Dew problem on a Newt reflector

[Keithp]

I've seen other posts where apparently you're not supposed to be subject to dewing problems with a Newtonian Reflector but a few nights ago, before the cloud arrived I experience heavy dewing on the secondary mirror and when I looked down the tube about 75 percent of the primary was covered as well.

I can reach the secondary and give it a quick wipe with a special cloth for cleaning glasses and that sorts it for about 10 minutes but the primary is obviously out of reach.

With regard to the 12v dew straps that are available would it be possible to attach one on to the secondary mirror holder and possible tape one to the base of the primary mirror to prevent dewing? And if it is would the gentle warmth on the primary cause any problems?

Regards

Keithp

[Aza]

I have made a dew shield from a camping mat!

Simply cut it to length, roll around the tube, and stick with duck tape.

Works a treat!

I will post a pic in a minute.

[Aza]

I also made one for my refractor with the left overs of the camping mat.

Never had a dew problem since

[Keithp]

Neat solution Arran, might hunt around for a black one though, purple might clash abit:)

Do you wrap that round flush with the top of the tube or have it extend beyond the top and by how much. Also makes me think about wrapping one all down the tube plus maybe extending it beyond the top edge.

Regards

Keithp

[DaveGibbons]

I've seen other posts where apparently you're not supposed to be subject to dewing problems with a Newtonian Reflector but a few nights ago, before the cloud arrived I experience heavy dewing on the secondary mirror and when I looked down the tube about 75 percent of the primary was covered as well.

I can reach the secondary and give it a quick wipe with a special cloth for cleaning glasses and that sorts it for about 10 minutes but the primary is obviously out of reach.

With regard to the 12v dew straps that are available would it be possible to attach one on to the secondary mirror holder and possible tape one to the base of the primary mirror to prevent dewing? And if it is would the gentle warmth on the primary cause any problems?

Regards

Keithp

Keith, Firstly stop wiping your secondary with any form of glass cleaning cloth it's an absolute no no:eek:. You will inevitably damage the ultra thin aluminium coating and it is impossible to avoid scratching .

Use a flexible dew shield on the top of the tube one thats made for a 10" sct will fit perfectly, they help delay the onset of dewing on the secondary mirror. Always keep the ota capped when left outside and not viewing through it and when dew does form use a hairdryer to gently blow an airstream across secondary and primary as needs be. It's a pain but something you learn to live with in our climate.

I know you can get specialised secondary mirror heaters that have to be fitted on the backs of the secondary mirror but involve taking the holder apart and reattaching the secondary , not as easy as it sounds.

using a standard 12v type strap would render the scope useless I think due to heat currents generated within the tube.

Probably the only really permanent fix is to build an observation shed or move to a Desert location:)

Cheers

Dave

[Aza]

My one just slips about an inch over the end of the tube.

Its all good as long as it is not windy!

Since using these I have never had a dew problem, and I live real close to a river (dunno if that gives you more dew or not).

[Sam]

I use a cardboard dew sheild made from very expensive corrogated cardboard (expensive because i had to use about 30p worth of clear tape to stick it together:D). Never had a problem with secondary dewing up since!

[Astro_Baby]

Agree with Dave - absolutely DONT go wiping the mirrors with anything. You shouldnt touch them at all.

You can get secondaey dew heaters but after seeing an accident with one on another forum (the heater got too hot, melted the glue on the secondary holder and the secondary fell off onto the primary ) I'd be cautious about using stuff like that.

The dew shield is an aboslute must for a reflector - it not only cuts down dew it act as a light sheild as well and prevents stray light going down the tube - it will also (allegedly) improve contrast.

[Keithp]

Thanks for all the replies and advice everyone, I'll try the shield route for now.

Duely noted about wiping the secondary.

Regards

Keithp

[Keithp]

Just re-read the whole thread I can't believe I posted

'Duely noted about wiping the secondary.'

[Aza]

Haha I love unintended puns

[DAVPPK]

Just re-read the whole thread I can't believe I posted

'Duely noted about wiping the secondary.'

LOL

[Vega]

The dew shield is an aboslute must for a reflector - it not only cuts down dew it act as a light sheild as well and prevents stray light going down the tube - it will also (allegedly) improve contrast.

I can vouch for the stray light and contrast improvement. My back garden in plagued by street light. I tested the scope with and without the dew sheild and I was shocked at the improvement in contrast (I'll admit it varied depending on which direction I was using it). With the shield I have never had any dewing or fogging of the secondary

Matt

[Steve922]

Why (How?) does the Dew shield prevent the condensation?

Steve

[Astro_Baby]

I have no idea but it does work.

Dew - as far as I know - is caused when cold air hits a warm surface but I always notice the scope tube dews up on the ouside while the mount and the tripod legs never do - not sure why that should be.

If its super wet and dewy the dew shield wont protect the secondary forever and I'm told at Kelling in April it was so bad even scope with dew shields suffered mightily which is why I have a 12v hairdryer as well as a backup.