Basic question: Dusty Mirror?

  • #1
sophiecentaur
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Main Question or Discussion Point

I was examining the insides of my 8" Dobsonian with my new Red Led torch last night and the mirror was looking decidedly dusty. What's the solution to that? A long fluffy duster on a stick? (With care, of course.) I am now thinking that it's probably best to store the scope with the tube horizontal.
Also, after my first view of Saturn and its rings (WOW) and the variegated patterns on Mars, the humidity went up and the whole thing was covered in condensation. What can you do about that? warming the tube up can't be good, optically but, there again, when you can't see through the finder scope and the tube is sopping wet . . . .
Any expert opinions?
 

Answers and Replies

  • #2
Simon Bridge
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You usually have to remove the mirror to clean it... you'll find videos online.
Dew is a tad tougher: basically hide as much as you can from the sky... you have dew-cap?
The manufacturer will probably sell you a dew shield (ask) or you can make one.
There are also heated dewcaps designed for use on telescopes (or use a hair-dryer).

See Sky and Telescope ... um...
Dew: http://www.skyandtelescope.com/astronomy-resources/dealing-with-dew/
Cleaning: http://www.skyandtelescope.com/astronomy-resources/caring-for-your-optics/
 
  • #3
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The torch test looks bad but unless the mirror is very heavily soiled just leave it alone. The light loss will be very small

Regards Andrew
 
  • #4
sophiecentaur
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The torch test looks bad but unless the mirror is very heavily soiled just leave it alone. The light loss will be very small

Regards Andrew
That's a relief; thanks. The mirror looked clean and shiny in daylight and that red torch picked out every speck of dust. I will hold fire on the cleaning job. (Sufficient unto the day is the evil thereof.) Storing the tube tilted downwards sounds a way forward.

Thanks also to Simon for pointing me to the Dew article. (Dewly noted, lol) I was staggered how everything got so drenched so quickly. The finder scope was suddenly impossible to use. I just took everything indoors and let it settle down on its own. I was ready to go indoors in any case. A foam sleeve sounds like an excellent idea for the tube (and virtually FREE!) I have a garden umbrella that can be instantly deployed, too. Plenty to think about.
 
  • #5
Fervent Freyja
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A long fluffy duster on a stick?
Any expert opinions?
Canned air with a long spray nozzle attached?
 
  • #6
Andy Resnick
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<snip>the humidity went up and the whole thing was covered in condensation. What can you do about that? warming the tube up can't be good, optically but, there again, when you can't see through the finder scope and the tube is sopping wet . . . .
Any expert opinions?
Glad to hear you are having fun! I have this same problem during summer vacation at the beach, and I don't think it's exactly the same as dew (great article, btw @Simon Bridge). For me, this happens when it's either (1) sunrise and I take my camera outside from the low-humidity air-conditioned inside into the warmer and high-humidity outside, or (2) at night (usually past midnight) after everything settles to thermal equilibrium but then 'something happens'(?) and the air temperature quickly rises a few degrees and the humidity goes up as well.

I'm comfortable with cleaning optics, so I simply wipe the front lens surface down with a clean paper towel (in a figure-8 pattern, not circles or straight lines) as often as I need to. In the morning I'll point the lens at the sun to intentionally heat the interior optics and evaporate off the water. To be sure, after 2 weeks the residual gunk builds up and when I get home I have disassemble everything for a thorough cleaning.

I like the idea of using a hair dryer to gently heat the mirror...
 
  • #7
sophiecentaur
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I'll point the lens at the sun to intentionally heat the interior optics
Is that safe? I was warned against doing that with my Newtonian because it could fry the secondary assembly. Could that idea have legs? The total power involved would not be more than 40W and probably only half that. Ot the actual focus, that could be harmful to a dust cap, for instance.
But if you have actually done it, it must be safe, I guess.
 
  • #8
Andy Resnick
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Is that safe? I was warned against doing that with my Newtonian because it could fry the secondary assembly. Could that idea have legs? The total power involved would not be more than 40W and probably only half that. Ot the actual focus, that could be harmful to a dust cap, for instance.
But if you have actually done it, it must be safe, I guess.
Well..... a few caveats. First, my primary aperture is 'only' 5.5". I only do this early in the morning, when the sun is (almost) able to be viewed by the unaided eye. I *never* look through the camera when I do this! Also, I don't think there's a focal point within the lens assembly.
 

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