David's Astronomy Pages Notes - CCD Session 15 (2002-01-11)

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Autoguiding Experience

Guide Session 144 (8 min guide, c. 1 sec guide)

Following image shows a 2min extract from an 8min auto-guiding session (only X-Axis guiding enabled, minimum move set to 0.010 secs, agressiveness 10 (100%) ). It shows the following problems

• X guide errors are exceeding 2 arc secs (equivalent to 1.6 imaging pixels at 1x1 binning), before correction kicks in.

• Overcorrection is occuring

Following graph shows presence of significant Drift in Y (Dec),indicating probable error in polar alignment

Adjusting the GuideErrY values for the overall drift, shows the remnant error in Y (Dec) guiding. It is measured as +/- 1 arc sec.  The error in X (RA), with auto-guiding is +/-3.5 arc secs.

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Guide Session 147 (9 min guide, c. 1 sec guide)

Graph shows the tracking errors over an 8min period (X and Y auto-guiding both disabled) . They show significant drift in both X (RA) and Y (Dec).

Subtracting the overall drift from the tracking errors shows the shorter duration errors :
Periodic error in X (RA) is approximately +/- 4 arc secs, with random error of +/- 1.5 arc secs. Error in Y (Dec) is in generally +/- 0.5 arc sec error but superimposed on occasional larger random errors of around +/- 3 arc secs. The large 6 arc sec change in GuideErrY at 5.1/2 minutes is concerning (unless it simply reflects an accidental knock of the scope)

5 min Guide Sessions

153 (Image #15026). The following 5 min guide session (Aggressiveness 8, 3 sec guide) shows guiding error  of +/-3 arc secs in X (RA) and +/- 2.5 arc secs in Y (Dec). Y corrections are under-correcting, whilst X corrections are over-correcting.

154 (Image #15027). The following 5 min guide session (Aggressiveness 8, 3 sec guide) shows over-correction / oscillation on X (RA) correction most notably during the interval 1 - 2 mins and a large (4 arc sec) random error in Y (Dec) at around 5 mins.

155 (Image #15028). The following 5 min guide session (Aggressiveness 8, 3 sec guide) shows fairly good guiding in Y (Dec) of < +/- 1 arc sec error aprt from around 0.5 mins where guiding corrections are struggling to keep up with a random drift in Y. X (RA) corrections are overcorrecting causing +/- 2.5 arc sec oscillations in position.
 

156 (Image #15029). The following 5 min guide session (Aggressiveness 8, 3 sec guide) shows X (RA) oscillation of around +/-2 arc secs. Two large random errors in Y (Dec) occur at c. 0.5 and 4 mins. It would seem that Backlash is causing the first 2 Y relay on points to have little or no effect on Y.

157 (Image #15030). The following 5 min guide session (Aggressiveness 8, 3 sec guide) shows reasonable tracking accuracy, with X (RA) and Y (Dec) guide errors of both <= +/- 2 arc secs, apart from a random Y (Dec) error at around 3.5 mins.

160 (Image #15051). The following 5 min guide session (Aggressiveness 8, 3 sec guide) shows reasonable `guiding accuracy in Y (Dec) with guide errors of around +/- 1 arc secs. X (RA) errors are quite large, around +/- 3 arc secs.

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FWHM Analysis of Horsehead Image

A 5 minute exposure of Horsehead Nebula (#15026) was analysed. It showed a FWHM of around 5.3 arc sec.

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Dew 

Towards the end of the observing session the front of the LX200 scope began to dew up heavily.  A flat field shows the impact : 

A 300 sec exposure of P/51 Povington (image #15051) was attempted however the impact of dew on light dispersion is event in the histogram which shows a broad background component :

Flat frame calibration does a reasonable job in restoring the background to a more uniform level, as the histogram below shows, but can not make up for loss of imaging capability of fainter objects - the comet P/51 Povington is still not visible in the image

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