Quote:
Originally posted by iceman
Excellent points Ian.. I'll definitely have to make a dew shield/tube extension for my dob.
What can be done about dew on the primary mirror?
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Mike,
I have a dew shield that I made that extends my tube by 320mm. It does an excellent job as a dew shield and also as a light baffle. Ian indicates the tube should extend past the secondary by at least 1 x the scopes aperture. That is consistent with my thoughts as well being that the tube should extend somewhere between 1 and 1.5 x the scopes aperture past the secondary. My secondary only dews under the most unfavourable conditions and even then it takes an awfully long time for the secondary to dew up. I actually forgot to take my dewshield with me to the April mini star party, you could have had a look at it and judged how effective it is. I really think it a better option than a secondary heater for several reasons including those that Ian mentioned.
In addition to the dew shield there are sound practices you can follow that also delay the onset of secondary dewing.
1. Return the scope to the horizontal position when its not in use.
2. Cap the focuser when you remove an eyepiece. A 35mm film cannister works well for this. Keep the cannister in your pocket and drop it in when you remove the eyepiece.
CS- John B