Wet behind the ears with Dew!

[Space Cowboy]

Trying to get to grips with dew forming on my Skymax 127 mak. I've found some plastic drain pipe for the dew shield. I understand the shield should extend 3-4 times the length of the aperture diameter..which would be about 20 inches. I'm aware that this will only delay the dew from forming and that a heat strip is the only way of keeping dew away permanently. So without having a heat strip setup if i put the scope outside to cool down with the dust cap on and only attached the dewshield when i'm ready to observe would that be ok for a decent length of dew free observing time? Or would that be a waste of time unless the dew shield is attached from the moment the scope is outside?

Would placing my eyepieces in front of a fan heater so they are kept luke warm keep those dew free also?

Any help would be greatly appreciated

[Ant]

A dew strap is really the way forward.

But letting the scope cool with the cap on and only removing it when observing will help (have to add the drain pipe).

But without a dew strap try something like this hair dryer, these can be used to undew dewed up optics (Corrector plates, lenses, EP).

Ant

[John]

One thing to think about is the weight that 20 inches of 5 inch diameter plastic drainpipe will add to the front of your scope - this might affect the drives and their accuruacy ?.

An alternative would be to use a foam camping mat (part of it anyway) rolled around and secured with tape or velcro - much lighter.

Ideally you want whatever anti-dew measures you are using in place from the start - with my 8" schmidt-cassegrain I used to find a heated dew band around the front of the scope, where the corrector plate (the front "lens") is, was enough on it's own to prevent dew.

With eyepieces, I tend to keep mine at room temperature while the scope cools down. I've not had any dewing problems with them.

John

[brianb]

With eyepieces, I tend to keep mine at room temperature while the scope cools down. I've not had any dewing problems with them.

Wow, I find it's a real problem - I have dew heater strips wrapped round the two eyepieces I use most & remove the others inside when not in use so they're dew free & warmed up if I actually need them.

[Space Cowboy]

Thanks guys!

[Taff]

Hi' When the dew starts misting the EPS and the finderscope out comes the hairdryer at low speed it does it's job.Mark

[The Warthog]

My 4" refractor has a dew shield of light plastic about 8" long, and I have not had any real problems with dewing. You would probably be better served by a cardboard tube that is light, and easy to make.

[adamsp123]

If I was making one on the cheap I would go with the camping mat, and I use a hair dryer, mains operated, good blast but is noisy.

[Space Cowboy]

Thanks again! Looks like i will be giving my scope a blow dry! May decide on a lighter shield, probably am going for overkill with the drain pipe

[Jakeyboi]

Some good ideas will try them out as dew is a real problem. Can I wipe the lens with an ordinary cloth or do I need to get a special one?

[brianb]

Can I wipe the lens with an ordinary cloth or do I need to get a special one?

Don't wipe it with anything when it's damp. Get it dry first, then blow away dust & grit using a rocket bulb blower - NOT a "compressed air" can (risk of staining from propellant, risk of cracking glass from sudden cooling) - then, if necessary (and it rarely is), remove dew residue with a very well laundered, but unironed, handkerchief - a cotton one which is practically threadbare is ideal, a "new" one which looks smart enough to pass muster by SWMBO isn't soft enough.

[ChrisEdu]

Them dews are a curse for viewing! grrr..... Got to get some heat on that scope to stop 'em forming.

[Talitha]

Them dews are a curse for viewing! grrr..... Got to get some heat on that scope to stop 'em forming.

I agree with that 100 percent.. preventing dew in the first place will eliminate the need to clean up. Moist optics act like glue for airborn particles like pollen.

[Psychobilly]

Prevention is definitely better than cure... I trickle the dew heaters right from the start of the session and as ramp them up as soon as there is any sign of any dew forming...

Matt black art card makes a cheap temporary dew shield but the foam camping mats will last a lot longer...

Peter...

[Macavity]

So is the truth of the matter that one can "get away" with just a heater strip and no dew shield? (Hoping!). Aside: Without a dew shield, a bright off-axis star once gave me the impression (shock!) something MAJOR had gone wrong with my precious scope...

[Psychobilly]

Heater strip works a lot better with a dew shield and it can literally just be made out of newspaper or anything you have at hand...

an A sheet of Matt black art card is about £1 but will eventually succumb to the dew itself... so thats why the foam camping mats are used...

You want to put the minimum amount of heat in that you can otherwise you'll be creating thermals in the OTA and in front of the scope...

Peter...

[Space Cowboy]

I've got a light green camping mat for my dewshield. Is it essential to paint it black?

[brianb]

So is the truth of the matter that one can "get away" with just a heater strip and no dew shield? (Hoping!)

Maybe one night in 3 in my experience .... usually the windier ones.

Is it essential to paint it black?

No, but it also works better as a flare shield if you do. The more artifical light or moonlight you have shining into the dew shield, the more it helps to have it blackened. Note, gloss paint is BAD, a matt finish is essential to prevent specular reflections!

[Talitha]

Hi Stuart, maybe a layer of black felt glued to the inside of the shield will cut down the glare? I used a medium blue camping mat for all of my dew shields and don't have a reflective problem but my sky is pretty dark (except for Luna, but she's so pretty i don't mind).

[Space Cowboy]

Thanks guys! Maybe i will take the felt route, rather wary of using paint if the stuff does not stick properly to the foam mat, don't want black paint on my scope lense :-)

[Keithp]

Anyone know what those 12v hair dryers sound like? I was thinking of getting one along with dew straps but wondered what they sound like at 2 in the morning...neighbours?

Keithp

[Talitha]

They're not too bad Keith.. much like a regular hair dryer on low fan speed. But TBH, you won't need one if you use dew tapes.

[jonathan]

Sorry to dredge up an old thread, but I have a newbie question about dew shields and the like..

If I have an 8SE SCT with a single dew heater on it do I still need to allow the telescope to cool down for 2 - 4 hours before it's usable? Assume I'm taking the telescope from a warm house out into the cold November night about 9 or 10pm.

[John]

I used to have a Celestron 8" SCT on a fork mount - a sort of forerunner to the 8SE. Taking the scope from my dining room outside, I found the scope OK for low-medum power after 20-30 mins then OK for higher power from 40-60 mins but still improving gradually for another hour after that.

I used to have a heated dew band which I switched on from the moment I started observing - it's so frustrating when the dew attacks and you loose visibility for a while. I rarely found I needed both the dew band and the dew shield but that might be because our house is 300 feet above sea level.

With SCT's I found the most important thing was to learn to collimate it. They tend to loose contrast and resolution fast if the collimation is out a bit because of the relatively large central obstruction.

[brianb]

I rarely found I needed both the dew band and the dew shield

Hmph, try my site ;-)

Seriously though - the dew shield isn't enough on its own, it just delays the corrector plate dewing over by half an hour or so. But, used in conjunction with the heater tape, it makes the heater tape's job much easier.

Most of the mass which needs to cool down is in the primary mirror & the other stuff at the back (heavy) end of the scope ... where the heater tape has zero effect (and is not needed because dew is "intercepted" by the corrector plate!) Cooling is unaffected. In fact it is common for an exposed corrector plate to overcool, that's why it dews over .... a bit of gentle heating at this end, restoring the corrector plate to ambient temperature, actually reduces tube currents (once the primary mirror etc. is in thermal balance ... which it gets to because it's shielded by the tube).