Newtonian dew heaters

[mccann73]

Hi all


I recently purchased a Bintel BT200 f/5 Newtonian scope with an Orion Mini Guidescope and Autoguider, with the cooler months coming I guess I need to start looking at dew heaters, from the controller side I am fine, I can build/mod one for myself, however I am not so certain what I need to protect on the scope, I have read a dew heater on the Newtonian primary is not usually necessary as long as fan is running, but I should have one on the secondary, and heater straps around the eyepiece/camera and ccd guider, sound correct?

Can anyone suggest what type secondary dew heater I would get for this telescope? For the dew straps does anyone have a preference to brand, I see bintel has Kendrick (standard and premier), google turned up Dew-Not Dew which seem to be cheaper or you can make your own, which I am not really interested in doing at the moment. :)


Thanks
Andrew

[barx1963]

Andrew
I have almost never had an issue with dew on a primary. Secondaries even in a solid tube can still be an issue and eyepieces.
I use the Kendrick Digifire 7 and it works really well, the ability to select 10% power works perfectly as you only want a tiny amount of heat in a secondary as to much sets up serious air currents and degrades the image. On the secondary on my old 12" I used this http://www.bintel.com.au/Accessories/Dew-Control/Newtonian-Heaters/Kendrick-Secondary-Heater--2--Stalk-/1059/productview.aspx just held in place with a rubber band and the wires running down the spider vane.

Malcolm

[mccann73]

Thanks Malcolm!

Will get the secondary heater from bintel, and grab a couple of straps from dew-not!

Cheers

[mental4astro]

The biggest thermal control issue there is with a solid tube is they are a giant heat sink. They will suck the heat out of something very quickly. You need to think a little more than just using straps. You need to think of the straps as maintaining heat, not adding heat

Big, cold eyepieces bunged straight into the focuser have no chance of resisting being dewed over if all you use is a dew strap. The mass of glass just will not be warmed by the strap, and any heat placed onto the casing of the EP will go straight out through the focuser. If the eyepiece is warm from the start, the strap isn't labouring to heat up a cold lump of glass - it will be keeping a level of heat within the whole eyepiece. There is a rate of heat exchange happening between the focuser and eyepiece. If this rate is greater than what the cold eyepiece can absorb from the dew strap, you are wasting your time. Heating is just like pushing a trolley - there is an initial mass that needs to be shifted, but once it is moving, keeping it going requires very little effort.

Secondary mirrors can be more problematic. First, the spider vanes are painted matt black - black painted metal drops in temperature faster and slightly lower than other colours - no, I'm not crazy. On my open truss scopes I've seen frost on the spider vanes while the rest of the scope is free from even dew!!! Then, if the actual housing of the secondary is metal, you are pushing the proverbial up hill. The heat here needs to bein the secondary from the start. Connecting the heat when the dew appears is too late.

BUT, here's the kicker - put too much heat into the secondary and you introduce heat haze into the light path. Not enough, depending on the assembly of the holder/spider, and you lose too. This is where multi channel controlers come into their own. You can regulate each strap individually.

Have fun!

Mental.

[Akwestland]

Andrew,

I am watching this thread with great interest as I am in virtually the same situation. I know nothing about dew heaters, it is all part of the learning.

In response to Alexs' post, and I may be right off track, is there benefit in pre-heating/warming your EP so as to alleviate the difference which will then allow for quicker stabilization. Has this been tried?

Fans on Newtonians, is this considered a mandatory item that if not fitted is something that is worth fitting? If this is the case what is the suggested method?

Cheers,
Andrew.

[barx1963]

Andrew
With fans, their main purpose is to assist with cooling the mirror prior to use. I never normally run mine while in use simply because I never bothered setting up a system to power it while moving the scope around. If dewing does turn into an issue with your primary then a fan can possibly help.
Some people have made up heated eyepiece boxes, I made one up but in the end went with a rack that held all my eyepieces with heater straps on them and then had them prewarmed prior to placing in in the focuser.

Malcolm

[Akwestland]

Malcolm,

Thank you for that reply. With our limited experience we have not come across the dew (due..lol) problem. At the moment I have an eyepiece box, I do have a rack on the tripod but I am always concerned about dropping an eyepiece, which is why I prefer the box on a table at this stage.

As for dew controllers, I am tempted to have a crack at making a dew controller as opposed to buying one. Some plans I have seen are very easy and cheap. I used to dabble in electronics years ago, so I know how to solder and build small circuits. The straps, I may buy, I am not sure yet as there are some simple plans for those as well.

As for the fan, that I feel would be the last one to attempt if needed.

Cheers,
Andrew.

[erick]

Andrew, yes make up the controllers but I suggest you spend some money on good robust heater straps. Many people prefer Dew-Not straps:

http://www.cloudynights.com/page/articles/cat/user-reviews/accessories/dew-heaters-and-controllers/dew-not-arent-your-typical-heater-straps-r22

http://www.dew-not.com/dew-not-desc-01.htm

They turn up occasionally for sale here but you have to be ready to pounce since they are popular :D

For the secondary, you really should do it properly with one of these:

https://www.astrosystems.biz/dewgrd.htm

But that takes a bit of work and depending on how your secondary mirror is mounted, might be a challenge to install. But they are superb at keeping dew off the secondary.

By the way, here is my experience of combatting dew:
http://www.iceinspace.com.au/forum/showthread.php?t=35705 (http://www.iceinspace.com.au/forum/showthread.php?t=35705)
http://www.iceinspace.com.au/forum/showthread.php?t=33207 (http://www.iceinspace.com.au/forum/showthread.php?t=33207)
http://www.iceinspace.com.au/forum/showthread.php?t=33500 (http://www.iceinspace.com.au/forum/showthread.php?t=33500)
http://www.iceinspace.com.au/forum/showthread.php?t=31846

[Akwestland]

Eric,

Thank you for the reply. Those dew heaters are a lot cheaper, am just starting to look at these so appreciate the links. I wasn't sure on making the straps but given this price it may have answered the question. Maybe a sill question, would you run more than one on a 200mm f5 Newtonian?

Cheers,
Andrew.

[mccann73]

Hi All

Thanks for all you info, there is a lot to think about!! I am heading up to Snake Valley in a few weeks so i will be looking at everyone’s setup, as Andrew and myself have seen, there is a lot of info to digest and a lot comes down to personal experience with their setup!

One thing I had never thought about, that Alex mentioned, was a way of keeping eyepieces warm before use, which now makes a lot of sense, will need to add that to my list of ever growing accessories.

I grabbed a secondary heater from bintel as Malcolm mentioned, Eric, I like the idea of the direct secondary mirror heating but I am not confident enough to install one, at least not yet! The Dew-Not straps I have ordered, a very good price compared to the others. As for controllers I will be using a couple of dumb pwm controllers for the time being, eventually I would like to buy/build one based on a microcontroller which can calculate dew points and adjust temps automatically..

Cheers
Andrew

[erick]

Hi Andrew

I would have a minimum of three heater straps. One on the objective of the finderscope, one that fits 2" eyepieces and one that fits 1.25" eyepieces. Beyond that, some heating on the secondary mirror and you are set up for the worst dew.

Additionally, if you have a fan or two blowing directly across the mirrored surface of your primary, that should keep that clear of dew as well. But, for the first several hours of the night, primary mirrors are usually a degree or two, if not more, above ambient. Not great for the optical path but it does keep the Primary free of dew.

All you need is enough heat to get the glass surface (eyepiece, finderscope, secondary mirror) a degree above ambient. I know the settings now on my Thousand Oaks controller, but with simple home built controllers I would adjust until the outside of the strap was just slightly warm on my lips (sophisticated measurement technology here!).

On my SDM setup I have the following to manage dew:

1. Two fans scrubbing the boundary layer on the surface of the Primary.
2. A built in Astrosystems heater on the Secondary
3. A Dew-not strap on the objective of the finderscope
4. Two Dew-not straps for the eyepiece, one that fits the 2" and one that fits the 1.25"
5. A home made resistor-chain "heater" on a filter slide which holds three filters and mounts inside the upper tube assembly
6. A long Dew-not filter in the bottom of a box in which I keep my eyepieces when not in use - keeping them a few degrees warmer than ambient.

3., 4. and 5. are controlled using the four separate channels of a Thousand Oaks controller. On other scopes, I have run all three (finderscope and two eyepieces) off the output of a simple one channel controller - works fine.

[ZeroID]

Fans are the easiest and work well from behind the primary. Mine is a 100mm muffin mounted in a fishing nylon spider to isolate any vibration and has a 5v\12v switch for slow\fast. Power supply is a modified PC unit. Suck or blow is up to your preference, you'll get conflicting advice on both. :P

P.S. The fan was rescued from an old PC at work as well.