View Full Version here: : Dew strap position on refractors

The Mekon
19-06-2016, 11:03 AM
Similar question has been posted on CN recently. I intend to buy new controller and dew strap for my 106 Sharpstar. When I had my AP 130 the position to place the dew strap was easy - around the cell on the main tube. With the Sharpstar the cell is covered by the sliding dew cap, being half way along it. Most posters on CN are stating the strap should be aft of the cell, but on the main tube? - would be a long way from the objective on the 106. I intending to place it on the dew cap, and where I place it, whether on main tube or dew cap, will determine what size strap I buy.
What thoughts?

19-06-2016, 11:24 AM
I place my dew heater strap on the main tube, just behind the (extended) sliding dew shield on my Esprit, same as I did on other OTAs with a sliding shield as well, works fine for me.

19-06-2016, 11:48 AM
I have placed the dew strap on both the dew shield and the OTA just behind the shield. Never had any issues with either location.

19-06-2016, 02:39 PM
I have always used the dew shield

20-06-2016, 08:18 AM

20-06-2016, 08:24 AM
I've often wondered. I guess it's common sense and it depends on the design on the refractor as well. You need to transfer the heat to the glass so the dew strap should be as close as possible and in contact with the cell or something that's linked to the cell. Most retractable dewshields have a small gap between the main tube and the shield so putting a strap on the dewshield directly won't do much IMHO. You're best putting the strap right after it on the tube? Again it depends how far the dewshield slides out and how close to the lens cell it comes at the back. If it's an alu tube good, CF tube well...

20-06-2016, 11:23 AM
On my refractor, I put the dew strap on the dew shield just in front of the lens. My theory is you create a warm pocket of air within the dew shield in front of the glass. I can't do this on the SN10 because the dew shield is plastic. In this case I put it on the OTA body just behind the corrector plate.

Both work - I've never had dew problems with either scope.

21-06-2016, 06:31 PM
I would put it on the objective or the dewshield where the objective is.

On the main body does not make sense because 1. Its the objective you want slightly warmer to prevent dew not the body and 2. Tube currents are destructive to sharp imaging.

I have wondered how much thermal currents from the dew heaters in the tube of a refractor degrades the image given how the images sharpen up once the refractor (or other types of scopes for that matter) equalise in temperature.

I think though without having done extensive testing its likely that putting the dew strap on the body would create thermal currents which would degrade the images to some degree.

I can't say I have noticed this on my AP Honders but that has an oversized tube to remove tube currents so its not a good example.

I used to use those wrap around insulation blankets like used on car windscreens. You wrap that around the objective area at the beginning of the night and it would keep the scope dew free for almost all night most nights.


21-06-2016, 07:03 PM
On the tube directly behind the dew shield, never had a problem. :thumbsup:

23-06-2016, 08:26 AM
I posted this question on the AP Yahoo Group and got some very informed replies.

The best was this:


It describes in detail where to put the dew heater strip and how to prevent the dew heater from degrading the view which it can if improperly located and setup.

Its worth reading. You don't want weaker results imaging with your expensive APO wasting some of its performance on incorrectly placed and setup dew heaters.