My ETX tips

Polar alignment tips

In working with the ETX and my Bogen tripod an head, I always seem to be striving for a more perfect polar alignment as well as ways to make the setup time as short as possible.

One of the most helpful things I've done has been to make a right-triangle template with one angle the same as my latitude. Your latitude is easily obtained either from a GPS unit or from a website that will let you zoom in on your location on a map and give you a very accurate value for your latitude. (Some of these are available through the Astronomy Links page on the Mighty ETX site.) My latitude happens to be 41.385 degrees.

With that in hand, it's very easy to make a template with a graphics program; I used Adobe Photoshop, which will giveme a readout of the cursor's position at all times. I opened up a new document with 150 pixels per inch, and drew a horizontal line 1000 pixels long. From the left endpoint of this line, I drew a perpendicular line (straight down) 881 pixels down. I then joined the two open ends for my triangle. The angle at the upper right of the is very close to 41.385 degrees. I printed this out, glued it to a piece of cardboard, and cut it out when dry.

Why 881 pixels? The answer is in the formula b = 1000 * tan(41.385). It's the trigonometry formula for finding a side of a right triangle when one side and an angle are known. Just plug in your latitude where I have 41.385. A moment with a calculator tells me that if I have one side of 1000 pixels and an angle of 41.385, I need the other side of the right angle to be 881.153 pixels long. 881 is close enough!

Anyway, with template in hand, I set up my tripod and carefully leveled the base, then attached the ETX to the Bogen head, and with the ETX's tube at 0 degrees, I used the various tilt axes on the Bogen head to level the base of the ETX. Having a circular level is a must for this! I got mine for $1.75 from the American Science and Surplus company.

With the base level, I then leveled the ETX's tube and reset my declination setting circles - I'd noticed they were about 3 degrees out at one point. I then placed the template's hypotenuse (long side) against the ETX's tube with the 41 degree angle pointing to the tripod. I raised the angle of the tube using the vertical axis of the Bogen head until the bottom of the template was level, which I tested simply by holding a regular level against the bottom of the template.

Now I can be polar-aligned in seconds! Since I have the Bogen head with the quick-release head, all I have to do is make sure the tripod is aligned to north, and snap the ETX on the head. Poof! Instant alignment.

The key to such a quick setup is definitely the fact that I was able to get the latitude angle set correctly thanks to the template. Since I hadn't been able to recall my trig classes and had to look up the procedure for making the template, I thought others might be interested as well.

John Deriso wrote a followup:
I found another circular level for an alternative to the one mentioned, a Sears Craftsman 939891 Acrylic Circular Level. Nice size, about 1" diameter, sells for a couple bucks. I lathe-turned an aluminum (plastic would also do) dust-cover plug to fit the 1-1/2" EP hole, with a step to a slightly larger diameter so it can't fall into the OTA. The level is glued to the top of the plug. Can be used to level the OTA and check the DEC circle adjustment, or rest the plug/level on the fork base to level the ETX housing.

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