POLAR ALIGNMENT AND ETX / LX "HOME POSITION" AT THE EQUATOR
From: email@example.com (Clay Sherrod)
At first glance, living at or near the equator is an astronomer's heaven....or so it seems. The planets pass nearly overhead during each apparition, the best of both the deep sky in the northern and southern hemispheres are visible...you have the Andromeda Galaxy, and you have the remarkable Magellanic clouds; looking "south" you see the Southern Cross (Crux) and to the "north" there is the Northern Cross (Cygnus).
Can there be ANYTHING wrong with this scenario? Yep. You can't see any bright reference stars to hone in on celestial north or celestial south. To set up your ETX or LX in "home position" requires that you know where the celestial poles are....how do you know if all the stars surrounding them are BELOW the horizons?
I have a correspondent who has a permanent pier in French Guyana, located enviously on the northern beaches of South America at Longitude West 52 degrees 39 minutes; Latitude +05 degrees 09 minutes. Can't get too much closer to the equator than that! He has constructed a fine pier on which the ETX 125 will be POLAR mounted and desires to make the alignment of the pier so precise that no tweaking will be required prior to each observing session.
But he has been baffled by a "good" method to establish "home position" for the ETX since celestial north cannot be seen. His first thought was to set up roughly "toward where north should be," slew - in Alt-Azimuth mode - to a bright star like Capella, center it and note the Azimuth with the AutoStar. Then add or subtract the difference in Azimuth (using Epoch 2000 or Sky Navigator for the exact time of observation) and return to celestial north's azimuth (360 degrees) by turning the entire wedge and scope assembly until AutoStar shows this difference.
In theory this can work...in practicality, you'd be lucky to even be in the right hemisphere on your first GO TO. So I have put much thought into this project and come up with a very precise, yet VERY easy alignment procedure for observers near the Equator. This is NOT a quick process and not something you would want to do each time you go outside; if you embark on this celestial alignment procedure, PLEASE arrange to leave your pier, tripod or platform set up once done and merely bring the ETX or LX in and out to observer, slipping into place on the pre-aligned mount when ready.
SET ONE - Roughing In Celestial North
First, any observer is such a condition (it may be YOU at latitude 37 degrees north, but you cannot SEE Polaris because your house is in the way....you can still do this exercise successfully!) must reckon to the BEST ACCURACY POSSIBLE "about" where north is located (or "south" in the southern hemisphere); since our correspondent is very slightly NORTH of the equator, northern alignment will be discussed here.
It is possible to get incredibly accurate in setting a home position azimuth merely by knowing EXACTLY your "magnetic deviation" - the amount, in degrees, that a compass is offset from true celestial north by the magnetic flux lines of the Earth's magnetic field. In many cases - just as in this case in the French Guyana - the offset can be HUGE! Figure 1 shows the lines of magnetic deviation for South America, and note that I have marked his approximate observing position (red circle and arrow) and I have marked the "magnetic deviations" (three "isobars" are marked with BLUE circles to show how rapidly the deviation changes!).
Magnetic Deviation (sometimes called "magnetic declination") can be downloaded from: www.geo-orbit.org/sizepgs/magmapsp.html for any continent worldwide; simply measuring on the map relative to YOUR geographical latitude and longitude, as I have done for the observer in South America, will allow you t quickly extrapolate your magnetic deviation.
As you can see, his magnetic deviation is: 15.2 DEGREES WEST. This is a significant - almost one hour of right ascension! - difference in TRUE CELESTIAL and MAGNETIC north. So you can realize his frustration with alignment, knowing that Polaris to reckon a visual sighting toward NORTH cannot be seen.
But in this FIRST step, we can get him VERY close to true AZIMUTH north. Keep in mind, he has very accurately determined his LATITUDE, and thus the setting for his polar wedge; this is not a problem since it is simply done from the level (as determined with a bubble level) and elevated using a protractor scale to the observer's latitude from the north horizon. So our discussion is appropriately AZIMUTH true north.
So What do I do with Magnetic Deviation?
Here is how you can get very close to true celestial north (this is only a starting point!) knowing your MAGNETIC DEVIATION:
If you look closely at both Figures 1 and 2 you will see that the flux lines are marked "10E", "16W", "12W", etc. There are BOTH "east" and "west" magnetic deviations and either direction affects your compass needle opposite the other. The rule for using the deviation is simple:
1) WEST DEVIATION (such at Long. W52 399, Lat. +05 09) - SUBTRACT THE AMOUNT (in his case "15.2 degrees") to the compass reading to obtain TRUE CELESTIAL AZIMUTH.
2) EAST DEVIATION (look far to the left of his location or EAST of the "Mississippi River" in Figure 2) - ADD THE AMOUNT to your compass reading to determine true celestial azimuth.
Now, using the compass - adjusted for the offset in degrees - merely rotate the wedge, tripod, pier or other mount until the axial north-south mechanical configuration of your system is in line with that CORRECTED compass reading! You are very, very close to true azimuthal north....true celestial north.
But you're not done. If this is merely for a one-night observation, you may stop here; your telescope system will be aligned pretty much in a north-south home position. But for exact alignment, you MUST use STEP 2 following only AFTER attaining the compass-corrected orientation just described.
STEP 2 - PERMANENT ALIGNMENT USING STAR DRIFT
Although this procedure may require days, or even a week of exacting adjustments, the results are most definitely worth it for piers, mounts and tripods that are to be left outdoors; this is particularly true in the larger LX 200 telescopes and the LX 90 which size alone suggests some permanent "home-based" or even observatory mounting might encourage you to use your telescope more often.
This technique is designed primarily for Polar aligned operation. Because very precise tracking and observation usually require observing in Polar mode any way to increase tracking efficiency and telescope operation control, such a detailed and exacting procedure is worth the effort if you are observing Polar.
The following drift method is extracted from my book "A Complete Manual of Amateur Astronomy" 1982, Prentice-Hall Publishers.
Accurate alignment through this method makes the MOST accurate use of your GO TO pointing, or setting circle use, and virtually can eliminate ALL declination drift even at high magnifications! The method can be used on ANY type mounting - German equatorial, fork, yoke, etc. You should USE A CROSS HAIR eyepiece at highest power (a barlow helps).
You've make a quick alignment using Step 1 above, since you could not see Polaris or any marker stars because of a terrestrial obstruction or you might be in the southern hemisphere and aligning to south.
1) Set up your telescope in the tracking configuration "Polar" under "TELESCOPE/MOUNT" on the Autostar and adjust your telescope as close as possible to "polar home position" (see my "Enhancement Guide..Part 3" on this web page), then set your latitude now as precisely as possible after leveling the base or tripod; if Alt-Azimuth (see below, as this technique can help a bit in alt-az mounting as well) have the telescope as close to "Home Position" as possible. I both cases, INITIALIZE the telescope (or use the standard hand controller - not AutoStar) to activate the tracking motors; if using AutoStar you may have to go through a "dummy" star alignment (use two-star "EASY") first, and just press "Enter" each time it slews to an alignment star....it will tell you after the second star "Alignment Successful" at which time the sidereal motors will be heard to engage; you are now reading to "drift align."
2) Point the telescope manually or with electric slewing (DO NOT USE GO TO at this point!) at a fairly bright star, using those listed in the table following. You MUST use a star that is both near the "meridian" passing overhead from north to south, and one that is VERY close to the celestial equator for accuracy. Once you have centered the star at very high magnification, CLAMP both axes securely so that tracking can be as accurate as possible. Adjust your cross hairs in an E-W and N-S orientation before beginning.
Celestial Equator BRIGHT STAR TABLE (Use for "Drift Alignment") Sadalmelek RA 22h 03m DEC -00 34' (Alpha Aquarii) Altair RA 19h 48m DEC +00 44' (Alpha Aquilae) Cheleb RA 17h 41m DEC +04 35' (Alpha Ophiuchii) Spica RA 13h 23m DEC -10 54' (Alpha Virginis) Regulus RA 10h 06m DEC +12 13' (Alpha Leonis) Alphard RA 09h 25m DEC -09 26' (Alpha Hydrae) Procyon RA 07h 37m DEC +05 21' (Alpha Canis Minoris) Betelgeuse RA 05h 53m DEC +07 24' (Alpha Orionis) Rigel RA 05h 12m DEC -08 15' (Beta Orionis) BELT STARS OF ORION: Mintaka RA 05h 29m DEC -00 20' (Delta Orionis) Alnilam RA 05h 34m DEC -01 14' (Epsilon Orionis) Alnitak RA 05h 38m DEC -01 05' (Zeta Orionis)
3) Your telescope at this point WILL NOT be accurately aligned to celestial or azimuthal north - close probably by not accurate by any means. So you will see some drift in these stars from misalignment. IGNORE ALL DRIFT IN RIGHT ASCENSION (azimuth).
4) If the star drifts SOUTH (down in your eyepiece of the ETX and LX scopes) then the POLAR AXIS is pointing TOO FAR EAST of true north; if the star on the other hand drifts NORTH (up) then the polar axis is pointing TOO FAR WEST of true north. The amount and speed at which it drifts is a good indicator of if you are close are far way from alignment. Rotate the entire mounting, wedge or tripod head in the appropriate direction just a small amount and check the drift again (DO NOT unclamp and move the telescope axes - only the mounting (tripod, pier or other mount)).
5) Double check to make sure that you are correct about the DIRECTION of the drift...do not trust your eyepiece. Make sure that if you think it is drifting north, it is indeed drifting in that directly; pressing lightly in that direction (to the north) will make star images move southward and visa-versa.
REMEMBER - IF THE DRIFT INCREASES RATHER THAN DECREASING LIKE IT SHOULD, then you have "overcorrected" in either altitude or azimuth of your polar mount. If so, reverse until you get to minimum or not drift whatsoever.
Although this method works only on Polar aligned (i.e., Polar Mode in ETX or LX) telescopes, using this technique in Alt-Azimuth mode can actually result in better home position as well.
6) Keep refining this same step over and over again, hopefully using smaller increments each time until no drift is seen FOR 20 TO 30 MINUTES even at high magnification...yes it is entirely possible and easy to do!
NOW WE ARE GOING TO ADJUST YOUR ALTITUDE!
7) After the north-south drift has been eliminated for AZIMUTH as describe above, we are ready to tweak even further. Chose a bright star CLOSE TO THE EAST HORIZON, at about 20 to 30 degrees above the horizon and MAKE SURE that this star, too is also close to the celestial equator as noted above.
8) Again using the crosshair eyepiece, clamp both axes and watch again for drift. Ignore any drift in right ascension entirely. If the star in this test DRIFTS SOUTH, then your polar axis pointing too LOW, so raise up the wedge altitude a bit or raise the tripod head some. If it drifts NORTH, the polar axis is aimed too HIGH, so you need to come down a bit.
9) Repeat as often as necessary.
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