Dew and dew heaters

[redstamp]

Total newbie question coming I'm afraid, but at least it'll be quick to answer:

A few nights ago I had the scope out and doing a spot of viewing and I noticed when returning back indoors, the outside of the scope was soaking wet with dew.  Firstly I hadn't noticed the problem whilst viewing, although maybe that's 'cos I'm a newb!  Secondly I wondered if it was damaging, particularly once back inside and wondering if I should leave the telescope end-cap off to let the insides dry out but also worried the longer the end-cap remains off the more dust and crud will settle on my beautiful mirror - how the conflict waged!

So searched and read a little on SGL and now (almost) more confused (I did say read 'a little'!)...

1) What am I actually trying to heat?

All the google images for dew heater seem to show a tube extender covering the end of the telescope tube, although threads on SGL talk about 'base plates' and other bits... although I was actually assuming the mirrors were the only issue?

2) Which types of scope get the problems?

I have a little newtonian reflector - is this any more or less susceptible than refractors or cats (that's me trying to sound like one of da 'crew')

3) Why am I taking the scope outside in advance of viewing to let it cool down so much it forms dew - surely the plan is rush outside from a hot house, get the viewing done and back inside for tea and crumpets before the dew realises you're there?

4) What's all this stuff about carry-mats?  I'm not planning on going camping with my scope!  Is it the insulating effect of the carry-mat that does the job or some other quality.  If so, why are these scopes made out of metal and not something less prone to dew, like wood!?

Thanks for help.

[astronymonkey]

Hi Redstamp,

welcome to the world of dew control !

Its something which we all encounter as the telescope cools and moisture from the air condenses out as dew. So first things first......

Letting the scope cool - this isnt essential but it does improve the image significantly by allowing the scope to cool down to the same temp as the surrounding air. Like the heat shimmer off a warm road, the same thing happens above a warm telescope mirror meaning any images are blurry and unsable due to the thermal currents. Add a buit of high magnification and this only becomes more obvious. As it cools though condensation can form so I tend to leave the optics covered and give it a hour or so to cool down.

Will the dew damage things -  in general  it wont be a issue and the dew often gets worse when you bring the scope in as the air in the house is warmer so can hold more moisture. I tend to leave my scope with the focusser cap off and the tube horizontal and let it air dry overight.

Prevention - in a reflector the smaller secondary mirror seems to dew up quicker, partly because it is small and can cool down quickly but also because it is near the open end of the scope. A cheap dew sheild can be made from a camping mat to give the secondary some protection from this - but foam is used usually bcasue its light so doesnt alter the balance of the sope that much, and also because it is cheap..... alternaives would be cardboard bu hats not very good in the damp air.

Many people use low powered heater strips and tapes to gently warm the optics as this will keep them warm enough to ward off dew. Its kind of like the rear window heater in a car which gives very little heat but just enough to keep things clear. With your scope a permamnet solution may not be worth it but the use of a 12v hairdryer connected to a leisure battery would be enough to give a quick blast if the optics began to dew up.

Dew can spoil a good nights viewing so thinking of how to tackle it  is worthwhile, even if just simple steps are taken to try to ward it off.

cheers

[redstamp]

So if I choose to invest in a heating strip or tape, then presumably I should target the secondary mirror with light warmth, but wont this get in the way of my viewing pleasure?

[astronymonkey]

Hi,

With smaller reflectors its often difficult to heat the secondary very well but the hairdryer option can work well and is less hasstle. Fitting a secondary heater tends to be a permament fix but requires quite a bit of work. What scope do you have ?

[redstamp]

Skywatcher Explorer 130P Supatrak Auto

[astronymonkey]

Nice scope , but with the secondwry being small id definately consider making a dew sheild out of a camping mat and getting a 12v hairdryer. The secondary heaters require the mirror to be removed and a heater pad affixing to the back before refitting , so a lot of aggravation.

Cheers

[Paul73]

Don't forget about your finder scope. That is the first thing that fogs up for me if I get a bit over excited observing and forget to put the lens cap on when not using.

Paul

[pyrasanth]

I use an Astrozap 4 port dew controller & an Astrozap flexible dew shield with heater. This works fabulously and has the extra 3 ports for eyepieces & finder if required. The controller is a four port rheostat so the heat intensity can be controlled.

A dew shield is vital if you want to get more than a few hours of observing as my optics are usually smoked with dew within a couple of hours if I don't use the shield.

[Astro Imp]

We've all had this problem to different degrees and have worked strategies to cope. 

A lot depends on the scope, your reflector is probably less of a dew magnet than Maks and Casses that have a large lens open to the elements right up front. A dew shield from a camping mat will probably suffice for you.

I also have a Newtonian and don't have too much hassle but always leave my scope overnight with the caps off and pointing down to stop anything dropping on the mirror.

[redstamp]

Thanks all - useful tips!

[pyrasanth]

On particularly dewy nights the primary mirror of my Celestron can begin to dew up. This is annoying as my Kendrick dew shield keeps the corrector plate clear but if the primary mirror dews you are back to to the same issue. I think the micro mesh air vents must allow moist air into the tube & I'm not sure how to tackle this issue- any ideas?