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Scattering in Refractive vs. Reflective Optics

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#26 azure1961p

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Posted 04 September 2014 - 10:16 PM

Unless the dust on the mirror is particularly reflective which it isn't then it'll not have any forward scatter of any consequence.  There's a lot of things in the light of day that might seem like it could play havoc however the light  levels at night are so low most of it is not visible (thankfully).  

 

Pete



#27 zeehas

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Posted 05 September 2014 - 12:01 AM

I looked up the albedo of desert sand, which I'm assuming comprises a good deal of the deposits on my mirror. It's about 40%. Compare this to the moon which has an albedo of about 12%. So far we've heard a lot of theory indicating that dust on optics shouldn't have much effect on contrast. (Thanks to those who posted the theory. It is very instructive.) The only one to report an actual observation so far did note a slight difference in contrast comparing prism with mirror diagonals and reflector vs APO. How much of this was due to dust is uncertain. I'd be interested to hear about any other experiments of this nature.

 

I've had my dobsonian out a couple times in the past few days. I'm trying to get some baseline observations before cleaning the mirror. I am happy to report that better collimation and the use of a fan have improved the view considerably. Although the 12 inch mirror still does not compare well with the 6 inch lens on Mars and Saturn.  I really would like to see what 12 inches of quality aperture is capable of, so I'm open to further suggestions. 


Edited by zeehas, 05 September 2014 - 12:13 AM.


#28 wh48gs

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Posted 05 September 2014 - 06:56 PM

I looked up the albedo of desert sand, which I'm assuming comprises a good deal of the deposits on my mirror. It's about 40%. Compare this to the moon which has an albedo of about 12%. So far we've heard a lot of theory indicating that dust on optics shouldn't have much effect on contrast. (Thanks to those who posted the theory. It is very instructive.) The only one to report an actual observation so far did note a slight difference in contrast comparing prism with mirror diagonals and reflector vs APO. How much of this was due to dust is uncertain. I'd be interested to hear about any other experiments of this nature.

 

I've had my dobsonian out a couple times in the past few days. I'm trying to get some baseline observations before cleaning the mirror. I am happy to report that better collimation and the use of a fan have improved the view considerably. Although the 12 inch mirror still does not compare well with the 6 inch lens on Mars and Saturn.  I really would like to see what 12 inches of quality aperture is capable of, so I'm open to further suggestions. 

 

There so many more other factors hurting 12-inch reflector more and much more than dust. The seeing error - whatever it is at a moment - is 1.8 times larger than in 6-inch aperture. Thermals are significantly more of a problem. Miscolimation, pinching, central obstruction, surface imperfections, including small scale toughness, all add to it. Reflective coatings scatter more than refractive coatings, and can be pretty bad. Baffling is generally better in a refractor, observer's body heat does not come in the light's way, etc. Dust. like anything, should be evaluated, but its effect is negligible compared to these.

 

Vla








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