Interesting home made dob in Seattle CL..
Posted 18 May 2007 - 10:26 PM
Posted 18 May 2007 - 10:51 PM
Posted 18 May 2007 - 11:09 PM
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.
Posted 18 May 2007 - 11:23 PM
Posted 18 May 2007 - 11:36 PM
Posted 19 May 2007 - 01:20 AM
Posted 19 May 2007 - 09:37 AM
how fast can that thing water the yard it kinda looks like a fancy sprinkler RAIN BIRD!!!!
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.
Posted 19 May 2007 - 03:11 PM
Posted 19 May 2007 - 03:12 PM
Posted 19 May 2007 - 04:47 PM
Posted 19 May 2007 - 04:52 PM
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?
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.
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).
Posted 19 May 2007 - 06:04 PM
Posted 19 May 2007 - 06:15 PM