Dew heater advice?

[browndog]

I would like some opinions on what dew heater I should get for my C8 SCT?

I currently have a dew shield, but I am finding that I am getting dew forming. So understand that my only alternative is a dew heater.

Do I make a DIY? or do I purchase??
Which is best?

[mick pinner]

l use a Digifire 7 with Kendrick straps on my SCT and refractors and have no dew problems at all. not the cheapest system around but does a great job.

[acropolite]

I did a DIY controller (http://www.iceinspace.com.au/forum/showthread.php?t=4720), and bought a strap from Scopestuff in the US for $35.00. Total cost around $50 for the lot. Since I made the controller, I haven't had a single night where I've had to use it. :P

You can also roll your own either with resistors or Nichrome wire. This site (http://www.backyard-astro.com/equipment/accessories/dewheater/dewheater.html#Anchor-49575)has a circuit diagram and details on how to make a heater with nichrome wire and silicone tubing. Some of the other guys have used an off the shelf kit from DSE or Jaycar, you should be able to get details by doing a search of the forum.:thumbsup:

[JethroB76]

Dew Not are an option if you decide to purchase either heater straps or the controller itself, can be bought in from the US or I think SDM scopes sell them in Oz

[sheeny]

I use a commercial (i.e. purchased) dew heater strap for my C8. I can't remember what brand, but I know it isn't a Kendrick... doesn't matter much which brand... My feeling is a strap is probably better than resistors for a large aperture like the scope, but for smaller apertures like your finderscope, EPs and camera lenses a DIY project with resistors is quite feasible in the long term. My main concern is handling over a long period of time and maintaining reliability.

If you want to build your own, have a look here:

http://www.iceinspace.com.au/index.php?id=63,292,0,0,1,0

and

http://www.iceinspace.com.au/index.php?id=63,285,0,0,1,0

Al.

[Omaroo]

Ian - here are a couple I've made for my 12" and 8" SCT's and all of my refractors. I made both of these straps as well as the pulse-width modulated heater controller described here:

http://www.backyard-astro.com/equipment/accessories/dewheater/dewheater.html

The number and type of resistors is determined for each application by a fantastic spreadsheet provided by the above poster - Al Sheehan. It's worth its weight in dew. :) I usually use 390 ohm, 1/2 watt jobbies.

For the straps themselves, I used the resistor method because quite frankly nichrome wire sucks. It's hard to work with and connect in a circuit (won't solder) and breaks without warning. Being a series circuit, one break and Nichrome's gone. The difference with my resistor-based heather strap system to most is that I lay the resistors down in parallel across two lengths of flexible copper braid - or solder wick. Soldered quickly, the joints remain small and wet, without solder spreading too far along the braid so that flexibility is maintained. This strip assembly is then laid down on a strip of flexible plastic about 12mm wide and all of it is then encased in a tube of heat shrink tubing.

I also sometimes leave out the plastic strip and encase just the resistor assembly in heat shrink so that it remains very flexible - so that it can be used wrapped around eyepieces (with velcro) as well as being used in places such as my red-dot finder optics.

Making these are fun - and effective. I've never-ever had a dew problem. Power usage is minimal - and even my 56-resistor 12" SCT strap only draws about 0.75 (measured) amps when in use at full bore. Their positioning - inside the flange, rather than outside the tube, makes them far more efficient and I use a fair amount less power than some of the professionally made units. You want to heat the AIR at the corrector plate interface - not the corrector plate or telescope. This way, you effectively achieve the same thing - keeping that air's temperature a couple of degrees above ambient, without having to heat the hardware itself - which is just a big heatsink.

[Omaroo]

Ian - as per my email back to you - here's my current 2-port heater controller. I've never found that you need a multi-port controller that allows individual settings - unnecessary expense in my book.

[westsiderailway]

Another question is ....


how many do you need for a 1000mm long scope.

[Omaroo]

For me - zero - I've only ever used a dew shield and a fan on my 8 inch Newtonian. A secondary heater for a newtonian is a bit of a specialised affair.

[PCH]

Guys,

At risk of posing a bit of a daft question, - what powers these dew heaters ?:P

Cheers,

[Omaroo]

Hey there Paul

Because I run 9 individual heaters on my imaging rig - power needs are quite high. For this reason I use my 80AH (amp-hour) 12vdc deep cycle battery that I've encased in a box and added all manner of different plugs to - from a large Anderson plug to ciggy, Merit and microphone plugs for the various uses I have for it. This large battery also lets me power the G11 mount and two laptops off it all at the same time.

If your needs are less, then you can get away with a 12v "motorcycle"-style battery such as the one I've shown. These are typically rated at 17AH and will give most people a night or two of heating - depending on he efficiency of their heaters.

[PCH]

Hi Chris,

thanks for that :thumbsup: All perfectly clear now.:D

Cheers,

[westsiderailway]

So what are the two heaters that you use.?

[Omaroo]

I have two output plugs, but only ever really use one. I then use in-line 12v plugs once the main lead reaches the OTAs and there I have double and triple-adapters branching off to everything. The main reason I do this is so that the cabling following the flexible line of the equatorial head is kept to an absolute minimum to avoid its bulk upsetting the balance and natural movement of the head.

[westsiderailway]

A nice ramberling reply:rolleyes::screwy:, but did not answer the question asked.:P;)

[Omaroo]

Not sure I deserve the emoticons you've used, but oh well...

Your question was "what two heaters do I use". I stated that I use nine heaters and I was under the assumption that you were asking about the two plugs at the end of my heater controller.

Otherwise, where did you get "two" from - or are you referring to my use of the word "secondary"? I was referring to the secondary mirror in your Newtonian scope (assuming that your signature gives me that information correctly) - not a second heater. I don't use heaters on my Newtonian - just the fan in the base behind the primary mirror cell. The draft from the fan works its way up the tube and keeps the secondary from dewing up as long as you have a dew shield on and your tube is covered (i.e. not an open truss design).

[westsiderailway]

Now that was the answer i was looking for...:cool:

Is your focal length shorter because of the fan inside the tube.?
What sort of fan is it,,,ie specialy made /custom job or one of the rack.

[peter_4059]

Chris,

Thanks for sharing your heater strap design - made one for the ED80 today - works a treat. Now for the finder and Telrad! Also picked up a 3A PWM controller from RSaustralia - $30 assembled.

Peter

[browndog]

Thank you for your advice Chris :thumbsup:.
I made a dew heater strap using your design and the spread sheet created by Al Sheehan. (It was all reasonably easy - I think that I surprised myself at how simple it all was.)

I tested it last night and the C8 was dew free all night :D. Yippy

The only problem was the guide scope :mad2: damn... I didn't make a dew heater strap for the guidescope and it was covered with dew. :doh: So I have another project this week.

[Omaroo]

Excellent Peter & Ian :thumbsup:

Any photies of your new masterpieces?

[peter_4059]

Chris,

Will do...

Couple of questions re the heater strap -

Where did you get the plastic that you used to keep the strap shape from? The only plastic I could find at Bunnings was quite stiff which was ok for the ED80 but not so good for smaller diameters?

Also how many watts did you use for the red-dot finder strap? I want to make a heater for my Telrad.

[Omaroo]

Hi Peter

I've also used plastic cut from many things - such as the funky coloured plastic covers that come on Marbig ring binders or "Pocket Display Books" from the stationary store. Anything slightly stiff will do - even plastic Coke bottles cut into strips. For the red dot heaters I use no additional plastic at all - just the resistor/braid assembly encased in heat shrink tubing on its own so that they remain really flexible. I used a cable tie to keep it on.

For my red dot heaters I used eight 390-ohm 1/2w resistors spaced 1 cm apart. This gives you:

390/8=48.75 ohms over the parallel which gives you about 3 watts at .25 Amps at full bore. Plenty - which is why you have a controller to turn it down.

[browndog]

It is not exactly a masterpiece - but it works!
Here is the completed dew strap heater for my 8" SCT.

Hi Peter - in regard to your question about plastic sheets. After a suggestion from Chris, I picked up a 1m square sheet PVC @ 1mm thick from a local plastic shop. It cost me $10. I just cut the strips as I need them.

Hi Chris - another question??? How do you connect multiple dew straps to your power supply? You indicated earlier that you only use one output plug - so how do you connect multiple dew straps? Am missing something really simple here? (I am also planning on making a PWM next week. I understand I can get a simple kit from Jaycar.)

[Omaroo]

Great effort Ian - looks good!! :thumbsup:

I use "double adapters" made from 2-pin Molex plugs from Jaycar. I always use a 2-pin female at the end of a cable. Into that I plug a male that has two sets of wires coming out in parallel to two new female plugs...so I essentially keep splitting the cable as I go to each device that needs it.

Well done on your build!

[peter_4059]

Here's mine - the controller and 2 straps - one for the 10x50 finder and one for the ED80.