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Equipment Discussions >> Classic Telescopes

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starman876
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Re: Elli Buk Cave 10" Cassegrain [Re: turk123]
      #5915122 - 06/11/13 01:58 PM

Could we change the spreadsheet to take these factors into consideration. I have not looked at the formulas in the spreadsheet, but I am sure we can make this work with the right input which in turn would benefit anyone doing a restoration and making sure they can get the most out of the scope once they put it back together.

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DAVIDG
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Re: Elli Buk Cave 10" Cassegrain [Re: starman876]
      #5915456 - 06/11/13 05:06 PM

Tom,
The best way to check secondary is to put a spherometer on it and measure the convex curve directly. This will tell you the physical radius but it won't tell you anything about the optical correction. As for the primary you can easily Foucault test it and get the radius to within a 1/16" or better and at the same time determine the optical correction.
I understand most people don't have these tools so what we can do is assume that the radius on the back of the primary is close to correct and take a guess at what the back focal length was meant to be. The back focal length, the radius of primary, the radius on the secondary and spacing all interplay with each other, so with the piece of information on the radius of the primary and an educated guess of the back focal length we can go from there. The simplest approach is to put the scope back together with the original spacing and use a Ronchi grating in the eyepiece that has at least 100 line per inch. Then adjust the grating so there no more then 5 lines showing, with three being the best. Then adjust the spacing between the primary and secondary until the Ronchi lines are the straighest you can get them. You'll find out that a small change in the spacing is going to make a large change in the back focal length so from each adjustment you'll need to refocus. Then use a star test and the break out point of when the shadow of the secondary becomes visible to fine tune the system. At that point we'll be able to tell by the inside and outside focus pattern of a star how well your optics are corrected.
Let me say again that you can have 1/2 wave or worse optics and the objects like the Moon at low power will look good and you'll be able to see the ring on Saturn. Things will not by a blurring mess and looking like someone smeared vaseline on the eyepiece. What will happen thou is when you start to push the power, the image will start to become less sharp and when the seeing snaps we'll never get that "space craft flyby" sharp image. Unfortunately very few telescopes that amateurs own have optics that are truely better then 1/4 wave so they are conditioned to believe 1/2 to 1/4 wave image are "great". When you look thru a telescope that really has 1/8 wave optics you'll be amazed at the image quality. Scopes like a typical 60mm f/15 refractor or a 6" f/8.3 newtonian have the laws of physics on their side and many times are operate at this level, that is why people enjoy the views in these scope some much.

- Dave


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DAVIDG
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Re: Elli Buk Cave 10" Cassegrain [Re: DAVIDG]
      #5915482 - 06/11/13 05:18 PM

Tom,
I did a calculation based on the radius of the primary being 80-7/8" and guessing that the back focal length was meant to be 10". That gave me a seperation between the primary to secondary of 30.262" and radius on the secondary of 27.234". Is the 30-1/4" close to what the original spacing was ?

- Dave


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starman876
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Re: Elli Buk Cave 10" Cassegrain [Re: DAVIDG]
      #5915741 - 06/11/13 07:50 PM

Kind of like looking through an 6" AP for the first time after looking through a Celestron CR6 for a while. The difference is mind bogling. I understand where Dave is coming from when he is talking about looking through 1/4 wave optics and then looking through something much much better. Kind of like looking through an average mirror and then looking through a Zambutti mirror. You just do not beleive the difference at first.

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turk123
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Re: Elli Buk Cave 10" Cassegrain [Re: DAVIDG]
      #5915878 - 06/11/13 09:32 PM

Quote:

Tom,
I did a calculation based on the radius of the primary being 80-7/8" and guessing that the back focal length was meant to be 10". That gave me a seperation between the primary to secondary of 30.262" and radius on the secondary of 27.234". Is the 30-1/4" close to what the original spacing was ?

- Dave




The scope is all apart but, doing a quick guesstament using the holes on the tube, the drop in the secondary and the amount the mirror is up into the tube, you are definitely within an inch. So I think you're VERY close!


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starman876
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Re: Elli Buk Cave 10" Cassegrain [Re: turk123]
      #5915889 - 06/11/13 09:38 PM

Wow within an inch. Talk about being good. Where does Dave live again? Talk about an optician you want in your corner when building or restoring a scope

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turk123
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Re: Elli Buk Cave 10" Cassegrain [Re: turk123]
      #5915894 - 06/11/13 09:41 PM

So what tools are used for these measurements. I would be very interested in looking into this just to understand it better. Can you provide links to maybe some of the tools that I could purchase to do the measurements? I guess if I'm going down this path, I may as well understand it better. Could be a good investment.

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Kevin Brose
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Reged: 07/03/07

Loc: Colorado
Re: Elli Buk Cave 10" Cassegrain [Re: turk123]
      #5915922 - 06/11/13 09:59 PM

Tom,

For a f/16 system the ratio of separation distance to backfocus is 16:1. So a change in separation distance of 1/16th of an inch will change the backfocus by one inch.

- Kevin


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Datapanic
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Re: Elli Buk Cave 10" Cassegrain [Re: turk123]
      #5915925 - 06/11/13 10:00 PM

Too bad you're not in Tucson - there's optical labs all over the place!

Looking good!


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Dave M
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Re: Elli Buk Cave 10" Cassegrain [Re: turk123]
      #5915999 - 06/11/13 10:34 PM

Quote:

So what tools are used for these measurements. I would be very interested in looking into this just to understand it better. Can you provide links to maybe some of the tools that I could purchase to do the measurements? I guess if I'm going down this path, I may as well understand it better. Could be a good investment.




You can build your own tester.
http://stellafane.org/tm/atm/test/tester-main.html
http://stellafane.org/tm/atm/test/tester-4.html
http://stellafane.org/tm/atm/test/software.html


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turk123
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Re: Elli Buk Cave 10" Cassegrain [Re: Dave M]
      #5916595 - 06/12/13 09:39 AM



You can build your own tester.
http://stellafane.org/tm/atm/test/tester-main.html
http://stellafane.org/tm/atm/test/tester-4.html
http://stellafane.org/tm/atm/test/software.html




That is very interesting reading especially the use of the device. I may build one of these to get acquainted with the technology.


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turk123
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Re: Elli Buk Cave 10" Cassegrain [Re: turk123]
      #5917070 - 06/12/13 02:14 PM Attachment (19 downloads)

I got a call from Ron the machinist. He finished the counterweights. I zipped over to the airport to pick them up.

To show you that I'm listening, I went with Terra's advice and left the grooving in them. He did make the grooving less pronounced and kept the bevel edge. They look spectacular. The picture just doesn't do them justice.

After reading more and more about how to finish them I was surprised to find out the best way to maintain them is with Gibbs or WD40. No clear coat for sure. Clear coat causes moisture to get trapped underneath and will create spyder webs of rust to form. I just need to remember to keep a can of WD40 in my bag.


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herrointment
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Re: Elli Buk Cave 10" Cassegrain [Re: turk123]
      #5917342 - 06/12/13 04:53 PM

Yow!

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turk123
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Re: Elli Buk Cave 10" Cassegrain [Re: herrointment]
      #5920063 - 06/14/13 06:51 AM Attachment (18 downloads)

There is a brass cover over the DEC worm drive. I didn't get a picture before I started, but it had quite a buildup of tarnish and dirt.

Edited by turk123 (06/14/13 06:52 AM)


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turk123
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Re: Elli Buk Cave 10" Cassegrain [Re: turk123]
      #5920065 - 06/14/13 06:53 AM Attachment (17 downloads)

Here is the polished version including the aluminum end cap.

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turk123
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Re: Elli Buk Cave 10" Cassegrain [Re: turk123]
      #5920069 - 06/14/13 07:01 AM

I recently purchased an Eastwood buffer and have been waiting for it to come (about a month). I found out a few days ago that the model I wanted is delayed another month! I tried to buy another buffer off them and found out they not only do not have my original choice, but that they have no buffers at all!

I found a 1 1/2 HP buffer at sears online and ordered it. It came in 2 days and boy is it nice! I got the metal stand for it also. It is awesome!

This thing is larger than it looks and is so superior to my bench grinder motor (it is too large and has short shafts).

http://www.sears.com/craftsman-8inch-bench-buffer-21181/p-00921181000P?prdNo=...

Edited by turk123 (06/14/13 07:02 AM)


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GalaxyCollide
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Reged: 03/23/13

Loc: NY, Long island
Re: Elli Buk Cave 10" Cassegrain [Re: turk123]
      #5920894 - 06/14/13 04:31 PM

Wow your works are amazing.... Very impressive....

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turk123
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Re: Elli Buk Cave 10" Cassegrain [Re: GalaxyCollide]
      #5925860 - 06/17/13 04:12 PM Attachment (15 downloads)

Today I built my paint booth. After being banned from the basement for ever painting again, I was able to come up with a nice solution for spaying the parts. I am using a generator out at my pole barn to power a 20 gallon compressor and the new paint sprayer. The "pop up" tent after getting holes eaten through it from mice, got wrapped in light plastic and cleaned inside. It's not going to have the ventilation of a regular paint booth, but I own a nice respirator that will work for the short time I'm in there. The tent is to protect the other item I have in the pole barn that I'd like to remain paint free.

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turk123
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Re: Elli Buk Cave 10" Cassegrain [Re: turk123]
      #5925865 - 06/17/13 04:15 PM Attachment (14 downloads)

Here is the new painting table. I can also hang parts from a wood frame I have at home that I use to store firewood. It is sturdy enough to hang the parts from and will fit in the tent just fine.

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starman876
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Re: Elli Buk Cave 10" Cassegrain [Re: turk123]
      #5925930 - 06/17/13 04:44 PM

So will you come out the same color as the mount

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