Dew

[noeliam]

Please can someone clarify the dew issue for me, I have a Skywatcher 200p dob and am heading to Kielder star fest in 2 weeks, Will i get Dew with a Dob? Do i need to make a Dew guard as others have done, and if i do get a problem what do i do with the hairdryer to clear it, Where do i aim the dryer as in down the tube or??????

 

Thanks

[laudropb]

I have a 200P Dob and have seldom had problems with dew. On the few occasions it happened it was the secondary that fogged up. I bought an astrozap dew shield from FLO and have had no problems since. I have never used a hair drier, but I imagine you would only need it for the secondary. I don't think it would be a good idea to aim it down the tube towards the primary.

[noeliam]

18 minutes ago, laudropb said:

I have a 200P Dob and have seldom had problems with dew. On the few occasions it happened it was the secondary that fogged up. I bought an astrozap dew shield from FLO and have had no problems since. I have never used a hair drier, but I imagine you would only need it for the secondary. I don't think it would be a good idea to aim it down the tube towards the primary.

What did you do to clear the 2nd mirror Laudropb please?

 

thanks

[rockystar]

the main problem I have is with the finder and and eyepieces - the recommendation I got, was to keep the caps on when not in use, or stick it in your pocket to warm it up.

the length of the tube should keep the primary from dew, so extending the tube helps prevent the secondary from dew. I use a camping mat (£5 from the supermarket)  cut to size. (shown in the before picture below)

[baggywrinkle]

You can make a little dew shield for the finder. Or wrap it with silver roasting foil, this helps reduce radiated heated loss which in turn causes the metal work etc to cool below ambient temperatures and possibly the dew point at that time. Acts much like those silver thermal blankets you see people using after running or used by mountain rescue as a first aid measure against excessive body heat loss.

On clear nights, which we always look forward too, the risk of dew formation is higher as heat radiates off the telescope more quickly. The dew shield helps by holding a column of still air which has a dew point higher than the outside ambient., however over time that effect will be lost. Dew heaters will help, you can get small ones for finders too.

[astronymonkey]

With a 200p if you get dew it will be on the secondary so ifnyou have 12v hairdryer then pointing it down the focusser tube helps direct it onto the  secondary.

As has been mentioned if you leave the caps on between viewing can help as can putting the scope in a horizontal position,

For the 200p a water butt stand helps raise the viewing height which is more comfortable but unrelated to dew but might be helpful on a trip to kielder as it gets the base off the damp ground and make sure you  have a wheely bin bag to cover the scope it  rains.

Cheers

[Charic]

On 10/10/2016 at 22:42, noeliam said:

 Will i get Dew with a Dob? Do i need to make a Dew guard as others have done,Thanks

Its possible, if conditions allow, but the Primary mirror is pretty well protected by the fact that it sits at the bottom of the tube, although the secondary could dew up?

I remember a  night when I first bought my scope, and during the short session away from home , in awe of seeing M31 in my 25mm,  and thinking I needed something with a wider field,  the images started to fail, thinking it was a dew problem, I wiped the eyepiece, then again, then went to change the eyepiece for one in the car when I discovered the door seals were frozen? I just didn't realize it was so cold in all the excitement of getting away to a darker site. I smartly packed away the scope and on return to the city lights, realized all the cars were covered in ice too.  I just saw a clear night ahead of me, I had not checked to see if dew would be present, but at least I could go home again, a little difficult if your out to Kielder.

I use a Dew Shield mainly for stray light reduction, camping foam underlay,  a little over  30cm long, 'stitched' with tie-wraps? and just tight enough to seat comfortably over the end of my scope,  but I  can't  actually remember a night  ( there have been very little this Year ) but overall, have not noticed a night whereby my scope was covered in Dew, water droplets, until the scope comes back indoors after a session.

As for an away fix, shielding by actually covering the end of the scope may work, but the obvious pitfall here is you won't see anything with the dust cap in place!
As mentioned above,   keeping the scope horizontal between viewing?  Don't point the scope towards the sky, another pitfall? as that's where we need to point in order to see the Stars, but the problem there is your  scope is looking  up into deep space, and the culprit,  thermal radiation of heat, even though your scope may have cooled to ambient! There is more info here on the subject http://www.astro-tom.com/tips_and_advice/dewing.htm

If local conditions allow for  the formation of Dew, then keeping your equipment above the dew point will alleviate dew from forming on your equipment. A 12 or 24 volt  hair dryer, something I don't posses,  or  maybe use a fan ( I use a PC fan with a 9 Volt battery ?)  dew  should not form under windy conditions.

I have given up maybe too easily in the past, but indoors is just a  warming few feet away, there's always another night, but different  no-doubt if away for a planned weekend, so something portable maybe required for my needs in future. I have a pocket hand warmer that uses charcoal, maybe sit one on a plate on the base of the scope under the OTA, but that would need some further testing, as it seems to defeat the process of cooling the scope in the first place! but might be essential in order to get the most of an evening away, when conditions for viewing could be at their best.

The dew point temperature can be obtained by using the Clear Outside app provided by First Light Optics https://clearoutside.com/forecast/50.7/-3.52    providing the app works for your area? I have mixed results, with their cloud cover readings,  due to the unique weather patterns that surround the Moray coastline, but overall the app is  quite informative.  

[scarp15]

At Kielder there is a very good purpose built warm room facility. If your eyepieces do become overwhelmed then you could potentially take them in here to dry out.  If planning on taking a 12V, you can charge the battery in here.  Otherwise advice as above regarding the scope will help. Others on the campsite might be able to assist if needed and when I have been to Kielder I have taken a 12V hairdryer (if plugging into the car mind you don't run down the car battery).  Kielder can become a dew magnet, if you have a pair of binoculars then take them along to.

[noeliam]

I have some thermal blanket double layered with like bubble wrap inbetween, also have a cover for the scope made out of it, would some of this be any advantage if i put it round the whole tube? Just a thought!!

 

Steve

[scarp15]

On 10/16/2016 at 20:01, noeliam said:

I have some thermal blanket double layered with like bubble wrap inbetween, also have a cover for the scope made out of it, would some of this be any advantage if i put it round the whole tube? Just a thought!!

 

Steve

No not such a good idea, as this will only trap and exacerbate thermal air currents inside the tube causing image distortion. The mirrors and tube needs to reach thermal equilibrium with the outside air temperature , this blanket might also have the disadvantage of trapping any moisture. In truth and in good observing conditions the scope should be fine, as said just cap it / point it horizontal in-between periods of use. An overnight scope cover would be required though.  A ground mat will also be very useful and provide some separation from the damp ground. 

[noeliam]

8 hours ago, scarp15 said:

No not such a good idea, as this will only trap and exacerbate thermal air currents inside the tube causing image distortion. The mirrors and tube needs to reach thermal equilibrium with the outside air temperature , this blanket might also have the disadvantage of trapping any moisture. In truth and in good observing conditions the scope should be fine, as said just cap it / point it horizontal in-between periods of use. An overnight scope cover would be required though.  A ground mat will also be very useful and provide some separation from the damp ground. 

Thanks Scarp15 great advice as ever, Now what are we gonna do about the weather, long range forecast doesn't look too promising 

[scarp15]

I'd take that long term weather forecast with a pinch of salt, besides weather conditions at Kielder can be very localised. 

[James]

Last autumn at Kielder one of the nights was predicted to be wall to wall cloud but we got several hours of clear skies - I took a series of timelapses and in them you could see the cloud skirting around to the north for part of the night, the south for another part. I took one from up the road leading into the campsite and I could see cloud all around, just not over the immediate area. So, yes, as per scarp15, take the long range with a pinch of salt

"as said just cap it / point it horizontal in-between periods of use. An overnight scope cover would be required though.  A ground mat will also be very useful and provide some separation from the damp ground. "

scarp15 again  Yep, that's exactly what I do. I have usually taken along a 12v hairdryer as well for the secondary just in case but I've gotten very used to just putting the cap on in between looking at objects.

A dew shield will probably help a little, before you spend your hard earned try making one first and seeing if there's an appreciable difference.

James

 

[Knighty2112]

I don't have a dob as I'm a 'frac man, but do have a finder that does dew up quite easily, so used some foam sheet I got for a pound to make a cover to protect it. I'm pretty sure you could make a decent one for other finders too that are exposed more to the cold, especially the Telrad, but with some ingenuity you could fashion one for a straight through or right angled  finder too perhaps, leaving a little flap part to lift up to view through the eyepiece when required. Here's some pics of my pound mod for the finderscope.