My old 8" LX200 Classic scope was brought into back action for the next few sessions. 
This is temporary arrangement whilst my 12" LX200R scope goes in for an electronic repair 

Optical Train :	F/10 8" LX200 / Optec TCF-S /  CFW10 / ST-10XME
Expected Values
Expected Focal Ratio :	f/10
Expected Focal length :	2000mm
Expected Image Scale :	1x1: 0.70 arc sec /px,   2x2: 1.40 arc sec/px,  3x3: 2.1 arc sec/px
Actual Values	(determined on 2010-01-17)
Actual Focal Ratio :	f/10.1
Actual Focal length :	2115mm
Actual Image Scale :	1x1: 0.66 arc sec/px,   2x2: 1.32 arc sec/px,  3x3: 1.98 arc sec/px

As part of setting up imaging equipment on 8" scope, opportunity was taken to undertaken some overdue maintenance and changes to my CFW-10 filter wheel.   This comprised

- removal of an annoying dust speck for the middle of camera window, which has been compromising the quality of images 
- cleaning/dusting of filters where necessary
- replacement of Red filter by H-alpha (Baadar, 7nm narrowband Ha) filter

As part of returning to imaging using my backup 8" scope, focus offsets were determined for each filter relative to the C (Clear) Filter. This was performed by taking images of a moderately bright star at different focuser step positions and determining the position of optimal focus from the resulting focus profile. 

It was expected that focus offsets would show a regular correspondence with the offsets previously generated for 12" LX200 operating at f5.7, but this was not the case.  For some filters which previously had focus positions inside the C filter with 12" setup where found to be outside the C Filter focus position with 8" setup.  Unless I had been previously carrying an offset error, it assumed that the difference is related to the different optical configuration between the two setups (with the 12" setup using an f6.3 focal reducer, whilst the 8" setup did not). 

Notes:
 1)  S Filter (#9) = Star Analyser 100 
 2)   Both setups used exactly the same camera to filter wheel distance.

 

Width of the profile at 10 arc secs FWHM is around 1080 steps (average)  +/- 60 steps

Theoretical CFZ (Critical Focus Zone)  =   (focal ratio)^2 * 2.2    (in microns)

Step size for Optec TCF-S focuser = 0.000085" (or 0.00216mm,  or 2.16 microns)

As part of setting up imaging equipment on 8" scope, new flat frames were taken for each filter after focus position details had been set.

 

Focus positions for the C Filter were collected over a 6 degC temperature range during the two observatory sessions on 2010-01-17 (+2 to +3 degC) and 2010-01-18 (0 to -3 deg C)
The dataset is plotted below. Best fit line indicates

• a temperature coefficient of  -107.5 steps per deg C (with a datum focus position of 3807 at 0 degC.)

The dataset is rather ambiguous and there appears to be a focus shift at around -1 degC. This might be due to a mirror shift (no mirror lock on 8" LX200 classic), uneven cooling of the telescope during the second session or atmospheric changes.  Until more data is collected the range in temperature coefficient is rather wide (-95 to -150 steps per degC). 

 

Although the 8" LX200 was mounted onto a wedge that was already well aligned to celestial north pole axis, it was quickly clear that some adjustment to the wedge was required.

Slewed to South Sky Point / Measured Star drift
  
  Time Interval :              5.13 mins
  Baseline Az :                176.1 degrees
  Declination Drift :          0.07 arc secs/min SOUTHWARDS
  Polar misalignment :      0.27 arc mins (approx)

  ** Star drift is very small and wedge mount is pointing more or less northwards ok.

Slewed to West Sky Point / Measured Star Drift

  Time Interval :              5.12 mins
  Baseline Az :                271.8 degrees
  Declination Drift :          3.47 arc secs/min SOUTHWARDS
  Polar misalignment :     13.24 arc mins (approx)
  Y Star change              397.4 --> 383.9   (confirm star drifts southwards)

    ** Star drifts south and therefore scope's axis points above the pole and needs to be lowered

Polar Axis Correction (altitude)
1) Slewed to South Sky Point
2) Centre a bright star
3) Jog south by 13.25 arc mins  
4) Use wedge adjustments to recentre star 

Although the correct step would be  to jog south by 13.25 arc mins (rounded from 13.24) ,  the actual step made was a jog north by 13.25 arc mins, which had the negative result of increasing the polar axis error to ~ 26.5 arc mins, instead of reducing it to ~ 0 arc mins.
 
Slewed to South Sky Point / Re-Measured Star Drift
  Time Interval :              5.12 mins
  Baseline Az :                185.0 degrees
  Declination Drift :          0.84 arc secs/min SOUTHWARDS
  Polar misalignment :      3.21 arc mins (approx)

  ** Star drifts south now even though no change to wedge azimuth has been made since earlier measurement
   (is this because the Polar Axis elevation is now out by over 25 or 26 arc mins ?

  ** In theory the southward drift implies that the scope's polar axis is pointing too far east and needs turning anti-clockwise 

Slewed to West Sky Point / Re-Measured Star Drift
  Time Interval :              5.12 mins
  Baseline Az :                271.6 degrees
  Declination Drift :          6.61 arc secs/min SOUTHWARDS
  Polar misalignment :      25.24 arc mins (approx)
  Y Star change              465.6 -- > 439.9 (confirms star drifts in southwards)

  ** Star drifts south and therefore scope's axis points above the pole and needs to be lowered by 25.24 arc mins

Polar Axis Correction (altitude)
1) Slewed to South Sky Point
2) Centre a bright star
3) Jog south by 25.25 arc mins  
4) Use wedge adjustments to recentre star 

Although a jog south of 25 arc min was commanded, the actual jog change made was 22.78 arc min (southwards). A second southward jog of 2.2 arc min was commanded, after which the net actual jog was measured to be 23.74 arc min (southwards). A third southward jog of 1.8 arc min was then commanded, after which the net actual jog was measured to be 25.65 arc min (southward). This was taken as being sufficiently close to the 25.25 arc min required,

Star was then re-centred using wedge altitude controls,  since stars needs to be moved southward (ie upwards for 180 deg rotated realtime image) the adjustment process involved turned the altitude control knob clockwise (loosening the wedge and moving the bright reference star back to the CCD field centre), with then a final anticlockwise adjustment to take out the slack and firm up the control knob tension.

Ideally South Sky and West Sky points would again be measured at this point, however imaging was begun directly without making a further check. 

Comparisons between Instrumental Magnitudes with different setups