Dobsonian Equitorial Platform

I own a type of telescope that consists of a Newtonian reflector on a simple, Dobsonian base.  Typically they’re just called Dobs for short.   Dobs offer a great value in that you get a lot of aperture, or size of the scope, for a relatively small amount of money.  The base is a very simple two bearing design that requires no alignment.  You go out, set the scope down, and start pushing it where you want it to go.  For bearings, you have an azimuth bearing which lets you move the scope side to side, and an altitude bearing that lets you point the scope up or down.  Here’s mine.

A downside to the azimuth-elevation style base is that you can’t follow an object in the sky by rotating about a single axis.    Imagine the sky is a big sphere (better known as the celestial sphere) rotating about an axis (known as the celestial axis).  This axis runs from North to South and is angled at whatever your latitude is.  See the crude sketch below to illustrate this.  The celestial sphere rotates around the red axis and the scope rotates about its green axes.

So, as an object rises in the sky at any latitude other than the equator or the poles, it will move diagonally up and over if you watch it.  To follow it with my scope I have to point it up and then over in a stair step fashion.  For the most part this isn’t an issue.  But, if you are at high magnification you’ll be nudging the scope a fair amount because the area you can see becomes smaller as you increase magnification.  If you want to take pictures or video through the scope this becomes big problem because you don’t want things moving in the frame.  There are several approaches to solving this issue.  First, you could put motors on both bearings and have a computer control the motion.  Second, you could buy a $1000+ equatorial mount to put the scope on and ditch the Dob base all together.  Third, you can build or buy a equatorial platform.

Alright, there is a fourth option to build a new mount of the fork, yoke, or split ring type.  I think these mounts would be very large compared to the scope and wouldn’t be a good choice due to the tube’s center of gravity location.

As you might have guessed I’ll be going for option #3 which is the cheapest.  What’s a equatorial platform you ask?  It does two things.  It allows the entire scope to rotate about the axis of the sky and rotates the scope about that axis with a motor to follow the sky.  The basic principle behind it is when two axes are parallel you can follow an object rotating about the first axis by a rotation about only the second axis once you’re pointing at it.  To do this we need to either orient the azimuth bearing’s axis parallel to the celestial axis or create a new axis parallel to the celestial axis and rotate about it.  Due to the nature of the Dob’s base it cannot be tilted to be parallel to the celestial axis because the optical tube will fall out.  If this problem were corrected then you’d still have the issue of keeping the weight of the tube from causing the base to tip over.  These issues have been overcome of course but there is a simpler way.  The equatorial platform works by creating a third axis to rotate about that is parallel to the celestial axis, keeps the Dob base supporting the scope without modification, and keeps the scope from tipping over.

Imagine that there is a cone that has its center axis aligned with the celestial axis and its edge parallel to the ground as seen in the picture below.  The cone could rotate without issue.   We could set the scope anywhere on or within the cone and it would result in the scope rotating about the cone’s central axis.  Let’s do that.  We want a level surface to put the scope on.  So, cut the cone parallel to the ground and trim the ends off anywhere that isn’t below the base of the scope.  If we do this we’re left with the blue object in the sketch below.

Because we don’t want our cone piece rotating on the ground we’ll also use a board with some rollers on it to support the cone piece.  On one of these rollers will be a motor that turns it at the correct rate to result in the cone making one complete revolution per sidereal day.  Since we only have a part of the cone, it will only be able to rotate for a portion of the day.  An hour is a good number people use.  The platform can be built to allow it to rotate for an hour and then be reset.  It is reset by physically moving the section of the cone back to the starting position.

If you’d like to see what others have built check out these sites:  Jan van Gastel’s  Chuck Shaw’s  Equatorial Platforms  Kurt Maurer’s

I’m still in the design phase with mine but I hope to start ordering parts and building soon!

The Moon and Venus

The moon, Venus, and Jupiter are currently all located in the western sky.  Venus is so bright right now that you can see it in the broad day light….if you know where to look.  Today was a good day to find it as it was near the moon.  I snapped a picture of them to share.

Here’s a similar one at night.

The Morning Sky

Six of the eight planets are up together in the morning sky.  So, I figured that since that doesn’t happen all the time I’d get up to see it.  All of them but Saturn are up in the morning sky.  Saturn is hanging out in Virgo currently and is about over head at midnight.  I was only able to get 4 (5 technically) planets with my camera.  The four I caught are shown below.   It is kinda hard to see them in this resized version of the picture so jump down to pic #2.

Here is a 100% crop of the picture above with the planets labeled for you.

Here’s a screen shot from Stellarium.

Four of the planets are bunched up near the horizon with Uranus and Neptune higher in the southern sky.

While I was waiting for the sunrise I took a couple other pics.  This is looking south at Scorpius and Sagittarius. The Milky Way runs through here.  Specifically, this is looking towards the center of the galaxy.

Here’s the same picture with the constellations outlined.

View of the same area from a different location.  I have a picture where the ground is level but it didn’t have the shooting stars.

That’s all for now.

Hubble Pass

Tonight the Hubble Space Telescope made a relatively bright, mag 1.9 pass from 20:03 to 20:11 in the southern sky.

Here it is rising heading from west to east (right to left in the pictures)

Here’s a picture from a few seconds later.  Upon reviewing it afterwards, I noticed a streak in the upper left hand corner of the image.  I’ve photoshopped it some to bring it out in the picture below.

Here is a 100% crop of it.  I went back to Heavens Above and found out it was “Cosmos 1898 Rocket”.  A little further research reveals that this is a rocket body from a Kosmos-3M which was used to launch a communications satellite.  A lucky catch.

As I had just set the camera down from moving it, the HST flared up by a couple magnitudes probably due to the solar panels reflecting the sun at just the right angle.  I clicked the shutter as fast as I could and caught some of it.  In the picture below, you can see the flare fading away. You can tell this because the track is “fatter” on the right and then thins out as it goes to the left.

Another picture of it about 30 seconds before it faded away.

I had really hoped to see some bright X-37 passes from the last couple of days but we were socked in with clouds.  This turned out to be a good pass though and I’m glad I caught it.

This Morning’s ISS Pass

Here are some pics of the ISS pass this morning that I wrote about in the previous post.

ISS is rising in the West between Saturn in the lower left and Arcturus (star in constellation Bootes) near the mid top.

Here it is in the North passing by the handle of the Big Dipper.

A picture of Venus in the sunrise since it was up too.

What ISS Up?

Unless you’ve been living under a rock for a while you know that there is a rather large space station in orbit above us at this very moment.  What you may not be aware of is that you can see it with your unaided eyes.

For the rock people, the International Space Station (ISS) has been in the works since the first module was launched in 1998. It is a joint operation between NASA, the European Space Agency (ESA), the Russian Federal Space Agency (RKA), the Japanese Aerospace Exploration Agency (JAXA), and the Canadian Space Agency (CSA).  It is about 167 ft in length, 357 ft in width, and 66 ft high.   Here is a picture of it from NASA:

Go here if you’d like a bigger picture:
http://spaceflight.nasa.gov/gallery/images/shuttle/sts-133/html/s133e010447.html

Most of the area of the space station is made up of solar panels which, as a side effect, aids us in seeing it from the ground.  To see it you first need to find out when it is passing overhead.  There are several good sites on the internet to help you with this but my favorite is
http://www.heavens-above.com.

Once you’ve gone to the Heavens Above site you’ll need to set your location by either selecting it from a map, database, or Latitude and Longitude.  For a more permenant solution you can register on the site.  Here is the home page with the area to set your location highlighted.

For this example, we’ll be viewing from here.  The track will be different from other locations and at other times.  Once your location is selected you’ll return to the home page but the Lat. Long. values will reflect your location.  Next, click on ISS and you’ll be taken to a page similar to this:

This page displays the visible passes over the next 10 days.  The meaning of most of the columns should be pretty obvious except for the “Mag” column. Mag, short for magnitude, is a measure of brightness for an object in the celestial sky.  The lower the value, the brighter the object is.  So, a mag -3.5 object is brighter than a 0 and a 0 is brighter than a 2.  For example, the Sun is a mag -26.7, the full Moon is -12.7, and the brightest Venus can appear is -4.4.  Looking at the chart above you can see that on 25 Mar the ISS will be at mag -3.2 which is very bright and easy to see.  Also note that the Mag values are proportional to the max altitude (Alt.) that the ISS will reach.    The altitude is measured in degrees from the horizon where 0 is the horizon and 90 degrees is straight up (Zenith).   For reference, if you hold your fist out at arm’s length it is about 10 degrees in width across the knuckles.

Since this may be your first time viewing we’ll click on the 25 Mar pass.  Doing so takes us to this page:

At the top of the page you’ll see a chart of the sky with the constellation lines drawn and labeled. On this chart, the pass is noted by the black arced line with the red arrow in it.  The arrow points in the direction of the pass. In the middle, you’ll see a chart detailing the time of the pass and on the bottom a zoomed in chart of the area around the max altitude (not shown here).   Be sure to notice that the East and West are swapped on the chart so that it matches the sky if you hold it up.  It helps if you are familiar with some of the constellations, but if not you can still manage.  On the chart, brighter stars are drawn larger than the dimmer stars so we can use these bright stars to find the general area to look.   For this pass we’re lucky because it tracks through the Big Dipper (I know…it’s an asterism not a constellation).  A lot of people are able to pick out the Big Dipper.  If you can already, you’re a step ahead.  If you cannot, go find it since it is up currently at night!

Once you’ve found the Big Dipper, locate the two stars at the cup end and extend a line out from the bottom until you hit a “W” shaped set of stars called Cassiopeia.   Now from the chart, you know that the ISS will pass through the end star on the handle of the Big Dipper and then into Cassiopeia.  All you have to do is start sweeping the area between these two points after the pass starts.  Feel free to find other stars along its track for extra points. Eventually the ISS will pass into this area and you’ll see it!  It’ll be easy to see because it’ll appear as a bright “star” moving in an arc across the sky very quickly.  If you see blinking lights on it, you’ve found an airplane so keep looking.

Now I know what you’re thinking, “Its not gonna pass through the Big Dipper every time.  What then?”.  Simple, look at the chart and find some objects that you can identify.  In this chart, Venus is up and it’s very bright so you can find it easily.  The Moon is also usually up and since my 3 year old can find it, I’m sure you can too.  Maybe you can use those to get going in the right direction.  But what if it doesn’t pass around any object you know?  Sorry (not really), you’ll need to learn some of the night sky!  To start, find a constellation that is pretty apparent, such as Lyra or Cygnus in the summer and Orion or Canis Major in the winter.  Once you’ve found one constellation, you can locate nearby constellations.  Pretty soon you’ll be able to pick out lots of constellations.

On a previous pass (2 Oct 2010) I took some 30 second exposure pics of the station passing.  Here are some of them.

Once you’ve seen the ISS pass a couple times you can start looking for other objects such as Hubble, the space shuttle (when appropriate), Iridium Flares (bright flashes), and the Air Force X-37 (when appropriate).

So, go forth and find the ISS!  Learn some of the night sky, which unlike all those combos you learned in Mortal Kombat, will actually still be useful knowledge in 10 years!