Introduction

Due to large and variable declination backlash on my LX200 I now exclusively use uni-directional Dec guiding.   The direction of Dec guiding for a given target is automatically selected by my guiding control program by initially examining the direction of drift in guide star with Dec guiding turned off. After determining the direction in drift (either North or South) the appropriate Dec Guide Mode is then automatically set.   Then after taking up any backlash and finishing off autofocussing of the main camera, the main set of images are acquired.   

Guiding data is automatically collated and a Guiding Chart is produced in realtime that reports guide parameters and guide results in terms of statistics,  Ra/Dec Error plots and History Graphs.   The quality of guiding for each target (shown by average RMS error, and av RA error and Av Dec error) is also collated and these are plotted on chart which illustrates how guide quality varies according to the telescope's pointing direction (Az/Alt).

Apart from general smaller RA guiding issues due to a potential sub-optimum PEC curve, and issues related to poor seeing,  cloud & loss of guide star, there is a more specific issue that affects random frames acquired for target observations typically those lying west of the meridian.

Data

The guiding data below relates to data collected by the guide camera on the guidescope which is attached firmly to fork-mounted LX200 Telescope.

The following example (S852, Guide Run 6, NGC 2782) shows a target lying east of meridian with a 'normal' quality of guiding over a 16 minute run (5x180s sub-frames). The average RMS of 1.08" is reasonable, giving the FWHM of 2.64" (Zenith FWHM 2.60") as determined by the target's focusing profile.  Guide Frames are 1.5s exposures.   Although this example comes from east of the Meridian,  equally good results are obtained for some observations west of the meridian, but more rarely so.

Example of a Target showing 'normal' quality of guiding over a 16 minute interval Declination drift (slightly to south) relatively well controlled (Dec), some residual Periodic Error/Random mechanical movement  (RA)  plus normal seeing variations ( RA & Dec)

Autofocusing Profile collected whilst the guiding was settling.  It shows that the seeing (FWHM 2.6") was fairly reasonable.

For some targets (most typically those lying in the western sky) the quality of autoguiding is distinctly poorer.  Typically the poor guiding is associated with intervals where guide star drifts away from the lock position in Declination despite guide output trying in vain to correct the error.  Sometimes there are also larger RA execusions but not usually.  Between intervals with poor guiding there are intervals when the guiding is ok/normal. The issue is well illustrated by the example below (S852, Guide Run 1, NGC 684). This was also a 16 minute imaging run with 5x180s sub-frames. The average RMS was 2.6".  Seeing  (FWHM 3.06") was slightly poorer than above 'good example' due to a low altitude but this can't explain the poor guide RMS, nor can the passage of any clouds (the guide star SNR is consistently high).

Example of a Target showing 'poor' quality of guiding over a 16 minute interval Declination drift ( to north) corrected over some intervals, but in other intervals Dec guide corrections have no effect (Dec) ,  some residual periodic error & random mechanical with larger unexplained -ve RA excursions over some intervals (RA),  plus normal seeing variations ( RA & Dec)

 

An earlier guide run from session S848 (2021-01-11) showing a particular anomalous 6 arc sec Dec Drift in it final (5th) frame and the case was analysed in detail (See Investigation - Autoguiding Issues (Uncorrected Dec drift during Guide Run 4) ).  After acquiring  4 x 180s with more or less acceptable guiding, the guide star drifted 6" away from Lock Position even though Dec Guide corrections where being made.  The small picture inserts on the guiding chart show expected (simulated) star shapes based on the guide error seen. Looking at the simulated star shape for Frame 5 It would be expected that Frame 5 would have been deemed unusable and would have been thrown away to make a 4 x 180s stack of the first 4 frames only.  In fact the effect on Frame 5 image was minimal and the frame was included in the original 5 x 180s final stack (after alignment of frames).

Re-examining the individual image frames shows that there is a minimal visual difference between frames. The original inclusion of frame 5 in stacking was surprisingly justified. Measured Av. Star Elongation in frames 1 to 5 is 1.15, 1.11, 1.13, 1.12 & 1.16. Frame 5 (1.16) is only marginally worse that the other frames and visually its star elongation is perceived to be very slightly more ovate, but the impact on star elongation is no where as bad as would be suggested by the guiding results and the simulated star shape.

Bad Guiding Results from Guide Run 4 (S848, 2021-01-11) Declination drift ( to north) corrected over some intervals, but during Frame 5 Dec guide corrections have no effect (Dec) ,  some residual periodic error & random mechanical movement  (RA),  plus normal seeing variations ( RA & Dec)

Re-examining the individual image frames from above run showed that there was only a minimal visual difference between frames. Measured Av. Star Elongation in frames 1 to 5 is 1.15, 1.11, 1.13, 1.12 & 1.16. Whilst elongation in Frame 5 (1.16) is marginally worse that the other frames and visually its star elongation is perceived to be very slightly higher,  the impact on star elongation is no where as bad as would be suggested by the guiding results and the simulated star shape.   The full frame Y pixel position of the centroid associated with star at the centre of above image has the following value in Frames 1 to 5  : 275.6, 275.2, 276.2, 276.4, 274.2 .  The largest Y position difference is between Frame 4 and Frame 5 where the 2.2 px difference corresponds to a distance of 3.1".   

Issue has previously been recorded as an Operational Issue in Session S756.

The guiding anomalies associated with this issue appear to have several different forms:

• Some develop gradually and then slowly dissipate
  
   
•  Some develop gradually and then end sharply
  
   
  
• Some develop sharply and end sharply
  
  Abrupt jumps could happen if the guider jumping to a second star that was very close by and of similar brightness or one that moved into the guidebox.  However it doesn't explain why guide output doesn't bring the second star back to the lock position and doesn't explain why the anomaly is always in one direction.   Star jumps would be expected to occur in both directions in equal proportion.
   
  
• Some develop as a train of anomalies
  
  
• Some have associated RA anomalies (but most don't)
  

Further examples
Further examples of guide runs from S852 session with poor guiding outcomes are shown below.
(these are again all asscoiated with telescope positions in the western sky)
 

More examples of Poor Guiding from S852 session.

Looking at variation in imaging quality across the sky illustrated by the summary charts below for observing nights during the last 2 -3 weeks, shows that poorest guiding perforamce (in terms of Av RMS, Dec RMS & Worst Frame RMS) are most typically associated with positions in the western sky.

Charts showing RA/Dec Guide quality for most recent session (S852) and a number of other recent sessions (graphs are in reverse chronological order, i.e. the more recent session chart is shown first Nights with cloud / unstable conditions and/or technical issue (S848, S847, S844) are not shown here.   The number against each point is simply the sequential target no. for the night (starting with 1) Green is considered good guiding (RMS score < 1") Red, Purple, Black is bad guiding (RMS Score > 2")

Charts showing Overall Guide quality and Worse Frame quality for most recent session (S852) and a number of other recent sessions (graphs are in reverse chronological order, i.e. the more recent session chart is shown first Nights with cloud / unstable conditions and/or technical issue (S848, S847, S844) are not shown here.

Analysis

From a review of the above data it is concluded that :

• the guiding issue is real and is persistant over multiple sessions
• the issue appears to most often affect those observations made west of the meridian
  and those targets that are lying roughly due west are the worst affected
• the issue isn't systematic, in that not all frames are affected (only some frames are affected)
  and some western targets observations can be completely unaffected, whilst some eastern targets are affected

The guiding anomalies are always associated with guide star drifting or jumping in the direction that the star naturally drifts when Dec Guiding is off. For whatever reason during a guiding anomaly the Dec guide output isn't creating any change or enough change in the scope's Dec position during the anomaly. Normally the Dec Drive should be able to bring the scope immediately back onto the guide star position. Between guiding anomalies this happens so the system is generally perfectly capable of guding.

Follow Up

2021-01-22
The following example taken from the following session (S853, 2021-01-22) shows that the guiding issue under discussion isn't restricted to positions West of Meridian but can also affect target observations East of Meridian (albeit more rarely). 

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This might suggest that the issue isn't simply due to loading on Dec Drive, but is related to particular positions where cables / cable guards are temporarily holding up /  rubbing against the equatorial wedge, fork-arms or pier.

In the following example, again from S853 Session (2021-01-22),  the start and end of the guiding anomalies are particularly sharp.
(In other cases one edge is ramped and the other sharp (like example above) or both edges are ramped )

Example of a Target showing 'poor' quality of guiding , Guiding Anomalies are abrupt (S853) Declination drift ( to north) corrected over some intervals, but in other intervals Dec guide corrections have no effect (Dec) ,  some residual Periodic Error/Random mechanical movement  with larger unexplained -ve RA excursions over some intervals (RA),  plus normal seeing variations ( RA & Dec)

2021-01-23
Session S854 (2021-01-23) had generally good guiding but again showed the occassional poor guiding result from observations west of Meridian.
Average RMS (Dec) west of the meridian doesn't seem to be inherently worse that the east. It just seems that western side is more likely to suffer one or more guiding anomalies and produce a worst frame RMS score.

Guiding Performance Results from Session S854 (2021-01-23)  The number against each point is simply the sequential target no. for the night (starting with 1)

2021-01-26: 
A visual inspection of the Telescope was made today (2021-01-26)

- Clamps on GuideScope's Orion XY Finder are firmly in place
- Guidescope securely attached to Main Scope.
- Cables normal, cable management is the most reasonable possible given current limitations & requirements

2021-02-01
A noticably regular repeating pattern of Guide Anomalies was seen during Guide Run 11 from Session S857 (2021-02-01).  Telescope pointing was to WNW.
Guide run had three very similar looking anomalies with duration of 2.3 to 2.6 mins each and occurring approximately every 5 minutes . Timings are subject to some measurement uncertainty. They occurred every 4.6 -5.4 mins when measuring from start of the anomalies or every 4.9 - 5.2 mins when measuring from end of the anomalies.

Guiding Performance Guide Run 11 S8547 (2021-02-01)

It doesn't appear that the observed 5 minute period can relate to any hypothesized up/down wobble in the fork arms caused by sidereal tracking of RA drive (as this would be expected to have an 8 minute period)

It doesn't seem that the issue can be caused by striction.  Striction involves the storing up of energy, and if a couple minutes of 'stored gear energy was released in one go it would be expected that declination error would suddenly jump to the south side at the end of each anomaly but it doesn't - it simply gradually comes back to the locked Y position (i.e.to the 0 error line).

Perhaps the dec gears are somehow coming apart on some regular interval and preventing the motor drive from turning the big Dec Gear effectively for a 2 minute period or so ?  One can imagine that due to loading/balance the effect might be greater on one side of the Meridian than the other and in certain sky positions. If the worm drive was uneven or worn in a particular way then it might not present a constant fixed angle to the cogs on the big Dec gear thereby producing less force at certain position of its rotation.

The time interval between the end of taking up Dec backlash and the end of the first anomaly was 4.3 minutes which is a little less than the 5 minute period for the guide anomalies, but it is not so dissimiliar to just make one wonder if there is some connection ?

Assuming a roughly constant star drift through the guide run,  then the anomalies are occuring after regular amounts of accumulated amount of movement of the Declination Motor/Gears.   It can be hypothesized that dependant on sky position the rate of star drift will be different and the period between anomalies might similarly vary as a consequence.  

If there is damage on the teeth of the big Dec Gear (none noticed when last inspected)  then it might be hypothesized that guiding anomalies might be more common when working at certain Declinations than other.  The dec drive knob is locked between sessions (and rarely released  manually and no particular pattern has been noticed to date but there hasn't been any detailed review to definitively say whether there is any relationship between guiding anomalies and telescope declination.

More Analysis

A number of possible causes have been considered  : 

• Differential movement / flexure between Guidescope and Mainscope might occur, but cannot explain the data.   Very sturdy system of dove-tail and clamps are used to attach the GuideScope to the Main Scope. There is no physical indication of any movement between the two scopes. Any issue of this type would typically be expressed by relatively normal guiding data but produce poor imaging results.
  
   
  
• Mirror flop in the main scope can not be the cause. Guiding anomalies is seen in data directly from the Guide Camera on the TS80 APO guidescope (a refractor).
   
• Differential Movement between the Guide Camera and Guidescope might occur, but cannot explain the data. The guide camera is attached to the guidescope via an Orion X-Y Finder and there is some inherent play between the two sides of the X-Y finder. This risk has already been minimised by the use of small but sturdy external clamps to firmly 'lock' the X-Y finder in position. It is difficult to see why these movements wouldn't be corrected by the Dec Guide Output and it doesn't really explain why guide anomalies that are almost always in one direction only.
  
   
• Dec backlash can not be the cause.  Uni-directional Dec Guiding is used and the guide plots for a given target demonstrate that there is either no backlash to take up (depending on which direction the scope came into the target and the centering jogs thereafter) or that the backlash is fully taken up after setting the Dec Mode to North or South (as appropriate), such that scope is guiding effectively before the first guiding anomaly occurs.
  
     
   
• LX200 Mount intermittently stops hearing Dec Guide Output commands or stops applying them.  Guide commands are sent from PHD2 to telescope via DeviceHub Telescope Driver, and Meade LX200 Driver and onto AutoStar II/LX200.  It is hypothesed that Dec guide outputs might intermittently not get to the Dec Drive Controller for some reason, so that Dec guiding effectively stops for a while and the guide star drifts in its natural direction (with Mount's current Polar Alignment a star lying on western side of Meridian drifts in a slight northward direction).   It is unclear what could cause this and why it would be worse for observations west of the Meridian.  RA guide outputs are clearly not affected by any outage in the Dec Guiding.
   
• Dec Movements caused by the RA Drive might occur, but cannot explain the data. It is hypothesed that the RA drive might produce some up/down movement of the telescope forks during its sidereal movement. However it difficult to see why these movements wouldn't be corrected by the Dec Guide Output and it doesn't really explain why guide anomalies are almost always in one direction only.  Any Dec movements generated by the RA drive would be expected to oscillate N-S in equal measure. The RA motor & gears were replaced in April 2018 and the Main RA Gear cleaned, inspected and regreased at that time (some damage noted on the main RA gear surface)
  A 5 minute period seen in Dec Guide Anomalies seen in Guide 11 on 2021-02-01 doesn't match the 8 minute period that might be expected to be theoretical consequence of  RA Drive movements.
   
   
• Dec Drive Issue due to loading/balance effect.  It is hypothesed that variable loading might cause the Dec Motor to be less effective in producing Dec Movements at certain times and positions .   The Dec Motor /Gear block is spring loaded against the main Dec Gear and was working correctly with no notable damage to gears or worm drive when last inspected (2019-10-12).  A difference in balance /loading could explain why one side of Meridian (west in this case) is more affected than the other, but it is difficult to see why the problem is so intermittent, with good guiding for 2-7 mins  followed by a 1-2 minute period of poor guiding  (maintainance and inspections of the Dec Drive in 2018 & 2019 are described further below).
   

  LX200 rebalancing with additional weights (2019-12-17)

  
   
• Dec Drive Issue might be due to some form of striction,  but it cannot explain all the data.  It is hypothesed that some form of striction might be occuring in the Dec drive whereby in certain positions an accumulation of force has to build up in the Dec Gear system before the main Gear moves.  This is likely to be affected by loading/balance and a difference in balance /loading could therefore explain why one side of Meridian (west in this case) was more affected than the other. Striction could also explain the intermittent nature of the issue.  Maintainance and inspections of the Dec Drive in 2018 & 2019 are described further below.   Striction would be expected to see the storage of 'gear enegy'  that would be released in one go at the end of the guide anomalies and cause the declination guide error to switch polarities but this isn't observed in the data.
   
• Some form of intermittent hang up due a restriction from Cabling.  It is hypothesed that some form of dragging of the cable or other build up in force in the cabling causes the Dec Guiding issues. It's possible to conceive that cable issue might be potentially be worse on one side of the meridian (west in this case) than the other due to different cable paths/tensions.  It is hypothesized that as Dec guiding occurs to offset the star drift associated with slight polar-misalignment, a resistive force builds up in the cable and eventually slow sdown or stops further Dec motion until the build up of force in Dec Gear system exceeds the force locked up in the cable.  The force in the cable is released and guiding continues ok. It is difficult however to see why RA guiding isn't more affected than it is. 
  
  Cables that by necessity have to come off the back of the telescope comprise a Power Cable for the ST-10 SBIG Camera (no option to power it from Pegasus UPB v1 Powerbox located on the Fork Arm) and a USB3 cable from the Guidescope (no option to route through current Pegasus UPB as V1 box is USB 2 only).   These cables are tied back to the handle on right hand fork arm and from there to a tie [point suspended from the pier that combine to prevent any drag associated with rotation around the pier on or from the floor.   The cables are stiff enough however that they could theoretically impart some force on Dec Guiding but not really in way that could quite explain the intermittant guiding behaviour seen.
  
  USB Cable from Main Cable, Serial Cables to Main Scope and Guidescope Focusers come off the scope to the Pegasus UPB located on the right hand fork arm.  Cabling to GuideScope Focuers is tighter than ideal , but is not expected to give any restrictive force to Dec motion unless at very low Dec (ie near southern horizon).   Cables to dew heaters also come off the telesscope to the Pegasus UPB on the fork arm but are loose and flexible.
  
  The power cable and USB cable to the Pegasus UPB are steered to the south side of the fork and then to the floor in a way designed to minimise cable snag when the fork-mounted telescope rotates. Whilst it could theoretically cause drag to RA motion it can't cause any restriction to Dec motion.
  
  

  Current Cabling Configuration (2020-01-26)
  Cable connections to the Telescope Possible impact on Dec Motion from a) the Guide Camera USB3 + ST-10 Power Cable that come from back off telescope b) the Serial Focuser Cables going down side of scope to Pegasus UPB box on the fork arm
  Guide Camera USB3 Cable (blue) and ST-10 /Filter Wheel cables on top of the Main Imaging Camera (Ignore blue foreground focuser cable ) (no possible impact on Dec Motion on these sections )		ST-10 USB2 Cable, 2 x Focuser Serial Cables and 2 x Dew Heater Cables coming off telescope and going to the Pegasus UPB box mounted on right-hand fork arm. (possible impact on Dec Motion from  relatively tight serial cables)
  ST-10 Power & USB3 Cable coming off the a tie point on right hand fork (can not impact Dec Motion in this section of the cabling)		Power & USB2 Cable Bundle coming off the Pegaus UPB box on right hand fork and directed to floor on south side of pier (can not impact Dec Motion)

  
  

Pictures showing 2018-2019 maintenance on the Declination Drive.

Declination Drive Cleaning & Regreasing (2018-04-27)  With my 12" LX200 indoors for a RA Motor Unit replacement in April 2019 opportunity was taken to clean and re-grease the Declination Drive at the same time ( See Notes ) The cleaning and re-greasing made an immediate improvement to the smoothness/sound of declination slewing.
Dec Motor Unit (picture shows main Dec gear removed with the wormdrive exposed for cleaning)		Main Dec Gear after cleaning (picture shows gear before re-greasing with Lithium Grease)
Dec wormdrive after cleaning		Dec wormdrive after re-greasing
Declination Drive Checks (2019-10-12) As part of checking out large and variable Declination Backlash (which was affecting bi-directional Dec Guiding)  the Declination Drive Cover was removed from the right hand fork arm in order to examine the Declination Drive.  The movement of the Declination Drive Unit and its tension against the large Declination Gear seemed normal.    It would have been nice to reduce the travel distance of the drive unit using travel limit screw and insert a small washer to increase the spring tension, but it wasn't possible to apply a enough leverage to move the screw using a small hexagonal allen key without risk of breaking the key or stripping the hexagonal slot on the screw ( See Notes )
Declination Drive Knob & Cover Removed (strapped to the fork arm below the Dec Motor is Pegasus UPB Powerbox and TCF-S Focuser Controller)		Declination Drive Unit Examination Location of the Tension Limit Screw is highlighted

Update 2021-03-14
As part of an attempt to better understand and improve autoguiding performance issues the SBIG Camera's power cable and the ZWO 178MC Camera's USB 3 cable were rerouted on 2021-03-14 ahead of session S870.  Instead of passing from the back of the imaging train to the right hand fork & then indirectly to floor the cables were rerouted to take them back up to the OTA and then down to the right hand fork & then indirectly to the floor (see Camera Cable Rerouting, 2021-03-14).

The benefits or othewise of the new arrangement are still to be determined
(Observing/Guiding opportunities were severly limited in session S870 due to wind/cloud)

Update 2021-10-20
A new Dec Motor was installed on the Observatory LX200 Scope on 2021-10-15 but initial sessions shown have show none of the improvement in Dec Guiding that had been hoped for. Adjustments in the Dec Motor linit screw and cabling have been tried but have still not reduced the problem. 

With still no resolution on Dec Guiding issue it is proposed to improve the mount's polar alignment so that the impact of poor Dec guiding is reduced  (see LX200 - Proposed Polar Alignment Change, 2021-10-25).

Update 2022-02-12
Dec Guiding has now been considerably improved (Q1 2022) due to a combination of improved telescope balancing,  modified/improved polar alignment and improved guiding management routines.  Dec RMS error values are generally on par with and even better than RA RMS error values. 

There is no longer any imbalance in guiding performance between targets lying west and east of the Meridian.

There are still some issues with Dec Guiding being only partially effective or non-effective at certain telescope positons, but the effect of this is mitigated in part by the better polar alignment (so lower rates of star drift) and by corrections to lock positions between frames.

A separate improvement that avoids scheduling targets where the dome aperture/telescope has to be pointed into strong wind, has also helped reduce overall Dec Guiding errors

Introduction

Storm Christoph was the third named storm of the 2020/2021 season and the first of this year, following Storm Aiden on October 30 and Storm Bella on Boxing Day 2020.  

The storm swept up the UK producing significant amounts of rain and flooding in many places. On higher ground it produced snowfall. The storm brought some heavy, and at times record breaking, rain to parts of the UK. Provisional figures fromMetOffice show Honister, in Cumbria, received 123.8 mm of rainfall on Tuesday (19th Jan), a new daily rainfall record for this winter and 2021. However, it was well short of England’s all time January daily rainfall record of 180.4mm set in 2005.

The following graphs record the passage of the storm based on Observatory's Weather Station, where it was seen to peak around midday on Jan 21st. (2021-01-21 12:00).

Main features observed.
- low pressure (962 mb minimum)
- high wind (30 mph* at peak) 
- little change in wind direction as storm came through  wind from NW leading in,  wind from WNW leading out
- moderate rain over a 30h period
- significant cloud cover (low clarity) for 72h period
- slight warming (steady 4 degC vs  0 +/- 2deg background)
- high humidity  (95-100%) leaading in,  lower humidity (85%) leading out

* note the weather station records lower wind speed than actual wind speed.

5 days Weather Graphs centered on Storm Christoph (2021-01-21)