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Setup and Alignment
Owners of Celestron Nexstar i series goto telescopes have mixed success with goto accuracy. There are both mechanical and logical reasons why alignments can be off. There are a number of members of the Yahoo based Nexstar Users Group have found the best procedures and documented them as a supplement to the Nexstar User Manual. I follow these procedures and have great success with goto accuracy. I do, however, take an additional step that some consider unnecessary, but I feel improves overall goto accuracy. These few extra steps take only minutes and they do improve the Nexstar's initial pointing to alignment stars.
First, I make sure that the tripod
head is level in all directions. This takes only a minute using a small
Second, after mounting the scope on the tripod, power on and site Polaris before going into alignment options.
Third, bring the scope down slightly below level then bring it up to level using a spirit level. Make sure that 'up' is your final move.
At this point, you know the scope is accurately pointed north and is level. Now begin your alignment procedure following the steps below. I prefer setting the scope level to using the index marks provided by Celestron. You can skip step 1 because you are already north and level.
Quick – start guide for “i” series telescopes
Jean C. Piquette
Before starting: Be sure to set the final GOTO approach directions in the hand controller (HC) to positive for both altitude and azimuth. Also, choose an eyepiece with a wide field of view, preferring one with at least 0.6-degree angular width, such as the 25mm eyepiece that comes with the 8i. (If you still are not satisfied with the quality of GOTO behavior even after trying these tips, you might try switching to a high-power eyepiece for centering alignment stars, then switching to a lower-power eyepiece for GOTO slews. Resort to this, though, only after you have tried using a single eyepiece, with a wide field of view, for both alignment-star centering and GOTO’s.) Finally, be sure to review the instruction manual directions for doing an auto-align. This quick-start guide only gives suggested changes to, and enhancements of, those directions. This is not a complete set of alignment directions.
1) When directed by the HC to set the telescope to “north and index,” use the direction buttons first to point the tube in the direction of Polaris (the North Star). This will get you pointing north. (If you cannot see Polaris, rough pointing to the north is adequate.) Then, use the “down” direction button to drive the tube downward until the index markings are well below the index position. Then press the “up” direction button to put the markers at the index position. It is not important that the index position is exactly set, but it is important that the final movement of the tube is up.
2) When the scope (HC)
starts selecting alignment stars, it is important to be sure to select two stars
that are very widely separated. This probably will mean going through the setup
process a few times (turning off the scope between each attempt) until you
become familiar with the selection of stars the HC will choose for you. The best
choices of alignment stars are ones where you need to spin around on your heels
to look at the second one after looking at the first. (The best alignment-star
choices are those that are at least 100 degrees apart.)
3) After you have
centered the first star in the finder, making sure that the star is in the field
of view (FOV) of the eyepiece, and have pressed “enter” (as directed in the
instruction manual), put the star far out of focus. It should become a giant
blob in the FOV. This makes centering of the star in the eyepiece easier.
4) While centering the
stars in the FOV, you must be sure that the final direction-button presses on
the HC are down and right. This is so important, in fact, that even if you think
a star is already well centered in the FOV, intentionally slew the tube by
pressing the up and left buttons, until the alignment star is in the upper left
corner of the FOV. Then, try to press only the down and right buttons to center
the star. If you overshoot, it is OK to back up. Just be certain that the final
direction-button presses you make are down and right.
5) Center the second
alignment star in the same way as you centered the first. And remember to
re-focus when done with the second star!
More information: GOTO quality will generally be best for targets that are not too far from the imaginary line connecting the chosen alignment stars. If the alignment stars are at least 100 degrees apart, you can expect to get good GOTO behavior approximately 70 degrees to either side of this imaginary line. If the alignment stars are only 70 degrees apart, GOTO quality is good approximately 50 degrees to either side of this imaginary line. Try to never choose alignment stars closer than 70 degrees apart. Always choose the most widely separated alignment stars that are available, but avoid choosing them in one area of the sky. (Choosing both stars near the eastern horizon, for example, is not good.) GOTO’s for targets close to the imaginary line connecting the alignment stars are usually more accurate. Thus, choosing alignment stars for which the imaginary connecting line forms a diagonal sweeping from one side of the sky to the other, and also passing close to the Zenith (i.e., directly overhead), is a good strategy.
1. Don't be afraid to do a realignment if you find objects are not as close as you would like. But, I don't recommend using Solar System objects (planets, moon, etc.). This is a good idea if you are moving to an area of the sky far from your original alignment stars.
2. I found that alignments can get 'off' if the tripod is setting on soft turf. I take a set of patio blocks with me to remote sites.
3. Before turning scope off, I goto Polaris. The scope is now ready for your next session. This helps me because of the way that I have the power and RS232 interface cables routed.