Some basic questions...

[Jasonb]

Hi all...

Am a little frustrated with my observing attempts tonight and am looking for some tips.

It's been cloudy for ages, and I've been dying to get out, especially to try my new UHC Filter on M42. However, the session turned in to a bit of a disaster, and I quickly had to give up due to clouds and dew! There's not much I can do about clouds, but the dew I'd like some advice about.

So, here's exactly what I did...

Firstly, it's cold tonight about 3 or 4 Degrees C and dropping. I had my tripod/mount set up outside for a couple of hours, so I wouldn't have to do it in the dark. I then brought out my Sky-Watcher Explorer 150P f/5 and left attached it to the mount, facing directly up into the sky with the main dust cover off, but the lens covers still on the Finder Scope. I left it for 25 mins.

Then I came out with my EPs (which had not been outside at all) and my Dew Shield, which I attached to the telescope. From the off though I could see I had dew issues, the view through the finder scope was clear, but through ANY of the EPs it was poor. All I could look at was the moon, clouds had ruined everything else (including using my UHC Filter), but the view was washy with condensation. At first I thought it might have been the variable polarising filter I was using, maybe it was dirty/scratched etc. (it's second hand), but even without that it was obvious. Oh-so-clear through the finder scope, murky through the main EP. And then the clouds covered the moon and that was that.

So, my questions. What did I do "wrong"? Did I leave it out too long before using it? Was pointing it upwards a bad idea? Should I have left the dust cover on while cooling down? Should I have had the dew shield on from the start? Should the EPs have been outside as well for those 25 mins so they would cool down too? I'm not sure if it was the EPs or the Secondary Mirror that dewed up. I *could* see dew on the Secondary Mirror after I brought the scope back inside, but that could be from bringing it back into a warm house?

Lots of Qs I know, but I'm currently looking for a dark sky site about an hour's drive away, and I don't want to go that distance and to have to stop after a few minutes! 

Which leads to more Qs. Looking at the situation I was in tonight, what do I do to solve it? I didn't want to go wiping any EPs or the secondary mirror or touching any surfaces. Do I just bring everything (EPs and Telescope) back inside again (or into the car if away from home) for a while, until the dew is gone, and then go back out again?

All advice welcome, thanks in advance!

 

[NGC 1502]

I’m assuming your 150p is a solid tube.  No need to leave the tube pointing up for cooldown. Removing the dust cover will allow air to circulate to help cooldown.

Having said that a 150p shouldn’t take long to cool unless stored in a heated room and it’s sub zero outside. And for viewing at low power hardly any cooling needed.

Al Nagler at TeleVue is on record as saying that eyepieces don’t need cooldown.

With a Newtonian a dew shield to extend the front end of the tube will help to reduce dewing of the secondary mirror.  Keep any eyepieces not in use covered in a box, or capped and in an inside pocket.  Those eyepiece racks on the side of a scope are ok for brief storage but not for extended use, that’s an invitation for dewed up eyepieces.

A 12v dew gun is handy for removing dew from a Newtonian secondary. Remove the eyepiece, fire the gun down the focuser.  That’s only useful for light misting up, if water droplets have formed it’s game over.

Next time out perhaps don’t use any filter at all to start with.  If all is ok then you could try filters later.

If during a session dew forms then stop and think it through. If eyepieces or secondary are dewed then the 12v dew gun mentioned earlier may sort it, at least for a while.  In severe conditions I’ve had Newtonian primary mirrors dewed up. Game over, unless it’s a truss tube and a dew gun can be carefully pointed at the primary mirror after removing truss tube and shroud.

Bringing cold gear into a warm house will greatly increase moisture forming on everything. If you have a shed/conservatory closer to ambient that possibly could help.

Hoping the above may help at least a bit, or others chip in with their thoughts.

Above all please don’t let these frustrations put you off. Your efforts to succeed is your gateway to a lifelong passion.

Cheers from Ed.

 

 

 

 

 

 

 

 

 

[Greymouser]

Hi, might be able to help a little.

Firstly I would say attach the dew shield straight away, I had this scope and had the same troubles with the secondary dewing up. The dew gun mentioned above would imo, be a 12v hair dryer, even a 240 volt one if you are feeling brave, ( or dim... ) I would be careful about pointing it up too, just imagine that million to one chance of a bird pooing down the tube! .

Do not wipe eye pieces, keeping them in a pocket helps, between use. You can even get small dew straps to prevent dew on eyepieces. You can even do this for the secondary, if you are practical. I always allow eyepieces to cool. though doubt it helps. Hair dryer and/or a warm pocket, as it dews up, does. Our breath sometimes inadvertently, causes dew on the eyepiece. Stopping breathing is not really an option.

Edited March 7, 2023 by Greymouser

[Cosmic Geoff]

Dewing up occurs primarily because telescope parts radiate heat to the sky, which (when clear) is very cold.  Moisture from damp air then condenses on the cold telescope parts.  Hence one can suppose that pointing your telescope straight up and leaving it was a bad idea. 

In the limited time I owned a Newtonian I did not put a dew shield on it, and it never dewed up.  My understanding is that a dew shield is not necessary except in extreme cases.  There will be a support between the secondary mirror and the sky, which should prevent dewing except in extreme conditions.

[Moonshed]

Lots of good advice given. I have not owned a Newtonian for many years, I have a Celestron SCT which are notorious for misting up on the inside of the corrector plate. However, some of my solutions may also help you hopefully.  1) Dew shield always fitted soon as set up. (Perhaps not so necessary with a Newt?) 2) A good blast with wife’s old hairdryer whenever necessary. 3) Never leave it set up with scope pointing upwards. 4) Set up in good time to allow to cool down, depending on where you store your gear. Mine is in a shed so that is not a problem. 
I have to say that my number one mist killer is the hairdryer, it is always close to hand when conditions demand. Nothing can survive it! Surprisingly it doesn’t affect observing or imaging after only a few seconds.

I Hope you get it sorted out and proceed to enjoy your views of M42 with your shiny new filter 😃

 

 

[Jasonb]

Thanks for your replies everyone...

It sounds like two things to definitely change is to put the Dew Shield on immediately, and also don't leave the Telescope pointing up. And yes, sorry, it is a solid tube Newtonian.

I can also try using the scope sooner, for lower magnifications at least.

Some sort of Dew Heater/Hair Dryer might also come in handy, but I'll see if the above steps help first.

I should have said, I have an EP "storage bag" that I use, so they're all kept in that and I take them out, put them back in as needed. So they're not left outside for long periods of time.

We'll see how the next sessions goes!

Edited March 8, 2023 by Jasonb

[Pixies]

Your scope suffered from  Nocturnal Thermal Cooling!

https://en.wikipedia.org/wiki/Radiative_cooling#Nocturnal_surface_cooling:~:text=energy to space.-,Nocturnal surface cooling

That's how a dew shield works - it shields the optics from direct view of the clear sky. Even a dew shield won't help when pointing straight up for too long.