Photo: Qhy9 mono + qhy 5x2" filter wheel and Baader 2" LRGB, Ha, O3 and S2 filters , Meade DSI Pro 2 as guider, TS 9mm off-axis guider Binoculars: Nikon Action 12x50 Telescopes: Skywatcher Evostar 120ED f7.5 APO + TS 2" flattener Mounts: HEQ5 Pro Eyepieces: Nagler 11mm type6, Pentax XW 7mm, Televue 2X barlow 1.25"
Quote:The dew heater strip should be wrapped around the OTA just behind the primary lens cell. .......-- Chris
George N Obsession 20 Optical Guidance Systems 10" F/9 R-C Cass 6" F/5 & 8" F/8 home-made Newts Explore Scientific 127mm ED MI-250 mount Denk II bino-viewer, with PowerX and Newt reducer, Member, International Dark-Sky Association
Quote:If the dew shield is of the sliding variety, with a soft material which acts like an insulator against hear transfer, the heater should be wrapped around the dew shield, perhaps at about the middle. This is to warm the dew sheild and thus compensate for the loss via IR radiation of the objective into the cold heat sink of the sky.
If the dew shield is fixed in place and has thermal contact with the cell, it doesn't so much matter exactly where the heater is placed, for thermal conductivity will distribute the heat fairly quickly. Myself, I prefer to always wrap around the dew shield. Some will choose to wrap around the cell region, and others just behind the cell. No big difference when all parts are metal and thus conduct heat readily.
One is not warming air; that would be inefficient anyway. Rather, the heater is redressing the radiative imbalance. Whether by adding energy to warm the objective conductively, or to irradiate the objective with sufficient IR to compensate for radiative loss to the sky, the heater is keeping the glass surface at a temperature above the dew point temperature.
I make my own heaters, and run them full time when deemed necessary (no duty cycle controller.) These heaters are of lower power consumption than commercial units, being 1/4 to 1/2 as energy hungry. I increase efficiency by making them slim and hence with good thermal contact, az well as wrapping in a thin insulating foam.
7x35 and 10x50 sears tower binocs, 3" f/10 edmunds reflector, 2.4" f/11.7 manon refractor, 6" f/8 jaegers refractor, 10" f/11 R30 Istar refractor, 3" f/15.8 sans&streiffe refractor, 3.1" f/15 selsi refractor(towa 339), 2.4" f/15 sears refractor, selsi 30x30mm spyglass, criterion 5-draw 25x45x75x spyglass(1957), 4.25" f/14.8 tasco 20te.http://cleardarksky.com/c/OmahaNEkey.html
Scopes: 190mm SW Mak-Newt @ f/5.3 & 90mm SV Raptor Triplet @ f/7Cameras: Atik 383L+ mono w/ EFW2 FW with Astrodon filters & Canon XSi 450D (Modded)Mount: Astro-Physics Mach1GTO mount guided with QHY5 KWIQ guiderSoftware: Sequence Generator Pro; PixInsight; Photoshop CS5Best investment: Stone Garden Observatory - Bortle 4 (Green)
Quote:Could I expand this interesting discussion? What about dew heaters on carbon fiber versus aluminum tubes? Would you have to change the settings because of the lower thermal conductivity of CF tubes? Or are dew heaters even effective on CF tubes? ...Keith
Warren - Stargazing since the 60's! Scopes: ETX-LS6, ED80T, AT6RC, Lunt LS60T, C9.25 Mounts: Atlas EQ-G, Vixen Portamount II Cameras: Atik 314L+,DMK31AU03,SSAG, ASI120MC Filters: Astrodon LRGB, Orion HA, SII, OIII Acc: Orion 5 place Filter wheels x 2, Flatman Primary Imaging site: Bortle Scale Class 6 Red Zone http://astrobin.com/users/rigel123/
Quote:Thanks Warren. Do you find you have to adjust the heat higher because of the lower thermal conductivity, or is it just business as usual? ...Keith