Sun, 19. February 2017
LX-90 Polar Alignment
1. ESSENTIAL / USEFUL TOOLS
Magnetic compass, clinometer, spirit level, allen keys, reel of tape.
Attach the allen keys to the mount with a length of string, so that they can be easily retrieved in the dark if dropped.
2. PREPARING THE MOUNT
Set up the tripod at a suitable height, with the top surface level and with the two small screw holes for the azimuth adjuster facing True South. Method: spirit level and compass.
3. FITTING THE WEDGE
Hold the wedge on top of the mount, raise the central threaded rod through the wedge and fit the compass to secure the wedge tightly to the mount. You will need three hands to do this. Tighten but do not over-tighten. Ensure there is sufficient lubrication to rotate the mount.
4. AIMING THE MOUNT
The wedge tilt-plate should be on the Northern side of the mount, directly over the tripod leg which is opposite the two small screw holes on the mount (for the azimuth adjuster).
Using the compass, readjust the mount, if necessary, to be exactly level (using the circular level and/or a spirit level) and to be facing True South, which for Campbelltown is about 11.5° East (i.e. to the left) of Magnetic South.
To aim accurately after dark, sit directly behind the telescope and locate Sigma Octans with binoculars, then pan down vertically to make sure the telescope is aiming in that direction. Adjust as necessary by moving the tripod legs and/or azimuth adjuster.
5. LATITUDE ADJUSTER
Using the small allen key, set the wedge elevation to the local latitude (34° for Macarthur Region). This may be done after fitting the telescope tube (see item 7).
Check again that the wedge is pointing towards True South and check again that the mount is level.
With the mount dead level and the wedge pointing True South, the axis of the telescope forks will be aiming towards the vicinity of the SCP.
6. AZIMUTH CONTROL ADJUSTER
If not already fitted, attach the spacer for the Azimuth Control Adjuster to the edge of the base of the Southern side of the wedge base (near the bubble level).
Fit the Azimuth Control Adjuster to the edge of the mount. It should be fitted with two screws but mine appears to have been incorrectly drilled. So to fit it, only one screw can be used, fitting it through the lower hole on the Adjuster to the upper hole on the mount.
To adjust in azimuth, loosen the large centre knob (compass) and rotate using the two side knobs on the adjuster. Lubrication prior to assembly is essential to achieve this.
7. FITTING THE TELESCOPE
It may be desirable to temporarily adjust the latitude down to about 20° to make it easier to place the tube on the wedge plate.
Insert the allen head bolt through the rear of the tilt-plate and tape or wedge it in place so that it does not fall out when positioning the scope over it. Note the three slots on the base of the telescope and align them with the three protusions on the wedge tilt-plate.
Lift the telescope tube into position. Push it gently over the bolt and then remove the temporary tape to secure the base of the telescope with the large allen key, tightening firmly.
Readjust the latitude to 34° after fitting the telescope.
8. FIT ACCESSORIES AND ALIGN THE FINDERSCOPE
Once the telescope is mounted on the wedge, fit all attachments such as finderscope, dew management system, power cabling etc.
Then align the finderscope with the telescope tube, using a distant object. This is crucial and it is best to do this before proceeding to align the tube to Polar Home. Whilst aligning the finderscope, do it with “Targets” set to “Terrestrial”. When finished restore setting to “Astronomical”.
9. REVIEW TELESCOPE SETTINGS
By this point: (a) the mount is dead level; (b) the fork axis is aimed at the same angle as the location's latitude; and (c) the axis is aimed in a True Southerly direction; (d) all accessories are fitted and functioning; and (e) the finderscope is aligned with the telescope.
Check that critical Autostar settings are correct, including:
Setup - > Targets - > Astronomical.
Setup - > Daylight Saving - > Yes/No.
Setup - > Cord Wrap - > On
Setup - >Telescope - >Mount - > Polar.
Setup - > Telescope - > Max Elevation – 80°.
10. POLAR HOME POSITION
One of the most difficult parts of the setup operation is aiming the telescope exactly 90° to the wedge and dead centrally between the forks, towards the SCP. Meade have not provided a method to achieve this.
The telescope tube is at the Polar Home position when it is accurately aligned parallel with the fork mount. In other words, when the telescope tube is positioned along the same axis as the mount and does not wobble when the tube rotates.
The following daylight methods were considered:
Pre-setting vertical: before mounting the tube on the wedge, set it on a horizontal surface and adjust to vertical with a spirit level. Tighten the clutch and lift onto wedge.
Visual assessment: the tube must look parallel with the forks.
Pre-setting Dec setting circle: Use the setting circle if it remains accurately set from a previous session.
Clinometer: set the tube angle to accurately match the axis.
Rotation: by checking the tube angle before and after rotation;
Currently I have settled on the following daylight routine, using a clinometer:
Set the two forks horizonally level, using a spirit level.
Record the angle of the tube using a clinometer.
Release the RA clutch and rotate the tube by 90°.
Record the angle of the tube again.
Both measurements should be the same. If not, adjust the Dec setting and repeat the measurements.
When done, reset the Dec marker to zero.
Observe the tube top end from behind, against a fixed background object.
Confirm the alignment by fast rotation of the tube in RA.
The tube should remain visually aligned with that object through a 90° rotation.
After dark, find Polar Home by rapidly slewing the tube on its RA axis, while observing through the eyepiece. If the stars rotate about the centre of the Field of View, the SCP is centred. If not, adjust the mount until they do. If stars at the edge disappear or appear, it is not centred. A camera exposure whilst rotating may help with this.
11. ROUGHLY LOCATING SIGMA OCTANTIS
From a skychart, determine the current orientation of the four star asterism in Octans: Sigma, Tau, Upsilon and Chi.
After dark, sit behind the telescope and use binoculars to confirm that it is aiming true South, by finding the asterism and checking that it aligns with the tube.
I am currently considering whether at this point it may be appropriate to go direcctly to drift aligning. If so, go to item 14.
12. ACCURATELY LOCATING SIGMA OCTANTIS
Locate Sigma Octans in the finderscope and manually adjust the mount to centre it, using latitude and azimuth adjusters, not the Autostar drives.
Assuming that the tube has been accurately positioned in Polar Home, after dark, with forks horizontally level, find Sigma Octans, using an eyepiece with an illuminated reticle. Turn the forks 90 degrees and if Sigma is misaligned, the forks must go up or down, so adjust the latitude.
The scope is in the Polar Home position when an exact alignment occurs in both positions.
When Polar Home is achieved, zero the setting circles and lock the two clutches tightly.
13. LOCATING THE SOUTH CELESTIAL POLE
Once Sigma Octantis is centred in both the eyepiece and the finder-scope, it should be a simple matter to make an adjustment using the manual altitude adjuster and/or azimuth controller to aim closer to the SCP, which is 66.4' away (or just over two Moon diameters). Experience will make this easier and (assuming it is in accurate alignment with the telescope), I use the finderscope to achieve this.
14. SKY ALIGNMENT OPTIONS
Available choices in Autostar Polar Mode:
1. “Easy”. This is quick. Autostar selects two stars for you.
2. “Two Star”. Useful if the sky is limited, selecting your own two suitable stars.
3. “One Star”. The dialogue appears ambiguous to me, so I do not use it.
15. POLAR ALIGNMENT – EASY
In Polar Home, centre Sigma Octans then SCP, MANUALLY with the Azimuth and Latitude adjusters.
AUTOSTAR DIALOGUE |
ACTION |
Select Setup - > Align - > One Star. A message appears: “POLAR ALIGN. Put the Telescope in the Polar Home position as described in the instruction manual. Use the keypad to set the declination to 90°. Adjust the ETX RA axis to within 180° of the counter clockwise hardstop. Next, adjust the tripod (do not use keypad) so that the tube is pointing at the celestial pole. Press Enter when done.” (ETX is a different Meade telescope model! What is the counter clockwise hardstop? There isn't one on the LX-90!)
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The tripod has already been set but use the azimuth and latitude controls on the wedge to readjust if necessary.
Do not use the Autostar Hand Controller for this.
Press ENTER. |
A message appears: Selecting Star: Rigel Kentaurus (e.g.) Slewing Press ENTER
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Centre the bright star in the eyepiece using the hand controller and press ENTER. |
A message appears: Selecting Star: Antares (e.g.) Slewing Press ENTER |
Centre the bright star in the eyepiece using the hand controller and press ENTER. |
A message
appears: Calculating
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Done. |
18.ACCURACY
At this stage the telescope should achieve polar alignment satisfactory for casual observing and short exposure photography. Sigma Octans is 66.4' from SCP. Alignment within one degree of the SCP will “keep objects in the FOV for 20-30 minutes”, according to Meade.
Drift aligning will further increase the accuracy.
Training the drives will improve accuracy.
19. DRIFT ALIGNMENT
Insert illuminated reticle eyepiece and Barlow lens and carry out the alignment procedure above.
A description, from IIS:
Latitude adjustment |
Work
out which is Nth/Sth in your reticle eyepiece, adjust reticle line
to match when rotating in RA. |
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1)
Point the telescope at a bright star low (20 degrees) on the
eastern
horizon.
Centre the star in a cross hair eyepiece. |
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Now
watch for up / down drift of the star. Don't worry about the star
drifting left or right. |
Northerly drift: adjust latitude higher. |
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If the star drifts South its heading towards the South celestial pole, so your polar axis is too high. Turn your latitude (Alt) rear wedge knob to lower your wedge plate. This will cause the star to move in the opposite direction. |
Southerly drift: adjust latitude lower. |
Azimuth Adjustment |
Next point the scope at a bright star that is close to directly above your head (the Meridian on the celestial equator). Repeat above steps. |
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Now watch for up / down drift of the star. Once again don't worry about the star drifting left or right. |
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Northerly drift: adjust azimuth towards West. |
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Southerly drift: adjust azimuth towards East. |
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You should repeat them again as one adjustment can throw the other one off.
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From the Meade manual:
Aim the telescope at a moderately bright star near the intersection of the N-S meridian with the celestial equator.
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Try to find a star within 30 minutes in RA of the meridian and 5 degrees of the equator. |
Disregarding RA drift, note the extent of Dec drift:
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Adapt the wedge direction in azimuth (left-right), using the azimuth control adjuster, to reduce the error. Track again for a period of time to be sure that Dec drift has ceased.
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Aim the telescope at another moderately bright star near the Eastern horizon but still near the equator. |
The star should be 20-30 degrees above the horizon and within 5 degrees of the equator.
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Again, note the Dec drift:
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Adapt the wedge direction in altitude (up-down) using the latitude adjustment knob to reduce the error. Track again for a period of time to be sure that Dec drift has ceased.
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LX90-06 rp