5" newtonian, dew and heater

[FrenchyArnaud]

Hi guys, 
I have not posted here for a loooong time...

As I am, after a long break, returning to AP, the Skywatcher 130PDS is out and about again! However, I have a minor problem with dew, which will soon likely turn in a problem with frost. I am struggling to find a definitive solution, ideally a ready made heating kit for primary/secondary. I considered a shield but it occured to me that the tube is already a shield for the primary and it still fogs up. 

Anyone knows of a proper solution that would allow both mirrors to stay actually dry and bright?
Thanks, 
Arnaud.

My latest image for tax (DSLR, coma corrector and CLS filter, celestron Advanced GT, 130PDS, bortle 7) 

Edited November 7, 2021 by FrenchyArnaud

[Zermelo]

Dewing up on a Newtonian primary is uncommon and suggests very damp conditions. As you say, it is shielded by the tube itself, so adding a further shield is unlikely to make much difference. The secondary, though, will certainly benefit from a shield, and that should be tried first.

Commercial heaters for secondary mirrors tend to be more suitable for larger scopes, but there are some made to fit smaller secondaries, like this one. There are other designs like the "split oval". Check the measurements. A home-made solution is not difficult if you're reasonably practical - a few power resistors glued to the back and a cable taken across one of the spider arms. Another suggestion is a hair dryer; 12V travel versions with a plug for a lighter socket are available, but your power tank would need to be up to the job. A 2-speed version would be better.

Some links:

https://stargazerslounge.com/topic/323378-really-bad-dew-on-secondary-newtonian-130pds-mirror-dew-shield-not-helping-help/#comment-3524033

https://stargazerslounge.com/topic/264748-secondary-dew-heaters/

 

[Carbon Brush]

Leaving a scope pointing at a clear sky for a long time means the mirror will cool below ambient temperature.
Looking around with an infra red thermometer will show the clear sky to be 20/30C cooler than your surroundings.
I don't know if this might be happening in your case?

If so, just a small fan forcing fresh air behind the mirror will prevent cooling below ambient.
Very little burden on a powertank.

As the air is not warmed, there is no woory about conenction currents disturbing the view.

I don't know if you scope is closed or open at the back of the mirror. Small newts tend more often to be closed so a fan is not a 5 minute fit.

If the non-skyward parts of the installation get dewed, this indicates the air is saturated so a small heater is called for.

HTH, David.

[Tiny Clanger]

1 hour ago, Carbon Brush said:

 

If so, just a small fan forcing fresh air behind the mirror will prevent cooling below ambient.
Very little burden on a powertank.

 

Totally coincidentally, I noticed these on the FLO new arrivals page a week or so ago https://www.firstlightoptics.com/misc/asterion-nt-active-cooler-for-sky-watcher-newtonian-telescopes.html while described as a cooling fan , they might be better thought of as an equalising fan  !

Dew point can be a very local thing, just a few paces away can make a difference because of airflow , changes in elevation , vegetation etc , if the setup is not fixed, it might be worth while OP experimenting with different locations.

Heather