Dew Shields/heaters - where to start?

[BiggarDigger]

I'm very much aware that I'm new to this and the more I read, the more I know I need to think about about practicalities, not just theory.

Last week, I had a really successful evening with my shiny new AstroMaster 130-EQM.  The sky was crystal clear, the air temperature was well below freezing and frost was everywhere.  After a couple of hours dew (and ice) became a real problem.   I've read a lot about DIY and manufactured heaters and shields, most of which should be within my abilities but I'm not 100% sure where the best place would be to start in combating the problem.

Should I knock up a shield extending the length of the tube, or is a heater a better method to protect the secondary optics?   If a heater is considered better, how is that fixed to the optics?  I've seen some comments about protecting the primary from dewing up, but is that generally considered a problem in a modest reflector like mine - doesn't the tube form a dew shield of it's own?  If it is considered necessary to protect the primary, how difficult is it to gain access - I'm a bit cautious about damage due to "novice fingers".

I invested in a Terlad to replace the Red Dot finder and it works far better for me, but towards the end of the evening, even with a diy shield it was misting up to the extent it was difficult to see through, so perhaps a heater would be good for that too?

How about eyepieces?  I noticed that my eyepieces were really heavily dewed at the end of the evening, even after keeping them in my pockets or the case.  The focus tube was dripping.  Again, I've read a few posts for both diy and manufactured heaters, but I'm not too sure exactly how they fix to the eyepieces.  Does the heater wrap around the barrel of the focus tube or somehow to the eyepiece itself?

I've even read about hair dryers, but I don't fancy tripping the house breakers.  I can build a DIY PWM power pack if needed, but maybe I should just give everything a blast of heat from resistors or nichrome on DC at first and work on the fancy stuff later?

Perhaps I've answered my own question, but I'm genuinely interested in your thoughts on the what would be "best bang for my buck" for an enthusiastic newbie like me.

 

[Marci]

You did, yes! Nichrome, DC, Velcro, PWM Controller... done. Doesn't get much fancier (unless you fancy automating the Pwm via 1-wire Ds18b20's + arduino/rasp pi via dew point math). Off you go... Crack on!

[Daz69]

Hi fella. I have the same scope, and same problems!

I'me still waiting on a few components to build my own dew bands and a power distribution box running from a 12v SLA deep cycle battery, using 2x PWM units each running 2 channels, so I'll be able to have 4 dew heaters if needs be, I plan on one for the secondary, one for the EP, and one for my Rigel finder. The remaining one will be to heat my EP case.

I will also have a power out (either 9v or 12v) to power a rear 12v DC PC fan, to softly blow a breeze up the tube, or pull it down. Will have to see which way is best. Some say up, some say down, who knows? 

My dew bands will be nichrome wire, held captive between 2 pieces of gaffer tape, and covered in a micro fleece "sock" which will have velcro for attaching. How to get this to fit on the secondary is yet to be worked out. These bands will take a bit of trial and error, but there should be a reference table somewhere on the www showing gauge and length to give ohms. 

[kendg]

Here is a pretty good site for dew it yourselfers.

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

Hope it helps

[cjdawson]

On a reflector, the dew shield will protect the secondary rather than the primary.

as to shield or heater...  do both.   The shield will slow the effects of due, the heater will stop it all together.

used in combination, the heater will require less power to be effective.

[laudropb]

I use an astrozap dew shield on my Dob and this lets me have dew free sessions for at least 4 hours without heating bands. I seldom view for longer than this anyway. I have a little hood for my Rigel finder and keep it covered when not in use. I also use a little battery operated travel hair drier on the finders if they do mist up. For the eyepieces I have hand warmers that you heat in a microwave. They are useful to keep in a jacket pocket. Keep the eps and me warm. You can also put them in or under the case or box that you keep your eps in.

This is a personal choice. I like to keep my set up simple. I use manual mounts to avoid trailing wires and power sources, but I know many others do use heating strips very successfully and if you have the DIY skills then go for it.

[BiggarDigger]

Thanks guys!

I think I'll put together a DIY shield for the secondary mirror using some camping mat material to extend the tube.  I read that it should be between 2 and 3 times the aperture, so that should be quite easy.  I'm not certain about a heater for the secondary just yet - I probably need to have a think how to mount so that it effectively delivers the heat to the mirror and not the mount, but also not to deliver too much heat in the tube.

I saw a few ideas for a Telrad dew heater using a resistor and powered from the internal batteries; that looks pretty straightforward and should make quite a difference.

Along with that, I think I'll be getting some nichrome wire and velcro to build an eyepiece heater.  I do like the idea of hand warmers too!

That should keep me going for now, fingers crossed!

[Daz69]

The boss for the secondary  mirror is the same diameter as the mirror itself, so you don't want a thick heater on there as it will likely interfere with your view. I will look to see if it's possible to route the wire in etched grooves around the boss itself, and then covering in black electrical tape and then covered with flock material. That way it shouldn't be any more wider than say 1 or 2mm than original. To wrap a thick dew band might be too much.

I would imagine the heat needed would be minimal as you only want to warm it, so shouldn't cause any melting of the plastic, however I will test this and see just how hot the wire gets, but another option is to encase the nichrome inside very narrow heat shrink tubing as a complete length, so that will shrink down nicely. 

A few more bits arrived yesterday, but I'm still waiting on my controllers. Once I have everything I'll start the build.

[BiggarDigger]

The bits for the dew shield and the Telrad heater should arrive this week.  I need a little more little more thought about the power source and controller for the eyepiece heater, but that shouldn't be too far behind.

Looks like the weather here will be closed in the for the next week, so I've got plenty of time to get stuff built.

[Daz69]

12 hours ago, BiggarDigger said:

The bits for the dew shield and the Telrad heater should arrive this week.  I need a little more little more thought about the power source and controller for the eyepiece heater, but that shouldn't be too far behind.

Looks like the weather here will be closed in the for the next week, so I've got plenty of time to get stuff built.

Hi buddy. Can you show me a pic of the Telrad heater please? I have the Rigel (same principle but upright) and the lens dews up quickly, and I'm forever wiping it. 

many thanks

[BiggarDigger]

Hi Daz, it's one I found in the DIY Astronomy section on this forum, described by M110 several years ago: 

It's a simple 10 Ohm 2 Watt resistor, plus a diffused red LED and a small SPST switch to take power from the batteries of the Telrad.  I've got a stack of Lithium AA cells in the garage so they should be able to supply plenty of current drain.  The LED indicates when it's powered preventing undue drain.  What appeals to me about this one is it's fully self contained running off the internal battery.

Richard

[Daz69]

Thanks Richard, that will be going on my tinker list for sure. I'm swapping out my original thick plastic vane and secondary hub for a curved vane assembly, and will be installing a dew heater on that, so I'm thinking to have that AND the Rigel heater connected into a small plastic project box attached to the OTA, and then pull out leads to my power pack/dew controller box. 

Some might think I'm wasting time, effort and money on such a beginner scope, but I can't afford my next step-up yet, and what I'm building can always be transferred to my next scope. Oh, and it gives me an excuse to not watch CBB and all that other tosh Mrs Daz watches in the evenings!

[cjdawson]

@Daz69 This doesn't sound like a waste of time and effort to me.  Allow me to for a start you'll get to understand the electronics of the heater devices. As you already mentioned you'll be able to transfer some if not all of it to a new scope. You'll have more knowledge about how to make a working heater setup.  Also, you'll quickly understand and appreciate that having separate controls for each heater element is worthwhile.  And finally, over time, there will be others asking about heaters, at that time, you'll be able to share your knowledge and save someone from having to do it the hard way. Just like you are saved from doing it the hard way because of this thread.

[Daz69]

On 30/01/2017 at 18:24, BiggarDigger said:

It's a simple 10 Ohm 2 Watt resistor, plus a diffused red LED and a small SPST switch to take power from the batteries of the Telrad.  I've got a stack of Lithium AA cells in the garage so they should be able to supply plenty of current drain.  The LED indicates when it's powered preventing undue drain.  What appeals to me about this one is it's fully self contained running off the internal battery.

Richard

Hi Richard.

Well I finally got around to making my heater for the Rigel. Unfortunately the Rigel case is glued shut at manufacture so I had to use the dremel wheel to cut open the back. I actually fitted 2x 10ohm 2 watt resistors in parallel to give 5 ohms at 1.152 watts. Might be sailing a little close to the wind for the resistors as I've always been taught that a resistor shouldn't go above 20-25% of it's capability, but it gets lovely and warm within 10 minutes. I've not tested it outdoors yet, but I'm hoping that short time usage will keep the lens from fogging up. I fitted a toggle switch, but didn't have any 3v LEDs to install, only 12v.

Thanks for the heads up on this one!

[BiggarDigger]

No problem Daz, as noted it wasn't my work; a lucky find from a few years ago.

I built my Telrad heater as per the guide mentioned earlier and it works a treat.  Enough to get fairly warm to the finger and it clears the mist on the Telrad within minutes.  For my heater I used a 3v LED with a 100Ohm resister as an indicator, but it's still a bit too bright, so I might increase that to 180Ohms.  The glass viewing window on the Telrad slides out (after a bit of judicious pulling) making insertion of the heater resistor pretty straightforward.  The black insulating tape is just there to give a little protection t the soldered joints.  Hopefully tho photos show the arrangement reasonably well.

I went out on Sunday evening and like several recent sessions everything pretty much was covered in dew within an hour and an hour later heavy frost was forming.  The camping mat dew shield for the secondary mirror worked very well, but unbalanced the scope a bit so that needed some extra thought.  The main problem I had on Sunday was the moon: it was so bright that it was very hard to locate any deep space objects.  I tried a few Messier objects to the north, but it was getting late and cold on a work night, and that story is for another time.

Well worth the cold and frustration with the moon to see the heater and shield working well and learning new tricks all the time.  Just need to find a solution for the eye piece misting up.