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Eric Mjolnir

Controling Telrad Dew

13 posts in this topic

The other night I was using my 20" Obsession and decided to do some star hopping without using the computer. I went to use my Telrad and it was a dewed up mess. So time to do some work on it. I have tried the dew shields and other tricks to keep this thing from dewing and they have worked to some extent.
My take on an idea came as I was going to work the other day. Early morning and the windshield on the truck was all dewed up, so kick on the defroster! Why not do the same for my Telrad!
Here is the Telrad from my Obsession.



It already has some modifications that I have applied in the past. The small switch on the side is for a blinker circuit that I installed. It was a small unit that I got at Radio Shack. The aluminum plate was made to slip over the plastic battery holder to keep the plastic box from splitting on the ends. This happens over time and can cause the Telrad to quit working due to a bad connection. The plastic battery holder is Velcroed to the bottom of the Telrad. The foam rubber packing seemed "Cheesy" to me.



I opened up the Telrad to see what I had for real estate, and just where I might mount the forced air heater I had in mind. Not much room in there, so my heater must be compact and not protrude into the optical path.



Checking the mirror intrusion into the area I had in mind for the heater.



I had a small 1.5" fan from a Peltier cooler that I could use to force air into the Telrad. It was going to have to be mounted on the outside and force air over a heating element without intruding into the Telrad much at all.

The small fan 40mm X 40mm X 10.5mm



Tested the fan and did not want any vibration, or noise. I found that the fan operated very quietly and vibration free at as little as 5 VDC. I picked through some of my stuff and found an LM7805 voltage regulator that I could use to provide voltage to the fan from a 12 volt source.



For the heater elements I had some 150 ohm resistors that could be series-paralleled to give me about 2.5 watts of heat. I would also be using the heat produced by the LM7805 and the fan motor itself.



I wanted to mount everything on the fan to keep it compact and as non-intrusive into the Telrad as possible. I made a copper wire buss ring and mounted the LM7805 to it and then glued the whole assembly to the back of the fan. This buss would carry the negative or ground voltage from the 12 volt source.



Now I added the 12 volt positive buss that would power the resistive heaters and provide the 5 VDC from the LM7805.




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Now it was time to build the heaters. I used 3 sets of series resistors. These were 1/2 watt resistor and each would be dissipating about a third of a watt of heat. I would bend them so that they would catch the negative and positive buses.

First set of elements.



Second set.



Third and completed fan and heaters.



Testing fan and heaters with a power supply. PS outputs 13.5 VDC and current reading indicates that the whole shebang is producing about 2.43 watts at that voltage. Certainly plenty of watts to keep the Telrad dew free.





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Well now for the fun part. Punching a big hole in a perfectly good Telrad! Hope this all works out. If it doesn't, I have 3 more of the dang things to experiment on. So I drilled the hole after finding a suitable place on the outside of the Telrad to mount it. I set it up on my milling machine and used a 1.5" hole saw to make the fan hole.





The hole is now permanent. I will clean it up with some emery cloth and machine the slot at the base of the clear glass window, for the defroster.



Emery clothed the fan hole to clean up the edges.



Set up the Telrad on the milling machine to cut 3/16" slot for "defroster".



Made several passes to take out material gradually without melting the plastic from bit friction.



Finished "defroster" slot required very little clean up.






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Mounted and ready for drilling.



4 holes drilled.



After tapping the holes for the 4-40 stainless steel screws I mounted the heater fan.



Checked it for intrusion into the reticle's light path. It is almost flush with the thickness of the plastic side of the Telrad. Excellent!



Installed switch.



Good clearance from diagonal and no intrusion into optical path.



Reticle image shows up nicely.



Finished unit ready for testing. Looks almost professional!





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I set up 3 Telrads. 2 with no dew control and my finished unit. The heated and forced air unit is on the right.


3 Telrads for testing.



Modified unit with "Dedewer" vent.



Telrad #2 and #3 with no vents or any dew control at all.




Got up at 4:15am to check results from over night test. Temp 69° and feels like 69°. Humidity at 86% but dew point had dropped to 65°. It never really produced heavy dewing like I expected. Will have to run test again. It did have some minor results though. My modded Telrad had remained clear. Telrad #2 looks usable still, and Telrad #3 was just beginning to show signs of dew formation. Maybe #2's proximity to the modded unit helped it stay clearer?



Re-checked the Telrads at 6AM. Temp 67° and feels like 67°. Humidity up to 93% and dew point at 64°. More dew had formed since the check at 4:15. Telrad #2 had begun to dew over. The battery cases were definitely dewed up with the exception of the modded unit being dew free in the area in front of the 45° optical window. So far I would call the test a success. I am going to test it quite a bit more and do some actual observing test with it and carry the other telrads with for comparisons.


For those that wish to DIY, a milling machine is not required. I would recommend a small drill press and a vise to drill the 1.5" hole for the fan though. Once the optical 45° glass window is removed you can hand drill several 3/16" holes across the front where I put the slot trying not to break into the previously drilled hole. Be very careful when drilling near the on/off potentiometer (pot). The plastic when drilled tends to want to pull the bit in when it grabs the last bit of the hole. This grab could damage the pot, or cut wires. You could remove the nut that holds the pot and push it inside of the Telrad until drilling is finished. 5 to 6 holes should fit, and give sufficient air flow. Start by drilling holes at the outside edges, then 1 in the center, and then see how many more will fit in between. The more the better. I did my wiring internal to hook up a switch. You could fore go the switch and wire to the fan from the outside using the old wire holder that was not used when the fan wires were diverted to the LM7805.

Please feel free to PM me if you have questions.






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Very impressed mate. And brilliant write up. Great mod idea. However not impressed you own a 20 inch dob. :angry:

I'll share this onto my timeline. The thread is worth sharing.  


Eric Mjolnir and poppasmurf like this

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On 7/27/2016 at 11:47 AM, Radar said:

Very impressed mate. And brilliant write up. Great mod idea. However not impressed you own a 20 inch dob. :angry:

I'll share this onto my timeline. The thread is worth sharing.  


Thanks all!

My last and finishing touch came in from Mouser Electronics.....the small fan guard. I have ordered enough parts to do all four of my telrads.

Looks pretty nice in my opinion and works great!





poppasmurf likes this

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Been a while since July when I was fighting summer dew with my Telrad. It's now dead of winter and the Telrad mod is performing excellent. This was one frosty night so I set the test up again to document it.

Start of experiment and the three Telrads were completely frost free as they had just gone out from inside. I waited over night and checked them the next morning. Temp was 17°F, humidity was 91% and dew point was 15°F. Modded Telrad optics completely frost free!






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Great result from the mod. It's certainly cold in your location. Its 1.30am here at the moment and its 18 degrees Celcius (64.5 F). Are you too far south to see the Auroras?


Eric Mjolnir likes this

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Hi Ray. I have seen auroras many times from my location. I have even seen them further south at the Nebraska Star Party during July. Some shots from town here from 2004 when there was a MCE.



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