First Light VI

Have you shared any of your Astronomy knowledge with anyone this month yet?  April is Global Astronomy Month– check this site out for more information.   Go ahead and do it.  Yesterday I had an opportunity to share some astronomy knowledge (what little I have) with a work colleague and it was a wonderful experience.  They learned some things they have never thought of  and I was able to better understand my thoughts.  Win-Win.  Then at home I had the kids pick their favorite planet, draw and color it and get some facts about it.  They both did a very good job (Jupiter and Mars were picked).

Finally!! The Televue AVT 1011 bracket came in and I was able to mount my telescope to the tripod!  It sure looks nice.  I was very impressed with the quality of the Televue mounting bracket.  But I’m wondering if I should drill a hole in the Celestron mount and tap some threads for a “Safety screw”.  You see the mount only has two screws which squeeze the flat side of the Televue mount into the tripod.  If you were to accidently bump one of the mounting handles it just may cause the scope to fall out of the mount.  That would be bad.  So I think a little modification is in order before I can totally trust my investment on top will not crash to the ground inadvertently.

Well no viewing last night- storm came in, but the HollyDome Observatory is showing some clearskies tonight although the Moon light (Full Moon) will interfere some, but hopefully the lunar viewing and Jupiter will turn out OK.  they say half or less moon is better as its not so bright and you can pick out details better- but what a  better first light test than looking at our closest neighbor in the night sky!

tonights clear skies

This chart can be a little confusing at first but let’s see if we can clear that up somewhat.

It’s the astronomer’s forecast. At a glance, it shows when it will be cloudy or clear for up to the next two days. It’s a prediction of when Ottawa, ON, will have good weather for astronomical observing.

The data comes from a forecast model developed by Allan Rahill of the Canadian Meteorological Center. CMC’s numerical weather forecasts are unique because they are specifically designed for astronomers. But they have 763 forecast maps. It can be a chore to find the one you want.

So, (Attilla Danko) wrote a script to generate the images like the one above which summarizes CMC’s forecast images just for Ottawa and the surroundings out to about 15 Kms.

There are charts for 5218 locations.

Summary: In the rows labeled “sky conditions”, find a column of blue blocks. You can probably observe then.

Details: Read the image from left to right. Each column represents a different hour. The colors of the blocks are the colors from CMC’s forecast maps for that hour. The two numbers at the top of a column is the time. A digit 1 on top of a 3 means 13:00 or 1pm. It’s local time, in 24hr format.

Transparency

Too cloudy to forecast Poor Below Average Average Above average Transparent

The line, labeled Transparency, forecasts the transparency of the air. Here ‘transparency’ means just what astronomers mean by the word: the total transparency of the atmosphere from ground to space. It’s calculated from the total amount of water vapor in the air. It is somewhat independent of the cloud cover forecast in that there can be isolated clouds in a transparent air mass, and poor transparency can occur when there is very little cloud.

Above average transparency is necessary for good observation of low contrast objects like galaxies and nebulae. However, open clusters and planetary nebulae are quite observable in below average transparency. Large globulars and planets can be observed in poor transparency.

A forecast color of white formally means that CMC didn’t compute the transparency forecast because the cloud cover was over 30%. So it may be possible to observe during a white transparency forecast, but the real transparency is usually yucky. CMC’s text page explaining this forecast is here.

Seeing

Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5

The line, labeled Seeing, forecasts astronomical seeing. (It’s an experimental forecast.) Excellent seeing means at high magnification you will see fine detail on planets. In bad seeing, planets might look like they are under a layer of rippling water and show little detail at any magnification, but the view of galaxies is probably undiminished. Bad seeing is caused by turbulence combined with temperature differences in the atmosphere. This forecast attempts to predict turbulence and temperature differences that affect seeing for all altitudes.

Bad seeing can occur during perfectly clear weather. Often good seeing occurs during poor transparency. It’s because seeing is not very related to the water vapor content of the air.

The excellent-to-bad seeing scale is calibrated for instruments in the 11 to 14 inch range. There are some more details in CMC’s seeing forecast page.

No computer model forecasts convective heating well, so consider the seeing forecasts for daytime hours to be less accurate. Seeing is forecast for 3-hour blocks, so triples of seeing blocks will show the same color. A white block on the seeing line means that there was too much cloud (>80% cover) to calculate it.

Note also that you may observe worse seeing though your telescope than what a perfect seeing forecast would predict. That is because tube currents and ground seeing mimic true atmospheric seeing. You may also observe better seeing than predicted here when observing with an instrument smaller than 11 inches.

Darkness

-4 -3 -2 -1 0 1.0 2.0 3.0 3.5 4.0 4.5 5.0 5.2 5.4 5.6 5.8 6.0

The line labeled darkness is not a weather forecast. It shows when the sky will be dark, assuming no light pollution and a clear sky. Black is a dark sky. Deep blue shows interference from moonlight. Light blue is full moon. Turquoise is twilight. Yellow is dusk and white is daylight. For those who prefer numbers, the scale is also calibrated. The numbers are the visual limiting magnitude at the zenith. (The brightness of the faintest star a standard observer can see straight up.) Mouse over a darkness block for details.

It is based on Ben Sugerman’s Limiting Magnitude calculations page. It takes into account the sun’s and moon’s position, moon phase, solar cycle and contains a scattering model of the atmosphere. It doesn’t consider light pollution, dust, clouds, snow cover or the observer’s visual acuity. So your actual limiting magnitude will often be different.

Wind

>72 km/hr 46 to 72 km/hr 28 to 45 km/hr 19 to 27 km/hr 9 to 18 km/hr 0 to 8 km/hr

This forecasts wind speed at about tree-top level. The wind forecast won’t determine whether or not you can observe, but it may affect your comfort and the type observing you might be limited to. In particular, long-focal length astrophotography, or observing with large dobsonians require light wind conditions. High wind may be particularly dangerous for larger truss-tube dobsonians which must be disassembled in the vertical position.

Humidity

<25% 25% to 30% 30% to 35% 35% to 40% 40% to 45% 45% to 50% 50% to 55% 55% to 60% 60% to 65% 65% to 70% 70% to 75% 75% to 80% 80% to 85% 85% to 90% 90% to 95% 95% to 100%

This forecasts ground-level relative humidity.

Humidity variations can indicate the likelihood of optics and eyepieces dewing.

But dewing is not simply correlated to relative humidity. Dewing tends to happen when the sky is clear, the temperature is dropping and there isn’t much wind. Being on a hilltop or in a small valley can make the difference between no dew and dripping telescopes. Unfortunately, the humidity forecast does not have the spatial resolution to know about small hills, valleys, or observatory walls. All of which can reduce dewing.

A sudden spike in the humidity forecast, an hour or so after the cloud forecast predicts a sudden transition from cloudy to clear, when there is no wind, means that ground fog will form.

Also, when the cloud forecast is opaque and the humidity forecast is 95%, rain is likely: a good time to cover the telescopes.

Since there are many different levels in this forecast, with similar looking colors, it’s best to activate the “explain colors when you mouse over” to interpret the colors.

Temperature

< -40C -40C to -35C -35C to -30C -30C to -25C -25C to -20C -20C to -15C -15C to -10C -10C to -5C -5C to 0C 0C to 5C 5C to 10C 10C to 15C 15C to 20C 20C to 25C 25C to 30C 30C to 35C 35C to 40C 40C to 45C > 45C

This forecasts temperatures near the ground. While temperature variations won’t determine if you can observe, the forecast can be handy choosing clothing for cold observing conditions. (In general, dress as if it were 20 degrees F or 10 degrees C colder than the forecast.) Observers with thick primary mirrors should take note of falling temperature conditions because their mirrors may require additional cooling to reach equilibrium and so prevent tube currents.

Cold temperatures also mean reduced battery capacity, stiffer lubricants, stiffer electrical cables and slower LCD displays. Camera sensors will have reduced noise. But, in general, electronics have a lowest temperature at which they will work.

 

Well, that’s all for now.

Roger

 

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