Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Miscellany

  • Please log in to reply
1022 replies to this topic

#1001 Terra Nova

Terra Nova

    ISS

  • *****
  • Posts: 23,083
  • Joined: 29 May 2012
  • Loc: Northern Kentucky

Posted 20 September 2020 - 02:50 PM

 Thanks. It is easiest unobstructed and perfectly achromat telescope that  ATM can make, in my opinion. 

 

                    - Dave 

It’s a beauty Dave! I could kick myself for not buying the one that was for sale at Stellafane last year! They are very cool scopes!



#1002 DAVIDG

DAVIDG

    Cosmos

  • *****
  • Posts: 9,739
  • Joined: 02 Dec 2004
  • Loc: Hockessin, De

Posted 20 September 2020 - 07:31 PM

It’s a beauty Dave! I could kick myself for not buying the one that was for sale at Stellafane last year! They are very cool scopes!

 I saw it as well Terra but it wasn't one of my design so I don't think you would have been impressed with it's performance.  I'm heading outside  in a few minutes to check out Jupiter, Saturn and Mars with mine. 

 

                           - Dave  


Edited by DAVIDG, 21 September 2020 - 09:27 AM.

  • steve t, Terra Nova and Bomber Bob like this

#1003 Terra Nova

Terra Nova

    ISS

  • *****
  • Posts: 23,083
  • Joined: 29 May 2012
  • Loc: Northern Kentucky

Posted 20 September 2020 - 10:29 PM

I just ran across this article Dave. You are mentioned quite a bit. I might just try my hand at building one!

 

https://www.cloudyni...fspiegler-r1656

 

If I do, I will be hounding you with lots of questions so be forewarned.


  • tim53, Bomber Bob and GreyDay like this

#1004 DAVIDG

DAVIDG

    Cosmos

  • *****
  • Posts: 9,739
  • Joined: 02 Dec 2004
  • Loc: Hockessin, De

Posted 21 September 2020 - 09:38 AM

I just ran across this article Dave. You are mentioned quite a bit. I might just try my hand at building one!

 

https://www.cloudyni...fspiegler-r1656

 

If I do, I will be hounding you with lots of questions so be forewarned.

 Happy to help in any way. I have a full size drawing of the layout as a PDF.  I built the scope like I built model airplanes as a kid ie I built it on top of the full scale drawing I made. The scope itself only cost me $75 to make and about $35 of it  was to have the mirrors coated.  One of the nice features is that both the primary and secondary are spheres but with opposite curves. So what this means is the tool used to grind the primary is also the secondary. Being spheres, the Foucault test is a null for spheres so you have the perfect test to make the primary and know when it is perfect. When the primary is done you use it as a test plate to figure convex secondary made from the tool used to make the primary. Now you have two very well figured surfaces and the image in the eyepiece matches what theory shows it should. 

Mars was very nice in it again last night. 

 

               - Dave 


Edited by DAVIDG, 21 September 2020 - 09:39 AM.

  • mattyfatz, steve t, Terra Nova and 2 others like this

#1005 Terra Nova

Terra Nova

    ISS

  • *****
  • Posts: 23,083
  • Joined: 29 May 2012
  • Loc: Northern Kentucky

Posted 21 September 2020 - 10:25 AM

Thanks Dave! Sounds like an interesting project. Are the primary and the secondary the same size and is the total or effective focal length 2 x f.l. of the primary?


Edited by Terra Nova, 21 September 2020 - 10:26 AM.

  • mattyfatz likes this

#1006 GreyDay

GreyDay

    Mariner 2

  • -----
  • Posts: 276
  • Joined: 28 Nov 2013
  • Loc: Southport UK

Posted 21 September 2020 - 04:28 PM

Being spheres, the Foucault test is a null for spheres so you have the perfect test to make the primary and know when it is perfect. When the primary is done you use it as a test plate to figure convex secondary made from the tool used to make the primary. Now you have two very well figured surfaces and the image in the eyepiece matches what theory shows it should.

I love the way Dave makes this sound so easy, like it's an everyday thing and so simple. The reality for me is I'd end up with two ash trays :)

 

Good luck with the project if you take it on Terra waytogo.gif


  • steve t, Terra Nova and Bomber Bob like this

#1007 tim53

tim53

    Hubble

  • *****
  • Posts: 15,133
  • Joined: 17 Dec 2004
  • Loc: Highland Park, CA

Posted 21 September 2020 - 06:04 PM

 Happy to help in any way. I have a full size drawing of the layout as a PDF.  I built the scope like I built model airplanes as a kid ie I built it on top of the full scale drawing I made. The scope itself only cost me $75 to make and about $35 of it  was to have the mirrors coated.  One of the nice features is that both the primary and secondary are spheres but with opposite curves. So what this means is the tool used to grind the primary is also the secondary. Being spheres, the Foucault test is a null for spheres so you have the perfect test to make the primary and know when it is perfect. When the primary is done you use it as a test plate to figure convex secondary made from the tool used to make the primary. Now you have two very well figured surfaces and the image in the eyepiece matches what theory shows it should. 

Mars was very nice in it again last night. 

 

               - Dave 

This is interesting!  So do I have this right?  You make the secondary out of the tool for the primary?  And is the reason for cutting out a smaller disk just for compactness?  

 

I need to discipline myself to get my half-baked optics projects completed, but I want to try making one of these!


  • Terra Nova likes this

#1008 DAVIDG

DAVIDG

    Cosmos

  • *****
  • Posts: 9,739
  • Joined: 02 Dec 2004
  • Loc: Hockessin, De

Posted 21 September 2020 - 06:39 PM

This is interesting!  So do I have this right?  You make the secondary out of the tool for the primary?  And is the reason for cutting out a smaller disk just for compactness?  

 

I need to discipline myself to get my half-baked optics projects completed, but I want to try making one of these!

  Tim,

   Yes the glass tool used to grind the mirror is then polished and figure to make the secondary. So you are grinding both at the same time. You core out the secondary from the center of the tool because it needs to be small enough not to block any light and this also avoids any turn edge problems.  

   Tim you cored the mirror blank for your Gregorian with  a diamond coring bits and that I  told you about so you know how easy and fast it is to core out the secondary. 

 

                         - Dave 


Edited by DAVIDG, 21 September 2020 - 06:57 PM.

  • tim53 and Terra Nova like this

#1009 DAVIDG

DAVIDG

    Cosmos

  • *****
  • Posts: 9,739
  • Joined: 02 Dec 2004
  • Loc: Hockessin, De

Posted 21 September 2020 - 06:56 PM

I love the way Dave makes this sound so easy, like it's an everyday thing and so simple. The reality for me is I'd end up with two ash trays smile.gif

 

Good luck with the project if you take it on Terra waytogo.gif

In all seriousness, a number of these  scopes of my design have been built without any help from me so it is not like I have some special skills and/or tools. My optics were made in my  kitchen and the OTA is a couple of cardboard mailing tubes. 

 We have  people here with more than enough skills to build one, especially when seeing  some of the amazing restoration projects that have been posted.

 

                     - Dave  


  • tim53, steve t and GreyDay like this

#1010 GreyDay

GreyDay

    Mariner 2

  • -----
  • Posts: 276
  • Joined: 28 Nov 2013
  • Loc: Southport UK

Posted 21 September 2020 - 08:00 PM

 We have  people here with more than enough skills to build one, especially when seeing  some of the amazing restoration projects that have been posted.

I agree Dave, the restorations and projects are the main reason i spend time here on CN, sadly in my area i'm yet to meet anyone who has ground their own mirror. I know there are quite a few amateur telescope makers here in the uk and i only live an hours drive from a company that do coatings, maybe i'll dig a little deeper :)


  • tim53 likes this

#1011 RichA

RichA

    Vanguard

  • *****
  • Posts: 2,228
  • Joined: 03 Jun 2010
  • Loc: Toronto, Canada

Posted 21 September 2020 - 10:34 PM

Expensive larger mounts prevent me from expensive larger scopes, in the same way as truss Dobs stop me going larger than 12" aperture.

The days of cheap large mounts are over I guess, at least GEMs.  Blame the imagers and their sub-arc second guiding needs...


  • GreyDay likes this

#1012 GreyDay

GreyDay

    Mariner 2

  • -----
  • Posts: 276
  • Joined: 28 Nov 2013
  • Loc: Southport UK

Posted 22 September 2020 - 02:20 AM

The days of cheap large mounts are over I guess, at least GEMs.  Blame the imagers and their sub-arc second guiding needs...

I also blame them for the death of the slow refractor and that moment of bitter disappointment a first timer feels, when they see the Andromeda galaxy is a smudge and Jupiter is a small white ball.

 

Great job guys...waytogo.gif


Edited by GreyDay, 22 September 2020 - 02:20 AM.


#1013 Terra Nova

Terra Nova

    ISS

  • *****
  • Posts: 23,083
  • Joined: 29 May 2012
  • Loc: Northern Kentucky

Posted 22 September 2020 - 07:58 AM

In all seriousness, a number of these  scopes of my design have been built without any help from me so it is not like I have some special skills and/or tools. My optics were made in my  kitchen and the OTA is a couple of cardboard mailing tubes. 

 We have  people here with more than enough skills to build one, especially when seeing  some of the amazing restoration projects that have been posted.

 

                     - Dave  

Dave, Is it a doable project in your week-long Delmarva mirror-making camp?



#1014 DAVIDG

DAVIDG

    Cosmos

  • *****
  • Posts: 9,739
  • Joined: 02 Dec 2004
  • Loc: Hockessin, De

Posted 22 September 2020 - 09:43 AM

Dave, Is it a doable project in your week-long Delmarva mirror-making camp?

 Terra,

    Unfortunately the Delmarva Stars Gazers disbanded so no more mirror making weekend.  Yes the optics could have been done in 3 days we held the class.  You made a mirror using Sam Browns 'All About Telescopes" so it is the same but you don't parabolize the primary, you leave it at a clean sphere. So that makes it easier. Making the secondary is the same polishing process as the mirror but you test it like you test an optical flat against a Master surface. In this case the Master surface is your primary mirror.  Only a little more then the 2" center section of the 4" glass tool used to make the primary is turned into the secondary so only this area needs to good. Since the primary and secondary have the same radii once you are done grinding the primary you are  also done grinding the secondary.  

   I have 4.25 f/27 system that is perfectly color corrected and with no central obstruction. It weights maybe 5 lbs so it is no problem for a CG-5 type mount. 

  Here is the design plans 

 

                     - Dave

 

delmarvaschiefdrawing.jpg


  • tim53, steve t and Terra Nova like this

#1015 Garyth64

Garyth64

    Fly Me to the Moon

  • *****
  • Posts: 5,093
  • Joined: 07 May 2015
  • Loc: SE Michigan

Posted 22 September 2020 - 10:04 AM

Hi David, is it possible to have that sheet with more resolution?  It's hard to read all the data.  Thanks.

 

This looks very interesting, and I may think about making one.


Edited by Garyth64, 22 September 2020 - 10:05 AM.


#1016 tim53

tim53

    Hubble

  • *****
  • Posts: 15,133
  • Joined: 17 Dec 2004
  • Loc: Highland Park, CA

Posted 22 September 2020 - 10:07 AM

I swear I'm going to make one, when I can get time.

 

Question:  Can it be scaled up to, say, 16"?  That would make a serious planetary imager.

 

-Tim.


  • Terra Nova likes this

#1017 DAVIDG

DAVIDG

    Cosmos

  • *****
  • Posts: 9,739
  • Joined: 02 Dec 2004
  • Loc: Hockessin, De

Posted 22 September 2020 - 10:15 AM

I swear I'm going to make one, when I can get time.

 

Question:  Can it be scaled up to, say, 16"?  That would make a serious planetary imager.

 

-Tim.

 Tim,

   It can't be scaled to 16" with out additional optics. The 4.25" aperture is the "sweet spot" for easy of construction and optical performance.

   I have 6" set of optics that gives amazing diffraction limited performance on paper  out to better then a 1 degree. Very few designs can do that. Minor issue it that it requires a toroidal shape secondary but like the 4.25" version one of the radii is the same as the primary and the other is only 1" different.  The result is that I can mechanically warp the secondary into the toroidal shape once the secondary is figured to a convex sphere like with the Yolo design but the warping is much less so easier in theory to do with a simple cell design. Another project I hope to finish before I turn into worm food.

 

                    - Dave 


Edited by DAVIDG, 22 September 2020 - 10:23 AM.

  • tim53 and steve t like this

#1018 tim53

tim53

    Hubble

  • *****
  • Posts: 15,133
  • Joined: 17 Dec 2004
  • Loc: Highland Park, CA

Posted 22 September 2020 - 11:32 AM

Maybe a tri-shief, then?  Like Oscar Knab built on a Springfield Mount (from Peter Manly's "Unusual Telescopes", google books online):

 

 

Attached Thumbnails

  • OscarKnab_trishief_springfield.jpg

  • steve t and Terra Nova like this

#1019 DAVIDG

DAVIDG

    Cosmos

  • *****
  • Posts: 9,739
  • Joined: 02 Dec 2004
  • Loc: Hockessin, De

Posted 22 September 2020 - 12:04 PM

Maybe a tri-shief, then?  Like Oscar Knab built on a Springfield Mount (from Peter Manly's "Unusual Telescopes", google books online):

 You don't  want a trischief. optical performance is not great  all the surface have different radii that  require aspherizing  and a lot of light lose with three mirrored system.  Your far better off making a folded Schupmann. 

There is a very good reason why I designed and built my 4.25" Schief since it is the "sweet spot" in optical performance and easy of construction. 

   My approach to optics and ATMing is I first look at what the theoretical  performance of the design can be then look at how difficult it is to make. It doesn't do any good to have an excellent design on paper  but it requires super tight tolerance and expensive glass  since you'll never get in the eyepiece what theory shows on paper. 

  

 

                - Dave 


  • tim53, steve t and Terra Nova like this

#1020 DAVIDG

DAVIDG

    Cosmos

  • *****
  • Posts: 9,739
  • Joined: 02 Dec 2004
  • Loc: Hockessin, De

Posted 22 September 2020 - 12:30 PM

Hi David, is it possible to have that sheet with more resolution?  It's hard to read all the data.  Thanks.

 

This looks very interesting, and I may think about making one.

 Gary and All,

   Attached is a PDF of the design that can be printed full scale so the telescope can be built on top of it. It also gives the layout for the teardrop shape cut out. People can get turned off by the accuracy of the angles but there is simple way around that. The optical path is just a big right triangle. You can easily get the lengths of sides of the triangle to within 1/8" of the needed lengths if not a 1/16" that is plenty good to get the angles close enough that aligning the optics will fix the rest. 

 

               - Dave 

Attached Files


  • tim53, steve t, Terra Nova and 2 others like this

#1021 Garyth64

Garyth64

    Fly Me to the Moon

  • *****
  • Posts: 5,093
  • Joined: 07 May 2015
  • Loc: SE Michigan

Posted 22 September 2020 - 01:45 PM

 Gary and All,

   Attached is a PDF of the design that can be printed full scale so the telescope can be built on top of it. It also gives the layout for the teardrop shape cut out. People can get turned off by the accuracy of the angles but there is simple way around that. The optical path is just a big right triangle. You can easily get the lengths of sides of the triangle to within 1/8" of the needed lengths if not a 1/16" that is plenty good to get the angles close enough that aligning the optics will fix the rest. 

 

               - Dave 

Much better! Thanks.

 

Dang, I sold my 4-1/4" f/13 newt about a year and a half ago.  I used another pyrex blank for the tool, so it has the same RC at the mirror.  I probably could have used them in making a Schiefspiegler.  But its RC was 108", so maybe not.


Edited by Garyth64, 22 September 2020 - 01:51 PM.


#1022 DAVIDG

DAVIDG

    Cosmos

  • *****
  • Posts: 9,739
  • Joined: 02 Dec 2004
  • Loc: Hockessin, De

Posted 23 September 2020 - 09:26 AM

 One other note on my Schief. since I have heard  people believe they are difficult to collimate the optics. In reality they are as easy or easier then a Newtonian. It takes me about 5 minutes to do mine. 

   You place a laser collimator in the focuser. When the scope was first built I adjusted the focuser so the laser hit the center of the secondary mirror so you only need to do that once. Now you adjust the secondary so the spot hits the center of the primary. Now you adjust primary so the spot goes back and hits the center of secondary. The primary is now adjusted so the side ways tilt is perfect. At this point the primary is looking at the secondary instead of the sky so you need to adjust it's  tilt in only one direction My mirror cells are designed so one of  the screws adjust the tip in the meridian plane of the optics. That is the plane that would divide the scope down the middle into two equal halves. So I look up the focuser and I can see the reflection of the secondary in the primary. Turning just the one screw I tip the primary away from the secondary until the secondary just becomes not visible. At this point the scope is almost perfectly aligned. I now center up a bright star and put it slightly out of focus. It is usually is  very slightly oval shaped. I just need another small adjustment of the meridian screw on the mirror cell of the primary to make it perfectly round. Done !

 

                     - Dave


  • Bomber Bob likes this

#1023 Terra Nova

Terra Nova

    ISS

  • *****
  • Posts: 23,083
  • Joined: 29 May 2012
  • Loc: Northern Kentucky

Posted Yesterday, 12:14 PM

Edmund still posts a lot of really good optical resources. Here’s one some might be interested in:

 

https://www.edmundop...jhsZFBMNUYifQ==


  • clamchip likes this


CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics