Quote:
Originally Posted by Akwestland
Eric,
Thank you for the reply. Those dew heaters are a lot cheaper, am just starting to look at these so appreciate the links. I wasn't sure on making the straps but given this price it may have answered the question. Maybe a sill question, would you run more than one on a 200mm f5 Newtonian?
Cheers,
Andrew.
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Hi Andrew
I would have a minimum of three heater straps. One on the objective of the finderscope, one that fits 2" eyepieces and one that fits 1.25" eyepieces. Beyond that, some heating on the secondary mirror and you are set up for the worst dew.
Additionally, if you have a fan or two blowing directly across the mirrored surface of your primary, that should keep that clear of dew as well. But, for the first several hours of the night, primary mirrors are usually a degree or two, if not more, above ambient. Not great for the optical path but it does keep the Primary free of dew.
All you need is enough heat to get the glass surface (eyepiece, finderscope, secondary mirror) a degree above ambient. I know the settings now on my Thousand Oaks controller, but with simple home built controllers I would adjust until the outside of the strap was just slightly warm on my lips (sophisticated measurement technology here!).
On my SDM setup I have the following to manage dew:
1. Two fans scrubbing the boundary layer on the surface of the Primary.
2. A built in Astrosystems heater on the Secondary
3. A Dew-not strap on the objective of the finderscope
4. Two Dew-not straps for the eyepiece, one that fits the 2" and one that fits the 1.25"
5. A home made resistor-chain "heater" on a filter slide which holds three filters and mounts inside the upper tube assembly
6. A long Dew-not filter in the bottom of a box in which I keep my eyepieces when not in use - keeping them a few degrees warmer than ambient.
3., 4. and 5. are controlled using the four separate channels of a Thousand Oaks controller. On other scopes, I have run all three (finderscope and two eyepieces) off the output of a simple one channel controller - works fine.