Symptoms of dew on secondary?

[Zermelo]

I have a 150mm F/5 Newtonian and I use a home-made foam dew/light shield when observing.

A couple of times, at the end of sessions, I've noticed what I'd decribe as a misty, diffused "flare" surrounding brighter objects, extending out for perhaps one third of the radius of the entire field.

Is this being caused by dew on the secondary? I've not been able to check it directly to see.

[wulfrun]

I was out about a week ago when there was serious dew, to the point that I gave up. Eyepieces, finder, Telrad, OTA, you name it it was dewed. I looked down the empty focuser and could see a dewy haze on the secondary, using a torch. It's the first time I've had that but my point is, just use a torch down the draw-tube and have a look.

[Zermelo]

7 minutes ago, wulfrun said:

I was out about a week ago when there was serious dew, to the point that I gave up. Eyepieces, finder, Telrad, OTA, you name it it was dewed. I looked down the empty focuser and could see a dewy haze on the secondary, using a torch. It's the first time I've had that but my point is, just use a torch down the draw-tube and have a look.

Thanks, I'll try that next time.

[Mark at Beaufort]

I have the same scope and occasionally have few on the secondary. I have 12 volt hair dryer which I use to clear it.

[Zermelo]

19 minutes ago, Mark at Beaufort said:

I have 12 volt hair dryer which I use to clear it

Yes, I do too - unfortunately I also have quite a small back garden and neighbours who all sleep with their windows open.
I've not felt justified using it a 2 a.m. !

Medium term plan is to make a resistance heater (I've bought all the bits).

Long term plan is to move somewhere with no neighbours.

[Jiggy 67]

These are well worth considering for an 8” their maybe smaller ones available somewhere 

https://www.firstlightoptics.com/dew-prevention/4tronix-secondary-mirror-heater-for-8-12-inch-newtonian-telescopes.html

Edited June 1, 2021 by Jiggy 67

[tuckstar]

I have a resistor on my secondary. Shares a port on my dew controller, via a splitter, with my telrad heater, also a resistor heater.

[Zermelo]

6 minutes ago, tuckstar said:

I have a resistor on my secondary. Shares a port on my dew controller, via a splitter, with my telrad heater, also a resistor heater.

Yes, I have a Telrad too, and that definitely mists up. I plan to do the same.