Am I suffering from dew formation.

[Roog]

Having got my first telescope, a 10” dobsonian back in September the majority of my viewing in the UK has been in cold conditions, it’s tough but it’s got to be done if only to catch my first proper glimpses of Orion’s beauty. However I have begun to notice that whilst every thing starts off ok, after say 30 to 40 mins I find it harder to bring stars into sharp focus. I have become quite adept at collimating my scope in the field so I don’t think it’s this, could this be due to build up of dew? 
Peering into the main mirror it isn’t obvious to me if it is dew.

Any ideas welcomed, and if it is dew, is there a popular kit of bits to deal with this for my 10” Dob? 
 

many thanks Paul

Edited February 13, 2022 by Roog

[wulfrun]

Assuming you're sure it's not the seeing etc, less likely to be the primary but quite possible to be the secondary or eyepieces dewing. I've had the secondary dew up in my smaller scopes, partly because it's less recessed in them. The 150PL, on the other hand, rarely suffers from it.

At the risk of ruining your night-vision, shine a small torch across the face of the secondary, look down the focuser tube - if it looks milky it'll be dew. If your dob is the truss-tube, do you have light shroud? A truss dob with no shroud has a pretty exposed secondary. You could also add a foam-matt tube "extension" on the front as well or invest in a portable low-power "hairdryer" type blower if nothing else works.

[Roog]

Thank you for your thoughts @wulfrun I am just jumping to conclusions really, but next time I will do the torch trick with the secondary mirror. 
my Dob is a metal tube type so I assume that this might offer some protection but I have seen a few set ups using heater blankets wrapped around the tube and the secondary mirror support and I wondered if this was what I need.

[Dave scutt]

I used to get my secondary mirror dew up all the time on my solid tube dob, as wulfrun said a foam camping matt not too thin works wonders as a dew shield and my secondary mirror hasn't dewed up since 

[wulfrun]

Firstly, identify the problem else you may only fix it by accident. It may help to put a plastic bag/shower cap or similar over the primary end (if it's open) but the chances are a closed-tube will dew the secondary long before the primary. You can get heaters for secondaries too. Heating the tube doesn't sound helpful, it's likely to cause air currents that will blur your view.

[Roog]

Many thanks for all of your advice, I will seek to find out if it is the secondary mirror, but it does seem the most likely issue from all of your comments. I will be on the look out for an exercise mat too.

[bosun21]

I have experienced some dew on my secondary mirror of my 8” Dobsonian from time to time, although not very often. I simply bought the Astro Essentials dew shield from FLO which took immediate care of the problem. These are competitively priced and good quality 

[Guest]

I have a 10 inch dob and almost certainly you are getting dew on your secondary.  It's just the nature of a reflector.  There are two schools of thought to defeating dew.  One is a dew shield, the other is a dew heater system.  The shield you can make yourself and are very inexpensive.  Heater systems (store bought) are pricey.  Here in Ohio dew is a very real thing, so is frost and ice.  Here, in my environmentals a dew shield only delays the inevitable.  The only way for me to beat dew is to stop it with heaters.  I run them year around and they have eliminated any dew/frost issues.

Edited February 14, 2022 by Guest

[Pixies]

I have an 8" dob and made a cheap and cheerful dew shield out of foam sheet.

Luckily, I don't suffer much from dew. I've probably used the shield 2 or  3 time in the past 2 years. I find that the screen of the Telrad finder acts like a dew-canary and is the first thing to dew up before anything else.

[Bruce Leeroy]

Could it be your eyepiece that is dewing up? Very easy to exhale/talk at the wrong time and mist up your eyepiece.

[Roog]

19 hours ago, Pixies said:

I find that the screen of the Telrad finder acts like a dew-canary

Ah yes I have noticed this too.

[Roog]

Thanks everyone, for your helpful advice and observations, next time I am out I will check the secondary as this seems to be the main target and I shall be more observant to minimise the risk of dewing up my eyepieces and telrad. 

[Roog]

23 hours ago, Mike Q said:

Heater systems (store bought) are pricey.

Hi @Mike QI note that you emphasised (Store bought) as being expensive, out of interest, is there a popular way of making these amonst amature astronomers?

[Guest]

14 minutes ago, Roog said:

Hi @Mike QI note that you emphasised (Store bought) as being expensive, out of interest, is there a popular way of making these amonst amature astronomers?

There are guys out there that make their own using resisters or wires and running just enough electric through them to make heat.  Just search for homemade dew heaters and you will find it.  Personally I don't have the skill set to attempt to do it. 

Edited February 15, 2022 by Guest

[Roog]

On 15/02/2022 at 09:31, Mike Q said:

There are guys out there that make their own using resisters or wires and running just enough electric through them to make heat.  Just search for homemade dew heaters and you will find it.  Personally I don't have the skill set to attempt to do it. 

Thank you for your reply @Mike Q, yes is did check "DIY dew heaters" in Google and found the resistor ladder one, and also nichrome wire too, I have used both these methods to make small heaters in other hobbies and whilst both are doable, its the packaging in a safe to use package which I find most challenging.  

[Guest]

17 hours ago, Roog said:

Thank you for your reply @Mike Q, yes is did check "DIY dew heaters" in Google and found the resistor ladder one, and also nichrome wire too, I have used both these methods to make small heaters in other hobbies and whilst both are doable, its the packaging in a safe to use package which I find most challenging.  

I get it.  I looked at it for a minute and decided to just buy a a complete system.  Yeah it was expensive but well worth it in the end 

[Roog]

I ended up buying a PCB based dew heater from FLO, by 4tronix, I had the choice of 2 sizes, one for 8" or for 12". As a have a 10" Dob and given that I didn't want the heater to overhang the mirror I opted for the heater intended for the 8" scope. It is a very tight fit on the secondary mirror support on my 10" scope and i bravely, or is that stupidly opted to file a small amount of PCB away from the inside of the 'C' shaped board, I had several fingers crossed that I didn't kill it by cutting through any tracks in the process.  Anyway I managed to bond the heater on to the back of the secondary mirror without great difficulty, although it would have been nice if the heater kit had included some cotton gloves to protect the mirror from my sweaty fingers, cables routed along one edge of the mounting star and I was nearly ready to fire it up!

I made up a  fused 16V battery pack from 4No. laptop batteries and bought a PWM small DC motor controller to manage the heater power  and connected it all up. From my initial dry tests I have power ratings for 0 up to about 2.4W.  Having not used such a device before I was concerned that the heat might debond the secondary mirror, so I started off with low power.

In real use I have found that the fog has gone, it runs at very low power perhaps in the order of 0.5 to 0.75W and my 1.8Ahr battery lasts for ages, certainly longer than I can stand out in the cold.

It seems that this was a good investment for my Dob. and until I find otherwise it seems to have put and end to my sessions being ruined by dew.  

[spike95609]

I hope your foggy problems remain fixed. It's an odd one - I have a 10" Skywatcher dob, and have used it for years in all manner of warm, cold and damp conditions, even when mist has been drifting in and out, and I've never once had this problem. The finder scope fogs up all the time of course, or it did until I hit on the highly technical solution of putting the lens cap on when I'm not using it, but never the primary or secondary. I'm usually only out for 1-2 hours though, so maybe it would if I was out there longer but luckily for me the human body gives way long before that happens.

[PeterStudz]

I very occasionally have issues with dew on my secondary,  although a simple dew shield is usually enough. But it takes a while. @spike95609 - my Dob is often out for 3 hours, sometimes more. Eg when camping out in the garden with my daughter it’ll be out all night. There’s nothing more annoying than waking up after a bit of kip and encountering dew issues. 

[Guest]

3 hours ago, spike95609 said:

I hope your foggy problems remain fixed. It's an odd one - I have a 10" Skywatcher dob, and have used it for years in all manner of warm, cold and damp conditions, even when mist has been drifting in and out, and I've never once had this problem. The finder scope fogs up all the time of course, or it did until I hit on the highly technical solution of putting the lens cap on when I'm not using it, but never the primary or secondary. I'm usually only out for 1-2 hours though, so maybe it would if I was out there longer but luckily for me the human body gives way long before that happens.

Sometimes the low tech options work well.

[bosun21]

For myself a dew shield is as far as I would go, as fitting a secondary heater surely must cause convection currents of air at the front of the scope. We are told to try and avoid observing directly above house rooftops for this very reason. A secondary heater must surely be worse than rooftops. That’s my two cents worth.

[Guest]

53 minutes ago, bosun21 said:

For myself a dew shield is as far as I would go, as fitting a secondary heater surely must cause convection currents of air at the front of the scope. We are told to try and avoid observing directly above house rooftops for this very reason. A secondary heater must surely be worse than rooftops. That’s my two cents worth.

A dew shield here in Ohio only prolongs the inevitable.  So a dew heater became a must.  Fortunately I only have to run at about 25 percent power to keep the dew away.  I read about these currents but have yet to really notice them.  They also work VERY WELL when frost and ice show up.

[bosun21]

9 minutes ago, Mike Q said:

A dew shield here in Ohio only prolongs the inevitable.  So a dew heater became a must.  Fortunately I only have to run at about 25 percent power to keep the dew away.  I read about these currents but have yet to really notice them.  They also work VERY WELL when frost and ice show up.

Well if you are actually using one and not noticing any I’ll effects that’s good to know. I am just thankful that I don’t have to contend with the levels of dew that you do Mike.

[Starwatcher2001]

I use long home made dew shields on my dob and SCT, which also cuts down the amount of stray light that gets into the tubes from neighbour's windows.

Edited May 28, 2022 by Starwatcher2001
typo

[Guest]

3 hours ago, bosun21 said:

Well if you are actually using one and not noticing any I’ll effects that’s good to know. I am just thankful that I don’t have to contend with the levels of dew that you do Mike.

There are a handful of nights a year I can run without them, but those nights are few and far between.  I think the key is to turn them up just enough to do the job and no more.  For me on most nights that is around 25 percent.  The only time I go beyond that is in the fall and winter

Edited May 28, 2022 by Guest