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General Astronomy >> Beginners Forum

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nikdangr
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Reged: 02/10/13

Loc: Annandale, VA
Dew shield
      #5687529 - 02/18/13 06:44 PM

For $25, is an "AstroZap Flexible Dew Shield For Meade ETX 90 and Celestron C-90" a good investment for my C90 Mak? I live in the Northern Virginia suburbs where the light pollution is pretty awful. Are there circumstance where it can cause vignetting?

(My Porta Mount shipped today. Can't wait!)


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WaterMasterAdministrator
Moat Keeper
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Reged: 02/17/10

Loc: Southeast Idaho, USA
Re: Dew shield new [Re: nikdangr]
      #5687534 - 02/18/13 06:47 PM

Hi Nik,

I think there's little danger of vignetting . I suspect that in VA dew is going to be more of an issue, and I've heard good reviews of the AstroZap gear.


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jgraham
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Reged: 12/02/04

Loc: Miami Valley Astronomical Soci...
Re: Dew shield new [Re: WaterMaster]
      #5687573 - 02/18/13 07:04 PM

Not a bad price for a commercial product. However, I make my own dew shields for my small scopes (5" and under) from thin (1-2mm) black foam rubber sheets from an art supply store and a bit of hot-melt glue. I cut the sheet to length, wrap it snuggly around the tube, overlap the edges about 1/2", then run a bead of hot-melt glue under the seam. These shields are durable, lightweight, and fit perfectly.

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GlennLeDrew
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Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
Re: Dew shield new [Re: jgraham]
      #5687742 - 02/18/13 08:30 PM

Light cut off will only occur if the sheild sags into the light path. As long as the sheild is coaxial, it can be quite long and still no vignetting will occur.

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Seldom
scholastic sledgehammer


Reged: 08/05/12

Loc: N of Cedar City Light Dome
Re: Dew shield new [Re: GlennLeDrew]
      #5688138 - 02/19/13 12:37 AM

Quote:

As long as the shield is coaxial, it can be quite long and still no vignetting will occur.




Infinitely long? Aren't the light sources are so far away the rays are considered to be parallel?


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GlennLeDrew
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Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
Re: Dew shield new [Re: Seldom]
      #5688241 - 02/19/13 02:37 AM

The maximum length depends on how much larger the shield's I.D. is compared to the objective aperture and the field of view. It's the latter factor which imposes the ultimate limit; if zero the cap length can be arbitrarily large. The wider the field of view, the shorter the cap length. To see this, think of a 'standard' camera lens whose field of view is some tens of degrees.

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Jon Isaacs
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Dew shield new [Re: jgraham]
      #5688350 - 02/19/13 05:55 AM

For larger scopes, the Walmart foam sleeping bag pads work nicely.

Jon


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nikdangr
super member
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Reged: 02/10/13

Loc: Annandale, VA
Re: Dew shield new [Re: Jon Isaacs]
      #5688554 - 02/19/13 09:38 AM

Quote:

For larger scopes, the Walmart foam sleeping bag pads work nicely.

Jon


I thought you were kidding until I looked at your scope.

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drbyyz
sage
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Reged: 11/04/12

Re: Dew shield new [Re: Jon Isaacs]
      #5688566 - 02/19/13 09:43 AM

Quote:

For larger scopes, the Walmart foam sleeping bag pads work nicely.

Jon




Exactly what I used! Here's mine plus a few other people posted some home brewed designs. Although $25 isn't a bad price for a commercial one, you can definitely save yourself a few bucks.

http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/5574274/page...


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nikdangr
super member
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Reged: 02/10/13

Loc: Annandale, VA
Re: Dew shield new [Re: drbyyz]
      #5689122 - 02/19/13 03:40 PM

Thanks for the input, guys. I'll go with the AstroZap. Over the years I've accumulated a fair amount of "stuff" from one-off home projects that never gets used again. Whether I've actually saved any money in the long run is questionable.

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GTBaker
member


Reged: 02/01/13

Re: Dew shield new [Re: nikdangr]
      #5689327 - 02/19/13 05:24 PM

Exactly what does a dew shield do and does every scope need one?

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GlennLeDrew
Postmaster
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Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
Re: Dew shield new [Re: GTBaker]
      #5689461 - 02/19/13 06:42 PM

A dew shield reduces the solid angle of the sky as 'seen' from the surface of the optical element being shielded. A clear sky (even on a sunny day) is a cold heat sink of some -30C or so. Upward-facing objects/surfaces radiate into that heat sink. A dew shield blocks a good portion of the sky, reducing the angular size of the heat sink. The dew shield's wall is at roughly the same temperature as the surrounding air (usually a tad cooler, due to its own loss via radiation into the sky), and so presents a warmer heat sink than that of the sky it's hiding.

The more exposed the optic, and the thinner it is relative to its diameter, the more urgent the requirement for a dew shield. SCT front correctors are particularly prone to dewing. MCTs are not so far behind, followed by refractors (the latter of which usually have a dew cap, which in at least some cases can benefit from an extension.)

Newtonian primaries inside solid tubes, or draped with a shroud if of the truss variety, are *usually*quite well protected. Their secondaries, however, due to being not far from the tube opening, and in spite of the reflecting surface not facing the sky, can be somewhat prone to dewing; a tube extension helps.

Because dewing occurs when there is a radiative imbalance which allows an exposed object to cool to the dew point temperature or below, the idea is to minimize the area of sky into which to radiate and at the same time not notably impinge on off-axis light contributing to image formation at the field edge. The latter factor is of concern for systems having field angles of view of greater than 10-20 degrees.

For telescopes, where the field angle hardly approaches 10 degrees, dew shields can be almost arbitrarily long. The common wisdom has it that a shield length 1.5X the objective diameter. This helps a lot, but a further lengthening is worthwhile, if said lengthening doesn't cause other problems, such as increased sail area, or sagging. For instance, a 3X length ratio shield reduces the solid angle of the visible sky by a factor of 3.7 compared to a 1.5X length ratio shield. Even a 2X shield is about 2.2 times better than a 1.5X shield in terms of sky area visible.

Another way to look at the gains afforded by a dew shield is to consider the fraction of visible sky with the shield in place vs that visible without. If we consider the worst case, where a fully exposed optic facing the zenith 'sees' as much as a 180 degree hemisphere of sky, that's a solid angle of 6.28 steradians (sr). Your 1.5X shield reduces the visible sky to 0.63 sr, which is a 10-fold improvement. A 3X shield exposes 0.17 sr of sky, which compared to 6.28 sr is a 37-fold improvement.

Incidentally, solid angle (steradians) equals

2 * pi * SIN^2(theta)

Where theta is the semi-angle.

For example, consider the 1.5X length ratio dew shield.

theta = ARCTAN(0.5 / length ratio)
theta = ARCTAN(0.5 / 1.5)
theta = ARCTAN(0.333)
theta = 18.4 degrees

sr = 2 * pi * SIN^2(18.4)
sr = 2 * pi * 0.0996
sr = 0.626

Now you can explore the gains afforded by any dew shield length. And other problems involving areas on a spherical surface.


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Turf1
member


Reged: 12/02/12

Loc: SW Michigan USA
Re: Dew shield new [Re: GlennLeDrew]
      #5691282 - 02/20/13 04:36 PM

I have a dew shield from Astrozap on my 11" SCT...it works wonderfully.

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GlennLeDrew
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Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
Re: Dew shield new [Re: Turf1]
      #5691490 - 02/20/13 06:31 PM

I looked up the Wikipedia entry for dew shield. It was pitifully brief, and made what is at least a confusing statement, or outright wrong if read literally. It suggested that a sheild "... provides a small amount of warm air in front of the objective..."

In a sense it could be said that this is 'technically' true. The shield, by reducing radiative loss, keeps the objective a *little* warmer than the more exposed parts of the scope. And so the air in contact with the objective will be a *little* warmer than the surrounding air. But with any kind of upward facing angle, that warmer air convectively chimneys up and out of the sheild, to be continuously replaced by cooler air pouring in and down.

This process is ongoing, the trend being the 'striving' for thermal equilibrium. Things continue to cool down, due to both radiation into the sky, and conduction into the surrounding air, the latter of which tends to cool during the night as the ground cools, especially when there is little or no wind.

The ultimate result, after sufficient time, is the potential for dew formation on the objective. *If* the dew point temperature is not too far below the ambient air temperature. For if the dew point is fairly well below the air temperature (low relative humidity), the objective (and perhaps even other more exposed objects) will not cool sufficiently for condensation to form due to the conduction of heat from the surrounding air.

An object radiating into a clear sky will always eventually cool to below the surrounding air temperature. The degree to which it cools depends on both its thermal conductivity and its radiative characteristics. This is an area in which my knowledge is weak, but I can state that metal will cool down more than will wood and cardboard. The surface treatment, such as bare vs paint/anodizing has a role to play as well. Perhaps even surface roughness.

In any event, an object cools until an equilibrium is reached between radiative loss into the -30C sky and the heat absorbed conductively by the immediately surrounding air. But if the air is still, it usually keeps cooling until sunrise. When there is a breeze, the stirred air resists the formation of an inversion layer (colder close to the ground than above) and so its temperature does not fall at anything near the rate as when calm. And so on breezy nights dew is much less likely, or at least is long delayed; the warmer air keeps things from cooling as deeply.

It should be pointed out that the hemisphere of Earth below the hemisphere of sky above factors into the radiative balance equation. While the ground is also radiating into the sky, and cooling, when warmer than about -30C it bathes your telescope radiatively, thus slowing its rate of cooling. The ground temperature to a significant degree affects air temperature near the ground, and so in most cases these two temperatures do not diverge appreciably. This suggests that radiation from the ground and conduction from the surrounding air are roughly equal factors. But there must be times when either one or the other will be the dominant contributor.

When you take your telescope outside on a cool night from the 20C indoors, initially it radiates somewhat into the cooler earth, and vigorously into the much colder sky. The scope 'wants' to settle toward an equilibrium temperature determined by the average of the various radiative sources and heat sinks which it 'sees' over the 12.56 steradian sphere surrounding it. If the air temperature is sufficiently warmer than this radiative equilibrium, conduction of heat from the air will slow or even halt further cooling to said equilibrium.


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RogueGazer
super member


Reged: 06/10/12

Loc: Central Point Oregon
Re: Dew shield new [Re: GlennLeDrew]
      #5691549 - 02/20/13 07:03 PM

I have the Astrozap for my 10" LX200 and I am happy with it. It goes on in about 10 seconds and off even faster. I make sure to apply it in a very slight cone shape so there is less chance of it sagging or shifting into the light path.

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nikdangr
super member
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Reged: 02/10/13

Loc: Annandale, VA
Re: Dew shield new [Re: RogueGazer]
      #5693053 - 02/21/13 04:22 PM

I got my AstroZap dewshield for the C90 today. The outside is a pebblegrained flexible plastic lined with black felt inside. It has about a three inch strip of fairly dense foam at one end for a snug fit on the OTA. It arrives flat and it's fairly stiff. I pre-rolled it to put a good curve in it so the velco doesn't have any problem holding it in a cylinder. It fits snugly enough that it won't move when slewing the scope even though it's nearly as long as the scope itself.

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ColdMotive
newbie


Reged: 12/23/12

Re: Dew shield new [Re: nikdangr]
      #5693817 - 02/22/13 12:02 AM



I've had more experience with this DIY model since Xmas and I can say it has not failed me yet. Even on a night at 20 F when frost covered my finder scope and frost was inching it way down the shield barrel; still the surface was clear.

Full instructions here DIY Dew Shield How To


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