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SteveG
Carpal Tunnel


Reged: 09/27/06
Posts: 2999
Loc: Seattle, WA
Make an Internal Telrad Heater new
      #3387883 - 10/13/09 07:28 PM Attachment (199 downloads)

I posted a picture of my Telrad with an internal heater that I built about 18 years ago. I was unable to find the directions I used (not my design), so today I reverse-engineered it by scraping the black paint. Please excuse my lack of electrical knowledge - the internet saved me here and I'm hoping that I got it right. Also please be warned that you make this heater at your own risk, and caution and testing are advised to avoid melt-down of your Telrad!

This is a 1.25 Watt heater that fits into the Telrad window and is powered by a 9V battery that fits in the Telrad Battery compartment. See the attached wiring diagram and pictures.

Items Needed:
(3) 22 ohm 1/2W resistors
(1) 9V battery connector
(1) small toggle switch
About 12" of wire - 18-22 Ga
Soldering Iron
Electrical Tape
Glue - Black paint
Drill w/small bits

Solder the (3) resistors together at the tips of their wire leads.
Bend them into a U-shape that fits within the Telrad window (lens side).
Remove the glass pane from the Telrad, and drill a small hole large enough to fit (2) wires through to the battery compartment.
Drill a hole and mount the toggle switch into the side of the Telrad.
Remove the foam filler in the battery compartment so that the 9V battery will fit.
Solder you wire to the resistors as a single curcuit which passes through the battery, resistors and toggle switch. I made a loop, where the wire comes up on the side of the window. Glue the glass window back in place. The resistors are glued to the window at the left side, bottom, and right side. The return wire runs accross the top, then back down the same hole into the battery compartment. Cover wire splices with black elect tape. I painted the resistors black before gluing them to the window. I added black electrical tape to the top side of the window to cover the heat wires for a cleaner look.

Radio shack source for 22 ohm ½ W resistors:
http://www.radioshack.com/product/index.jsp?productId=2062311

--------------------
WO Megrez 120 ED on GP Mount & Surveyors Tripod
WO ZenithStar 80 ED II
10" Meade LightBridge/Paracorr
Orion ED 100 on Polaris Mount
Astro-Tech 8" f6 Dob
Bushnell Ares 5
Meade 226 2.4" f11.7
SpaceScope 2.4" f15


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SteveG
Carpal Tunnel


Reged: 09/27/06
Posts: 2999
Loc: Seattle, WA
Re: Make an Internal Telrad Heater new [Re: SteveG]
      #3387888 - 10/13/09 07:29 PM Attachment (137 downloads)

Outside picture

Attachment

--------------------
WO Megrez 120 ED on GP Mount & Surveyors Tripod
WO ZenithStar 80 ED II
10" Meade LightBridge/Paracorr
Orion ED 100 on Polaris Mount
Astro-Tech 8" f6 Dob
Bushnell Ares 5
Meade 226 2.4" f11.7
SpaceScope 2.4" f15


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SteveG
Carpal Tunnel


Reged: 09/27/06
Posts: 2999
Loc: Seattle, WA
Re: Make an Internal Telrad Heater new [Re: SteveG]
      #3387895 - 10/13/09 07:31 PM Attachment (120 downloads)

Inside wiring

Attachment

--------------------
WO Megrez 120 ED on GP Mount & Surveyors Tripod
WO ZenithStar 80 ED II
10" Meade LightBridge/Paracorr
Orion ED 100 on Polaris Mount
Astro-Tech 8" f6 Dob
Bushnell Ares 5
Meade 226 2.4" f11.7
SpaceScope 2.4" f15


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SteveG
Carpal Tunnel


Reged: 09/27/06
Posts: 2999
Loc: Seattle, WA
Re: Make an Internal Telrad Heater new [Re: SteveG]
      #3387897 - 10/13/09 07:32 PM Attachment (111 downloads)

Window top side

Attachment

--------------------
WO Megrez 120 ED on GP Mount & Surveyors Tripod
WO ZenithStar 80 ED II
10" Meade LightBridge/Paracorr
Orion ED 100 on Polaris Mount
Astro-Tech 8" f6 Dob
Bushnell Ares 5
Meade 226 2.4" f11.7
SpaceScope 2.4" f15

Edited by SteveG (10/13/09 07:33 PM)


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SteveG
Carpal Tunnel


Reged: 09/27/06
Posts: 2999
Loc: Seattle, WA
Re: Make an Internal Telrad Heater new [Re: SteveG]
      #3387902 - 10/13/09 07:34 PM Attachment (111 downloads)

Window bottom side

Attachment

--------------------
WO Megrez 120 ED on GP Mount & Surveyors Tripod
WO ZenithStar 80 ED II
10" Meade LightBridge/Paracorr
Orion ED 100 on Polaris Mount
Astro-Tech 8" f6 Dob
Bushnell Ares 5
Meade 226 2.4" f11.7
SpaceScope 2.4" f15


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SteveG
Carpal Tunnel


Reged: 09/27/06
Posts: 2999
Loc: Seattle, WA
Re: Make an Internal Telrad Heater new [Re: SteveG]
      #3387906 - 10/13/09 07:34 PM Attachment (87 downloads)

Wiring Diagram

Attachment

--------------------
WO Megrez 120 ED on GP Mount & Surveyors Tripod
WO ZenithStar 80 ED II
10" Meade LightBridge/Paracorr
Orion ED 100 on Polaris Mount
Astro-Tech 8" f6 Dob
Bushnell Ares 5
Meade 226 2.4" f11.7
SpaceScope 2.4" f15


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kaaikop
scholastic sledgehammer


Reged: 07/13/08
Posts: 941
Loc: Saint-Donat, QC
Re: Make an Internal Telrad Heater new [Re: SteveG]
      #3387912 - 10/13/09 07:38 PM

Steve!

Thanks a million for taking the time to explain this to us!
This is a brilliant idea, and a great little "rainy-day" project, something almost anyone with a soldering iron can do.

Most of all, it cost close to nothing to make, I thing it should be an "available option" on future Telrads!

--------------------
Benoit, RASC Montreal

-C9.25 on EQ6 Pro - C6/ED80 on Portamount
-TV Plossls, Radians, Nags, Pans/SWAs, UO Orthos.


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GlennLeDrew
Postmaster


Reged: 06/18/08
Posts: 6562
Loc: Ottawa, Ontario, Canada
Re: Make an Internal Telrad Heater new [Re: SteveG]
      #3387923 - 10/13/09 07:43 PM

I've done the same in the past. But I further simplified things by using a single, suitably power-rated resistor epoxied into the 'nook' at the base of the reflector plate. If I recall, I settled on 1 Watt of power consumption and used a resistor rated at 1 Watt also.

Personally, I would not use a 9V 'transistor' battery because of its *very* limited capacity. At 1.25W, the current draw is about 140 milli-amps. If that battery's capacity were to be 500 milli-amp-hours, it would last for just over 3.5 hours.

Of course, I'm going on very sketchy memory regarding 9V battery current capacity, but I do know they're pitiful.

--------------------
Home-made and modified binoculars
Mallincam VSS+

My Gallery (mostly DIY stuff) **UPDATED Feb 7 2013**

Simple minds discuss people. Good minds discuss events. Great minds discuss ideas. - Hyman Rickover



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Solar Ken
professor emeritus


Reged: 02/07/06
Posts: 648
Loc: Oregon
Re: Make an Internal Telrad Heater new [Re: GlennLeDrew]
      #3388259 - 10/13/09 10:41 PM

Excellent. Thanks for the detailed explanation.

--------------------
Lunt LS60THa/B600C (solar)
Stellarvue SV102ED 4"
Meade LX90 SCT 8"
Orion XX12i Intelliscope Dob 12"
_____________________________________

There is a moment after you move your eye away when you forget where you are
because you've been living, it seems, somewhere else, in the silence of the night sky.


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propelller fan
member


Reged: 08/08/09
Posts: 33
Loc: PNW...the wet side
Re: Make an Internal Telrad Heater new [Re: Solar Ken]
      #3388351 - 10/13/09 11:35 PM

I recently made a telrad heater using 3 100 Ohm resistors, covered with heat-shrink, (at least I THINK they were 100 Ohm)for use with a 12 VDC power source. I also made an EP heater in a similar manner.

I have severe dewing conditions to deal with as most of the time the optics are completely saturated with dew after only about 10 minutes. With the dew heater on, it keeps the Telrad's glass dry all night.

If anyone decides to build this type of heater, keep in mind that you only need to heat the Telrad window slightly above ambient temperature to be effective.

To conserve battery power, use the highest resistance that will effectively prevent dew. Also don't go above the individual resistors' power ratings. Here is an Ohm's law calculator that will compute power usage. Enter the total resistance and the volage you will use. This link

--------------------
Meade 5.25" ETX-125
Meade 8" LX200
Meade 10" LX200
Meade 6" LXD55 Refractor
Orion 10" classic DOB
Custom built DEW prevention system
**5.6 magnitude backyard skies**


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Nathan F
sage


Reged: 10/10/08
Posts: 329
Re: Make an Internal Telrad Heater new [Re: propelller fan]
      #3390640 - 10/15/09 05:04 AM

Thanks so much for the info, I am definitely making this sometime this winter.

--------------------
12 inch f/5 Deep Space Observer Dob
31mm Nagler, 13mm, 6mm Ethos, 1.6x Antares 2 inch Barlow
Celestron SkyMaster 15x70 Binoculars
Homemade Maple Parallelogram Binocular Mount


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davidpitre
Post Laureate


Reged: 05/10/05
Posts: 3183
Loc: Central Texas
Re: Make an Internal Telrad Heater new [Re: Nathan F]
      #3619609 - 02/11/10 09:08 PM

A few questions from an electonics dummy.
I've been reading that dew-heaters made from resistors should be wired parallel and not in series. The above one is in series. Wiring in this case is simplified which I like. How does it act differently ?

--------------------
David



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YetAnotherHobby
super member


Reged: 09/02/09
Posts: 158
Loc: Central CT
Re: Make an Internal Telrad Heater new [Re: davidpitre]
      #3622221 - 02/13/10 08:05 AM

The key is power dissipation. In a parallel circuit of identical resistors the voltage across each resistor is equal to the supply voltage, and the current draw is divided equally across each element. In a series circuit the current is equal through each resistor, but the voltage is divided equally across each resistor.
One big benefit of the parallel configuration is that you can add more "heaters" to a circuit and only increase the current draw from the source. In a series circuit adding another element will raise the resistance of the entire circuit, which reduces the current flow so the chain will run cooler. To dissipate the same heat in the adjusted series chain, you either have to raise voltage, or change the values of the rest of the resistors in the chain to restore the original amount of current flow so that the same power is dissipated in each element.

--------------------
Too many telescopes, not enough time.


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hudson_yak
Carpal Tunnel


Reged: 11/15/07
Posts: 1548
Loc: Hyde Park, NY, USA
Re: Make an Internal Telrad Heater new [Re: YetAnotherHobby]
      #3622301 - 02/13/10 09:14 AM

Questions to anyone who's done this...does it also keep the lens clear? Or even if not perhaps lens fogging doesn't matter all that much anyway?

Mike

--------------------
Tele Vue-85 + Quarter Hitch + Berlebach UNI 24
Meade 12" LightBridge
Orion ED100 + Losmandy GM-8
Orion SVP
Meade 8" LX10


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b1gred
Enginerd


Reged: 04/01/04
Posts: 16902
Loc: Castle Rock, CO 6677' MSL
Re: Make an Internal Telrad Heater new [Re: hudson_yak]
      #3623071 - 02/13/10 06:10 PM

The lens isn't as susceptible to dew, as it's shielded by the flat plate.

--------------------
"Dark Skies & Great Viewing"

RandyR / W0RDR
GPS 9.25 XLT/Sky Align /FeatherTouch
TV85 w/FeatherTouch





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hudson_yak
Carpal Tunnel


Reged: 11/15/07
Posts: 1548
Loc: Hyde Park, NY, USA
Re: Make an Internal Telrad Heater new [Re: b1gred]
      #3623458 - 02/13/10 11:01 PM

Sorry, I guess I should have said that I was asking this question because mine does.

Mike

--------------------
Tele Vue-85 + Quarter Hitch + Berlebach UNI 24
Meade 12" LightBridge
Orion ED100 + Losmandy GM-8
Orion SVP
Meade 8" LX10


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b1gred
Enginerd


Reged: 04/01/04
Posts: 16902
Loc: Castle Rock, CO 6677' MSL
Re: Make an Internal Telrad Heater new [Re: hudson_yak]
      #3623473 - 02/13/10 11:20 PM

I would think the bit of warmer air trapped in the wedge shaped space should solve that.

--------------------
"Dark Skies & Great Viewing"

RandyR / W0RDR
GPS 9.25 XLT/Sky Align /FeatherTouch
TV85 w/FeatherTouch





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SteveG
Carpal Tunnel


Reged: 09/27/06
Posts: 2999
Loc: Seattle, WA
Re: Make an Internal Telrad Heater new [Re: b1gred]
      #3626183 - 02/15/10 01:05 PM

Yes - the lens and glass plate stay completely free of dew when the heater is on.

--------------------
WO Megrez 120 ED on GP Mount & Surveyors Tripod
WO ZenithStar 80 ED II
10" Meade LightBridge/Paracorr
Orion ED 100 on Polaris Mount
Astro-Tech 8" f6 Dob
Bushnell Ares 5
Meade 226 2.4" f11.7
SpaceScope 2.4" f15


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sealcove
member


Reged: 06/12/13
Posts: 46
Loc: Mount Desert Island, Maine
Re: Make an Internal Telrad Heater new [Re: SteveG]
      #6005326 - 08/03/13 08:23 PM

This is necro post, but I like this solution and thought I would revive this thread with a question:

Does anyone here know enough about electronics to modify this design to utilize 2 or 4 1.2 volt AA batteries rather than a 9V? Maybe that is a bad idea, but I already have a good charger and lots of 2600mAh NiMH batteries that would seem more practical (a rechargeable 9v is lucky to be 300mAh so not much capacity). I just have no idea what resisters to pair with either 2 or 4 1.2v AA's.

Any guidance would be greatly appreciated.


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n1wvet
member


Reged: 04/22/11
Posts: 42
Loc: Northern Virginia, USA, Earth
Re: Make an Internal Telrad Heater [Re: sealcove]
      #6005704 - 08/04/13 01:22 AM

This is pretty easy with Ohms law, v=IR.
From the original post above, SteveG used 9V and a total of 66 ohms,
so the current I is 9V/66R=0.136A.

That 9V battery is only going to last a few hours at best. Not good.

So for two nicads V=2.4. Current from above is 0.136A.
R=2.4V/0.136A = ~18 ohms and divide this by 3 for 6 ohms for each resistor.
The closest value to 6 ohms is actually 6.2 ohms.
Many companies like digi-key, radio shack and mouser sell these value resistors.

For four nicads, V=4.8, R=4.8V/0.136A = ~36 ohms and divide
by 3 = 12 ohms for each resistor. 12 ohms is standard value.

All Resistor dissipation is well under 1/2 watt each, so use 1/2 watt
resistors which fit well in the Telrad viewer.

I have done this conversion myself using a 12V source.

It works great. No dew.

--------------------
Thanks!
Meade 10" Classic
Meade 12" GPS UHTC


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