David's Astronomy Pages CCD Image Notes - Session 94 (2005-09-18)

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Sky Condition Graph

Sky Conditions Chart (S94)

Notice  1) Generally poor seeing (FWHM < 5 arc secs) 2) Patch of dark thin cloud of lower light transmissibility at around 20:55 h UT

See explanation of these graphs

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Optical Artifact

Diffraction Artifact (bottom of image) associated with  glare from Altair (Altair lies beyond top of image)  [ Note : Image is slightly out of focus ]

CCD Image 20sec exposure (C Filter) 2004-09-18 (# 94001)

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Positioning Error Analysis / Jog Correction Benefits

During the session automated slew operations were carried out to 24 targets in a 4 x 6 arc deg area in constellation of  Cygnus, each was followed by an automated operation that attempted to correct mis-slew errors by performing N-S and E-W jog corrections (LX200 telescope/1995 vintage, initial sync on Sadr (Gamma Cygni), no TPoint).   

The benefit of jog correction is evident in the following graphs. Based on telescope slews alone position errors of up to 4 arc mins were recorded. After performing automated jog corrections the position errors reduced to under 1.5 arc mins.    It is noted that the automated jog correction appears to over correct the W position (ie  +ve position errors before correction translate to slightly -ve position errors after correction).

Comparison of N and W Position Errors before/after jog correction Session S0094,   2005-09-18

Comparison of Total Position Errors before/after jog correction Session S0094,   2005-09-18

Further analysis Session S0094,   2005-09-18

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Elongated Star Image Analysis

Several images during the session appeared to show elongated star images. Often this was accompanied by stars appearing more bloated (ie deterioration in FWHM values).  Image analysis scripts were updated to reported star elongation information (calculated within Bisque's CCDSoft application) and an Excel Spreadsheet tool was developed to display elongation information. The potential value of the technique is still to be evaluated, but it may offer potential value in understanding nature of the variations in seeing conditions,  and quantification of mis-collimination. 

Reasons behind the elongated star images during session are still not fully explained.  Some of it appears related to variable seeing.  Wind or poor telescope balance cannot be ruled out. (The observatory was unmanned at the time of imaging and therefore the movement by observer can be ruled out).   A focus variation between images in C filter and R filter is noted, with R filter producing images of generally lower FWHM (see Parfocal test from 2005-06-06)

Analysis of 4 images of the starfield surrounding  NSV13395 (Cygnus) 2005-09-18 21:41 to 21:43 hUT Note distinct image quality difference between 1st and 2nd image taken through (1st  is worse)

Analysis of 3 images of the starfield surrounding  NSV13378 (Cygnus) 2005-09-18 21:57 to 21:59 hUT

Analysis of 4 images of the starfield surrounding  NSV13353 (Cygnus) 2005-09-18 22:07 to 22:09 hUT

CCD Image 30 sec exposure,  C Filter 2005-09-18  22:08h UT (#94130)

Elongation analysis of 4 x 30s exposures taken over 2 minutes shows  that the dominant elongation axis in each image is not fixed, but  variables from image to image. This suggest that variation is not related to a constant or  uniformly varying factor (such as mis-collimination or polar misalignment),  but is more random, most probably related to atmospheric turbulence.

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Variable Star Patrol Tool

During the September development work was carried out on workflow and an Excel Spreadsheet tool to aid analysis of suspected variables (eg stars from NSV catalog) and potentially detect new variable star behavior.    Due to limited nights for observing due to cloud/weather in NE Scotland,  it is likely that I will be unable to mount a dedicated patrol at present time, however the toolkit may be used in a more limited role, and in case the software learnings proved useful for undertaking Elongated Star Image analysis 

Example star field (containing NSV 13347) 30s exposure, R Filter

CCD Image 30sec exposure (R Filter) 2004-09-18 21:07h UT (# 94037)

Magnitude Analysis (image #94037) Star instrument magnitudes (GSC stars) are plotted against instrument magnitudes  measured  in a baseline image, and a best fit line calculated. Stars which differ from best fit by a  specific threshold (0.15 mag in this example) are highlighted by red symbol. Specific 'watch' stars can be set and are highlighted by blue circles.

S/N quality control plot for same image S/N values for the potential 'anomaly' stars are all less than 20. It is probable that the  error bar on magnitude measurement is greater than the threshold.   This suggests that this star field would benefit from using longer exposures

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