21 replies to this topic

[BrickInTheSky]

My setup is a Vespera 2 with a hygrometer sensor and frequently a filter. I am in sub tropical humid climate and the temperatures are not falling down to a dew point easily (only towards the morning). I had noticed that occasionally I get some spots on the filter that I think is a tiny dew. Vaonis is not vey clear on when exactly the heating of the lens is activated (they say when it is close to a dew point but do not say what ‘close’ means) and where will I see that it is or was activated (I see the sensor reports the temperature and humidity but not the activation of the heating).

Now to the main question: I see people attach a plastic hood to the lens and that supposedly helps with the dew. Please explain how? I can understand that if there is some water droplets falling down and the scope is pointing at a lower target some of the droplets will be blocked. But of the scope is pointing up or it is a dew that is formed everywhere in the air because temperature had fallen to a dew point: how a piece of plastic helps? My outside furniture gets dew everywhere: top surfaces, bottom surfaces, etc…..

[former lurker]

Dew doesn't "fall"... That would be drizzle rain or mist. Dew condenses on cooler surfaces that are below the "dew point". The primary cause of a surface being below the dew point at night is radiational cooling. That is when the surface is radiating heat to the cold of deep space thus getting the lens surface cooler than the airtemperature.

[former lurker]

Oops. Premature message sending...

The dew shield blocks radiational cooling of lens surface.

[BrickInTheSky]

Oops. Premature message sending...

The dew shield blocks radiational cooling of lens surface.

Btw: you can edit your previous replies :-)

As I mentioned in the original post, I agree with you on the dew: it is not falling from above. And I guess we agree that the plastic hood is not effective against anything that is falling down (especially when the lens is pointing up).

So you are saying that the plastic hood helps by diminishing radiational cooling? How? By shielding from the wind? I am looking for an explanation of the process, not just the fact that I have to take on faith. :-)

[kabes]

Btw: you can edit your previous replies :-)

As I mentioned in the original post, I agree with you on the dew: it is not falling from above. And I guess we agree that the plastic hood is not effective against anything that is falling down (especially when the lens is pointing up).

So you are saying that the plastic hood helps by diminishing radiational cooling? How? By shielding from the wind? I am looking for an explanation of the process, not just the fact that I have to take on faith. :-)

It reduces radiation to the cold sky. This aspect is obviously greatly reduced if you are looking at the zenith. It also reduces air exchange by extending past the glass, it cuts down on convective currents of humid air reaching the lens. That column of air in front of the lens also acts like a bit of an insulator, keeping the lens a bit warmer than the surrounding air for longer. Don’t want to emphasize that part too much though, reducing radiation to the sky is really the main thing it does. 
 

I use one on my Seestar mainly to block ambient light from cars, neighbours, etc. as it has the dew heater.  In that case it’s important the interior absorbs light as plastic can be shiny. I sanded the interior of mine and used ultra black paint. 

Edited by kabes, 14 June 2025 - 08:43 AM.

[former lurker]

Yes, the plastic hood helps reduce radiational cooling by limiting the angle of deep sky exposure. The lens surface is radiating heat directly to a much smaller area of deep space. And any condensation on the dew shield that does happen will change the dew point near the lens surface. And less heat from the scope will escape to deep space.

Plastic is a much better thermal insulator than metal, or nothing at all. So slightly more heat is retained on the lens surface.

And shielding from the wind helps too.

[Don W]

I live in the high desert of Arizona.

 

What is this dew thing you are talking about 

 

LOL

[PFJs Customs]

Here is another thread by Flexydad where he looks into the benefits of a dew shield both for the dew factor and other factors including stray light.

 

https://www.cloudyni...#entry14168223 

 

Hope it helps the discussion here. 

[BrickInTheSky]

I live in the high desert of Arizona.

What is this dew thing you are talking about

LOL

Yea, probably not an issue for you. How about dust?

I am in the hot and humid place but the temperatures are quite stable. Here are a few temperatures (according to the Vespera hygrometer sensor) of my latest observations. Yep, its gets hotter during the night :-)

Start Temperature / End Temperature
25.8 / 25.6
26.5 / 28.5
28.1 / 29.3


Start corresponds to about 10 pm and end at about 3 am. The dew point is around 23 degrees that I do get occasionally :-)

Edited by BrickInTheSky, 14 June 2025 - 10:35 AM.

[Don W]

Dust is only a problem when it’s windy. I would rather clean off dust than chase dew all the time.

[sanford12]

I won't run without one. On nights with really heavy dew you can have some run down the outside of the tube and get on the lense when removing it especially if it's pointing straight up. I use a camera lens microfiber cleaning cloth.

Edited by sanford12, 14 June 2025 - 04:08 PM.

[bradhaak]

They're very effective, but not so much for dew as for stray light.

 

A few fortunate folks live in places that are fully dark. But the majority of us live in urban/suburban areas with street lights, car lights, landscaping and house lights, and just about any other form of ambient light pollution imaginable. plastic dew shields block a lot of it, and a few (like mine) have baffles to reflect some of the stray light back out rather than down into the scope. 

[gus1989]

They're very effective, but not so much for dew as for stray light.

 

A few fortunate folks live in places that are fully dark. But the majority of us live in urban/suburban areas with street lights, car lights, landscaping and house lights, and just about any other form of ambient light pollution imaginable. plastic dew shields block a lot of it, and a few (like mine) have baffles to reflect some of the stray light back out rather than down into the scope. 

Did you 3d print this yourself with the baffles?

[Amazed]

If you use a snap on plastic shield, be careful the arm does not close while the shield is still attached. 

[bradhaak]

If you use a snap on plastic shield, be careful the arm does not close while the shield is still attached. 

There are a number of dew shields being sold that will safely remove themselves. There is also a printable STL for a dew shield on Thingiverse that has that feature.

[Astronotrip]

For my Vespera 2 I 3D printed a dew shield (also covers lights ofc). Very useful I always use it.

Here I wrote sthing about it and you can see pics also (english translate widget on top of the article):

https://www.astronot...pour-votre.html

 

and direct link to the dew shield files for pinting (it includes a filter holder): https://www.thingive...m/thing:5455237

[CraigR]

I can only speak about the Seestar S50 (and I assume the S30). It has an effective dew heater that you can manually turn on and off.

 

The most prolific Seestar users are those who are hosted at Starfront Observatories. Those Seestars stay powered up 24/7, with the arms closed when not in use. They're in buildings with retractable roofs, so they're basically outdoors 24/7/365. I had an S50 there for about 5 months and I know people who have been there for 10 months — again, with the Seestar sitting basically outside all the time. In the time I was there, I had many nights where the the temperature/dew-point spread was very small. I never had any trouble with dew on the lens, or spotting from dew/dust, or anything else dew related. And I never had a dew shield.

 

With this in mind, on the Seestar at least, I would argue that a dew shield is unnecessary for prevention of dew-related problems. If it's good for anything, it's stray light hitting the lens. So they more correctly should be called "light shields", and I would be in favor of those if you're in an environment where street lights and other light sources are a problem.

 

My refractor is also at Starfront. It has an extendable dew shield, and I have a dew heater strip just behind the objective. While I power down the mount and camera during the day, the dew heater controller and dew heater stay on 24/7 since the lens is continuously exposed. My experience is the same as others there; we don't have problems with dew even though our scopes are outdoors all the time. And lest you think it's relatively dry there, this is Central Texas, and as I type, the humidity is 96%. :-)

 

If you want to be especially cautious, check the forecast for times when the temperature is within about 10°F (5°C) of the dew point and bring your scope indoors during those times.

 

So — dew shields if you need them for stray light. Otherwise, the dew heater does the trick.

[AhBok]

My experience is similar to CraigR. I live in the sultry south where dew is a big issue. If I am using the S50 more than an hour, zi turn on the dew heater and use an external battery. A dew shield will slow, not prevent dew formation for the reasons already stated (especially slowing cooling of the boundary layer of air in front of the objective). The shield doesn’t help with a light polluted sky obviously, but if you are getting stray direct light from an annoying source, then I find using one to be useful,

[sanford12]

I've used a dew/light shield everytime with the S50. The only time dew made it onto the the lens was when it was pointing straight up and then it was just a small drop. Last night the S50 was dripping with dew after shooting all night but it was on the horizon the whole time, not a bit of dew on the lens. Most here aren't going to send their scope to a telescope farm. One question is who cleans the lenses of the scopes that are there. There's going to be dust and pollen.

Edited by sanford12, 20 June 2025 - 05:42 AM.

[BrickInTheSky]

One question is who cleans the lenses of the scopes that are there. There's going to be dust and pollen.

I was wandering same and guessed it is part of the maintenance fee to have it periodically cleaned. I think, personally, I am way to pedantic about cleaning every little speck: it probably does not matter as much as I think it is.

Edited by BrickInTheSky, 20 June 2025 - 02:36 PM.

[Dart106A]

If you use a snap on plastic shield, be careful the arm does not close while the shield is still attached. 

Good advice. I have definitely come close a couple times on my S50! Now I try to just set it lightly in the recess of the lens just in case I forget so it will break free. I would rather replace that, than the scope.

 

73, Mark

[stevecourtright]

My S30 has moderately elongate stars in the upper left corner and I found when I applied a printed dew shield to the scope that the star shapes became more distorted.  The star shapes went back to the previous amount of distortion when the shield was removed.  While I need to test this more, I would advise caution when attaching anything to your scope that you don't also shift the position of the objective.