David's Astronomy - Notes 2007-09-10 (S00211)

A refined polar alignment was attempted during session 208 (2007-08-27) based on Drift Align Analysis, however the result was only partially successful. Further drift align images suggested that imperfections in the telescope's mount were limiting the accuracy of drift align results and their dependability for polar alignment.

Analysis of a TPoint mapping run conducted during last session (S210), indicated a polar misalignment of between 3.3 and 7.8 arc minutes depending on terms used. There was also some uncertainty on the results as it was later determined that the computer time was out by 5 seconds during the mapping run, which could have produced an error equivalent to 1.25 arc min (75 arc sec).

Reminder of Results from TPoint Mapping (S210)

TPoint fit 1 - RMS 86 arc secs

9 terms : 6 basic (equatorial) terms + Tube Flexure(TF) + two Harmonic terms

Polar Alignment Information :

************* AZIMUTH ************ MA: -82 seconds (-1.4 minutes),

************* ALTITUDE ************ ME: -181 seconds (-3.0 minutes).

For latitude 57.32° :
Mount should be rotated axis East (clockwise) by 2.5 minutes.
The polar axis should be lowered by 3.0 minutes

TPoint fit 2 - RMS 89 arc secs but lower ME/MA error

10 terms : 6 basic (equatorial) terms + 4 Harmonic terms

Polar Alignment Information

************* AZIMUTH ************ MA: -84 seconds (-1.4 minutes)

************* ALTITUDE ************ ME: -462 seconds (-7.7 minutes).

For latitude 57.32° :
Mount should be rotated axis East (clockwise) by 2.6 minutes.
The polar axis should be lowered by 7.7 minutes

Based on general indication that Alignment required that the Polar Axis be both rotated clockwise and lowered, but also in recognition of remaining uncertainties it was decided to make part of the indicated changes by rotating the mount clockwise by 1.6 arc min and lowering the polar axis by 4.0 arc min. In the event it is believed that the mount was actually rotated clockwise by 1.8 arc min and the polar axis lowered by 2.9 arc min. The alignment modification should be in the right direction and will be analysed again during a future TPoint Mapping Run.

Polar alignment corrections were made using CCD Image assistance. This utilised the method whereby a bright star, near to the intersection of Meridian and Declination 0 was first centered in the CCD Field of View. The telescope scope was then moved in the opposite direction to the change required (using appropriate jog or move commands) and then the mount/wedge controls were used to re-centre the star.
[more detailed description of CCD Assisted Polar Alignment Correction)

CCD Assisted Polar Misalignment Correction (TPoint)

This procedure records the steps used by correcting Polar Misalignment, based on knowledge of misalignment information from TPoint Mapping (ie where both azimuthal and inclination corrections are known)

Step	Procedure	Comment
1) Determine Required Polar Alignment Corrections	Determine Polar Misalignment by performing and analysing a TPoint Mapping Run (see TPoint example, 2007-09-03)	TPoint is considered to be preferable method of determining the Polar Misalignment. In this example the telescope mount requires to be rotated clockwise by 1.6 arc min and polar axis requires to lowered by 4.0 arc min (see Polar Alignment example - 2007-09-10)


2) Correct Polar Axis Misalignement (azimuth)	Slew to South Sky Point (Dec 0 / Due South, and centered a reasonably bright star	The South Sky point is a convenient position to understand the operations being made. Also the accuracy of azimuthal correction is greatest at Declination 0 deg.

	Check that star lies in a area of sufficient stars that 20 sec exposure can be successfully image linked	This provides a backup in case the bright target star is accidentally moved out of the FOV and is 'lost'.

	Identify suitable exposure for tracking the bright star, and turn on cross-hair feature on CCD imaging software (if available0	A 2 sec exposure (at 2x2 binning) was found appropriate to show the target star and a couple of adjacent fainter stars (useful later as a ID check to ensure the correct target star is being followed). Key '6' turns on centre cross-hair in CCDSoft

	Take out any backlash in RA	I need to make an E jog, so I jogged the scope W and then back E to recentre the bright star. (not sure if this step is necessary for RA drive, but performed anyway)

	Take 20 sec baseline image	Image linking the image records the initial position

	Jog scope E/W by required amount in the opposite direction to the change required. a) For a Clockwise axis change jog the scope East b) For a anticlockwise axis change jog the scope West	I needed to make a 1.6 arc min clockwise rotation of the mount, so I jogged the scope East by 1.6 arc mins

	Take 20 sec check image and confirm that required jog has been successfully applied	Analysis of check image showed telescope had actually moved by 1.8 arc mins (depending on accuracy required this could have been nudged closer to 1.6 arc mins) Note: It is possible to abort and start again since no change to wedge position has yet been made.

	Set CCD to continuous snap images	2 sec exposures, Cross Hair turned on

	Loosen wedge manual knob (and 3 additional knobs if applicable), rotate wedge in fine increments in required direction whilst watching CCD frames, stop when target star is perfectly recentred.	After 2 years I found that wedge was initially very stiff to turn / minor corrosion between wedge and pier plate, and the azimuth control knobs couldn't exert enough leverage to move the wedge, requiring me to manually made to 'force' the wedge (loosing the target star, a 20 sec exposure /imaged linked then allowed me to find out how far I had overshoot). Loosing the wedge knobs causes a slight drop/rise in Dec position (ignore this for the moment)

	Tighten Wedge knobs(s) and ensure that target star remains centred	Loosing the wedge knobs causes a slight drop/rise in Dec position, however this is normally recovered when knobs are re-tightened. Adjusting the order in which knobs are tighten allows the target star to be perfectly recentred.

	Take 20 sec check image	Image linking the image records the final position. Position should ideally match the initial position.

3) Correct Polar Axis Misalignement (inclination)	Take out any backlash in Dec drive and ensure that star is centered	I need to make a S jog, so I jogged the scope N and then back S to recentre the bright star.

	Take 20 sec baseline image	Image linking the baseline image records the initial position

	Jog scope N/S by required amount . Movement is in direction opposite to the direction of the change required. a) For raising the polar axis jog the scope North, b) For lowering the polar axis jog the scope South	I needed to lower the polar axis by 4.0 arc mins, so I jogged the scope South by 4.0 arc mins

	Take 20 sec check image and confirm that required jog has been successfully applied	Analysis of the check image showed telescope had actually moved south by 2.9 arc mins (depending on accuracy required this could have been nudged closer to the target figuire of 4.0 arc mins) Note; It is possible to abort and start again since no change to wedge position has yet been made.

	Set CCD to continuous snap images	2 sec exposures, Cross Hair turned on

	Slightly loosen the 2 tilt angle adjustment knobs on side of wedge. Then adjust Latitude Control knob in fine increments in required direction whilst watching CCD frames, stop when target star is recentred. To raise the Polar Axis turn Latitude Contol knob in clockwise direction. To drop the Polar Axis turn Latitude Control knob in anti-clockwise direction.	I needed to lower the Polar axis which required Tilt Plate to be made more steep, achieved by means of anti-clockwise turns of the Latitude Control knob. Loosing/tightening the tilt angle adjustment knobs causes small change in RA position, and a larger change in Dec position. The Dec position change may need to be allowed for when making the Latitude changes. Any RA position change should be noted but otherwise ignored for the moment. It was evident that the process of re-centering the star somewhat of an fine art and requires a delicate interplay of pressure from Latitude Control knob and the plate's two side locking knobs. (In a previous session when trying to raise the polar axis (by making the Tilt Plate less steep), quite a few clockwise turns of the laitiude knob were seemingly needed even though I was only raising the polar axis by just a few arc mins0.

	Tighten tilt angle adjustment knobs and ensure that target star remains centred	Loosing/tightening the tilt angle adjustment knobs causes small change in RA position, and a larger change in Dec position. Tilt plate adjustments may need to be repeated to ensure that the target star lies in the central cross-hair position after knobs are finally tightened/locked.

	Take 20 sec check image	Image linking the image records the final position. Position should ideally match the initial position.

4) Check Improvement in Polar Alignment	Determine residual Polar Misalignment by performing and analysing a new TPoint Mapping Run, or my cheking star drift	Results to be inserted following next session.

This Web Page:	Notes - Session 211 (2007-09-10)
Last Updated :	2015-05-16

David's Astronomy Pages
Notes - Session 211 (2007-09-10)

Polar Alignment (2007-09-10)

CCD Assisted Polar Misalignment Correction (TPoint)


	Polar Alignment (2007-09-10)
	CCD Assisted Polar Mis-Alignment Correction

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David's Astronomy Pages Notes - Session 211 (2007-09-10)

Polar Alignment (2007-09-10)

CCD Assisted Polar Misalignment Correction (TPoint)

David's Astronomy Pages
Notes - Session 211 (2007-09-10)