Planetary imaging and dew heaters?

[Sammyb]

Hello

I read on these forums a suggestion that dew heater tapes may affect the quality of planetary images, presumably by causing air currents within the tube.

Would this be the case with an SCT?

After I read this the other night I switched mine off but didn't notice much of an effect - the seeing was pretty poor anyway. Fortunately it was a dry night and not much dew about but on other nights it will be needed so I'm unsure as to how tackle this.

Any thoughts would be welcome

Thanks

Sam

[jambouk]

This is interesting. I'll be watching this with interest as my recent attempts at planetary have been dire and i use some dew heater bands....

James

[hobsey]

Yes they can effect the viewing if they are too hot. This is why it is advisable to have a dew heater controller instead of going directly into the power pack.

You only need a small about of heat.

[Laurie61]

I did try a few experiments with home made dew control system and used varying wattage outputs to see what happened. I think I went up to around 75/80w output, more than a typical dew band, before seeing a problem. This was fitted to a 10" sct. I use an Astrozap set up on my C11 and have not seen a problem with that so far with one dew band working at Max output.      

[Sammyb]

Thanks for the responses. I'm using a dew-not heater tape that plugs directly into a 12v supply.

Do these tapes provide a level of heat that could be a problem?

Sam

[IanL]

Thanks for the responses. I'm using a dew-not heater tape that plugs directly into a 12v supply.

Do these tapes provide a level of heat that could be a problem?

Sam

Pretty hard to say as there are too many variables to make an accurate prediction of the appropriate level of power output.

- Assuming your heater band is a good fit around the OTA you are looking for a typical maximum output of about 0.3 watts per cm of length (if the band is a lot longer you'd need less, so if it was double around you would want 0.15 watts per cm of length).

- Bear in mind that is likely to be the maximum output you will need (unless you are imaging in the Arctic or something).  Most of the time you will need something less than 0.3W per cm, which is why you really need to use a dew heater controller to dial down the power output to an appropriate level.

- Determining the appropriate level is an art rather than a science.  It depends on the ambient temperature and dew point, both of which will usually change through the night, plus the size and construction of the OTA, which determines how effective it is as a heat sink.  The ideal power output is one which keeps the lens, corrector or mirror (depending on the type of scope) just above the dew point at all times.  Too low and dew will start to form, too high and you can create currents in front of the scope.

- You need to monitor the amount of heat by feel really.  If you touch an unheated metal part (e.g. tripod leg) it should feel cold.  Next touch the heater band or the part of the scope that is being heated, and it should not feel as cold, but it should not feel warm or hot to the touch (otherwise you are probably overheating it).  After a while you can usually tell whether it is a night where you need a low setting or one where you need to crank it up to 11.

To determine the power output of your existing band you would need to measure the resistance (with a multi-meter) and do a bit of maths.  All the information and calculations you need to do that are here (plus links to a spreadsheet to do the maths for you):

http://www.blackwaterskies.co.uk/2013/05/making-your-own-nichrome-dew-heater.html

If you need a heater controller, you can buy a commercial one or build your own for not very much money at all (if you can work eBay and a screwdriver, then you can make the most basic version for a few quid - go to the DIY section of the forum for more help, since this is where the idea came from):

http://www.blackwaterskies.co.uk/2013/05/a-cheap-multi-channel-dew-heater.html

A few other things to bear in mind:

- The best strategy for dew heating an objective or corrector plate is not to heat the front cell of the OTA.  There is a lot of metal and it is connected to the rest of the scope, so most of the energy you put in will sink away in to the scope rather than going in to the objective/corrector.  Instead you need to radiate gentle heat on to the front of the objective/corrector.

- If you have a metal dew shield (like on many small refractors), wrap the heater band around the dew shield a few cm in front of the objective.  The thin metal will heat up fairly quickly and radiate heat on to to the lens.  You can get much more effective dew control with less drain on your battery and hopefully less waste heat creating convection currents.

- It is a bid harder with an SCT.  Most people seem to wrap the band around the front cell in conjunction with a foam dew shield, which requires a lot more heat input to be effective (and thus heats the whole OTA and can create tube currents - see below).  Large metal dew shields are usually not recommended since they quickly get cold and are thought to be no more effective than no dew shield at all after a short while - I haven't got one but I do wonder if heating the dew shield in front of the OTA would work in the same way as for a small refractor.

- You definitely need a dew shield of some sort, ideally a foam one (made from camping mat) since preventing radiative heat loss from the corrector to the sky is essential - the slower heat is lost, the less you have to replace with your heater band.  Some people recommend cutting a small hole in the dew shield at the bottom just in front of the corrector plate.  It is said that cold air will tend to 'fall' in to the dew shield to replace the slightly warmer air that is leaving due to convection, and if there is a place for it to drop out of the shield you can keep the air moving in front of the corrector.  (Moving air will reduce or prevent dew formation, since it is the layer of air in contact with the surface that cools below the dew point and then dew condenses on the surface - keep replacing the air and it should stay above the dew point).  Whether this works or whether it is just creating more moving air and distortions I could not say.

- Personally I've found that tube currents inside the SCT are a much bigger problem for planetary imaging.  You really need to let the tube cool down for a long time to minimise them, and if it is a night where the temperature keeps dropping, the tube will not reach thermal equilibrium with the environment and your images will be affected throughout.

[Sammyb]

Thanks IanL for the comprehensive response! I'll see how I get on.

Sam