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Offlinewilshire
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i want to build a telescope
    #5242361 - 01/30/06 02:36 PM (18 years, 3 months ago)

it's going to be a reflector 'dobson' type. i've been looking at plans and reading how these work and it looks very doable. pretty simple really. anyone know about this stuff?

i can get a good 4.5 inch mirror pretty cheap, but the price goes up sharply for a good 6 inch or larger. every one of these i've seen or plans i've read talks about using a 6 inch or larger. is 4.5 inch wide enough? what will i be able to see (in good skies) if i use a good 4.5 inch mirror and construct my telescope well?

and about grinding my own. i'm attracted to this idea, but probably not for my first scope. anyone have any experience with this?


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OfflineGomp
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I want to build a pseudoscope! [Re: wilshire]
    #5242548 - 01/30/06 03:19 PM (18 years, 3 months ago)

This is totally; ..

:offtopic:

...but!

(this) Might be interesting?

http://pseudoscope.blogspot.com/

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Offlinewilshire
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Re: I want to build a telescope [Re: Gomp]
    #5243589 - 01/30/06 07:14 PM (18 years, 3 months ago)

""So when you view objects with this scope, the convex becomes concave. So a brick lying on the ground appears to be a brick-shaped hole sunk into the ground. A tree turns inside out as it were - it's front branches appear at the back, whilst the back branches come out in front, hanging or suspended in mid-air as their support is eclipsed by branches in front. The scope will suggest to the viewer that a person's face is hollow or concave but your brain refuses to accept such nonsense and so peoples' heads merely appear odd or uncomfortable to look at. It is best then to choose to look at a landscape or tree or anything which may suggest ambiguous perspective. "

wow. i'm building one of those ASAP!

on telescopes, i'm surprised from learning how a newtonian reflector works at how... primitive these really are. it's so simple. i suppose that the real art of it is learning how to grind that perfect parabolic mirror by hand, which is something i can see myself getting into, but not for my first telescope.


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Edited by wilshire (01/30/06 07:19 PM)

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InvisibleDieCommie

Registered: 12/11/03
Posts: 29,258
Re: i want to build a telescope [Re: wilshire]
    #5247812 - 01/31/06 05:25 PM (18 years, 3 months ago)

Quote:

wilshire said: is 4.5 inch wide enough? what will i be able to see (in good skies) if i use a good 4.5 inch mirror and construct my telescope well?


Thats the size of my mirror. Its good, but kind of small. I can see, i think, 4 of Jupiter's moons. Two bands of clouds on jupiter, sometimes I think I can make out the red spot, not sure though. You will be able to see Saturn's rings, but probably not the cassini gap. Although I tend to view in a moderatly light polluted are.

Keep us updated on your progress!

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OfflineLearyfanS
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Re: i want to build a telescope [Re: wilshire]
    #5249350 - 01/31/06 11:11 PM (18 years, 3 months ago)
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That sounds pretty cool.







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Mp3 of the month:  Sons Of Adam - Feathered Fish


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InvisibleDiploidM
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Re: i want to build a telescope [Re: wilshire]
    #5252372 - 02/01/06 06:59 PM (18 years, 3 months ago)

I built the telescope on the left, including grinding the mirror, in the pic below. It's a 32 inch f/6 Newtonian design on a dob mount with an equatorial clock for tracking during photography. I also own an apochromatic 7" f/4 refractor made by Takahashi and an 8" Meade Schmidt-Cassegrain. Here are some pics I've shot over the years. I hope you like.  :wink:

No, I'm not in the pic, I shot the pic. I used a deep red filter on the flash to preserve my friends' dark adaptation. :grin:





This is the Omega Centauri globular cluster. It's a collection of about a million stars that are part of our galaxy. Almost like a mini-galaxy inside the Milky Way. I shot this through the Meade 8" Marathon Key just north of Key West, Florida a few years ago:



This is Saturn shot through the 7" apochromat from deep in the Florida Everglades as far from Miami as you can go driving before you get to the ocean off the extreme southern tip of Florida:


The Orion Nebula (M42) shot though the Meade 8" from the Florida Keys:


This is a fixed tripod image of the Southern Cross in the constellation Crux. Instead of clock driving the telescope and tracking the stars, I fixed the telescope to one position and time-lapsed the focus to get the weird effect.


This is one of my favorites. It's the Andromeda Galaxy shot from the Florida Keys. It's the only galaxy other than ours that you can see naked eye from a dark location:


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Republican Values:

1) You can't get married to your spouse who is the same sex as you.
2) You can't have an abortion no matter how much you don't want a child.
3) You can't have a certain plant in your possession or you'll get locked up with a rapist and a murderer.

4) We need a smaller, less-intrusive government.

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Offlinewilshire
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Re: i want to build a telescope [Re: Diploid]
    #5252378 - 02/01/06 07:02 PM (18 years, 3 months ago)

did you shoot those pictures!?

that one of the andromeda galaxy is incredible!


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InvisibleDiploidM
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Re: i want to build a telescope [Re: wilshire]
    #5252383 - 02/01/06 07:04 PM (18 years, 3 months ago)

Yes, thank you.  :mushroom2:


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Republican Values:

1) You can't get married to your spouse who is the same sex as you.
2) You can't have an abortion no matter how much you don't want a child.
3) You can't have a certain plant in your possession or you'll get locked up with a rapist and a murderer.

4) We need a smaller, less-intrusive government.

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Offlinewilshire
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Re: i want to build a telescope [Re: Diploid]
    #5252393 - 02/01/06 07:06 PM (18 years, 3 months ago)

i'm really interested in griding my own mirror, and it would certainly put a larger one within the reach of my budget. is that overly ambitious for a first telescope?


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InvisibleDiploidM
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Re: i want to build a telescope [Re: wilshire]
    #5252444 - 02/01/06 07:20 PM (18 years, 3 months ago)

As long as you keep the size to 8" or less, you'll have no trouble. Even so, the weight of the mirror and the blank against which you grind it will weigh ~30 lbs.

Also important, don't attempt a mirror faster than about f/10 focal ratio. The faster the mirror, the steeper the parabola you will have to grind, and steep parabolas take a lot of skill and experience to learn to grind correctly. You're much better off with an accurate f/10 than a curve error on an f/6.

Good luck!  :thumbup:


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Republican Values:

1) You can't get married to your spouse who is the same sex as you.
2) You can't have an abortion no matter how much you don't want a child.
3) You can't have a certain plant in your possession or you'll get locked up with a rapist and a murderer.

4) We need a smaller, less-intrusive government.

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InvisibleDieCommie

Registered: 12/11/03
Posts: 29,258
Re: i want to build a telescope [Re: Diploid]
    #5252598 - 02/01/06 08:02 PM (18 years, 3 months ago)

what sort of apparatus does one need to grind a mirror?

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InvisibleDiploidM
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Re: i want to build a telescope [Re: DieCommie]
    #5253078 - 02/01/06 09:28 PM (18 years, 3 months ago)

You need the mirror blanks, two of them. One functions as a tool, and the other one eventually becomes the mirror. As you slide (grind) the mirror over over the tool over and over again for several days, the tool takes on a convex shape and the mirror a concave one. The blanks are usually Pyrex and they're annealed (heat treated to remove intermolecular stresses). You grind with progressively finer abrasives as the curve develops.

When you start to get close to the final curve, you start testing the shape using a remarkably primitive but extremely accurate device called a Foucault Tester which you can make yourself. It's basically a knife placed at the focal point of the mirror. If the mirror is a perfect parabola, all the light rays reflecting parallel to its axis will meet at a single point in space right at the focal length.

If you place the knife edge just barely below this point and look along the mirror's axis, you will see light reflecting off the upper half of the mirror. Move the knife infinitesimally up, and the mirror will appear to become totally dark the instant the knife covers the focal point.

If there is the tiniest flaw in the parabola on the mirror surface, the mirror will darken gradually rather than all at once as you move the knife edge. By 'reading' the gradual darkening, you can diagnose where on the mirror surface the parabola is not quite perfect and grind in a correction then test again. Repeat until you're happy with the curve.

More on the theory here:

http://www.jlc.net/~force5/Astro/ATM/Foucault/FoucaultTester.html


--------------------
Republican Values:

1) You can't get married to your spouse who is the same sex as you.
2) You can't have an abortion no matter how much you don't want a child.
3) You can't have a certain plant in your possession or you'll get locked up with a rapist and a murderer.

4) We need a smaller, less-intrusive government.

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OfflineChuangTzu
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Re: i want to build a telescope [Re: Diploid]
    #5253951 - 02/02/06 01:54 AM (18 years, 3 months ago)

:eek:

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OfflineChuangTzu
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Re: i want to build a telescope [Re: Diploid]
    #5258149 - 02/03/06 02:10 AM (18 years, 3 months ago)

How many mirrors did you grind before that 32 inch f/6?

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InvisibleDiploidM
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Re: i want to build a telescope [Re: ChuangTzu]
    #5259235 - 02/03/06 01:09 PM (18 years, 3 months ago)

My first two mirrors were 6" made 20 years ago. I FUBAR the first one. :smile:

More recently I made a 14" then finally the 32" which was 90% ground by a machine; I just did the final figuring by hand. There's almost no way to grind such a large mirror entirely by hand unless you're a competition body builder!  :whoa:


--------------------
Republican Values:

1) You can't get married to your spouse who is the same sex as you.
2) You can't have an abortion no matter how much you don't want a child.
3) You can't have a certain plant in your possession or you'll get locked up with a rapist and a murderer.

4) We need a smaller, less-intrusive government.

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InvisibleDieCommie

Registered: 12/11/03
Posts: 29,258
Re: i want to build a telescope [Re: Diploid]
    #5259694 - 02/03/06 03:25 PM (18 years, 3 months ago)

and where do you get these mirrors, and how much do they usually run?

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InvisibleDiploidM
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Re: i want to build a telescope [Re: DieCommie]
    #5259739 - 02/03/06 03:45 PM (18 years, 3 months ago)

Newport Glass is very reliable and been in the business a long time:

http://www.newportglass.com/ngwkitp.htm


--------------------
Republican Values:

1) You can't get married to your spouse who is the same sex as you.
2) You can't have an abortion no matter how much you don't want a child.
3) You can't have a certain plant in your possession or you'll get locked up with a rapist and a murderer.

4) We need a smaller, less-intrusive government.

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InvisibleTODAY
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Loc: Metropolis City, USA
Re: i want to build a telescope [Re: DieCommie]
    #5259741 - 02/03/06 03:46 PM (18 years, 3 months ago)

Quote:

DieCommie said:
and where do you get these mirrors, and how much do they usually run?




i'd like to know this too. i need large concave mirrors for a project i'm thinking about working on but i have no idea how/where to get them. i guess i could rig a bunch of flat mirrors if large concave ones aren't practical.


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ca'rouse (k-rouz)
intr.v.
To engage in boisterous, drunken merrymaking.

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