Dew prevention for secondary mirror

[MartinB]

I haven't had a problem yet but I just know I will, at a critical moment. Does anyone have any opinions on the best way to prevent dew on a newtonian secondary. Does anyone have any experience of Kendrick's system or suggestion for something better?

[russ]

Hi Martin, I found a dew shield (homemade from camp bed roll) was all i needed in the end for the 12" newt. And if the night was really bad the primary would go too and i'd give up.

You could always get a 12v hairdryer for those rare moments it does go?

Regards

Russ

[Nostromo]

Hi there you can get several different types of Kendrick heater from your local dealer.

There are secondary heaters of various sizes so if you can measure your secondary I can tell you which model you need - I'm the distributor. Nick

[acey]

In the past I used to cup my bare hand round the mirror for a minute or two - the mirror warms and begins to clear slowly (continuing after you remove your hand). The disadvantage is that you waste time and get a cold hand. And a marked mirror if you're not careful.

I tried a camping-mat dew-shield but found it a faff in various ways: hard to keep from blocking the light path because of a tendency to droop, also it made it harder for me to the aim the scope after years of sighting it without the shield.

I got a 12V hair-dryer and jump-start power unit (Maplin's have been selling them for £20) and have only had a chance to use it once so far, but it seems to work, and is currently my favoured option. And of course it works on eyepieces too, which in my experience suffer dewing more frequently than the secondary. Though the hand-warming method works on them too.

Andrew

[russ]

The good thing about the hairdryer is it blows nicely through the 2" focuser and clears the secondary in no time. On the downside it canes the powertank.

Another downside to the dew shield/camping mat is the extra weight on the front of the tube which can upset the balance on the dob.

[MartinB]

Thanks for the suggestions. I'm thinking of something for my 8" newt which I'm using for imaging. The problem here is that I can be away from the scope for long periods. I can certainly give the camping mat a go. A hair dryer is one of my favourite dew tools but I'm not always there to see the dew forming.

Nick, does the secondary dew prevention system need a Kendrick controller as well? Also, do Kendrick or anyone else do a narrower/lower profile strap - say 1 cm approx that I could use on the nose piece of my QHY8?

[ChrisEdu]

Any recommendations for where to get a 12v hairdryer?

[russ]

Got mine from Amazon for £7

[Nostromo]

Nick, does the secondary dew prevention system need a Kendrick controller as well? Also, do Kendrick or anyone else do a narrower/lower profile strap - say 1 cm approx that I could use on the nose piece of my QHY8?

Hi there yes all Kendrick dew heaters need a controller to run them... But then we would say that! The narrowest heater is the 1.25" Premier heater which is a bit more than 1cm wide. (not the blue Firefly as it is too thick). There's also the 2" Premier version, which is just as narrow if it's a 2" nosepiece.

Regarding your secondary, what size is your secondary and is it glued on, or does it have silcon blobs or a metal shroud?

Best regards,

Nick

PS: I wouldn't recommend a hairdryer guys - it blows dust onto a moist sticky surface and the dust sticks much better this way. Hence the profusion of dew control systems out in the marketplace (I have a vested interest )

[Doc]

If you are viewing from outside your house then a normal hairdryer on extention lead is the way to go. As said above bllow it through your focuser and secondry is defrosted really quick.

If you use 12v then I personally find 12v hairdryers a bit under powered.

[beamish]

:nono: Just be aware of the possibility of blowing crud from the elements of the dryer onto a damp 2ndry mirror !!

[Doc]

:nono: Just be aware of the possibility of blowing crud from the elements of the dryer onto a damp 2ndry mirror !!

Good point.

Not happened yet but you never know.

[MartinB]

I've always used dew straps but sometimes, in extreme conditions such as the Friday night at Kelling this year, they just aren't enough on a 10" SCT. My astro kit is too precious NOT to use when the skies are clear so a hairdryer it is, specks or not!

[Nostromo]

Hi Martin, you may find a higher wattage heater is more suited for extreme conditions.

If you use a low wattage heater, then it's fine for average/normal conditions

Yes Kelling Heath was seriously dewy. We did a "dew dance" before the event.

Best regards Nick

[MartinB]

Yes Kelling Heath was seriously dewy. We did a "dew dance" before the event.

If your clear sky dances work as well you can earn more money than you ever could selling dew prevention gear. If not the lynch mob will be round later!

[MikeD]

I was thinking about trying a DIY solution , resistors and thermal glue,

See here http://astro.neutral.org/eq/dew.html

mike.

[MartinB]

Thanks Mike, only trouble is any DIY I attempt turns into a disaster. The wife runs for cover when she sees me with a screw driver.

[Nostromo]

If your clear sky dances work as well you can earn more money than you ever could selling dew prevention gear. If not the lynch mob will be round later!

Never fear mate dew and clear skies go hand in hand. Fortunately it's been such a while since the big bang that the background temp of the sky is nice 'n low. Perfect for dew to form when you expose anything to it. I couldn't have arranged it better meself.

[themos]

Any ideas how many watts would I need for the secondary of the 12" Skyliner? Some people on the net suggest 100mW is enough for prevention. Most say no more than 1W. I'm aiming for a design that does not require a PWM controller but can be connected to the "12V" DC. What's the consensus here?

[barkis]

If the secondary mirror is in a cell, would it not be possible to enclose a small 12v 3w auto bulb inside. It could be painted black, or ensure the cell is light sealed. Various wattage sizes could be experimented with, to find the most suitable.. A twin cable could easily be routed to an external supply.

Ron.

[themos]

I'm thinking of having a 680? resistor in parallel with a reed-switched 220?. That would provide about 0.25W when the switch is open and about 1W when a small magnet is put close to the reed switch. The latter would be for "de-misting" rather than prevention and would be done when the tube is horizontal for safety. Or maybe I should use a tilt switch...