Jump to content


Photo

? MYSTERY ? EDMUNDS ? 6" CASSEGRAIN ?

  • Please log in to reply
129 replies to this topic

#51 TOM KIEHL

TOM KIEHL

    Apollo

  • *****
  • Posts: 1383
  • Joined: 17 Nov 2012
  • Loc: N.E. Ohio

Posted 27 June 2013 - 08:36 AM

A HA, Thanks Fred, I thought so, Maybe were on to something here....Hope Johann can find his parts to his Cass. and post pictures so we can compare . :question:

#52 DAVIDG

DAVIDG

    Aurora

  • *****
  • Posts: 4762
  • Joined: 02 Dec 2004
  • Loc: Hockessin, De

Posted 27 June 2013 - 08:58 AM

As far as I know Novak never sold Cassegrain optics or any type of telescope kits to assemble one. He did sell newtonian mirrors and secondary but I bet they were from Coulter. I have Sky and Tels back into 40's and the only companies that had ads for Cassegrain optics were Coulter, 3B and Mars Optical. Mars turned into 3B. Novak Cass parts were being designed for mostly Coulter Cass optics. As a kid a had a couple of Novak catalogs and they never mentioned a Cassegrain telescope kit. Novak's "Cassegrain Notes" was first published by him in the early 70's, I believe, which is when Coulter started to sell cassegrain mirror sets.
Since your primary mirror is made from a slope sided Corning blank, it was most likely made in the 60's. In the 70's most companies selling telescope mirrors were making them from United Lens blanks were have squared sides and both faces of the disk were the same diameter. So my guess it that it is either a homemade set of optics or 3B made them.

- Dave

#53 amicus sidera

amicus sidera

    Skylab

  • *****
  • Posts: 4049
  • Joined: 14 Oct 2011
  • Loc: East of the Sun, West of the Moon...

Posted 27 June 2013 - 09:20 AM

The first mention of Cassegrain Notes in a Novak S&T advertisement is in the April 1978 issue, page 73 (Newtonian Notes predates this by several years), but his ads from at least 1975 mention Cassegrain baffle tubes and mirror mounts... as you said, Dave, he almost certainly wasn't producing mirror sets, but reselling those made by others.

Fred

e.t.a.: just on a side note, the feeling of nostalgia I get from going through old issues of S&T can be quite strong at times; ah, for a simpler era... or at least, one in which I'm in my early twenties again! :grin:

#54 Dave M

Dave M

    Cosmos

  • *****
  • Moderators
  • Posts: 8170
  • Joined: 03 Aug 2004
  • Loc: Ohio

Posted 27 June 2013 - 09:28 AM

Not that it means much but didnt you tell me the guy you bought the scope from bought it from a camera shop in Houston 30-some years ago.

#55 DAVIDG

DAVIDG

    Aurora

  • *****
  • Posts: 4762
  • Joined: 02 Dec 2004
  • Loc: Hockessin, De

Posted 27 June 2013 - 12:14 PM

Hi Tom,
I did some calculations on the setup of your scope based on some assumptions of the focal length of the primary being 24", that is an F/15 system ie 90" focal length and the back focal distance being 10". If those are right then secondary to primary distance needs to be 16.84" and the baffle tube for the primary needs to be about 1.5" ID and 8.35" long ( from the surface of primary to top of the tube) The secondary baffle needs to be 2.38" in diameter and 1.90" long.
The secondary to primary spacing is critcial because the correction for spherical aberration is detemined. It also chances the back focal length and small changes in the secondary position are going to make a large change in back focal length. If you can get the exact focal length or radius on the primary and the convex radius on the secondary I'll be happy to recalculate the spacings and baffle dimensions.

- Dave

#56 TOM KIEHL

TOM KIEHL

    Apollo

  • *****
  • Posts: 1383
  • Joined: 17 Nov 2012
  • Loc: N.E. Ohio

Posted 27 June 2013 - 03:32 PM

Thats what he told me.It was in used cond. then, when Dan bought it in Texas.

#57 DAVIDG

DAVIDG

    Aurora

  • *****
  • Posts: 4762
  • Joined: 02 Dec 2004
  • Loc: Hockessin, De

Posted 01 July 2013 - 10:51 AM

Without proper baffling cassegrains suffer badly from sky fogging since light from the sky goes directly into the eyepiece. The cass. I'm assembling from vintage parts doesn't have any coating on the secondary at this time, so the image is dim. Without the baffle tube it is almost impossibe to see the image buried in the all the glare from the sky fog.
Here is a shot of the baffle I made from the Stables mailing tube. I just cut it to length and then took another section about 3" long and split down the slide and cut out a strip so it would fit tightly inside the main baffle and glued it in place. The fit was a little loose in the hole in the primary so I wrapped a couple of turn of printer paper around the end that were saturated with glue. The fit is snug now and I'll give the tube a couple of coats of polyurethane to seal it well and spray it flat black on the outside and line the inside with black velvet flocking paper.

- Dave

Attached Files



#58 DAVIDG

DAVIDG

    Aurora

  • *****
  • Posts: 4762
  • Joined: 02 Dec 2004
  • Loc: Hockessin, De

Posted 01 July 2013 - 10:53 AM

Here is another shot of the tube showing the end that fits in the center of the primary.

- Dave

Attached Files



#59 Dave M

Dave M

    Cosmos

  • *****
  • Moderators
  • Posts: 8170
  • Joined: 03 Aug 2004
  • Loc: Ohio

Posted 01 July 2013 - 06:46 PM

Dave, Nice work on the baffle, will it just be pressed in or will you put a tiny dab of silicon on it for final assembly ?

#60 TOM KIEHL

TOM KIEHL

    Apollo

  • *****
  • Posts: 1383
  • Joined: 17 Nov 2012
  • Loc: N.E. Ohio

Posted 01 July 2013 - 07:36 PM

Very nice Dave, The scope that I have is my first attempt at restoreing a vintage cassegrain, I will need all the help I can get. So far yourself and everyone has been very helpfull in getting me throgh this project that I will start this fall, So I quess I will follow your lead, and this thread when the time comes to do my resto. Buy then hopefully you will have all the bugs (so to speak) worked out of your project, and can give me more tips that can help me on my scope restoration. :bow: :grin:

#61 TOM KIEHL

TOM KIEHL

    Apollo

  • *****
  • Posts: 1383
  • Joined: 17 Nov 2012
  • Loc: N.E. Ohio

Posted 01 July 2013 - 08:48 PM

Dave, I notice that your mirror retaining tabs are thicker and wider than mine. Is that a thicker and wider "BACKING TAB" for a thinner sheetmetal tab that goes on first to hold the mirror in place? :question: I don't have that backing piece, will have to fab. 3 of them, good idea, glad i seen the inside of your cell. :idea:

#62 DAVIDG

DAVIDG

    Aurora

  • *****
  • Posts: 4762
  • Joined: 02 Dec 2004
  • Loc: Hockessin, De

Posted 02 July 2013 - 10:29 AM

Hi Tom,
Here is a close up of a backing tab. It's about 3/16" thick and like you said goes over the thinner tabs and they prevent the mirror from moving sideways.
You need to know the radius of curvature of the primary and secondary. With those pieces of information you can calculate the size and length of the baffles needed and also the spacing between the primary and secondary. Is there any writing on the mirrors that might indicate those measurments ?
If you have done mirror testing then the best way is with a Foucault tester and measure the primary mirror. Lacking that, then just using the primary mirror, project the image of the Moon onto a piece of the paper. For the secondary your going to need a spherometer. Since the secondary amplifies the focal length of the primary, a small change in it's position makes a big change in were the image comes to focus behind the primary. It also effects the optical correction of the system.

- Dave

Attached Files



#63 TOM KIEHL

TOM KIEHL

    Apollo

  • *****
  • Posts: 1383
  • Joined: 17 Nov 2012
  • Loc: N.E. Ohio

Posted 02 July 2013 - 10:42 AM

Thanks Dave , Great picture, as far as the mirror's go , all I have is what you see,, and I don't have access to that kind of testing equipment. I will probably start with the figures that you worked up for me, and work with it from there.

#64 DAVIDG

DAVIDG

    Aurora

  • *****
  • Posts: 4762
  • Joined: 02 Dec 2004
  • Loc: Hockessin, De

Posted 02 July 2013 - 01:30 PM

Tom,
For me to calculate the baffling and spacing I'll need the radii on both the primary and secondary. You can tape a card to the end of yard stick and put the yard stick through the hole in the primary. Aim the assembly at the Moon and move the stick back and forth until the Moon is sharply focused on the card. Then read the distance from the card to the surface of the mirror. You should be able to get to within 1/8". I'm guessing that it will be very close to 24".
As for the secondary, if you can get the use of set of machinist calipers you can measure the thickness of the secondary at the very edge and also in the exact middle. That will tell us the sagitta or depth of the curve and from that we can calculate a good approximation to the radius. From there you can assemble the scope and have enough adjustments in the cells to find the exact spacings.
Without some data on the radii you can be adjusting things until the cows come home and never get a good image. I wonder if this happened to the original builder and why he gave up on the project ?

- Dave

#65 Dave M

Dave M

    Cosmos

  • *****
  • Moderators
  • Posts: 8170
  • Joined: 03 Aug 2004
  • Loc: Ohio

Posted 02 July 2013 - 03:07 PM

Dave,
I have a Foucault tester Tom can use, dont have the caliper though. My tester is identical to the Stellafane tester, and iirc it has a 120 line ronchi screen in it.
So Tom, there ya go, come get it :)

#66 DAVIDG

DAVIDG

    Aurora

  • *****
  • Posts: 4762
  • Joined: 02 Dec 2004
  • Loc: Hockessin, De

Posted 02 July 2013 - 03:24 PM

Dave,
I have a Foucault tester Tom can use, dont have the caliper though. My tester is identical to the Stellafane tester, and iirc it has a 120 line ronchi screen in it.
So Tom, there ya go, come get it :)


Dave,
That is great. Tom will also need to borrow the Ronchi grating from it once the optics are mounted in a tube so he can fine tune the position of the secondary using a star.
One can purchase a set of digital calipers from Habor Freight for under $15.
Dave, to answer your question about the gluing the baffle tube in place or not. I'm hoping that the fit will be snug enough once it is sealed with polyurethane and painted that I won't have to glue it in place.

- Dave

#67 Dave M

Dave M

    Cosmos

  • *****
  • Moderators
  • Posts: 8170
  • Joined: 03 Aug 2004
  • Loc: Ohio

Posted 02 July 2013 - 04:06 PM

Dave, Thanks!, i was just curious :)

Tom, the top of your pool table would make a nice test bed.
You`l need to make a mirror stand, pretty simple, just a couple pieces of plywood and a couple 1" dowels.
http://tinyurl.com/qgr7a8z
I know your going to ask :grin: how to use the tester and
ronchi.
Go here
http://stellafane.or...t/tester-4.html
and see the section " How do i use the Ronchi Grating"

#68 TOM KIEHL

TOM KIEHL

    Apollo

  • *****
  • Posts: 1383
  • Joined: 17 Nov 2012
  • Loc: N.E. Ohio

Posted 02 July 2013 - 05:18 PM

Sounds like a joint effort to me. :help: :help: :help:

#69 Dave M

Dave M

    Cosmos

  • *****
  • Moderators
  • Posts: 8170
  • Joined: 03 Aug 2004
  • Loc: Ohio

Posted 02 July 2013 - 05:36 PM

Now dont be scared :scared: its really fairly easy.

#70 TOM KIEHL

TOM KIEHL

    Apollo

  • *****
  • Posts: 1383
  • Joined: 17 Nov 2012
  • Loc: N.E. Ohio

Posted 02 July 2013 - 05:41 PM

But MASTER , I am but a lowly Grasshopper :bow: :lol:

#71 Dave M

Dave M

    Cosmos

  • *****
  • Moderators
  • Posts: 8170
  • Joined: 03 Aug 2004
  • Loc: Ohio

Posted 02 July 2013 - 06:08 PM

I can help you get the readings, then maybe we can get Dave to do the calculations..Dave`s the Master :ubetcha:

#72 tim53

tim53

    Cosmos

  • *****
  • Posts: 9216
  • Joined: 17 Dec 2004
  • Loc: Highland Park, CA

Posted 02 July 2013 - 06:15 PM

But MASTER , I am but a lowly Grasshopper :bow: :lol:


Such knowledge is worth more than any fortune, Cookie! :grin:

-Tim.

#73 starman876

starman876

    Fly Me to the Moon

  • *****
  • Posts: 5952
  • Joined: 28 Apr 2008
  • Loc: VA

Posted 02 July 2013 - 06:18 PM

Soon you too will be able to carry the hibachi grill with your forearms.

#74 TOM KIEHL

TOM KIEHL

    Apollo

  • *****
  • Posts: 1383
  • Joined: 17 Nov 2012
  • Loc: N.E. Ohio

Posted 15 July 2013 - 02:44 PM

If I find everything for that 6" cassegrain I will post a pic. I bought it from the guy who made it in 1978. Looks like Toms, but it in a aluminum tube. Came with a 16X50 finder. No idea where that went.


Johann do you still have this? http://www.cloudynig...5665943/page...

#75 starman876

starman876

    Fly Me to the Moon

  • *****
  • Posts: 5952
  • Joined: 28 Apr 2008
  • Loc: VA

Posted 15 July 2013 - 02:47 PM

Yep, still do. However, that is not that cassegrain I was refering to.






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics