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Installed a cooling fan on my telescope

I finally got around to installing a mirror cooling fan on my 8″ f/6 Dobsonian telescope. Its a project that I’ve been putting off (for no good reason) for quite sometime. I had some time off over the Christmas break so I thought I’d finally give it a go.

So the first question would be why install a fan to cool the primary mirror of a telescope? My scope is essentially a 48″ tube with an 8″ diameter primary mirror at one end. If the scope is stored indoors then the air inside that tube is, of course, roughly at room temperature. Take the scope outside and you have a tube full of warm air surrounded by the cooler (and sometimes colder!) ambient air of the great outdoors. When light passes from the cooler denser ambient air to the warmer less dense air inside the tube it will bend (refract) and cause image degradation. A temperature difference of just a few degrees is all it takes for the effect to be noticeable to the experienced eye. The idea behind the fan is that it blows cold ambient air on to the back of the mirror and up through the telescope tube thereby equalizing the mirror temperature and tube air temperature to the ambient air temperature.

Fan mounted in Styrofoam cut to fit back of telescope tube

It doesn’t take a very large fan to effectively cool everything down. A small 12 volt computer cooling fan will do the trick just nicely. I mounted mine in a piece of Styrofoam cut to fit over the back end of the primary mirror.  To reduce possible stray  light reflections, I painted the Styrofoam with water based black paint. Be sure to use water based paint as solvents found in ordinary paint will melt the Styrofoam! The Styrofoam baffle is mounted to the telescope tube using Velcro strips to hold it in place. This configuration is OK but the Velcro doesn’t stick to the Styrofoam very well so a better solution is necessary.

Power source and on/off speed controller mounted on the side of the telescope

I then had to mount a power source and, although probably not 100% necessary, an on/off switch and a speed control knob. My power source is a bit of overkill in that it’s a 20 AH rechargeable car battery booster with one of those cigarette lighter sockets that provide a 12v power outlet. The 20 AH battery (the smallest one I could find!) will theoretically keep the fan running for over 30 hours! At some point I will downgrade the power source to something that’s not quite capable of delivering 400 amps! The on/off switch and the speed control are simply a rheostat and an on/off micro-switch mounted in one of those electronics hobby store project boxes.

The first test of the unit was on January 4, 2011 under less than ideal skies (urban setting, average to below average seeing, but average for the location). On that night Jupiter and the Orion Nebula were by far the best targets and I was impressed with the views of both! Jupiter, during periods of intermittently good seeing, was clear and crisp with good contrast. Same with the Orion nebula – more ‘Texture” in the nebula than I’ve ever observed from this site. I can’t wait to give the whole thing the electric Kool-aid acid test at a local dark sky observing site on a crystal clear night.


Observing Log – January 4, 2011

Weather Conditions

Temperature -4C, Windchill -11C, Humidity 68%, Wind SW 24 kph, Sky was clear, seeing was less than average, transparency was less than average.

Objects Observed


Observing equipment: 8″ Dobsonian with 14mm Meade EP giving approximately 85x. Seeing was not good enough for higher magnifications.

In the early evening, through periods of intermittent reasonably good seeing,  Jupiter was sharp and clear. From left to right alignment was Callisto, Io, Jupiter, Ganymede. Europa was eclipsed by Jupiter. Equatorial belt clearly visible and one bright band between two dark bands were visible in the south.

Perseus Double Cluster (NGC 884 and NGC 869)

Located just a few degrees off the pointy head of Perseus the double cluster is one of my favorite open clusters (or 2!). Together the clusters nicely fill the field of view in my 14mm eyepiece. The clusters are somewhat unique in that they are both close to us (NGC 869 is roughly 6700 light years distant and NGC 884 is about 7600 light years distant) and, at roughly 5.5 million years old for NGC 869 and 3.5 million years old for NGC 884, quite young as clusters go .

M36 and M38

Two open clusters with Messier designations in the constellation Auriga.  I enjoy finding M38 because its near two of my favorite asterisms – the Leaping Minnow and the Cheshire Cat. Look northwest of Elnath to the asterism Leaping Minnow. The minnow is “leaping” toward the smiling face of the Cheshire Cat. M38 is located just where you might imagine a dimple on the cat’s face if cat’s had dimples! M38 reveals a definite pattern of stars that some say it looks like the math symbol pi. M36 was easily found by scanning toward the horizon from M38.

Star Atlas Development

Well, its been well over a year since I’ve  posted anything on this site.  So, where does the development of Star Atlas stand, you ask? Its now in its second beta incarnation. After some consideration I decided to re-write the whole project;  the objective being to make the basic engine a little more robust. Now that winter approaches I’ll be putting more time into the project so stay tuned for more updates.

Saturn at Opposition

Saturn reaches opposition on February 23rd! Rather than prattle on about it for paragraphs and paragraphs I’ll point you to the Universe Today blog site. The Universe Today is one of the better astronomy blogs. They also publish regular and very interesting podcasts. Check them out!

Star Atlas Update

Since last post I’ve had to back off a tad from new developments with Star Atlas. Reason:- My Sidereal Clock wasn’t keeping proper time! Now, it wasn’t just out a constant few seconds or even a constant few minutes. No, my Sidereal Time seemed to be further and further off the mark the longer my program ran! Not a great amount but enough to make the star positions inaccurate after just a few minutes.

Now, I’m glad you’re asking, “What is Sidereal Time and why is it important?”. Sidereal Time is time measured by the stars. The time we use everyday (the time kept by our watches) is called Local Mean Time (LMT) and is derived from Local Apparent Time and… well, its all based on the position of the Sun. Sidereal Time, on the other hand is time based on the position of the stars. If you were to look at a bright star in your night sky (say Alpha Orionis or Betelgeuse in Orion) and record it’s altitude and azimuth (position in the sky) at a given time you would find that on the next evening the star would reach that same point 4 minutes earlier (by your LMT or your “watch” time). So, your Sidereal Clock is running approximately four minutes faster than your LMT clock.

Now, to calculate the instantaneous position of a star in your night sky you need to know the Right Ascension and Declination (sort of the Lat and Long) of the star, your Longitude, your Latitude, and your Sidereal Time (derived from your Coordinated Universal Time (UTC) and your Julian Date). In computer speak, here are the formulas:-

Julian Date = 367 * CurrentUTC.Year – Math.Floor(7 * (CurrentUTC.Year + Math.Floor((CurrentUTC.Month + 9) / 12)) / 4) + Math.Floor(275 * CurrentUTC.Month / 9) + CurrentUTC.Day – 730531.5 + (CurrentUTC.Hour + CurrentUTC.Minute / 60 + CurrentUTC.Second / 3600) / 24

Mean Sidereal Time = 280.46061837 + 360.98564736629 * Julian Date + Longitude

(Math.Floor simply means take the nearest integer (non-decimal) value.

So, you can see why I might have goofed a bit on originally coding them. Anyway, all is well now; well, almost. This particular version of the Mean Sidereal Time formulas will keep my Sidereal Clock accurate to within 0.001 seconds for the next 10 years but will only be accurate to 0.1 seconds a hundred years from now. Such is life; more accurate formulas are needed. Stay tuned.

So, after fixing up the Sidereal Time issue I added one new feature and one new improvement. I redesigned my splash screen (the screen that displays on program start) to look a tad niftier and I coded the feature that allows the program to be used from anyplace on the face of the earth! Yep, you can now select one of 850 cities (facilities for user defined locations and locations read from a GPS will be added soon) and you will see the night sky configuration for the current time in that city.


Star Atlas Planetarium Software Update

The latest updates added to my Star Atlas Planetarium sofware:-

User Selectable Date and Time

The time and date can now be changed by the user. Up until now the application would read the system time and display the position of the stars, in real time, based on the time offered by the system clock. The user can now select any date/time combination from January 1, 1753 to December 31, 9998. The caveat being, of course that for dates far from the epoch date (year 2000) of the stars in the star catalog the less accurate the star positions are. But, for reasonable deviations from the year 2000 this is not a problem. Future versions of Star Atlas will recalculate the epoch date if the deviation from the default epoch is too great. Once the user date has been selected the sky will, of course, change in real time as if it were actually that day and time.

The usefulness of this feature is that the user can now choose to see the sky at the time of their choosing. Its really cool, for e.g., to look at the sky on the date and time you were born, or to create a star chart based on some evening next month, etc, etc.

Sky Animations (Time Warping)

This feature is really cool. From the currently selected date and time the user can choose to advance the sky any number or combination of years, months, days, hours, and minutes. The refresh rate of the display is selectable from a fast update setting of 10 milliseconds to incremental update settings from 1 to 10 seconds. The upshot is that the user can advance (or reverse) time as the need arises. So, for example, the user may be viewing the sky on January 22, 2008, at 5:00 PM but might be interested in finding out when the belt stars in Orion cross the southern meridian. The user would simply configure and run an animation to advance the sky in say 4 minute steps every 1 second and then watch for the time when Orion’s belt crosses the meridian. Running the animation would cause the sky display to change every second with the time (and the star positions) advancing by 4 minutes with each display refresh. Its hard to describe in words but essentially the operation gives an animated view of the stars moving through the sky in “sped up” time.

I tried to make some movies of these new features but, alas, YouTube strips out too much detail and makes the videos to small for full screen views of Star Atlas to be meaningful. You’ll just have to wait for the first beta release of Star Atlas to experience the joy and elation brought on by Sky Animations!

Clear Skies!

Planetarium Software Update

Things are moving along nicely with my planetarium software project. Some improvements since last post:-

  • Constellation names are now being displayed and can be turned on and off.
  • Constellation lines can be turned on or off.
  • The local horizon can be turned on or off and can be displayed separately from the Alt/Az grid.
  • The local meridian can be turned on or off (with 10 degree tick marks) and can be displayed separately from the Alt/Az grid.
  • Zoom in and zoom out are enabled.

Following is a screen shot of Star Atlas looking south from Halifax at approx. 1:45 AM on December 29. The Local horizon (curved green line at the bottom) and the local meridian (vertical line, very faint in the screen capture, with the tick marks) are visible.

A beta release may soon be available for friends and family!

(Click for a larger image)

Looking South from Halifax