David's Astronomy Pages
Notes - Session 208 (2007-08-27)

Previous Notes
Notes
(S199)
  Home
Notes
(Main)
  Home
Home
Page
  Next Notes
Notes (S210)
 
Bullet Need for improved Polar Alignment
Bullet Polar Alignment Checks from Star Drift Measurements
Bullet Some comments on the Drift Align Method
Bullet Polar Check/Align Diary
 
>

Goto Images from 2007-08-27


Need for improved Polar Alignment

From checking the drift of stars during recent sessions it was established that the Scope's Polar Axis was an estimated 3.8 arc min too low and 5.8 arc min too far west.  (ie out by 7 arc minutes).   Whilst not a barrier to acquisition of star targets (due to TPoint modelling and automated star centering tools) the polar misalignment was causing stars to drift across a CCD image at rates of up to 1.5 arc sec/minute. This is equivalent to 0.6 pixel per minute drift at my normal CCD imaging setup (2x2 binning, F6.7).    

Drift rates are lower away from the Dec 0 deg, and therefore I've been generally able to get away with unguided image exposures of up to 3 minutes, without significant star trailing.   Whilst I can employ autoguiding for particular targets,  it is not generally suitable for automated acquisition run or when trying to acquire images through filters (esp B Filter).  Where a series of long exposure images are taken (eg 10 x 180 secs) or a long run of shorter images (eg 200 x 15 secs), there is a tendancy for a target star/object  to drift away form the image centre, such that subsequent averaged/summed aligned images have to be cropped to a slightly smaller area.  

For very long runs (eg a star transit) a particular star drift could theoretically drift from the centre to the edge of my CCD field of view in a 3-4 hour period. Reference stars closer to the edge might drift out of view within just 20-30 minutes, requiring regular manual correction or autoguiding.  Problem with standard autoguiding with my ST7 camera is the difficulty of finding a suitable guide star in small FOV and insufficient robustness to the passing cloud where guide star becomes lost.

For highly accurate differential photometry (necessary for any successful attempt at exoplanet transit detection)  it is vital that target and reference stars are kept fixed on specific pixels for the duration of the imaging session.    For this purpose I have developed my own software routines for autoguiding (which employ guiding corrections between image frames and are robust to both cosmic ray artefacts and passing clouds. Despite this procedure a residual amount of field rotation can still cause movement of reference/target stars between CCD pixels. Also it still doesn't allow for exposures of > 3 minutes, since star trailing may still occur between guide corrections, which are only made between frames.  Overall there is still an advantage for to minimize the number of guiding corrections, due to unwanted guiding effects (eg where a guide in Declination introduces a small unwanted error in RA). This is due to the weaknesses in the Dec drive system on my old style LX200 and where perfect backlash correction can't be achieved.

Back to Top


Polar Alignment Checks from Star Drift Measurements

Polar Alignment checks were made on 2007-08-14. The involves firstly measuring the N-S drift of a star near to Meridian & Declination 0 (South Sky Point) and then the N-S star drift on a star close to either western horizon (West Sky Point) or alternatively close to eastern horizon.  Only N-S drifts are considered. 

N-S drift of a star at South Star point indicates an issue with East/West pointing of the telescope's polar axis, whilst N-S drif of a star at the West Star point indicates an issue with the inclination of the scope's polar axis.

For the northern hemisphere (where I am), the following rules apply

- South sky point 

    star drifts north --> scope's polar axis pointing too far west and needs turning clockwise 

    star drifts south --> scope's polar axis pointing too far east and needs turning anti-clockwise 

- West sky point

    star drifts north --> scope's polar axis pointing too low and needs raising

    star drifts south --> scope's polar axis pointing too high  and needs lowering 

With a little trigonometry the rate of drift can be used to calculate the approximate amount of polar misalignment. 
From stra drift checks during recent sessions it  was estimated the the scope's polar axis was pointing 3.8 arc min too low and 5.8 arc min too far west.  (ie out by 7 arc minutes). 

Images during Polar Alignment Check (2007-08-14)

South Sky Point
Animation of 2 images taken 3 minutes apart 
illustrating northwards drift of 1.6 arc sec/min caused 
by 6.3 arc minutes misalignment in polar axis azimuth.
(the slight drift in RA is either due principally to the inclination misalignment 
in the polar axis or Periodic Error and is ignored - it's the N-S drift that is measured)
Image
West Sky Point
Animation of 2 images taken 3 minutes apart 
illustrating northwards drift of 1.0 arc sec/min caused 
by 3.8 arc minutes misalignment in polar axis inclination
(the obvious drift in RA is due principally to the azimuthal misalignment 
in the polar axis and is ignored - it's the N-S drift that is measured)
Image

 

Control Tool used to perform Slews to 
South & West Sky Points
Image
Control Tool used to perform 
Automated Image Capture / Drift Analysis 
Image
 
Log file associated with Polar Alignment Check

Slew to South Sky Point
  Slewing to Azimuth: 180.0deg, Altitude: 33.0deg
Operation...                   Completed

Measuring N-S Drift
Two pictures will now be taken separated by approximately 3 minutes
Do not move the scope during the operation.

  Taking 20s image (C)...      Ok        [00204327] 01:57:56UT 2x2 20s C
  Linking image (GSC)...       Ok        Stars 11, Scale 2.610

  Taking 20s image (C)...      Ok        [00204328] 02:00:59UT 2x2 20s C
  Linking image (GSC)...       Ok        Stars 12, Scale 2.611

  Time Interval :              3.05 mins
  Baseline Az :                180.1 degrees
  Declination Drift :          1.64 arc secs/min NORTHWARDS
  Polar misalignment :         6.28 arc mins (approx)

  Baseline Dec : +00 13 44.90 
  Second Dec   : +00 13 39.89 

Operation...                   Completed

 


CCD Assisted Polar Alignment (2007-08-27)

CCD Assisted Polar Alignment

To help address the above issues it was decided to make an improved polar alignment attempt. This was not done without some hesitation due to the 'danger' of loosing the reasonable accurate alignment that I already have.  The controls to allow my scope wedge to be raised/lowered and rotated are somewhat stiff and prone to significant re-movement when locking down after alignment. 

After reviewing my planned procedure and confident that the potential benefits outweighed the risks, an improved polar alignment was attempted at the end of Session 208  (2007-08-27).  The steps taken are tabulated below, together with comments and results.

(In the end it was concluded that the drift align method still left residual Polar Misalignment and it was subsequently decided to improve polar alignment using information form TPoint mapping - Improving Polar Alignment with TPoint)

Step   Procedure     Comment
1) Determine Polar Misalignment    Determined Polar Misalignment by analysing N-S drift of star in southern and western skies (at approx Dec 0)   Used measurements made on previous observing session

Required action : Turn polar axis clockwise by 5.8 arc mins. Raise Polar axis clockwise by 3.8 arc min

 
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 being at Dec 0 increases the accuracy of azimuthal corrections.
         
    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 needed to make an E jog, so I jogged the scope first 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 

for clockwise axis change jog scope East, whilst for a anticlockwise axis change jog scope West
  I needed to make a clockwise axis change of 5.8 arc mins, so I jogged the scope East by 5.8 arc mins
         
    Take 20 sec check image and confirm that required jog has been successfully applied   I checked that scope has correctly jogged East by ~ 5.8 arc mins.

Note 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   I need to make a N jog, so I jogged the scope S and then back N 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 

For raising the polar axis jog scope North, whilst to drop the polar axis jog scope South
  I needed to raise the polar axis by 3.8  arc mins, so I jogged the scope North by 3.8 arc mins
         
    Take 20 sec check image and confirm that required jog has been successfully applied   I checked that scope has correctly jogged North by ~3.8 arc mins.

Note 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 need to raise the Polar axis which required Tilt Plate to be made less steep, achieved by means of clockwise turns of the Latitude Control knob. Quite a few turns of the knob were seemingly needed even though I was only raising the polar axis by 3.8 arc mins.

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

         
    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) Re-Check Polar Misalignment    Determine Polar Misalignment by analysing N-S drift of star in southern and western skies (at approx Dec 0)   Star drift was measured in each case by analysing two images taken 5 minutes apart.
 
5) Correct Polar Axis Misalignement (azimuth / inclination)   Repeat Step 2     

 

         
    Repeat Step 3    
 
6) Re-Check Polar Misalignment    Determine Polar Misalignment by analysing N-S drift of star in southern and western skies (at approx Dec 0)   .

 


Images during Refined Polar Alignment (2007-08-28)
showing Polar Axis Inclination Correction in progress
Image
Image
Image
Annotated CCD Images 
20 sec exposure, 2x2 binning, C Filter
2007-08-28  (#208106,111,113)
 
Control Tool used to perform telescope Jog
Image

Back to Top


Some comments on the Drift Align Method 

In theory the drift align method provides an accurate way of achieving very good polar alignment and is certainly very handy when the polar star is either obscured or can't be viewed/imaged with CCD equipment due to fork construction on an SCT like the LX200.

In practice a number of issues are noted that make it difficult to achieve a precise polar alignment, or make it particularly tedious.

Uncertainty over the optimum Western Sky Point for Drift Analysis
Image

Back to Top


Polar Check/Align Diary

Diary of Drift Check / Drift Align Results

2007-08-13 (S204)

Drift Checks. (3 minute periods)

    South Sky Point :  Northwards Drift at a rate of 1.64 arc secs/min 
        (--> polar axis pointing too far west by 6.3 arc mins) 

    West Sky Point :  Northwards Drift at a rate of 0.81 arc secs/min 
        Due West/Dec +16 deg
        (--> plar axis pointing too low by 3.1 arc mins)

Polar axis i) needs turning clockwise by 6.3 arc min and ii) needs raising by 3.1 arc min

 

2007-08-22 (S206)

Drift Checks. (4 minute periods)

    South Sky Point :  Northwards Drift at a rate of 1.41 arc secs/min 
        (--> polar axis pointing too far west by 5.4 arc mins) 

    West Sky Point :  Northwards Drift at a rate of 1.12 arc secs/min    ( Due West / Dec +16 deg)
        (--> polar axis pointing too low by 4.3 arc mins)

Polar axis i) needs turning clockwise by 5.4 arc min and ii) needs raising by 4.3 arc min

 

Average results (S204/S206) :

Polar axis i) needs turning clockwise by 5.8 arc min and ii) needs raising by 3.8 arc min

 

2007-08-27 (S208)

Polar Alignment Adjustment (using CCD-assisted Polar Axis Correction)

Polar axis i) turned clockwise by 5.8 arc min and ii) raised by 3.8 arc min

 

Drift Checks. (5 minute periods)

    South Sky Point :  Northwards Drift at a rate of 0.40 arc secs/min 
        (--> polar axis pointing too far west by 1.5 arc mins)  
        (= improvement of ~ 4.3 arc mins)

    West Sky Point :  Northwards Drift at a rate of 0.3 arc secs/min    ( Near Dec 0  W/WSW)
        (--> polar axis pointing too low by 1.2 arc mins)
        (= improvement of ~2.6 arc mins)   

Polar axis i) needs turning clockwise by 1.5 arc min and ii) needs raising by 1.2 arc min

Polar Alignment Adjustment (using CCD-assisted Polar Axis Correction)

Polar axis turned clockwise by 1.5 arc min 

[ aim of raising polar axis by 1.2 arc min could not be completed before end of session)

[follow up drift checks in the next session showed that the above change to Polar Axis Azimuth had seemingly had an effect on Polar Axis inclination - it is unclear how or why this occurred)

 

2007-09-02 (S209)

Drift Check.  (6 minute period)

    South Sky Point :  Southwards Drift at a rate of 0.05 arc secs/min 
        (--> polar axis pointing too far east by 0.2 arc mins) 

Drift Checks. (5 minute periods)

    West Sky Point :  Southwards Drift at a rate of 0.80 arc secs/min    ( Azim 270 (W) / Dec +16 deg)
        (--> polar axis pointing too high by 3.1 arc mins)

    West Sky Point 2:  Southwards Drift at a rate of 0.52 arc secs/min    ( Azim 249 / Dec +3  deg)
        (--> polar axis pointing too high by 2.0 arc mins)

   East Sky Point :  Northwards Drift at a rate of 0.31 arc secs/min    ( Azim 104 (E) / Dec +7 deg)
        (--> polar axis pointing too high by 1.2 arc mins)

Polar axis i) needs turning anti-clockwise by 0.2 arc min and ii) needs lowering by 1.2 - 3.1 arc min

 

2007-09-03 (S210)

Polar Alignment Adjustment (using CCD-assisted Polar Axis Correction)

Polar axis: lowered by 1.6 arc min 

 

Drift Checks (5 minute periods)

    South Sky Point :  Northwards Drift at a rate of 0.25 arc secs/min 
        (--> polar axis pointing too far east by 1.0 arc mins) 
       (Alignment seemingly worsened by 0.8 arc mins)

    West Sky Point 1 :  Southwards Drift at a rate of 0.40 arc secs/min    ( Azim 270 (W) / Dec +16 deg)
        (--> polar axis pointing too high by 1.5 arc mins)
        (Improvement of 1.6 arc mins ?)

    West Sky Point 2:  Northwards Drift at a rate of 0.50 arc secs/min    ( Azim 249 / Dec +3  deg)
        (--> polar axis pointing too low by 1.9 arc mins)
        (Opposite drift direction to West Sky Point 1 - confused !! )

Polar axis i) needs turning anti-clockwise by 0.2 arc min and ii) needs lowering by 1.5 or raising by 1.9 arc min

It would appear that Drift Align method has reached as far as it can, and that further refinement of Polar Alignment requires the statistical information available through TPoint Mapping.  

Alternatively I need to use longer baselines (i.e. a time longer separation between the two images used to measure drift rate and direction). 

  

TPoint Mapping Run

Polar axis i) needs turning clockwise by 2.5 arc min and ii) needs lowering by 3.0 or 7.7 arc mins 

TPoint results dependant on terms used, and subject to 1.25 arc min error associated with a 5 sec error in computer clock.

 

2007-09-10 (S211)

Polar Alignment Adjustment (using CCD-assisted Polar Axis Correction)

Polar axis turned clockwise by 1.8 arc min and lowered by 2.9 arc min 

(target change had been clockwise rotation of 1.6 arc min and a lowering of 4 arc min)

 
 

 

Back to Top


This Web Page: Notes - Session 208 (2007-08-27)
Last Updated : 2015-05-16
Site Owner : David Richards
Home Page : David's Astronomy Web Site