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The fight against Telrad dew


Moonshane
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Telrads are great and I use them exclusively as the RDF instrument on my telescopes. Their Achilles heel though is that even with a few shield on them, the glass screen does tend to dew on even vaguely damp nights. I started to think about how I could hold off dew without resorting to electrics - I like it simple.

I have previously owned a Rigel Quikfinder and even without a dew shield the perspex screen stayed dew free for much longer than the Telrad screen. Therefore I ordered a small (150mmx150mmx3mm thick) sheet of perspex on ebay (cost about £2) and cut this to size. I made three so that I have two for my two Telrads and a spare for my pocket. That way if they ever dewed, I could swap them over in turn and retain one in my pocket.

The first trial was not a particularly damp night but it did stay dew free all night and the view was exactly the same as with glass.

I'll report back when I have a scope dripping in dew and confirm if it improves matters at all. If possible, I'll use one with perspex and one alongside with glass to gauge the difference. Thought I'd share this for now though.

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I don't wish to hijack this thread, but may I share my way of getting round the dripping wet Telrad.

First I power it off my dew heater supply, then a few resistors under the window work up to a point.

post-6754-0-24679800-1368028283_thumb.jp

Then to really kill any tendency to dew, a transparent dew shield works wonders. It was the plastic packaging from a pack of Christmas candles, and just happened to fit. I'd tried the commercially made solid black dew shield, but it just got in the way

post-6754-0-66092500-1368028539_thumb.jp.

Huw

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Hi Huw - all great suggestions. I could of course buy a dew heater for the Telrad (or make one) but I really cannot be bothered with wires on my dob. would it be possible to power the resistors from the 2x1.5v AAs in the telrad? I presume you'd need some sort of voltage control though? (more wires).

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A neat idea Shane - I will look forward to seeing your comparison results.

Until I looked at my Telrad again I hadn't realised that the glass is just a push fit!!

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I use a sheet of 'fab foam'. It cost 50p from hobbycraft. Initially held in place by carefully trimmed Velcr, but since the Velcro has lost its stickiness, I use an elastic band, which I should have just used in the first instance. I never see dew on my Telrad since I did this.

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The dew shield probably has more effect than the heater, my resistors are 100 ohm total, but probably overkill with the shield, try 200 ohm for starters, should give you up to 100 hours or so off a pair of AA batteries. Switching between one battery and two would give you two power settings with very simple circuitry. Or switching resistors in parallel/series would do the same thing, but even out the battery consumption.

Huw

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I make my own red dot circle sights and the best way I have found to stop dew is to place a 3volt bulb wrapped in foil within the mirror carrier , turn on with separate switch clears in 30seconds the residual heat rising also clears the glass and lens

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No probs moonshine I have one in bits at the moment will photo over the weekend and post, the view through it is similar to telrad ie, 4 degree 2 degree and in my case a dot. I also use dotted not solid circles as you can see more stars. They have a dimmer. They also tend to be larger than the telrad as I like a big screen. The cost to make depends on if you have old lens lying around I use 50mm bino optics.

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I just dab mine with a tissue, there's no optical glass in a Telrad, so no probs. I've been doing this since I bought it (4years ago). It has had no detrimental effects on mine, It still works as good as the day I bought it. Incidentally it's still on the same batteries too :)

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I just wipe the glass with my finger when it dews up, though I am sure tissue would be better. I have also wondered if anti-mist coating would work.

Although I have always used a Telrad on my Newtonians, it would not fit on my Equinox. My wife kindly bought me one of these red dot finders http://www.rothervalleyoptics.co.uk/skywatcher-red-dot-finder_d3149.html (other similar models are available at lower cost), which is much less prone to dew. I miss the ring targets of the Telrad, but not a lot.

On the 70mm I simply squint down the tube, but I do not find this possible with tube rings.

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