Dew heater strips

[jaygpoo]

Up until now I have not really used the dew heater strip with controller that I brought from Modern astronomy. I have a celestron 9.25 and the strip is wrapped just around the very front edge of the scope. The other night in the cold before the frosty mornimng the heater proved inefective. I have since brought a second one from w&w astro for my ed80mm. Now as a test I used a thermometer to test the heat rise when the strip was on full using the controller. To my surprise the temp rose from 57f to only 69f  ( control full). Then I connected the strip directly to the battery and tested it again. Hell the temp rose from 57-170f.  There appears to be no flattening out of the temp when connected directly as both the strips I have acted the same. They may have come from different suppliers but look the same. My question is this. a 10F rise cannot be enough to act as a dew deterent. However a 100f rise feels great but will it drain the battery ( quicker if using 2 strips) and can the heater strips overheat?   Do you need to use a controller?   Jay

[ChrisLX200]

Yes you need a controller, and you don't need much heat - but you do need to put it in the right place (into the glass). If you put the strap around the very front edge of an SCT the heat has to be conducted through the side of the scope, down the tube a bit, and then into the edge of the corrector in the hope of raising the temperature of that half a degree or so. Most of the heat will be lost unless you have it placed as close to the edge of the corrector plate as possible. Also, it's a good plan to use insulation over the top of the strap to stop 90% of the heat disappearing into the sky   Use too much heat and it will wreck your seeing for sure.

ChrisH

[YKSE]

I agree with Chris, the tape should be placed as close to the corrector as possible.

I use a controller, it's set to median power, the heater tape felt just little warmer than my fingers, you may need to check your controller again if it only raises temperture to 69f at max.

[Northern Soul man]

Hi Jay the heating is very subtle, I thought my dewzap heated shield wasn't working, I have my hitec astro controller set only to half way with the 11" CPC, it just needs a degree or so above the "outside" temp of the scope so very subtle - I just place my lips where the heat should be on the dew shield and you can certainly feel the warmth, try the lip test theyre very sensitive and shows the heaters workin.

Paul.

[RayD]

19 minutes ago, Northern Soul man said:

I just place my lips where the heat should be on the dew shield and you can certainly feel the warmth, try the lip test theyre very sensitive and shows the heaters workin.

Just make sure it's warm enough that your lips don't stick and you end up looking like a scene from Dumb and Dumber 

[jaygpoo]

Thanks for replies an I will use my controller which has two outlets to see what ahppens on the next cold night. One point to question is that on a few star parties I have seen and been told that heaters were used without controllers an I still do not have the answer as to if that is a silly practice or not ?

 

[Northern Soul man]

Hi Jay, It really depends on you, how long do you want to observe for, at some point, your going to get dewed up without the heaters, so with your two channels, you can run one strip for the corrector and one strip for the EP's.

Your 925 will, I think, need a dew strip whether its a combined heater and dew shield or not, SCT's above 8" will probably draw a couple of amps an hour - larger correctors need a little more, my 11" is usually set to half power, if its a damp night I will turn it up to full power - its all about managing the power you have from your supply, If its a power tank then you need to just be careful with a full drain of the tank if its 7aH - so a couple of amps for the scope and a couple of amps for your dew heater - that gives you about 2 hours on full whack - if you have a 17aH power tank, it will prolong the amount of time before it drains - so a good 3-4 hours on full whack.

If you have a powered, regulated power supply, then you can run full all night, just be careful about running electricity outdoors in the damp - I've seen observers use these regulated supplies inside a breathable containers whilst outdoors.

If you go to a dark site or travel to do your observing, don't worry about the corrector dewing up because as soon as you pack away and begin the journey home, your going to put the heaters on in the car - so the scope will dew up anyway - its just a common problem with SCT's and don't go around thinking that if the scope dews up its the end of Civilisation!!! - its just how things are - I just leave the caps off the scope if I have any dew on the corrector - wait for it to clear - then cap and place away - or there has been times where I have packed away, leaving the cap off the back of the visual back and just placed a silica pack over the hole to draw out any moisture - then just cap the next day - been doing it this way for years with all of my SCT's and never had a problem.

Its just how things are with SCT's - so if you don't use some form of heat - it will shorten your session - hair dryers can be used - but make sure you get one for outdoor use - I've never bothered with one, because I think if your heating the corrector too quickly with a BLAST of warm air - then you could get pollen or dust sticking to the corrector - I think the heater strips are great - get the strip as close as you can to the side of the corrector and away from the casing - so I place on the metal tube just behind the casing which extends in front of the corrector - if that makes sense.

During your observing, you may want a break to go indoors - just park (Hibernate) the scope with the corrector facing slightly below level with the heaters left running - the rule of thumb is the more you point the scope up into the sky - the quicker the dew will form - so horizontal and below when having a break

All the best Jay and just enjoy your observing - there's no right way or wrong way - and definitely don't get too hung up on the dew thing!!!

Paul.

 

[jaygpoo]

Proto star. Wonderful reply so warm and informative I felt you sitting next to me!!!!  Great info and will act as instructed by all above. Very helpful but I still do not have the answer to the question. What happens if you connect the strip direct to the battery . Will it heat up to an unregulated point or will it flatten out at some point. Is it a total no no ????

[SonnyE]

I'm so fortunate that I live in a place where I don't have to heat my doo...

I mean DEW.

In either case one needs to be in control. Losing control in either case could get you sent home.

[RayD]

21 hours ago, jaygpoo said:

Proto star. Wonderful reply so warm and informative I felt you sitting next to me!!!!  Great info and will act as instructed by all above. Very helpful but I still do not have the answer to the question. What happens if you connect the strip direct to the battery . Will it heat up to an unregulated point or will it flatten out at some point. Is it a total no no ????

It won't blow the end of your OTA off, but it's not ideal.  The controller is really nothing more than a simple variable resistor which reduces or increases the voltage to the strip.  Running directly from the battery will do the same as having the resistor set at closed (resistance = 0), but the idea would be to get things just warm enough to prevent dew, not heat the end of your OTA so you can warm your hands on it 

Personally I would say for the cost of a single channel 2 port controller, use one, as in most cases you will only be running on half load which will be enough to keep the dew at bay.

[jaygpoo]

Question Answered. Thanks