts Achievement5+ Referrals Achievement!365 Days+ Registered Achievement!
    Join Date
    Dec 2010
    Location
    Delaware River Valley, Western New Jersey
    Posts
    328
    Points
    7,670
    Level
    61
    Thanks
    91
    Thanked 198x 106 Posts

    Default Hold Everything!

    Hi Corey!

    After considerable browsing I've come to the conclusion that the best deal is at

    6 Inch Telescope Mirror Kit - Amateur Telescope Making & More

    They offer a complete kit for just over $50. That includes the Pyrex mirror blank. all the abrasives and the pitch needed for polishing.

    All you need are a spherometer, a small, waist-high table, and a couple of 6 inch tools made from Portland Cement. (one for grinding and then a second for the pitch lap needed to polish)

    It's one source. No fuss. No muss and they have some great books too.

    Art

  3. The Following 3 Users Say Thank You to Art Bianconi For This Useful Post:

    admin (02-02-2011),Ali8bongo (01-05-2012),cawilliams1983 (02-02-2011)

  4. #13
    cawilliams1983's Avatar
    cawilliams1983 is offline Bright Giants
    Points: 5,972, Level: 53
    Level completed: 11%, Points required for next Level: 178
    Overall activity: 0%
    Achievements:
    Got three Friends20+ Friends Achievement!200+ Posts Achievement!First 1000 Experience Points400+ Posts Achievement
    Join Date
    Nov 2010
    Location
    Bright, IN
    Posts
    312
    Points
    5,972
    Level
    53
    Thanks
    30
    Thanked 58x 50 Posts

    Default

    I'm sorry, Al, I didn't recognize you. You're doing some impressive work with your gregorian optics... Way to challenge yourself. I'm just playing it safe with a newt... I'll have to catch up on your thread; the suspense of your secondary caused me to look away like a scare child watching a scary movie. Where do you plan on your mirrors finished?

    Thanks again, Art. You've successfully ended my die hard plan on ordering from WB... These prices are great!
    Corey

    Meade DS 2114
    To view links or images in signatures your post count must be 5 or greater. You currently have 0 signatures.
    ,
    To view links or images in signatures your post count must be 5 or greater. You currently have 0 signatures.
    SkyQuest XT10 Dob, Zhumell EP kit, Orion Min-EQ Tabletop mount to be used with wife's Canon 350D

    KADIAN member

    To view links or images in signatures your post count must be 5 or greater. You currently have 0 signatures.

  5. #14
    Art Bianconi's Avatar
    Art Bianconi is offline Banned
    Points: 7,670, Level: 61
    Level completed: 40%, Points required for next Level: 180
    Overall activity: 0%
    Achievements:
    First 1000 Experience Points200+ Posts Achievement!400+ Posts Achievement5+ Referrals Achievement!365 Days+ Registered Achievement!
    Join Date
    Dec 2010
    Location
    Delaware River Valley, Western New Jersey
    Posts
    328
    Points
    7,670
    Level
    61
    Thanks
    91
    Thanked 198x 106 Posts

    Default

    Quote Originally Posted by cawilliams1983 View Post
    Art. You've successfully ended my die hard plan on ordering from WB... These prices are great!
    Years from now your friends and family will curse me from having made you into a glass pushing addict. You will disappear daily into the grinding area of your home and pump away at a 17" blank!

    BTW, when ordering pitch #73 is best if the work area is reasonably warm. Softer if it's not.

    I believe they have a temperature guide to help you choose.

    Do you still have the suggestions I made for forming cement tools?

    I'll get dimensioned drawings to you on the spherometer.

    Art

  6. #15
    cawilliams1983's Avatar
    cawilliams1983 is offline Bright Giants
    Points: 5,972, Level: 53
    Level completed: 11%, Points required for next Level: 178
    Overall activity: 0%
    Achievements:
    Got three Friends20+ Friends Achievement!200+ Posts Achievement!First 1000 Experience Points400+ Posts Achievement
    Join Date
    Nov 2010
    Location
    Bright, IN
    Posts
    312
    Points
    5,972
    Level
    53
    Thanks
    30
    Thanked 58x 50 Posts

    Default

    I've always wanted to be the focus of a good intervention!

    Err... The cement tool you suggested required me to get a pre-concaved mirror.. Can I not just make one from broken ceramic tile or is there a better way?
    Corey

    Meade DS 2114
    To view links or images in signatures your post count must be 5 or greater. You currently have 0 signatures.
    ,
    To view links or images in signatures your post count must be 5 or greater. You currently have 0 signatures.
    SkyQuest XT10 Dob, Zhumell EP kit, Orion Min-EQ Tabletop mount to be used with wife's Canon 350D

    KADIAN member

    To view links or images in signatures your post count must be 5 or greater. You currently have 0 signatures.

  7. #16
    Art Bianconi's Avatar
    Art Bianconi is offline Banned
    Points: 7,670, Level: 61
    Level completed: 40%, Points required for next Level: 180
    Overall activity: 0%
    Achievements:
    First 1000 Experience Points200+ Posts Achievement!400+ Posts Achievement5+ Referrals Achievement!365 Days+ Registered Achievement!
    Join Date
    Dec 2010
    Location
    Delaware River Valley, Western New Jersey
    Posts
    328
    Points
    7,670
    Level
    61
    Thanks
    91
    Thanked 198x 106 Posts

    Default The tool curves itself

    If you take two flat disks, regardless of materials and using an abrasive to speed things up, simply run them back and forth in a straight line, rotating the tool periodically, one will become convex, the other concave.

    TOT, (tool on top) deepens the sagitta. MOT, reverses the process and reduces the depth of the mirror.I think that's it. It's been so long since I worked with flat blanks and tools I have to stop and think about it.

    By alternating back and forth between TOT and MOT, you control the depth and speed of the finished mirror (the F).

    When you have arrived at the desired depth, or close to it, you migrate to the next finer abrasive and work the tool until the remaining pitts are equal to or smaller than the abrasive currently being used, then you clean off the tool and mirror, check the depth with your spherometer and then migrate to the next finer abrasive.

    When you have gotten to an almost polished state and at the finest of the grit provided, you switch to pitch.

    You can use the existing tool with it's convex surface or use a new tool which is still flat. Given the shallow depth of the mirror you are making, it won't make too much difference.

    I personally prefer to set the abrasive tool aside in case I need to do some corrective grinding. This happens sometimes. You get down to a fine abrasive and polish and something happend while correcting and you don't catch it until it's way over. In those situations you need to go back to course grinding to get rid of the error and if the tool has pitch on it, you can't. I suggest you make seperate tools.

    Making the tool:

    Bathroom tiles are epoxied to a cement tool using the same tiny spacers found in the tile & ceramic dept of your local home center. They look like 6 pointed stars. and insure that the gap between tiles is constant.

    When the epoxy cures, you start grinding with the tile tool and whatever course abrasive you have. Periodically, you do an El Marko test. That's where you draw a series of wide lines up and down and left to right or radially, criss crossing the tool and the mirror.

    When the felt tip lines dry, apply some abrasive to the glass, and run the tool back and forth several times and then inspect the lines to see if they are all wearing uniformly. Continue using the course abrasive until the tool passes the El Marko test. (see Pics of my 15" tool)

    Now proceed with the aggressive grinding using the twist and turn regimen which we'll talk about another time.

    BTW, Loews has some really great tiles, half inch square, made of colored glass. (see pics) The color is a glaze and doesn't matter. These are perfect for rough grinding although some of the pros I know are using 5/8th inch steel nuts embedded in their tools and getting great results. (see pics)

    The blue on black photo is a Ronchi test of my 15" f-4.4 during final figuring. It's not done yet.

    Happy grit!

    Art
    Attached Images Attached Images
    Last edited by Art Bianconi; 02-03-2011 at 02:37 AM.

  8. #17
    tinkerer's Avatar
    tinkerer is offline Main Sequence
    Points: 5,067, Level: 48
    Level completed: 59%, Points required for next Level: 83
    Overall activity: 0%
    Join Date
    Jun 2010
    Location
    minneapolis
    Posts
    108
    Points
    5,067
    Level
    48
    Thanks
    14
    Thanked 57x 45 Posts

    Default

    Corey

    I decided on a gregorian as I did not want to grind a convex mirror for one of my first projects and I did want a folded optical design to keep things physically short. As for when the mirrors will be done? I think about a year from now as I only have a couple of more months before I begin a major summer project that will consume most if not all of my spare time. However I am going to be interested in how much faster the primary mirror goes through rough and fine grinding as I have learned a lot on the secondary that will definitely help speed things along. ( speed things along is a very relative term)

    Also I think that the lapping and figuring process will represent a learning curve every bit as steep and more so than the rough and fine grinding. I have an engineering background with a lot of work on process development for machining parts, so I find the whole grind, lap, figure process to be endlessly intriguing and at times frustrating. However the frustration so far has always yielded to ornery persistence and a willingness to learn. The a-ha moments are rewarding.

    Al Powers

  9. #18
    DAVEG's Avatar
    DAVEG is offline Main Sequence
    Points: 1,712, Level: 25
    Level completed: 12%, Points required for next Level: 88
    Overall activity: 0%
    Achievements:
    10 Days registered365 Days+ Registered Achievement!First 1000 Experience Points50 Posts Achievement!750 Days+ Registered Achievement!
    Join Date
    Jan 2011
    Location
    Delaware
    Posts
    77
    Points
    1,712
    Level
    25
    Thanks
    1
    Thanked 84x 45 Posts

    Default

    Corey,
    As an instructor for the Delmarva mirror making whose images have been posted here, in this thread, having close to 30 years experience in making optics and teaching mirror making, and 16 Stellafane awards for telescopes I have made, let me say, for your first mirror I HIGHLY recommend that you grind in the curve. For a 6" mirror at f/10 it will take you less then one hour to grind it to the proper depth with 80 grit. A 6" f/8 maybe one and half hours. It will take you at least 2x that time to make a cement tool and then it has to cure for few days. Also if you have never made a cement tool there is a technique to it and if not done correctly it can cause you headaches. The Willmann Bell kits are excellent. They contain all the materials you need and I know of many excellent mirrors that have been made by them. The books written on the subject, give the instruction for grinding a mirror against a glass tool so you have clear path forward using a proven technique.
    At the Delmarva Mirror Making class we do use pregenerated glass tools and blanks. The reason is that most people are making 8",10" and 12" mirrors that are in the F/5.-F/6 range. It takes a considerable amount of time to grind a mirror of that size and that depth of curve and we have a limited amount of time ( 2 1/2 days) for the class. So we do it to save time. In you case, a pregenerate blank is not worth the money since you can grind so quickly.
    You also don't need a spherometer or anything fancy like that. If you make a 6" f/10, the depth of the curve is 0.0375", so all you need is a #63 drill bit which is 0.037" in diameter. You place a metal straight edge across the mirror and see if the drill bit will fit under it in the center of the mirror. Simple, cheap and also of accuracy you need.

    All the Best,
    - Dave
    Flexie likes this.

  10. The Following 3 Users Say Thank You to DAVEG For This Useful Post:

    gfphoto (05-05-2011),ghswen (02-03-2011),pfarmb (08-12-2012)

  11. #19
    cawilliams1983's Avatar
    cawilliams1983 is offline Bright Giants
    Points: 5,972, Level: 53
    Level completed: 11%, Points required for next Level: 178
    Overall activity: 0%
    Achievements:
    Got three Friends20+ Friends Achievement!200+ Posts Achievement!First 1000 Experience Points400+ Posts Achievement
    Join Date
    Nov 2010
    Location
    Bright, IN
    Posts
    312
    Points
    5,972
    Level
    53
    Thanks
    30
    Thanked 58x 50 Posts

    Default

    Dave,
    Thanks for the response. Impressive accolades! I appreciate your input.

    I had already to decided to grind in the curve myself, earlier. I will be ordering the flat blanks soon.

    This project is just for self-gratification and I'm not concerned about wasting time on making a tool; that's part of the fun. Are the techniques for grinding with a tool made from cement and a glass blank so different that it would render a book that assumes you're working with the other useless?

    Using the drill bit method you suggested for measuring the depth, how am I to measure the curve? Also, I'm having a hard time imagining how this can even precisely measure the depth as it would be akin to trying lay a nail flat in a a bowl... there would be room underneath it. Besides, I've wanted a spherometer since I was a small boy.. j/k but again, making the spherometer is another enjoyable part of the process..
    Corey

    Meade DS 2114
    To view links or images in signatures your post count must be 5 or greater. You currently have 0 signatures.
    ,
    To view links or images in signatures your post count must be 5 or greater. You currently have 0 signatures.
    SkyQuest XT10 Dob, Zhumell EP kit, Orion Min-EQ Tabletop mount to be used with wife's Canon 350D

    KADIAN member

    To view links or images in signatures your post count must be 5 or greater. You currently have 0 signatures.

  12. #20
    DAVEG's Avatar
    DAVEG is offline Main Sequence
    Points: 1,712, Level: 25
    Level completed: 12%, Points required for next Level: 88
    Overall activity: 0%
    Achievements:
    10 Days registered365 Days+ Registered Achievement!First 1000 Experience Points50 Posts Achievement!750 Days+ Registered Achievement!
    Join Date
    Jan 2011
    Location
    Delaware
    Posts
    77
    Points
    1,712
    Level
    25
    Thanks
    1
    Thanked 84x 45 Posts

    Default

    Corey,
    As for the drill bit method, all you need to do is see if the very end of the shaft of the bit will fit under the straight edge, in the middle of the disk. Any error from having it slightly tilted will be very small and give an uncertainty in the radius of less then 1/4". You can easily adjust the radius in fine grinding using other very simple techniques to get the finally radius to a 1/16" or better if you wish.
    As for the cement tool, if not made correctly, tiles can come loose and/or material can break free and scratch your mirror. I have seen them break apart and also when tried to be used as the base for a pitch lap, have the pitch slide right off ! Even if made correctly, it will need to be cleaned very very well after each grit size to prevent any course grit particles from scratching the finer ground surface. Some people apply a coat of shellac to seal it after each grit size. I have made and used cement/tile tools many times and when done correctly they do work great, but as I said there is a learning curve with them that you will need to master along with everything else. A glass tool doesn't present these problems and you can spend your time making your mirror.
    A spherometer is a great tool. I have made a number of them. To use it correctly though you will need a very flat surface to zero it on, you will also need to calibrate it and like any tool there is technique to using it correctly. All very doable, but again something that takes away from mirror making and for the type of mirror your making it is simply not needed. As an example I had a friend who made his first mirror using a spherometer. The spherometer had steel balls for the feet. My friend would rotate and push the spherometer around on the ground glass surface. He kept getting readings that were changing and didn't make sense. I ask him to try the drill bit method as a double check. They didn't agree and the error was pretty large. As a third check I told him to just wet the mirror and measure the focal length directly by using the Sun. It agreed with the simple drill bit method. He couldn't figure why his very expensive high precision spherometer didn't agree with the simpler, easier and cheaper methods. He sent me the spherometer and I saw right away that he had worn small flat spots in the metal balls. Using a spherometer was new to him and he just didn't understand exactly how to use it. The sad part is that he almost gave up making his mirror because of this and he didn't need a spherometer in the first place. I'm not trying to stop you from making a spherometer, but I believe that it should be a task within itself and not part of making your first mirror.
    I have been doing this for a long time and I keep seeing time after time, beginners getting caught up in things that either don't matter or make things more difficult. At the Delmarva Mirror Making class which I have help teach for the last 10 years we have a simple procedure that works very well and a success rate of over 95%. With most of mirrors being made in the 10" size. In the last few years we have had 100% of the mirrors that were started, were finished in the 2-1/2 days of the class and some students have made two mirrors in that time. As for the quality of the mirrors, at this years Stellafane, 2 of 3 optical awards in the newtonian class were given to mirrors that can be traced back to our class and there are many other from years past.
    Mirror making is alot of fun, especially if you understand what matters and what doesn't.

    All the Best,
    - Dave

  13. The Following User Says Thank You to DAVEG For This Useful Post:

    BABOafrica (09-26-2011)

 

 
Page 2 of 4 FirstFirst 1234 LastLast

Similar Threads

  1. Making a large homemade telescope mirror
    By Charlie123 in forum ATM DIY Telescope Making Forum
    Replies: 3
    Last Post: 01-29-2011, 09:20 PM
  2. quartz; SiC mirror grinding
    By Nova419 in forum Astronomy Beginners Forum
    Replies: 0
    Last Post: 08-18-2010, 01:05 AM
  3. Mirror grinding process
    By Uri in forum ATM DIY Telescope Making Forum
    Replies: 13
    Last Post: 06-14-2009, 06:44 AM
  4. Replies: 1
    Last Post: 10-04-2006, 01:43 PM
  5. So, grinding a mirror ??
    By JAS in forum Amateur Astronomy Forum
    Replies: 7
    Last Post: 12-15-2003, 12:27 AM

Tags for this Thread

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
Powered by vBulletin® Version 4.2.0
Powered by vBulletin®
All times are GMT. The time now is 11:50 AM.