Advice needed

[advinnie]

Hi all I'm totally new on here and just want to say hello and ask for some advice. I hope I'm posting this in the correct place if not I am sorry.

I have a sky watcher 200p and I'm wanting to know how does everyone stop the main mirror from misting up ?

I left my new toy out side for the 30 minutes like the book I have told me to but the maim mirror just got covered in mist and is now very wet. Is there a way to stop this ?

Cheers all

[bosun21]

When I leave mine out to cool down I leave the cap on and open up the focuser and luckily mine has a primary fan which circulates the air. Also don’t leave your scope pointing upward when cooling as it’s easier for dew to settle on your primary. I point mine level, even slightly downward.

[M40]

Hello advinnie and welcome to the site.

The 200p is basicaly a long tube with the mirror at the bottom, so, try to avoid facing the tube straight up. All the best.

 

bosun21 beat me to it 👌

Edited January 18, 2023 by M40

[Guest]

I run a 10 inch and 16 inch dob and have yet to have the first dew hit the primary.  Now the secondary.....thats another matter altogether.  You can make or buy a dew shield that might work for you, in my environmentals its a dew heater.   Now my scopes live in my unheated garage so they are always at or near ambient air temps.  I do all the alignment and collimation and then put the caps back on.  

Edited January 18, 2023 by Mike Q

[westmarch]

Hi there and welcome to the site,

dew is a perennial problem in our green and pleasant land. Some nights are especially prone to it and the forecast on Clear Outside is a good guide to it. You can find this in the resources menu at the top of the website.  

You want to cool your scope to reduce heat currents within the scope but when the scope becomes colder than the moisture laden air, dew starts to condense on the scope. 

In Newtonian scopes, the primary mirror is usually protected by the column of air trapped within the tube it is often the exposed secondary mirror which gets colder first and mists up. The finder scope is also relatively exposed and will cool quickly.

There are a few things you can do:

1. When you leave your scope to cool, point it horizontally.  Pointing the mirrors at the deep cold of space is the quickest way to radiate off heat  from the mirrors.

2. Fit a dew shield. This can be as simple as a foam camping mat around the end of the scope.  If you do a search on dew shields you will see some examples. This extends the column of stable air beyond the secondary mirror and slows heat loss.

3. You can fit dew heaters - these are electrical heaters which maintain the temperature of the scope a few degrees above dew point but don’t generate distorting heat currents. These can also be fitted to finder scopes.

4. Keep eyepieces in your pocket when not in use. Since these are some of the most exposed pieces of your optical train, this will keep them warm and help prevent misting.

5. Use a portable hair dryer on cool setting. This can evaporate dew but its effect is relatively short lived.

Hope the above helps and don’t become discouraged. Not all nights are dew magnets.

John

 

Edited January 18, 2023 by westmarch

[discardedastro]

Just to add to the above - which is all right - leaving the cap on is good advice while it cools down but once you're observing a dew shield will go a long way. Astrozap ones work very well and aren't too dear but a bit of foam wrapped round the end will do.

Dew heaters are probably excessive for the primary on a 200P; I've used mine in very humid conditions for all-night imaging without any issues (with a dew shield). However, they are useful for other optics like any coma corrector/eyepieces, finderscopes, and your secondary, which is all much more prone to dewing up.

[bosun21]

On heavy nights of dew I also use a dew band around my eyepieces.

[Guest]

Whether you need a dew band is 100 percent based on the environmentals where you are at. Where i live in Ohio...its pretty much a necessity.  A dew shield will work for a while, but after a couple hours heat is definitely the best option.  

[advinnie]

Hi everyone, I can't thank you all enough for help and advice, I will now go have have a look at these dew shields and heaters. What a fantastic hobby this is, I always wanted to get in to this but just never did until just last week. 👍

[Zermelo]

Even if it's not a dewey evening, you may find some advantage from the dew shield in controlling stray light entering the tube, bouncing around and reducing the image contrast. It depends on how much local light pollution you have, its direction, how well the inside of your tube is blackened , etc.

I always fit mine, dew or not.

[Carbon Brush]

Welcome to SGL.
Another consideration is the need for cooling, and how you go about it.

If you want to cool the scope before use, leave it in a garage or shed, rather than the garden.
That way if you change your mind, or the rain clouds do it for you, the scope is in a safe place.

Does the scope need cooling? Yes you will read about getting the best performance from a cooled scope.
You don't get thermal currents in the tube to limit magnification, etc.

But in reality you go outside, setup , decide where to look, spend a bit of time finding what you want.
Then you use a low power view to establish the target.
Then finally you may increase magnification. By this time the scope has cooled significantly.
But if the 'seeing' composed of air transparency and stability isn't up to taking high magnification.....

A fan in the tube can keep air moving to aid cooling and delay dew.
A small heater at the base of the tube will prevent dew on the main mirror. Overdo it and you get the thermal currents spoiling the view.
Both take time to fit and money to buy. But once you get long sessions in, you will appreciate these.

fine for a long term slution. But what do I do tonight? Take a hair dryer outside.
Obviously either a 12V camping/caravan dryer, or make sure you use an RCD protected mains lead.
A quick blast (a few seconds) will dry off any scope parts for many minutes and not put in enough heat to create thermals - unless you over do it.

If you look at a planet (Jupiter) or the moon at high magnification and the view looks good, wave the hair dryer around.
You will get a feel for how the hot air disturbs the view, and how the effects are gone in seconds.

Keep asking the questions and enjoy the journey.

David.

[Guest]

No doubt about a long dew shield working as a light shield too.  When installed mine extends the overall length to six foot and change.  It really does help more then one. Might expect. 

[Orange Smartie]

Apologies if this already been mentioned or if it is too obvious, but just to say don't be tempted to wipe dew from primary or secondary mirrors. They are very easily scratched.

And welcome to the hobby!

Pete

[PeterStudz]

I also have a SkyWatcher 200p Dob and I’ve never had dew form on the primary even when it’s been outside for several hours on a dewy night. However, without a simple dew shield the secondary mirror (being closer to the opening) can dew up. But condensation will form very quickly once the telescope is brought inside. When you were observing are you sure that dew formed on the primary and not just the secondary? I can never really see my primary in the dark when outside. 

But I’ve found that a dew shield will practically prevent dew issues on my mirrors. With a dew shield I’ve had the telescope out for up to 4 hours and it’s stayed dew free. Doesn’t mean it can’t happen but it’ll delay it. 

I made mine out of a cheap yoga mat. However, the mat I got was a bit thin and on its own and would not hold shape. So I stuck on some veneer that I had down the shed. You don’t need to go to that length. A thicker camping mat will work perfectly on its own and is very simple to make.

 

Edited January 20, 2023 by PeterStudz

[Spile]

Mirrors on my 200P don’t dew up but my eyepieces, finder and Telrad suffer although the Telrad shield does help.

[great_bear]

On 18/01/2023 at 23:17, bosun21 said:

On heavy nights of dew I also use a dew band around my eyepieces.

Another tip is to not use the tripod eyepiece tray for eyepieces (nor a Dob eyepiece rack) on cold nights - keep them in a foam-lined case with the lid closed when not in the focuser. I also keep my bulb-blower handy for eyepiece de-misting. 

Edited January 21, 2023 by great_bear

[bosun21]

13 hours ago, Spile said:

Mirrors on my 200P don’t dew up but my eyepieces, finder and Telrad suffer although the Telrad shield does help.

Buy the relatively cheap Telrad heater. Keeps my telrad clear on even the heaviest nights of dew

[Spile]

8 hours ago, bosun21 said:

Buy the relatively cheap Telrad heater. Keeps my telrad clear on even the heaviest nights of dew

I like to keep as low tech as possible and the diy shield does a pretty good job on most nights https://astro.catshill.com/telrad-dew-reduction/

[bosun21]

4 minutes ago, Spile said:

I like to keep as low tech as possible and the diy shield does a pretty good job on most nights https://astro.catshill.com/telrad-dew-reduction/

Whatever works for you is best for you. That’s my motto👍

[Ratlet]

It won't be much use for bigger newtonians, but for my 130pds I cut a cheap XL sized t shirt (New Orleans Saints) and cut the arms off. 

The tube goes through the arms and my quickfinder goes through the neck hole and the focuser inside the t-shirt.

The shirt helps keep a warm air pocket around the eyepiece and also doubles as an observing hood which helps for dim DSO if the neighbours have left lights on.

[bosun21]

34 minutes ago, Ratlet said:

It won't be much use for bigger newtonians, but for my 130pds I cut a cheap XL sized t shirt (New Orleans Saints) and cut the arms off. 

The tube goes through the arms and my quickfinder goes through the neck hole and the focuser inside the t-shirt.

The shirt helps keep a warm air pocket around the eyepiece and also doubles as an observing hood which helps for dim DSO if the neighbours have left lights on.

Ingenuity at its best.

[powerlord]

Lots of good a device here. I'll tell you what works 100% for me on my 200 and 300, both of which I got dew issues on primary and secondary.

15cm 12v pc fan. Bit of card, wood or correx, duct tape.

Make a shroud out of the above to fit on the bottom. Make sure fan is positioned to blow OUT/down. not in to tube. So it is sucking air down through tube. Duct tape it all in place so air tight. The fan should be only touching the shroud, so u don't get vibration issues. I use correx, its easy to make a more domed shroud, to keep fan from touching base.

Any time it's out with cover off, fan is on.

I've been doing this for a year now with zero due issues since fitting.

Stu

Edited January 22, 2023 by powerlord

[bosun21]

3 hours ago, powerlord said:

Lots of good a device here. I'll tell you what works 100% for me on my 200 and 300, both of which I got dew issues on primary and secondary.

15cm 12v pc fan. Bit of card, wood or correx, duct tape.

Make a shroud out of the above to fit on the bottom. Make sure fan is positioned to blow OUT/down. not in to tube. So it is sucking air down through tube. Duct tape it all in place so air tight. The fan should be only touching the shroud, so u don't get vibration issues. I use correx, its easy to make a more domed shroud, to keep fan from touching base.

Any time it's out with cover off, fan is on.

I've been doing this for a year now with zero due issues since fitting.

Stu

Any chance of a photo of this please. I am very interested for my 12” dobsonian. Thanks.

[powerlord]

Here u go.

 

Not pretty, but it does the job.

[Dave Lloyd]

Hi and welcome.