How to Find Things with the ETX 90RA (7/7/2012)

There are two secrets to using the ETX 90RA to find things. These secrets are also true for most other telescopes so they are worth learning. The first is:  use the setting circles. The second is:  you don’t have to understand why setting circles work to begin using them. Just start using them.  Like many things in life they are easier to use than to explain or read about. Use them first. Understand them later.

In this age of intelligent telescopes that beep and purr and smoothly move to any object in the sky, it may seem silly to consider any other way of finding things than simply entering their name into a keypad or clicking their location on a computer or smart-phone screen. Maybe it is… and maybe its not. The good news about these automated wonders is that there is a whole new generation of star gazers that are spending more time looking at the objects in the night sky as opposed to looking for them. The bad news is that for some of them, they  are entirely dependent on the digital brains to do the job and never really learn the night sky. Again, there’s nothing wrong with that but they are missing an aspect of the hobby that is not only satisfying but also enabling.  Everyone has had their batteries run down during the middle of an observing session at one time or another. To the star-hopper and setting-circler this is a minor inconvenience. To someone who relies entirely on the go-to going to, it can be a night ender.

I must admit that setting circles have probably run off more would-be amateur astronomers than mosquitos or cold feet. But for those who persevere, the rewards are worth it.  And despite what you may have read or heard – the ETX 90RA is well equiped to find things in the sky – if you use the setting circles. To do so will take a little more effort than when using the robo-scope… but not much more.

To find things with the ETX 90RA you need to learn a few things:

1.  a few bright stars

2. how to polar align the scope

3. how to read the setting circles

4. how to adjust the setting circles

 

The first item you need to know anyway. Every season has very recognizable constellations and there are always a handful of brighter stars visible at any time of the year. You need to learn at least a dozen of them by name and by sight. These are the markers you will use as a starting point to find everything else. Get your star charts out, find your planisphere, or fire up your favorite planetarium program and go through the seasons. List twelve bright stars that are visible from your lattitude throughout the year and record their right ascension and declination. I have my list printed out and taped to my copy of Sky and Telescope’s excellent Pocket Sky Atlas. The stars I use are: Aldeberan, Rigel, Capella, Betelgeus, Sirius, Procyon, Pollux, Regulus, Spica, Arcturus, Antares, Vega, Altair, Deneb, and Fomalhaut. Like I said you need to know at least 12 of these by sight. When you look up their coordinates you’ll find they are listed by increasing right ascension. Learn which of these stars can be seen in the different seasons.

The second item -polar aligning the telescope – is really easy because the ETX 90RA is a fork mounted scope as opposed to a german mounted scope.  Search for “Kochab Method” of polar alignment and follow the directions. What you are doing is lining up on Polaris and offsetting the telescope by a small amount. the North Celestial Pole (NCP) is within 3/4 of a degree of Polaris and when you move the telescope’s mount just a little off-center from Polaris, you are close enough to polar alignment to find and track objects all night long. And the mount doesn’t have to be perfectly level to do this – although it helps. Neat.

Before we go any further we need to discuss the straight through finder that is original equipment on the ETX 90 series. It is useless. Replace it with a right angle finder or simply use rubber bands to hold a Rigel Quick Finder on the tube and use it. A green laser held against the finder bracket can work also ( unless you’re at a star party that has prohibited their use! Check this.) Don’t even try to use the straight through finder. This is the only thing Meade did that completely missed the mark  when they developed the telescope. You may still be able to find a kit ( sold by Meade of course) that would convert the straight through finder to a right-angle one.

The third item is easy to do but does take some practice. All you need to remember is that the units of right ascension are hours, minutes, and seconds.  The units of declination are degrees, minutes, and seconds. There are sixty minutes in one hour, sixty seconds in one minute. There are sixty minutes in one degree and sixty seconds in one minute. For what we will be doing you can round all the numbers off to the nearest minute and that will give you plenty of accuracy to find anything you want.

Look at the following pictures. The first one shows the right ascension and declination settings for Vega. The second pair of pictures shows the right ascension and declination settings for Capella.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The fourth item involves adjusting the setting circles. Again, the fork mounted ETX series has simplified things because you usually do not have to adjust the declination settings.  You do need to verify that when the telescope is set on a level surface and set on 900 of declination, that it is straight up. You can verify this by placing a level across the top of the telescope. If its not, you can loosen the large nurled nob in the center of the declination setting circle and adjust it so that the declination reads 900. A really good explanation of this and an alternative method for calibrating the Declination Setting Circle can be found here  on Weasner’s Mighty ETX site/Telescope Tech Tips/Performance Enhancement/Part 3.

Now lets put it all together.

Check you declination circle first. You don’t have to do this everytime but the declination setting circle can slip from time to time so if things don’t look right as described below, you probably need to check it. Next polar align the scope using the Kochab method. Once this is done, select a bright star from your seasonal list and manually center the star in the field of view. From your list of stars, note the RA and DEC of the star you’re observing. Check the declination against the declination shown on your ETX’s setting circle. It should be the same but within one degree is close enough for observational star gazing. Next slide the RA setting circle  around until the RA of the scope matches the RA of the object. Sometimes on dewey nights the RA setting circle may stick. Simply run your finger around the setting circle until you feel it loosen up from the moisture. Your done.

To use the setting circles to move to another object, note the coordinates of the new object and move the telescope to the coordinates being careful not to touch the RA setting circles as you do this. The object your looking for should be either in the FOV of your 26mm eyepiece or within one FOV of the eyepiece. Once you have the object centered you can lock the RA axis and the telescope will track the object.

There is one other thing. Unlike the ETX’s wealthy cousin, the Questar 3.5, the RA setting circles do not utilize a more user friendly and pricey friction clutch arrangement. This means that after a few minutes you will notice that the object is still in the center of the eyepiece but that the RA setting has moved and is no longer showing the correct coordinate. The declination setting has not moved and is still correct. The only thing you have to remember is to move the setting circle back to the correct coordinate before you move to the next object. The celestial objects move approximately one degree every four minutes. That means if you let your scope track an object for 16 minutes that the RA setting circle will have an error of 4 degrees. Again, simply reset the setting circle before you move to the next object.

After you have used this method a few times you will find that you can polar align the telescope in, literally, three or four minutes and have the telescope ready to go and viewing objects in less than five minutes. This is no exageration. This is also why I don’t get upset when some well meaning new observer ( or myself!) accidentally kicks the tripod out of alignment. In no time at all I’m back in alignment and back to observing. This is simply not so simple to do with the robo-scopes.

Learn this method. With minor variations it applies to any telescope that is equipped with setting circles.

It has the added benefit of giving  you a new way to look at the sky. You will begin to envision the lines of RA and DEC. You will know what the Siderial Time is. You will instinctively look to the section of sky for an object based on its coordinates alone… OK, that one is going to take some time… but it will happen. You will develop a feel for the Celestial equator and how the Ecliptic moves north and south through the seasons. You will also rely on something that is very basic and not subject to the whims of the digital grimlins that seem to plague some nights over others.

And… its fun.

Not bad for a scope that’s not supposed to be very good at finding things….

 

Fred

7/7/2012

 

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