I am hoping someone can help me with the correct place to fit my home made dew heaters to my LX 200 Meade scope.
I originally fitted them close to the corrector plate, but this siginificantly reduced the image quality.
I am presuming the drop in image quality was due to the difference in temp. between the scope tube and the corrector plate which was warmer than the mirror / tube.
Q/ Where do the commercially available ones fit to the scope ?
Youve got it on the right place, and its supposed to make the corrector lens warmer than ambient (the tube), thats how it stops dew forming on the lens ;-).
The only way it can effect image quality is if its way too hot and causeing thermals. It only needs to be 1-2 deg higher than ambient, 15-20 watts should do the trick.
The heaters I designed are two at 6 watts each. They can be independantly switched on / off.
Both on at once when dew is heavy, and then one one by itself when not so heavy.
And were designed to fit one inside the end of the LX200 near the corrector, and the other on the outside of the tube near the corrector.
However, when I tried them out for the first time last time I had the scope out all night, the image quality while viewing Saturn was very very poor.
I then turned the heaters off and allowed the corrector temp to drop compared to the tube, and the image was dramatically better just before the dew formed again. : (
I think you have answered my question though with repect to positioning. Thanks.
I will try just one heater next time on its own and see if that makes a difference, and doesnt heat up the corrector too much.
I may also monitor the temp to see that its only 1-2 degrees warmer.
I think the one closest to the corrector plate will cause the most trouble.
Do a test, don't use it for now. Wrap around the heater on the tube with some cloth like a strip of old beach towel. Go around many times so there is no way much heat can escape directly into the air.
Run the heater. See if it happens again. I leave mine running all the time regardless of season, time or temp and have never dewed up or felt there were heat shimmers.
Ideally you should have something that is permanent and just set it and forget it.
I think many people wait until they see dewing starting then they turn on their heaters. It's a bit like saying "I'll get some insurance after I crash the car". Don't let the correctot get too cold to start with and you will not need much heating at all.
Also insulate your heaters well. I see people calculating their heater consumption to the third decimal place to determine how long their batteries will last. Then at the same time they have most of the heat from their heaters escaping into space.
Your loss of image quality sounds like you have the system running too hot. I suffered this until I built a pulse-width controller for it. Being able to turn the heat down to where it was doing the job, but no more, was the key.
You do NOT want to heat the corrector plate. You want to WARM the air in front of the corrector and semi-trap it inside the dew shield area. You only need a differential of a couple degrees to prevent dewing.
Here's a photo of where I installed my home-made strip, and it works fabulously well and does not affect image quality because the heat is set to the bare minimum for the effect to work. I turn it on at the beginning of the evening and let it reach temperature well BEFORE it is needed.
From my experience, I have found the best position for the heater is to be just behind the corrector plate housing. The corrrector only needs to be no more than 1 deg above ambient.
Placing the heater in front of the corrector will require more power as the heat is lost to the open air quicker before it gets to the middle of the corrector and is replaced by cold air, this can result in the middle of the corrector still dewing up while making the outer edges too warm as you crank up the power to try to compensate.
Using a good dew shield and as Monte has said, insulate your heaters from the cold outside air will bring your power consumption down to minimal levels and makes heating the corrector evenly to about 1 degree above ambient easier.
I think the one closest to the corrector plate will cause the most trouble.
Do a test, don't use it for now. Wrap around the heater on the tube with some cloth like a strip of old beach towel. Go around many times so there is no way much heat can escape directly into the air.
Run the heater. See if it happens again. I leave mine running all the time regardless of season, time or temp and have never dewed up or felt there were heat shimmers.
Ideally you should have something that is permanent and just set it and forget it.
I think many people wait until they see dewing starting then they turn on their heaters. It's a bit like saying "I'll get some insurance after I crash the car". Don't let the correctot get too cold to start with and you will not need much heating at all.
Also insulate your heaters well. I see people calculating their heater consumption to the third decimal place to determine how long their batteries will last. Then at the same time they have most of the heat from their heaters escaping into space.
Your loss of image quality sounds like you have the system running too hot. I suffered this until I built a pulse-width controller for it. Being able to turn the heat down to where it was doing the job, but no more, was the key.
You do NOT want to heat the corrector plate. You want to WARM the air in front of the corrector and semi-trap it inside the dew shield area. You only need a differential of a couple degrees to prevent dewing.
Here's a photo of where I installed my home-made strip, and it works fabulously well and does not affect image quality because the heat is set to the bare minimum for the effect to work. I turn it on at the beginning of the evening and let it reach temperature well BEFORE it is needed.
Thanks Chris, Your heater is in exactly the same place as I fitted mine ( yours looks a lot neater than mine though )
May I ask the wattage of your heaters ?
From my experience, I have found the best position for the heater is to be just behind the corrector plate housing. The corrrector only needs to be no more than 1 deg above ambient.
Placing the heater in front of the corrector will require more power as the heat is lost to the open air quicker before it gets to the middle of the corrector and is replaced by cold air, this can result in the middle of the corrector still dewing up while making the outer edges too warm as you crank up the power to try to compensate.
Using a good dew shield and as Monte has said, insulate your heaters from the cold outside air will bring your power consumption down to minimal levels and makes heating the corrector evenly to about 1 degree above ambient easier.
Thanks Daniel.
I may be using too much power and need to make a PWM for it.
I think the corrector infact was too warm ...much warmer than 1-2 degrees above ambient. Is 6 watt heaters too much ?
From my experience, I have found the best position for the heater is to be just behind the corrector plate housing. The corrrector only needs to be no more than 1 deg above ambient.
Placing the heater in front of the corrector will require more power as the heat is lost to the open air quicker before it gets to the middle of the corrector and is replaced by cold air, this can result in the middle of the corrector still dewing up while making the outer edges too warm as you crank up the power to try to compensate.
Using a good dew shield and as Monte has said, insulate your heaters from the cold outside air will bring your power consumption down to minimal levels and makes heating the corrector evenly to about 1 degree above ambient easier.
Thanks Daniel.
I may be using too much power and need to make a PWM for it.
I think the corrector in fact was too warm ...much warmer than 1-2 degrees above ambient. Is 6 watt heaters too much ?
Thanks Chris, Your heater is in exactly the same place as I fitted mine ( yours looks a lot neater than mine though )
May I ask the wattage of your heaters ?
Thanks for the tips.
Geoff.
Geoff - I ran 50 390 ohm, 1/2 watt resistors in parallel. If you take 50x390 ohm resistors you have a total of 7.8 ohms over the whole lot (divide the resistance by the number of parallel resistors in the circuit). This results in an 18 watt unit at 12 volts DC. I then use my home made PWM controller to limit that to 0, 1/8, 1/4, 3/8, 1/2, etc, etc to get a nicely controlled output. At 1/8th of a power cycle I can linearly estimate that it's running at 2.25 watts at that setting.
I have to respectfully disagree with putting heaters INSIDE your OTA, I'm sorry. It's not required in my experience, and introduces warm air tube currents where you don't want them. It doesn't matter that you lose heat if installed on the outside where we have ours, because you merely compensate by cranking it up a tad more. We aren't looking for efficiency - just a small layer of air that's slightly warmer NEAR the corrector. Remember - it's the temperature of the air near the corrector which is the important thing - irrespective of the temperature of the corrector flange ring, which will be much warmer than 1-2 degrees above ambient. Dew shields, by themselves, almost work well - without any heating devices. They work by trapping air close to the corrector (or objective on a refractor) which settles at the transmissive surface and is hopefully unaffected by circulating ambient air at the end of the tube/dew shield assembly. Because this air is trapped it remains slightly (and I mean slightly) warmer than the outside air above it. That's all that's needed. The heater just helps by raising that same air another notch or two.
Here's another one I made for my C8. Works a treat - super efficient and only needs to run at around 1/4 power on the controller - even on the foggiest, dewey nights. This one even has a layer of felt in between its outside edge and the corrector flange to limit the heat transfer. I want to warm the air, remember, not the flange itself.
1) Lay resistors down and solder to 3mm solder wick braid - outer edges set 20mm apart. If you are careful, you won't get solder spreading too far.
2) Cut the excess legs off.
3) Cut a strip of plastic (from a Coke bottle or something similar) 2 cm wide and lay the resistor strip on top before slipping 25mm shrink wrap tubing over the lot.
4) The other side. Join both sides at the right circumference by overlapping 2 inches and then put yet another heat shrink over the join. I also usually wrap a 2cm strip of felt over the OUTSIDE so that it sits snugly inside the flange. The felt stops quite an amount of direct heat being transferred to the metal - and serves to heat the air in proximity instead.
