The AstroSpotter Device

The Easy Way To Find The Stars! 

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The AstroSpotter is a fairly simple device that eliminates the frustration experienced by people trying to use a small flat star chart to find stars in the very large spherical night sky.  It would serve as an outstanding astronomy teaching aid and my hope is that people will be able to use AstroSpotter to make star finding as well as star identification as easy as possible.

These days with digital phone/devices there are programs to point out parts of the sky whereas this was invented before those devices existed so it is 'Old Skewl' approach.  If nothing else this device will help people visualize the way the coordinates of the heavens are laid out and maybe help to explain the dynamic way the stars move over the night as well as from season to season.

The AstroSpotter makes a great tool for the beginner, the student,  as well as advanced binocular watchers of the night sky.  The  functionality of AstroSpotter can also be had with equatorial telescope mounts but that sort of hardware can easily be far out of the financial reach of the casual star-gazer or student who is just starting out in star gazing.

Picture 1:  The AstroSpotter

I invented AstroSpotter because all the other devices that try to do this job either cost a bundle or just get beginers frustrated when they finally try to go out at night and try to find stars from  documents or devices.  Once the AstroSpotter is aligned, stars and constellations are easy to spot given their coordinates taken from any number of references listed in the reference section of this web page.  

This device is greatly enhanced if you use it with a green laser pointer.  Please note that in some places laser pointers are illegal and you must obey applicable laws.   Also note that at a serious astronomy star party it is highly discouraged and often not allowed to use a green laser pointer because you may ruin photographs in progress at that time or perhaps just agrivate those around you at the time.

The AstroSpotter device  is made to attach to a standard camera tripod in order to hold it steady on the north star.  Because this tool requires allignment with the north star, the AstroSpotter device is only useful for  the northern celestial hemasphere viewable from the United States and most of Europe.

The AstroSpotter is not being sold here at this time so you need to be a bit of a do-it-yourselfer in order to get one of these devices at this time.  

And oh yes ... the legal stuff:  All rights to the AstroSpotter belong to the original designer and cannot be used for profit without permission from myself, Mark E. Johnston,  in writing.

How To Align AstroSpotter (Simplified Method)

For star gazers who know where the north star is and who know how to figure out their local sidereal time, the basic idea to align the AstroSpotter is as follows.

1)  Secure the AstroSpotter to a camera tripod and point it directly at the north star by either sighting the north star along the top of the fin or by sighting the north star through the middle of the plastic pipe that the fin is attached to.

2)  Assure that the very top of the large dial on the AstroSpotter always indicates the correct local sidereal time.   Re-adjustment of the dial every 5 minutes or so using a little turn of the big dial will do the trick nicely.    If the sidereal time is 11:00 then adjust the dial so 11:00 is at the top of the dial.  You may find it handy to have a second watch set to sidereal time along when you use the  AstroSpotter.   Please see the References section for a website with sidereal time or your astronomy program usually will tell you  your current sidereal time  (Which is the current Right Ascension directly above your head at any given time.   You may proceed to using the AstroSpotter  now if you wish.

Detailed Description of how to setup the AstroSpotter:

The baseplate can be attached to a standard camera tripod using the mounting hole and a standard 1/4 20 tripod mounting screw. The north star must be visible if very precise measurements are to be done and this description will only explain how to use the device when the north star is visible. If you know how to find the north star skip the next paragraph.

The north star is about 40 degrees off  the horizon in the United States and is ALWAYS   NORTH (How clever).  If you know how to spot the 'Big Dipper' you may have heard the trick about how the two end 'pointer' stars on the dipper's 'cup' point right at the north star if you follow the line from the bottom of the 'cup' to the tip of the cup furthest from the handle.  If this does not help, back to the compass approach.  Keep in mind that a compass in central California will point about 10 or more  degrees east of the true north. This correction is called the "Declination adjustment" (Has nothing to do with the declination of the celestial sphere) and varies widely within the United States since the magnetic north pole is actually somewhere in Canada. One way to determine the correct "Declination adjustment" for your compass is by calling or writing to the U.S. Geological Survey. Many compasses give you a chart that shows the adjusted values to find true north if you know in the USA you are at a given time.


                           Parts Of The AstroSpotter  

To align the AstroSpotter, it is first important to get the north star to be pointed to through the hole in the middle of the axis tube.   Or sight the north star along the top of the big fin that holds the two star pointers.

The axis tubes are aligned to point to the north star by sighting the north star along the top edge of the sighting fin when the fin is in a vertical position. In general, this will be good enough to do plenty of good star gazing. If even better alignment is desired, a little practice will allow the north star to be centered directly through the axis tubes once the rough alignment has been done using the sighting fin. Hint: Stay back a foot or so from AstroSpotter and keep your focus point at infinity as you sight through the tube. The only error here is you will sight about 4" away from the center of the north star, but that should be fine (  joke ).

After sighting the North star along the top of the fin, The disk with the scale for right ascension should then be aligned.  


IT IS CRITICAL THAT YOU RE-ADJUST THE ASCENSION DISK EVERY 5-10 MINUTES BY 1 MINUTE PER MINUTE. THIS IS MOST EASILY DONE BY NOTING THE TIME WHEN YOU FOUND SIDEREAL TIME AND ADDING TIME TO THAT EACH TIME YOU RE-ADJUST.  You can also set a second watch to local sidereal time from some computer program that you may have and that will be fairly good for the next hour to use on AstroSpotter before your watch should be re-adjusted to match actual sidereal time.

Align the AstroSpotter to point towards the North star as mentioned earlier. Find the 'date' scale on the disk and align the current date to be at the top of the disk pointing directly overhead.

Move the fin to align with your correct LOCAL time. Local time varies from the time on your watch by as much as an hour so see the AstroSpotter fin or earlier section 'Your Local Time' for an explanation. If it is not possible to align the fin to local time because the fin does not go that far, move the fin to align with 12 hours ahead of the correct time of day and remember this trick for later.

Rotate the disk to align the current date with the present location of the fin (or the 12 hours from present time if you had to leave the fin at that point from the last step).

The correct sidereal time is now pointing directly overhead. I suggest you either set a second watch to the time on top of the disk or remember the number of hours and minutes the sidereal time is displaced from your own watches time. YOU WILL NEED THIS LATER TO KEEP THE AstroSpotter Aligned.   A watch will be fine for all normal situations but  but you should know that on the next night the same watch will be about 4 minutes ahead of sidereal time because sidereal time is a bit faster than our standardized clock time.  A link to get your local sidereal time (LST) is in the references section.

A Quick Review of  basic setup of the AstroSpotter:

1)  Secure the AstroSpotter to a camera tripod and point it directly at the north star by either sighting the north star along the top of the fin or by sighting the north star through the middle of the plastic pipe that the fin is attached to.

2)  Assure that the very top of the large dial on the AstroSpotter always indicates the correct local sidereal time.   Re-adjustment of the dial every 10 minutes using a little turn of the big dial will do the trick nicely. 

Locating A Star With Known Celestial Coordinates

When the celestial coordinates of a star are taken from a book or the Planisphere, the AstroSpotter device can quickly locate the object and eliminate the difficult task of converting a flat drawing of stars to the actual position of the star or constellation that you want to see in the night sky. This is one of the 2 main functions that the device was designed to perform. Keep in mind that the AstroSpotter must be aligned first and the right ascension disk must be updated if the readings are to give correct ascension.

The Figure below shows the sighting of a typical star of interest. After aligning the device as stated above, rotate the sighting fin to align with the right ascension of the star as shown on the disk. Then set the pointer on the declination scale to align with the correct declination of the star or constellation. This star is at a right ascension of about 3 hours and at about +40 degrees (This is the star Algol which is visible in winter)


             Typical Sighting Of A Star

The scale closest to the clear ascension disk is useful for stars just above the north star that would be difficult to sight using the other scale due to the position of the tripod that the AstroSpotter is mounted upon. For some stars that are within +40 degrees to +90 degrees and are below the north star, the sighting fin will not rotate to the correct right ascension because  the mounting plate gets in the way. The figure below shows the way in which the AstroSpotter pointer can be used in reverse to sight such stars. For these stars, the extra scale on the full 180 degree scale can be used to sight the star along the pointer in a reverse manner.


    Sighting A Star Below The North Star


Using AstroSpotter To Identify A Star

The other great use for the star AstroSpotter is to identify interesting stars or constellations that may be noticed while gazing at stars.

Here the idea is to twist the sighting fin and turn the pointer on the declination scale so that the pointer is pointing right at the star or Constellation in question. Once this has been done, simply read off the right ascension from the disk and read off the degrees of declination from the scale on the sighting fin.

After doing this, consult your planisphere or The Field Guide To The Stars And Planets (See References at the end of this paper) to determine the identity of the star or constellation. Keep in mind that the AstroSpotter must be aligned first and the right ascension disk must be updated if the readings are to give correct ascension.

Making Your Own AstroSpotter Device

If you wish to make an AstroSpotter device and would like better directions on making one or some sort of kit of parts to be offered, let me know and perhaps if it seems a big enough need, I can figure out some way of getting some sort of parts kit together.  It is ok for an individual to make his or her own AstroSpotter for personal use.   I have in my garage a lot of the required parts that are available on the cheap.   Contact me via the email on the  about me page.

It would be wrong and unjust to take this idea and profit from it without contacting me and reaching an agreement. 
Besides my origional ideas used in this device, many hours were spent in order to get all these original  ideas into the AstroSpotter.  Not described in these pages yet are more ideas that I wish to incorporate into the AstroSpotter at some future date.

The  parts used in an AstroSpotter device are all available in one way or another but it took me personally months to figure out just how  and where to get all the parts that both look and fit nicely together.  The process went through at least 4 versions of the AstroSpotter and ended up in what is described in these web pages.

It is my hope to help folks enjoy an AstroSpotter on their own but for now only the basic idea of my design is shown below.  Once again this idea is ok for individuals to use to make their own AstroSpotter to use for themselves or explaining the night skie to others such as school situations.

This page was last updated on 2/4/2008