Posted 18 February 2013 - 06:44 PM
(My Porta Mount shipped today. Can't wait!)
Posted 18 February 2013 - 06:47 PM
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.
Posted 18 February 2013 - 07:04 PM
Posted 18 February 2013 - 08:30 PM
Posted 19 February 2013 - 12:37 AM
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?
Posted 19 February 2013 - 02:37 AM
Posted 19 February 2013 - 05:55 AM
Posted 19 February 2013 - 09:38 AM
I thought you were kidding until I looked at your scope.
For larger scopes, the Walmart foam sleeping bag pads work nicely.
Posted 19 February 2013 - 09:43 AM
For larger scopes, the Walmart foam sleeping bag pads work nicely.
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.
Posted 19 February 2013 - 03:40 PM
Posted 19 February 2013 - 05:24 PM
Posted 19 February 2013 - 06:42 PM
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.
Posted 20 February 2013 - 04:36 PM
Posted 20 February 2013 - 06:31 PM
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.
Posted 20 February 2013 - 07:03 PM
Posted 21 February 2013 - 04:22 PM
Posted 22 February 2013 - 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