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Interesting home made dob in Seattle CL..

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#1 Darenwh

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Posted 18 May 2007 - 10:26 PM

Seller states was built with direct help from John Dobson. That would definately make it a classic. Of note: seller also has another ad for a solar scope that is home made, classic design, and being a solar scope probably quite rare. I think both of these would qualify as being classics.

Dobsonian telescope.

Solar Scope

#2 ngc2289

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Posted 18 May 2007 - 10:51 PM

That solar scope is better made then the ones John D. used to bring up to Fremont Peak back in the 70's. Mike Traub :)

#3 trainsktg

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Posted 18 May 2007 - 11:09 PM

For some reason, the pictures of the scopes aren't loading for me.Hmmm.

I'm glad to see the seller rates John Dobson so highly, but one thing is for certain, he most certainly did not invent the solar newtonian as described in the ad. Sam Brown details it construction in 'All About Telescopes'. There are others built many decades ago. Here's my 4.25" f13 built in the 60's, using an uncoated primary, herschel wedge and welding filter.

Keith

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#4 Darenwh

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Posted 18 May 2007 - 11:23 PM

True but they are still a rare and very novel type of scope. They are also very unique and this is a very well made version of these scopes. It may or may not be considered a classic depending on the persons view but any dobsonian scope that was assembled by or with the help of John Dobson would, in my book, be a classic. Johns design has had such a dramatic effect on the industry that any cope using his mount design and worked on by him would qualify. The only question on this would be, how do you prove the scope is one that John assisted in the construction of?

#5 trainsktg

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Posted 18 May 2007 - 11:36 PM

I agree with your assignment of a scope built by a famous ATMer as 'classic'. Definitely desirable. I was just disagreeing with the claim of invention as made by the seller.

Keith

#6 clintwhitman

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Posted 19 May 2007 - 01:20 AM

Hey Keith how fast can that thing water the yard it kinda looks like a fancy sprinkler RAIN BIRD!!!! sorry just messin around!!!! The scope. DOB he is selling sounds like a Oddessy 3.5 there kinda a pain to move around I have an 76 blue tube F4.5 his is an F6 so its a long sono tuber

#7 BHunt

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Posted 19 May 2007 - 06:29 AM

Keith,
How does that work.
Bill

#8 trainsktg

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Posted 19 May 2007 - 09:37 AM

how fast can that thing water the yard it kinda looks like a fancy sprinkler RAIN BIRD!!!!


:lol: :lol: :lol:

As the ad describes, this is used only on the Sun. An uncoated primary reflects about 5-10% of the light incident on it. The secondary can be a herschel wedge or an uncoated secondary, again reflecting about 5-10% of the light striking it. Now, you have eliminated over 99% of the light, but its still too strong. Depending on the size of your primary, you need to select another filter to take that down several more thousand times. 'All About Telescopes' has a nice chart that helps you calculate this. I've never found any specific 'how-to' plans anywhere, even in the pages of ATM 1,2,3 or A. I presume its a word of mouth thing. In reality, this design has probably been around for since they started using glass for mirrors.

The white light views through a scope of this type are much more 'crisp' than I've seen with glass or film filters.

Keith

#9 refractory

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Posted 19 May 2007 - 03:11 PM

Could one simply run the light through more uncoated reflections, which would be easy enough, say a chain of mirrors?

Jess Tauber

#10 refractory

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Posted 19 May 2007 - 03:12 PM

Or two longish parallel mirrors with the light at a high angle?

Jess Tauber

#11 twhite

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Posted 19 May 2007 - 04:47 PM

I've seen other solar scopes built on the same principle as the one in the craigslist ad, though I don't recall having seen one that suggested using one-way mirror as the optical window/secondary. I would be concerned about the flatness of such a piece of glass introducing distortion into the image -- or maybe I'm wrong?

#12 ngc2289

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Posted 19 May 2007 - 04:52 PM

John D. used optical windows that were pretty good opticaly. John had the windows aluminized on one side. :)

#13 trainsktg

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Posted 19 May 2007 - 05:57 PM

Could one simply run the light through more uncoated reflections, which would be easy enough, say a chain of mirrors?

Jess Tauber


I suspect collimating such a chain of mirrors would be a futile mess in a larger aperture telescope. According to the Sam Brown charts, 3 reflections gives you a 1/8,000 reduction, suitable for a 2" aperture. 4 reflections gives you a 1/167,000 reduction, OK in a 3" aperture, but he recommends a 1/330,000 reduction for that size. Since a 5.5" mirror gathers about 3.4x as much light as a 3" mirror, I suspect a reduction of about 1/1,100,000 is right for that size aperture. This is reached with as few elements possible using 2 reflections (1/400x) and a 5 neutral density filter or 5.1 welding filter.

Keith

#14 trainsktg

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Posted 19 May 2007 - 06:02 PM

I've seen other solar scopes built on the same principle as the one in the craigslist ad, though I don't recall having seen one that suggested using one-way mirror as the optical window/secondary. I would be concerned about the flatness of such a piece of glass introducing distortion into the image -- or maybe I'm wrong?


Well, I guess it works, but I agree with your assumptions. John Dobson is synonymous with 'cheap', so perhaps this was his recommendation as a material in the beginning. Perhaps later he switched to the custom optical windows?

Sam Brown lists the following secondary materials...black mirror (1/20), black penta mirror (1/400...must be a bear to mount and align), right angle prism (1/20), penta prism ( 1/600)and herschel wedge (1/20). Mine uses a standard uncoated secondary (1/20).

Keith

#15 BHunt

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Posted 19 May 2007 - 06:04 PM

:graduate: So when are you writting the book Keith! You need to come east so I can pick your brain.
Bill

#16 trainsktg

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Posted 19 May 2007 - 06:15 PM

Its already written. Sam Brown's All About Telescopes. I'm just a parrot here. :blah:

Keith


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