David's Astronomy Pages Notes - Session 599.3 (2017-01-17)

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Notes (S600)

	2017-01-17
	Periodic Error Measurement & Correction
	RA Tracking Rate Check
	Polar Alignment Check (Azimuth Term)
	Autoguiding Attempt using PHD2
	Observatory Software Issues - TPoint - PemPro/TheSky6
	Updating Comets in TheSky6
	2017-01-16
	ZWO ASI178MC - Read Noise Tests
	Images from 2017-01-17 >>

Periodic Error Measurement & Correction

Periodic Error data was collected from a 2 hour run covering 4 Worm Period Cycles using PemPro with CCDSoft5 collecting 0.2s image exposures on a star located near Declination 0 and the Meridian

Beside the main periodic error relating to each 8 minute turn of the worm gear drive, there were also the semi-regular 10-13 arc sec bumps in RA that occur every 104.2 secs or so that have seen in other recent sessions (2017-01-02, 2017-01-08). The latter occur with a non-integer number of cycles (13.78) per worm period which can't be elimated using PEC. Collecting data across 4 full cycles of the worm drive allows these bumps to be largely cancel themselves out and allow a Periodic Error Curve to be derived.  

The resultant PEC curve was updated to the LX200 Mount and tracking errors were monitored over a 50 minute interval. This confirmed that the main periodic error was corrected, but its highlighted the uncorrected residual errors with 104s period.

There was less drift in RA than on previous assessment attempt which suggest Telescope RE-Balancing has helped with tracking.

PemPro Notes: 
- LX200 PEC must be turned off using PemPro's Handbox before a new PEC Table can be uploaded to the mount.
- During data acquisition and later on analysis points up to a total of 999 are plotted as small filled circles joined by lines. After reaching 1000 points the graph refreshes and points are simply joined by lines only.

PemPro plot showing Tracking Errors in RA Drive (2017-01-17, All Cycles) 4 complete cycles (around 1 hour 40 minutes of data)

PemPro plot showing Tracking Errors in RA Drive (2017-01-17, All Cycles) - Corrected for RA Drift Data shows first order periodic error with a period of 8 minutes  corresponding to one rotation of the worm drive gear.   This is overlaid with 10-12 arc sec fluctuation related to bumps in RA every 104-105s. The latter has a non-integer number of cycles per worm period and can't be eliminated by PEC (The peaks in one cycle don't line up with the peaks in the next cycle, but aref shifted along by some 23 seconds or so )

PE Curve created by PemPro (2017-01-17). Curve shows the regular periodic error associated with the RA worm drive gear The curve has an opposite polarity to the tracking error reflecting that it is curve to correct the periodic errors in tracking

PE Curve loaded to Mount (2017-01-17). Tracking errors are based on CCD image views where South is at Top of Image (ie rotated 180 degs) Above PE curve needs to inverted to produce the correct PEC curve for sending to the Mount

PemPros plot showing Residual Tracking Errors in RA Drive after uploading PEC Curve (2017-01-17) First 25 minutes of data Graph shows the RA bumps in RA occuring with a period of around 104-105s, that can't be eliminated using PEC.

Full 51 minutes of data

Residual Tracking Errors in RA Drive - detail Unless the origin of these residual error can be found and fixed mechanically the only way of correcting these residual RA tracking errors is through autoguiding.  The residual tracking error shows typical maximum rates of change of  0.84 to 0.97 arc sec/sec, but can reach 1.42 arc sec/sec.   Autoguiding using 2s exposures will be challenging to correct these rates but will be better than no guiding at all. (Aim is not necessarily to perfect 10-30 minutes subs,  but to get more 1 - 3min subs that are adequate for stacking)

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RA Tracking Rate Check

The same PemPro dataset used to assess Periodic Error also contains information about overall error in the RA Tracking Rate.

Beside the main periodic error with a period of 8 minutes (one revolution of worm drive gear) the following graph below shows the monitored star is falling behind the average tracking rate of the mount.  This indicates that the tracking is faster than sidereal rate.

After 2 hours the scope is 45 arc secs or so ahead of the star (an error of 0.375 arc sec/minute)

Based on sidereal rate of 15.04 arc secs/sec, the motion of a star in 2 hours will be 15.04 * 60 *60 * 2 = 108288 arc sec
The LX200 mount is therefore moving 0.042% faster than sidereal rate  ( from 45/108288 * 100)

Previously (2016-12-27) the mount was tracking at a rate that was 0.24 to 0.28% slower than sidereal rate. This was prior to Adjustment of RA Drive Spring and Telescope's Balance.

The new value suggests a significant improvement from the early measurement.  In 10 minutes this newly established tracking rate error would equate with a displacement of 3.75 arc secs. This is quite a bit less than the residual RA Tracking Error due to 10-15 arc sec bumps that occur with a period of around 104 secs and which can't be eliminated by PEC corrections in the Mount, and a little bit less than the measured declination drift due to the Polar Alignment error in azimuth (5.3 arc sec drift in 10 minutes)

More data is needed to establish if this tracking rate error is still present and/or of the same magnitude since the PEC curve was uploaded to the mount, and whether the tracking rate error varies according to different hour angles.
 

Tracking Errors in X (RA) direction - 2017-01-17 Dataset collected after adjustment to RA Drive Spring and Telescope Balance but prior to PEC upload

Tracking Errors in X (RA) direction - 2016-12-27 An earlier dataset that was collected prior to adjustment to RA Drive Spring and Telescope Balance

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Polar Alignment Check (Azimuth)

The same PemPro dataset used to assess Periodic Error also contains information to assess the precision of the Polar Alignment (at least in terms of Azimuthal Error) since the monitored star lay near to the Meridian (due South) and close to the Celestrial Equator (Dec 0 ). The dataset shows that the star drifted northwards with an average drift rate of 0.53 arc sec/minute. 
Since altitude of star was around 33 deg or so atmospheric diffraction effects will be small and can be ignored.

The direction of drift indicates the Polar Axis of the Mount is too far west. (and needs to be moved east ie clockwise): 

based on equations in http://celestialwonders.com/articles/polaralignment/PolarAlignmentAccuracy.pdf

    Alignment Error {in arc mins} = 3.8197 * Drift Rate {in arc sec/minute} / Cos (Declination of Star {in degrees})

So for drift rate of 0.53 arc sec/min and Dec 0 the alignment error is  2.02 arc minutes.

Therefore to achieve Polar Alignment (in azimuth terms) the mount needs to be rotated clockwise by 2.02 minutes.

This particular dataset provides no information regarding the correctness of the Polar Alignment in altitude terms. This needs to be measured based on drift of a star lying in eastern or western sky.

PemPro plot showing Tracking Errors in Y direction (ie N-S) (2017-01-17, around 1 hour 40 minutes of data)

Drift rate is highest during first hour where star lay closest to the Meridian and then begins to tail off. Note:  The drift direction of the star is based on CCD images where South lies at the top of Image. Consequently the data shows that the star is drifting in a North direction.

N-S Drift for Star Close to Meridian (above dataset rotated to show North at the Top)   During first hour of data (32 arc sec drift in 60 minutes) the average rate of drift is 0.53 arc sec/minute [in northward direction]

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 Autoguiding Attempt using PHD2

Guide rate was set to x0.50 on Autostar II handbox from an earlier value of x0.66

Note: Guide Rate is not stored during Power-Off/Power-On cycles, and will reset to the value x0.66 at the start of the next session.

An attempt was made to autoguide 12" LX200R scope using PHD2 software using SBIG ST-10 Camera.  The attempt was unsuccessful as the calibration step failed each it was tried due to very large differences between the RA and Dec rates.  The graph seemed to show that the software was unable to induce effective stepwise nudges in a north or south direction.  

It was initially thought that Dec adjustments were somehow switched off, but setting the Dec. Guide Mode to 'Auto' didn't resolve things.

It's possible that the problem could be due to a large Declination backlash, which isn't worked through before the main set of N (or S) nudges are made. The Dec Backlash value is still set to 0 on the Meade Autostar II handbox (a new Front Board was only put into the scope in December).

Calibrating Autoguide in CCDSoft showed no such problem. This uses the same leads to telescope etc, but uses a different calibration method involving single large (10s) steps rather than multiple small steps (100-250ms). The noticeable difference between +Y and -Y motion values suggest there is uncorrected Dec backlash.

It's proposed to try PHD2 again in near future.
- test that PHD's commands are getting through to scope by using Manual Guide
- try again but with a much larger step size. 

The use of Metaguide will also be investigated.

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Observatory Software Issues

During the session on 2017-01-17 (S599) a critical software problem was encountered involving the use of TPoint,
which prevented a plans to make a TPoint mapping run at the end of the session. There was also problem connecting to ST10 camera after PemPro program crashed

TPoint
Problem: Unable to insert a TPoint Model item into TheSky6, and thus unable to perform a TPoint Mapping run
to improve Telescope Pointing. A TPoint Model is not listed/available when using the 'Insert New Object' option in TheSky6.
Reinstalling TPoint didn't fix the problem.  (absence of an Uninstall option suggested that something wasn't quite correct).

(A TPoint model is ideally required before using new ZWO ASI178MC camera with 12" LX200R due to it's
small sensor (smaller than ST10) and a plate solution workflow for images from the new camera is not yet ready).

Cause: Although TPoint software had beed installed on current Windows 7 laptop at some point, it hadn't become registed
in the Windows Registry. The installation process should should do this, but it doesn't happen automatically on WIndows 7
machine. The installation doesn't produce any information message to warn the user. (Note TPoint software pre-dates Windows 7)

Solution: TPoint needs to be 'Run as Adminstrator' on the first occasion after installation. Doing this gets the software
registered in the windows registry and a TPoint Model can then be inserted into TheSky6 documents.  This was retrospectively performed on 2017-01-19 after reinstalling the TPoint software (Vn 1.00.505) with upgrade to Vn 1.00.511.

PemPro / TheSky6
Problem: Unable to connect to LX200 scope from TheSky6 after crashing out from PemPro program.   TheSky6 reports that 'another connection to the telescope is still open'

Cause: PemPro crashed leaving an active ASCOM process with connection to LX200.

Solution:  Solved by going to list of Processes in Task Manager and killing the relevant ASCOM process.

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 Powershell Useful Things

Problems: SharpCap produces FITS files with the ".fits" extension. However AIP4WIN doesn't automatically recognise this file extension as an image file (have to use Show All Files option in order to to open/select such a file). Hence there is a need to be able to quickly change a collection of ".fits" file to  ".fit" files

In just the folder:
Dir *.fits | rename-item -newname { [io.path]::ChangeExtension($_.name, "fit") }

In the folder and in all subfolders below
Get-ChildItem -Recurse -Include *.fits | rename-item -newname { [io.path]::ChangeExtension($_.name, "fit") }

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Updating Comets in TheSky6

The option to get 'Observable' Comets from the Web from within TheSKy6 unfortunately stopped working sometime ago.

To now get latest Observable Comets into TheSky6 program I follow the following procedure.

1) Download Soft06Cmt.txt from http://www.minorplanetcenter.net/iau/Ephemerides/Comets/Soft06Cmt.txt

2) Rename file as Comets.cmt

3) Goto to 'Data / Comets and Minor Planets / Comets' in TheSky6.
    - Select all comet rows and click 'Remove'
    - Now Click 'Import' and navigate to and select the Comets.cmt file
    - Select all the rows in the list of comets and then click ok to bring them into TheSky.

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