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Insulated Dew Shield

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#1 JimK

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Posted 09 January 2011 - 03:27 PM

I use my Questar in my yard, which is about 6100 ft above sea level. Because of the corrector cooling when pointed at the sky, in late fall and winter ice crystals form on the corrector as frost even though the dew shield is used. It is very hard to see objects clearly through ice crystals.

I first tried using just a cloth covered heater strip, but the aluminum dew shield dissipated the added heat too fast and frost still appeared on the corrector - ambient temperatures when observing were in the 30s (F). So I decided to use the same techniques that I had used for my orange tube C8 SCT (as posted here on CN):

1) a heater strip was added to about the middle of the dewshield, just after the corrector (I used the low-power Dew-Not brand from Scopestuff).

2) I made a dewshield blanket out of metal-coated plastic bubble wrap (Reflectix) using Ron Keating's instructions. [http://www.dewbuster...dewshield.html] Insulation strips were used on either side of the heater strip for keeping it in the right place and for blocking air flow along the dewshield.

A portion of the insulation was extended a little to protect the top of the optical tube, and entire top of the dewshield and optical tube have about two layers of insulation. I used Velcro strips to keep it together and kept it just a little shorter than the Questar's aluminum dewshield.

The heater compensates for the thermal loss to the sky when viewing, the dewshield makes the heater more efficient (less heat loss, less power needed), and the double insulation thickness on top helps keep the upper tube heat losses to a minimum, helping to stabilize any internal air currents.

Results - no more corrector icing, even though I have been observing in the 20s and low teens (F), and defocused star images don't show thermal plume spikes. This telescope is used to look at the stuff in the sky and is not just kept as a display piece, so it has to remain functional for me.

Drawbacks:

1) The shiny insulation is ugly (in my opinion), but at least it works well and it may prevent ice damage to the corrector finish and coatings. And the insulation/heater combination can be easily removed when the weather is warmer, without any damage to the Questar.

2) The dewshield must be pulled off the optical tube in order to remove or install the threaded protective cover for the corrector. This is not a problem for me because I remove the dewshield anyway when allowing the telescope to cool down for better viewing.

I have attached a composite picture of this modification. No plans - everything was just cut to size when making the insulation parts...

Attached Files



#2 greedyshark

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Posted 09 January 2011 - 10:31 PM

Nice. :waytogo:

CS,
Charles

#3 Erik Bakker

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Posted 10 January 2011 - 05:48 AM

the double insulation thickness on top helps keep the upper tube heat losses to a minimum, helping to stabilize any internal air currents.

Results - no more corrector icing, even though I have been observing in the 20s and low teens (F), and defocused star images don't show thermal plume spikes. This telescope is used to look at the stuff in the sky and is not just kept as a display piece, so it has to remain functional for me.


Hi Jim,

Thanks for your superb article. The points you make about the importance of thermal insulation are extremely important to note by all visual observers, not only Questar users. I find myself obtaining good images the quickest when I set up with the dewshield off, and putting it back around the tube when observing. Better insulation between the warmer air inside the tube and the cold air outside means very little temperature differences between the tube and thus calm air inside the tube which translates into very stable images. Of course, my climate is far more moderate than yours, so I can get away without an extended dewcap on most nights. I am inspired by your report and will try your insulation myself.

CS,

Erik

#4 JimK

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Posted 10 January 2011 - 09:16 PM

Thanks, Charles and Erik.

Trying this type of insulation should be rather easy - even without the heater element for milder situations. In fact, I just used some rubber bands to hold it together at first - the Velcro was later.






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