SCT Dew

[Westmoorland]

I thought pointing an SCT downwards gets rid of the dew? I also pointed the eyepiece downwards and that's fogged up.  I shined a torch at the corrector plate and on top of the dew it's covered in obscene looking smudges despite me never touching the thing. I don't think I'll bother putting the dew shield on again. How do you get past this stage?

[Stu]

2 hours ago, Westmoorland said:

I thought pointing an SCT downwards gets rid of the dew? I also pointed the eyepiece downwards and that's fogged up.  I shined a torch at the corrector plate and on top of the dew it's covered in obscene looking smudges despite me never touching the thing. I don't think I'll bother putting the dew shield on again. How do you get past this stage?

No, pointing it down helps prevent dew forming when it is cooling or you are not using it.

I would look into getting a heated dew shield if I were you, plus potentially a 12V hairdryer to clear it if it gets bad. Some nights are much worse than others for dew too, but with a big SCT you generally always need to be prepared for it.

[Mr Spock]

I'd go for the hairdryer option. With SCTs dew heaters tend to create tube currents and low level 'tremor' in the image.

I saw a good lecture a few years ago by Damien Peach where he detailed all this. With a hairdryer the effect on the scope is only temporary.

[Elp]

If it's a Celestron I use their custom dew heater rings which replace the ring sitting on the outer edge of the corrector plate. Works wonders, removed the dewed over plate in less than five minutes, combined with a dew shield didn't dew over again.

[Westmoorland]

Thank you, all. Looks like a hair dryer until I can be sure my Celestron lithium power tank can power the scope and a dew heater at the same time: I've got a nasty feeling it won't.

[Elp]

The ring uses a 12v input, if you were using a power unit box or an Asiair pro/plus it would work via the 12v outs.

[Westmoorland]

7 minutes ago, Elp said:

The ring uses a 12v input, if you were using a power unit box or an Asiair pro/plus it would work via the 12v outs.

The Celestron lithium battery ought to be all I need for visual.  I'll resist extra box/units for as long as I can. I won't have anywhere to put them.

[Cosmic Geoff]

I have found that dew settles preferentially on upward facing surfaces.  I always use a dew shield and find this is adequate 99% of the time. More condensation-prone locations may require adopting heated solutions in addition.

[Merlin]

The point is to make the shield long enough. Three times the aperture works for me.

[bemak187]

On 19/09/2022 at 09:36, Elp said:

If it's a Celestron I use their custom dew heater rings which replace the ring sitting on the outer edge of the corrector plate. Works wonders, removed the dewed over plate in less than five minutes, combined with a dew shield didn't dew over again.

Next on my list for my 11 inch.  Glad to hear they work well.

[Elp]

1 hour ago, bemak187 said:

Next on my list for my 11 inch.  Glad to hear they work well.

Bear in mind mine is a C6 so it clears up faster than a larger scope I presume.

[beamish]

On 19/09/2022 at 09:36, Elp said:

If it's a Celestron I use their custom dew heater rings which replace the ring sitting on the outer edge of the corrector plate. Works wonders, removed the dewed over plate in less than five minutes, combined with a dew shield didn't dew over again.

Are you able to throw some light my way...
Ive bought one of the celestron heater ring to work with my Kendrick controller but my plugs are in the opposite position to images on the manual pdf. I dont want to blow it 😉
So plse confirm which is the "power socket"

[beamish]

Fwiw, when i was using the C11 i had a custom heater band placed around the front cell and a shield, never once had a problem with dew again.

 

[Elp]

1 hour ago, beamish said:

power socket

It's the one with the male upstanding pin in the centre, it's physically impossible for a  5.5mm/2.1mm jack 12v cable to plug into the thermistor control one looking at it.

[beamish]

Cheers, 👍👍

[LukeSkywatcher]

On 19/09/2022 at 08:27, Mr Spock said:

I'd go for the hairdryer option. With SCTs dew heaters tend to create tube currents and low level 'tremor' in the image.

I saw a good lecture a few years ago by Damien Peach where he detailed all this. With a hairdryer the effect on the scope is only temporary.

That's actually really good advice. I'd never of thought about it like that.

I do have a dew heater, but will buy a small travel hair dryer.

Good old Damien Peach.

Edited November 8, 2022 by LukeSkywatcher

[dweller25]

@Westmoorland
Try wrapping the OTA with 2 layers of reflectix from B&Q 👍

 

 

Edited November 8, 2022 by dweller25

[cjdawson]

I'm a bit late to this party, but want to give my experiences anyway.   SCT's are very likely to dew up if prevention isn't applied straight away.   Here's how I do it.

1. Dew Shield.   This is a must, don't use the scope without one, it will slow the heat radiation by a significant amount.   Which will mean that part two doesn't need to work as hard.

2. Dew heater.  You can get or  make (I made) a dew heater that sits in front of the correctly plate, and provides enough heat to replace the energy lost even with the shield in place.   This will prevent dew from forming, but at the same time allow the scope to cool to get stable images.

3. We all get caught out, so get a 12v hair dryer.  This can be used in the event that you had the heater on too low, and the the optics misteded up anyway.    It's a last resort, but one that's handy to keep on hand should it be needed.

 

I find that the combination of 1 and 2, sorts out 90% of observiing sessions, even when everything else gets soggy.    As for eyepieces, keep them in your pocket when not in use and that'll keep 'em warm.

[knobby]

@cjdawson

Hi, do you have any images of your dew heater ?

I'm DIY ing something at the moment and had considered placing it inside the front lip but thought it would stop the cap fitting and also make electrical connections difficult.

Cheers 

Edited December 18, 2022 by knobby

[newbie alert]

Corrector plates are a magnet for dew,  dew shield and a dew heater sorts it out... Found hair drying method is only short term and your focus changes for a few mins after applying, so you have to wait a while it sorts itself out, then repeat once dew forms again

 

[bosun21]

For the 12v hairdryer make sure you have a heavy enough 12v supply. Around 15A

[cjdawson]

On 18/12/2022 at 22:33, knobby said:

@cjdawson

Hi, do you have any images of your dew heater ?

I'm DIY ing something at the moment and had considered placing it inside the front lip but thought it would stop the cap fitting and also make electrical connections difficult.

Cheers 

 I’m sure that I have some pictures somewhere.   I’m on holiday at the moment so can’t take any fresh pictures at the moment.

From what I remember the heater that I made is constructed like this….

 

1. NiChrome wire (resistance wire) as the heating element.  Run 12v through, at 100% duty cycle, I think it worked out to be about 30 watts of heating.  (About 2.4 amps). Though in reality, I run it from a PWM, and normally at less than 50% duty so keeps my scope clear even on the soggiest nights.

2. The wire is threaded through heat shrink tubing, with the ends poking out,  the ends are connected to another wire, think I used a solder, but it’s mainly a mechanical joint.  Wasn’t happy with this and would do something different next time… though it’s lasted for years without issues.

3. The wire with heat shrink is then threaded through copper brake pipe.

4. The pipe was then bent to form a circle.

5. finally spray painted matte black.

 

The whole thing is held in place on the SCT by pressure alone.

the ends of the heater wire are simply taped to the side of the Ota, and run down to a phono plug which connects to the PWM (Heater controller).

 

If I find out a piccy, I’ll post it here later