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Equipment Discussions >> ATM, Optics and DIY Forum

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Reged: 11/30/09

Loc: Midwest, USA
Re: Uncoated Newtonian lunar scope new [Re: Pinbout]
      #5637833 - 01/22/13 09:43 PM

Yeah, sweet in its elegant simplicity. Got a couple years before that solar eclipse...may be enough time...

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Carpal Tunnel

Reged: 02/18/11

Loc: Rancho Cucamonga, CA
Re: Uncoated Newtonian lunar scope new [Re: Pinbout]
      #5637888 - 01/22/13 10:27 PM


from groski's white light newtonian page


For safe viewing, the image of the Sun needs to be reduced by 100,000to 1 in brightness across the full spectrum. This is an Optical Density of 5
( LOG(100000) = 5). Bare glass reflects about 5% of the light striking it. Thistranslates into an OD of about 2. The two reflections in a Newtonian would
results in an OD of 3.5. Still too bright for comfortable viewing so a third
reflections is needed. I used a homemade Herschel wedge to achieve this. I also
took advantage of the fact that when light is reflected off of bare glass at an
angle it becomes polarized. The closer the angle of reflections gets to the Brewsterangle of 57 degrees, the more complete the polarization. With both the diagonal
mirror and Herschel Wedge placed at 45 degrees the light becomes crossed polarized
and the image brightness can be farther reduced.

so can you use a ND filter and a polarizer to get from 3.5 to 5 OD instead of a wedge?

100,000* reduction is the number you are looking for, but a transmittance of 0.05 is an optical density of 1.3, and two reflections is 2.6, leaving you 2.4 or so short. He probably did need the polarization trick to bring the brightness down with just three reflections as a light reduction mechanism.

Shade 7 (0.27% transmittance) or greater welding filters would provide enough reduction.

*I find a range of 30,000 (minimum for safety) to 300,000 (depending on how bright you find optimum for comfortable observation) cited. 100,000 is right in the middle.

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