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Old 12-09-2019, 02:40 PM
eclectiK (Roger)
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Dew shield vs dew heater vs dew shield + heater

Hi all,

There is so much conflicting information.

I understand dew/condensation forms on the telescope when the temperature of the telescope is lower than the dew point of the air - which is what I found out spectacularly last night as I might has smeared Vaseline over my telescope.

However, some sites say a dew shield is enough, some sites say you need a heater with a dew shield.

So 2 questions:
- will a dew shield typically suffice? (for a Sydney winter) i.e. kmart yoga mat.
- do i really need to go the heated route?

Cheers
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Old 12-09-2019, 02:49 PM
Wavytone (Nick)
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1. What’s the scope ? SCT/refractor/newtonian ?
2. Where did you set up ?

Things to note:

A. Dew should not be an issue if there is a breeze so no need for dewcaps etc if there is one.

B. The local microclimate around you has a lot to do with dew - setting up on lawns, ovals or anything else with grass and black soil that has been watered is going to have dew. Conversely light sandy soils don’t hold water and even better sandstone outcrops... won’t have dew on these. Ditto tarred or concrete pavements.

Simply picking the scope up and moving a few metres to a different surface can make a significant difference.

C. Colour of the OTA affects the rate of heat loss - black is the worst.

D. If the scope has a closed tube (SCT or maksutov) consider applying an insulated dewcap that extends back over the OTA and/or insulation around the OTA to retain whatever heat it has acquired earlier. This has proven very effective against both dew and the internal tube current these scopes often develop, despite being contrary to the mantra of “must cool the scope”. OTOH if the scope is an open-tube type (newtonian or Cassegrain) this won’t have any effect and you’re stuck with using fans.

E. Install a small fan to blow air across the surface you want to keep dry. It works.

F. Add a heater strap around the OTA, particularly effective if this is underneath some insulation (eg EVA foam or yoga mat). Which brings us to...

G. Don’t cool the scope using forced ventilation (fans).

H. Use a handheld warm hair dryer to evaporate the dew and warm up the optics a tad. There are portable ones that work off 12VDC.

I. Any combination of the above.

Basically don’t let the scope cool in the first place, insulate the OTA and use a dewcap. If however dew does form a blower is probably the best way to remove it.

Last edited by Wavytone; 12-09-2019 at 03:15 PM.
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Old 12-09-2019, 03:05 PM
eclectiK (Roger)
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Hi thanks for replying.

Quick answers,

1. It is a SCT 10inch meade
2. I set up in the backyard on grass, but its sheltered by the carport and neighbor fences so there is typically no breeze unless its really windy
3. It sits outside under the carport all day under a plastic bag as a dust cover so its generally is whatever temperature it is outside. (the sun cannot shine directly onto it)

4. my last resort was to grab a hairdryer on low heat and blast the surface.

Does that mean there is no point to a dew shield?

Cheers
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Old 12-09-2019, 03:26 PM
Startrek (Martin)
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When in Sydney, Iím located quite low at the bottom of a hill near river level. I observe and image with newts in the backyard
The only system that works for me are dew heaters , couldnít do without them
I previously tried dew shields and fans etc. but my dew heater system allows me to observe and image all night even in a heavy dew ( drips off the OTA )
I guess itís trial and error and all sites are different
Obviously try a low cost method first
Cheers
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Old 12-09-2019, 06:56 PM
N1 (Mirko)
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SCTs are probably more likely to require active heating than say a newt. Very exposed surface. There are some good threads on here discussing the merits of insulation as mentioned below.
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Old 12-09-2019, 10:51 PM
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billdan (Bill)
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My experience is that Dew shields were not very effective, they were OK at 45 - 60į elevation, but when getting close to looking straight up, then they were useless.
I also stopped using hair dryers, they blow dust onto the optics and you also keep chasing focus for the next 2-3 minutes, and it dews up again after 15 minutes.
Dew heaters are the way forward I think.
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Old 13-09-2019, 07:36 PM
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The_bluester (Paul)
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My experience is with the C925, no dew heater, forget it except in quite warm weather.
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Old 13-09-2019, 07:40 PM
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ChrisV (Chris)
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I found a dew heater did a good job on it's little brother, a C8
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Old 13-09-2019, 08:36 PM
RyanJones
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Quote:
Originally Posted by The_bluester View Post
My experience is with the C925, no dew heater, forget it except in quite warm weather.
Totally agree Paul. I have used home made dew shields on my 4SE MAK and my C5 with no dew issues at all. With my 9.25 itís a completely different story. I have a Celestron dew shield for it and itís only really good as a light shield. With such a large piece of glass in the corrector itís no surprise. My Newts on the other hand Iíve had major issues with the filter on the end of my camera dewing up in the focuser tube. Iíve addressed this with a small dewstrap around the focuser.
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Old 14-09-2019, 11:36 AM
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ChrisV (Chris)
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Quote:
Originally Posted by RyanJones View Post
Totally agree Paul. I have used home made dew shields on my 4SE MAK and my C5 with no dew issues at all. With my 9.25 itís a completely different story. I have a Celestron dew shield for it and itís only really good as a light shield. With such a large piece of glass in the corrector itís no surprise. My Newts on the other hand Iíve had major issues with the filter on the end of my camera dewing up in the focuser tube. Iíve addressed this with a small dewstrap around the focuser.
I know I'm off topic but thanks Ryan. That's a great idea for dew on the newt.

Chris
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Old 15-09-2019, 10:59 PM
eclectiK (Roger)
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Hey all, thanks for the responses, after much internal deliberation with my wallet, I find an old hairdrier works wonders for the 3 minutes it takes to film Jupiter etc

Plus side, Saves me from having 4 million cables dangling from the sct.

Rog
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Old 16-09-2019, 08:24 AM
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mental4astro (Alexander)
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I've been insulating my Cats for a few years now, both SCT and Mak.

The dual purpose insulation/dew shield does two things:

1, stops internal heat plumes from forming from a warm primary & baffle tube as the insulation stops the tube from getting cold, so the temperature differential and gradient between tube and internals is much reduced to close to nil, so you can start using the scope straight away without needing to wait for the scope to cool - you can use high magnification from the very start.

2, keeps the corrector dew free for longer.

See here:

Thread about insulating a Cat


My Mak, naked: Click image for larger version

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My Mak with the insulative dew sheild: Click image for larger version

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This does not mean the corrector does not dew up. If conditions are right dew will form, but much later. I live across the road from a lush park in Sydney, and not far from the coast, dew is ever present even with the weather apps say the dew point is too low for dew to be forming, everything is soaked here... the microclimate at my place is a bugger. I'm finding the corrector of my 7" Mak holds out well for between three and four hours before the corrector begins to mist up (and I've been using 300X and higher from the start) if at all, and the SCT for a couple of hours. A hair dryer is all I use to gently evaporate the dew, but I am not imaging.

Problem with dew heaters is they introduce a heat source that is an issue for a few reasons, 1, Majority of the heat goes into the metal tube and out to the atmosphere, very little into the glass, 2, the perimeter of the glass is heating (expanding), while the glass deeper in the glass away from the heat source remains cooler, and you end up with developing strain in the glass which distorts it, the last thing we want to do!

...

A better solution would be to insulate and use a dew strap together if you plan to be out for an extended period. The heater could be switched on to a very low setting from the very start. The insulation retains heat for longer inside the OTA, and the dew heater works with the insulation to make for a much more efficient thermal system that vastly reduces any strain inside the corrector due to heat differential. Refractors would also benefit from the same insulation method for the same thermal dynamic reasons as the corrector of a Mak and SCT.

Best of all would be to use a small fan inside the dew shield assembly instead of a heating strap - no heat-induced distortion in the optics or OTA.

Alex.
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