dew shielddoes it make the tube longer ?

[todd8137]

my scope is a 12"dob so its 1500mm .so if a add my dew shield wich is 300mm ,thats how much it sticks out as (1500mm+300mm) is my lenght ,1800 mm?as try in to work eps with it on so does it had lenght and make the tube longer a no its a crazy idea but am sure some one will tell me because if so does this also change the mag of eps?ie 1800mm divided by 32 =56 or am a wrong ?thanks

[msinclairinork]

Adding a dew shield will make no difference to the focal legnth so will not change and 'magnifications' or anything. As the secondary mirror and focuser hasn't moved.

HTH

Michael

[Ad Astra]

Precisely!

The 'tube' is really just there to hold the primary mirror, secondary mirror, and focuser in a fixed position relative to each other, and keep them lined up properly.

A dew shield is usually for a refractor - it helps keep the objective lens at the front from loosing heat and cooling too rapidly which can cause dew to form.

A reflector, on the other hand, has most of the optics farther down inside the tube anyway, so a dew shield is rather irrelevant. Most people who have a truss-type scope (no solid tube) use a light shroud - a simple black cloth usually held on with velcro - to help keep stray light out of the optical path which really messes up your contrast.

All that said, I'm not the real dew-master. We don't get much wet stuff (usually) where I live. If it isn't actually raining - humidity is usually between 0% - 25%, rarely over that.

Of course, I just bought a new telescope mount - so we've been having rain for a week.....

Dan

[Moonshane]

Precisely!

The 'tube' is really just there to hold the primary mirror, secondary mirror, and focuser in a fixed position relative to each other, and keep them lined up properly.

A dew shield is usually for a refractor - it helps keep the objective lens at the front from loosing heat and cooling too rapidly which can cause dew to form.

A reflector, on the other hand, has most of the optics farther down inside the tube anyway, so a dew shield is rather irrelevant. Most people who have a truss-type scope (no solid tube) use a light shroud - a simple black cloth usually held on with velcro - to help keep stray light out of the optical path which really messes up your contrast.

All that said, I'm not the real dew-master. We don't get much wet stuff (usually) where I live. If it isn't actually raining - humidity is usually between 0% - 25%, rarely over that.

Of course, I just bought a new telescope mount - so we've been having rain for a week.....

Dan

totally agree with both comments above but would add that for newtonians dew shields can act more as light shields if you live in an area where people e.g. turn on their lights when you are observing. there's a chance that at certain angles light can enter the base of the focuser causing a loss of contrast. BTW one way my newt secondary dewed up once was when I left the plug out and went in for something to eat. an hour later I could not work out why everything was fuzzy! so always keep the plug in when not in use (I don't tend to leave eyepieces in the scope when I'm not there).

[Astro_Baby]

Clearly you two have never used a Newtonian under average uk conditions

Without a dew shield on a newt in the UK you may as well try and use a newt under a waterfall.

Everytime I have gone to Salisbury star party I see the tragedy of people under a beautiful sky who can see nothing because they believed a dew shield on a newt was a waste of money and now have mirrors soaked as perfectly as if they were under a shower.

At my firts SSP with a newt I was told in advance that a dewshield was a waste of time, I ignored tyat advice and bought one anyway and lo on the first night I had about the only usable newt on the campsite.

The UK can suffer very damp conditioms, out in California its probably very dry.

Years ago a dewshield wasnt quite such a must but newts back then generally had longer tubes with the secondary mirror set quite far down the tube so the tube itself provided protection. Modern fast newts tend to have the secondary mirror only just about inside the tube and as a consequence dew can take the secondary out of commission very fast.

I have had plenty of nights in the UK where the dew has been so bad even the primary has lost the plot but thats rare and usually only happens after quite a few hours observing, on the other hand regardless where I observe from within a 70 mile radius of where I live dew is a regular curse.

[Moonshane]

Clearly you two have never used a Newtonian under average uk conditions

Without a dew shield on a newt in the UK you may as well try and use a newt under a waterfall.

Everytime I have gone to Salisbury star party I see the tragedy of people under a beautiful sky who can see nothing because they believed a dew shield on a newt was a waste of money and now have mirrors soaked as perfectly as if they were under a shower.

At my firts SSP with a newt I was told in advance that a dewshield was a waste of time, I ignored tyat advice and bought one anyway and lo on the first night I had about the only usable newt on the campsite.

The UK can suffer very damp conditioms, out in California its probably very dry.

Years ago a dewshield wasnt quite such a must but newts back then generally had longer tubes with the secondary mirror set quite far down the tube so the tube itself provided protection. Modern fast newts tend to have the secondary mirror only just about inside the tube and as a consequence dew can take the secondary out of commission very fast.

I have had plenty of nights in the UK where the dew has been so bad even the primary has lost the plot but thats rare and usually only happens after quite a few hours observing, on the other hand regardless where I observe from within a 70 mile radius of where I live dew is a regular curse.

Very interesting. Just measured my secondary to top distances and the 12" is 8" down the tube (this has never dewed) and the 6" is 6" down the tube (this has dewed once). Based on your comments though I'll be taking my home made dew shields to SGL6!

[todd8137]

great news and thanks a was not sure a do have the astrozap dew shield use it a lot but was not sure ,but thanks for clearing that up for me cheers pat

[AndyH]

Only place I've experienced bad dew was in Kielder, and that happened only once. It totally threw me and took me about a quarter of an hour to figure out what the hell was going on

An utter pain in the bahookie

Andy.

[John]

I don't suffer from dew much either (our home is 300 feet above sea level so I wonder if that is anything to do with it ?) but I did notice dew being much more of an issue at the SGL6 site so I'll pack accordingly.

I still can't quite understand why dew shields are not standard equipment with SCT's and Mak's though, as they are with the other front end mounted glass design, the refractor

[AndyH]

I still can't quite understand why dew shields are not standard equipment with SCT's and Mak's though, as they are with the other front end mounted glass design, the refractor

110% agree.

Andy.

[todd8137]

thanks all for your response,a got the dew shield as a "just in case " and some times my next door leaves the toilet light on and shines down my height above sea level is around 198 meters but a least a no no now that it does not alter the focal lenght .

[Earl]

I don't suffer from dew much either (our home is 300 feet above sea level so I wonder if that is anything to do with it ?) but I did notice dew being much more of an issue at the SGL6 site so I'll pack accordingly.

I still can't quite understand why dew shields are not standard equipment with SCT's and Mak's though, as they are with the other front end mounted glass design, the refractor

Added Value sales ploy.

aka, get the custy to spend more

[Mr Spock]

Never in 35 years of using Newts have I had one dew up. My SCT has only dewed once and that has no dew shield...

Maybe it's because I'm on a hillside and not in a valley bottom?

[twotter]

I live in Fenland, which is as flat as a very flat thing, actually I may even be slightly below sea level come to think of it.

Anyway, my 200P dob has an AE dew shield fitted since new and although the shield has been very wet on a number of occasions I haven't had any problems with dew on the mirrors. Very worthwhile piece of kit, and it looks smart as well

[Astro_Baby]

Well all I can say is that at every SSP I have been to the dew has been hell.

Its down to the owner oif course, if your happy to lose your viewing for a few nights for want of a few uid to make a dewshiueld or buy one thats the users own call.

I never get dew problems but thats cos I ALWAYS use a dewshield. SSP 2 was enough to cos convince me as most people had the first night ruined through horrendous dew which was a pity because it was the best night there.

ps I just measured my Skywatcher 250P and the centre of the secondary mirror is barely 4.5" from the end of the tube, which means the nearest edge of the secondary is closer to 3" from the edge of the tube. Thats not a lot of tube to keep the dew down when refractors typiccyly have a dew shield thats 1.5x the apertire of the scope. That would make the 250s optimum dew shield around 15" in depth not the measly 4" as built. Bearing in mind the secondary mirror is almost as big a piece of glass as a small 60mm refractor you can see that dew can be a big issue potentially , and where I live in actuality as well, and to be honest everywhere I have observed has been no better which leads me to think that the counties of Wilthsire, Surrey, Hampshire and East Sussex have abnormally high dew irrespective of height above sea level. The dryest place oddly is my sisters which is right next to the sea. The worst is easily Salisbury plain where even the primary can cath a serious dose of dewing up.

[Davidv]

Never in 35 years of using Newts have I had one dew up. My SCT has only dewed once and that has no dew shield...

Maybe it's because I'm on a hillside and not in a valley bottom?

Must be. I've never seen a SCT not dewed up if it has no dew shield.

DIY Newtonian telescope designs always recommend placing the flat/focusser the same distance from the end of the tube as the tube's diameter. You don't seem to get this with commercial scopes 8 inches or over, maybe to keep the length down a bit.

[jamespels]

Very interesting thread - last time I was out (Feb!), the secondary (and the outside of the ota) iced up in about an hour. Took me a while to work out why I couldn't see very clearly after half an hour...

So, to make a dew shield, is this literally just a tube to go on the end of the scope? Is there any advantage to one made of metal (will codense water vapour quicker) than plastic?

Thanks,

[AndyH]

Mmmm. I shouldn't think so. I've used plastic and metal dewshiels and haven't noticed much if any, difference.

Thin aluminium sheeting(eg:0.5mm) is cheap and easily pliable to a tube shape. Drill a few holes for low profile nuts n bolts, to hold it together(or use velcro) and either paint the inside matt black or use flocking material.

Very cheap and simple to do, in my humble opinion.

Andy.

[AndyH]

Oh yeah, plastic sheet can be bought just as easily and it's edges may not be as (potentially) sharp.

Andy.

[bcfcciderhead]

Well, used my 200P dob before xmas about 6am to view saturn, not long had it so had no dew shield, after about 15mins the telrad was very foggy and had trouble seeing the red projections, the primary also very bad and if you wiped your hand on the OTA you can wipe water of onto the floor.

I used my 130P in the same conditions (time, temp etc) with dew shield and the mirrors were fine, but yet again the OTA very wet.

Had the dob out last friday night , it had about 45mins cool down time, when i went out to it the telrad had some but was still usable (it now has a dew shield), the OTA yet again very wet but all mirrors ok, i now have a light shroud as i have the truss type dob and a small dew shield at the open end.

So from my experiance a dew shield is a must, deffo in the uk winters.

For those who may like to know the locations i used was from hills in S.Wales down to sea level near the river severn.

To be honest for what it will cost to make one yourself from a camping mat i think it's worth doing just to be covered ...thats not a joke by the way

[jamespels]

Cheers all.

Andy - will grab the next suitable bit of sheeting I find and make something up then.

J.

[Uranium235]

Try this instead (if you have any laying around).

Foam underlay for wooden flooring. Its bendy, easy to cut, and very very light. Mine is grey in colour, so I blacked the insides using some black paper. Its also really easy to form around your OTA, all you need to do is leave enough "give" in it so when you add/remove it youre not disturbing the OTA so much (eg: you need to remove if using a focusing mask).

All you need is the underlay, masking tape, superglue, black paper and some good scissors. Its greatest asset it its weight, less than 100g im sure (ive never actually weighed it). But i know the alu dewshield you get with the SW ED80 is 200g, and the underlay feels a lot lighter than that, and its 2.5x longer.

[jamespels]

Excellent idea U235 - I think I even have some underlay lying around, and I don't think there's a room in this house without a roll of Duck tape hidden somewhere...

Thanks,

J.

[RikM]

I find a dew shield is an absolute must with my 150P. I used a section of Asda black foam camping mat and sticky velcro pads.

[todd8137]

ive never had a dew problem but brought a dew shield a while ago it does what a wanted stray light, i did try a black camp mate but am **** when it comes to things looking tidy and proper and that did not flo got it to me in good time