David's Astronomy Pages CCD Image Notes - Session 81 (2004-12-28)

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Introduction

One of the weaknesses in my CCD Imaging Setup is the flexure of the CCD Camera Position relative to the Telescope's Optical Axis. This is due to the significant weight of the ST7 CCD Camera + CFW-8A Filter Wheel (~1.5 kg total weight) and the weakness in the bearings holding the drawtube within my JMI NGF-S focuser.   This causes variable vignetting effects as the scope is pointed at different parts of the sky. 

It is appears that the problem has got worse between 2002 and 2004, suggesting either wear or loosing of the bearings.  An attempt to tighten the focuser bearings hasn't been made, but to see if problem can be reduced an attempt will be made in near future.

In recent sessions I have experimented with using a 'correcting' flat frame to try to correct the gradient caused by the difference between the vignetting pattern in the master flat and the raw light frames.  This has been created by comparing differences at the 4 corners of an image and calculating a simple  gradient surface.   This has had some success,  but has not completely got around the problem -  residual uneven lighting still compromises the display of faint  deep sky objects.

Following Session 81, some software experiments were carried out to find an improved corrections process.

Sagging of the LX200's primary mirror may also contribute to the warping of the optical axis, but appears to be less significant. It is noted that the JMI NGF-S focuser is now discontinued and is replaced in JMI's product line by the new NGF-CM focuser.

Flat Frame Components

As part of an investigation into variable vignetting as my LX200 scope is pointed to different parts of the sky,  two software rountines were written into my AstroImage Workbench Software Tool. These were a 

i) a routine to create a smoothed mean surface through an image (normally a flat).
ii) a routine to calculate a normalised difference between an image surface (normally a flat) and its smoothed mean. (routine is a bit like unsharp masking, but allows 

Together the two routines allowed the vignetting component of a flat frame to be separated from the component associated with intra-pixel sensitivity + dust shadows.  The following example demonstrates the splitting of these components using the Master Flat from an earlier session.

Normal Flat  - all components		Unsharp Masked
Master Flat from Session S79 15 x 0.5 sec exposure  (average combine) 2x2 binning 2004-12-11(#79091-105, Set 1)		Normal Flat with Unsharp Mask Processing (this highlights the intra-pixel sensivity & dust shadows,  but does not split these components from vignetting)
Vignetting Component (Flat1 Routine)		Intra-pixel Sensitivity  + Dust Shadow Components (Flat2 Routine)
Smoothed Mean Processing of Normal Flat		Normalised Difference between Normal Flat  and Smoothed Mean

It should be noted that the Vignetting Component Flat also includes any Sky Gradient.

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Image Reduction - Dual Flat Method including Smoothed Night Sky Flat

As part of an investigation into variable vignetting as my LX200 scope and ways to improve correction of this effect on images, a new method of image reduction was carried out on 2x2 binned images from Session 81.

Firstly images were batch reduced using Dark Frames (Medium Combined Frames of same exposure and temperature) and a Special Master Flat which represented just the intra-pixel sensitivity and dust shadow components (see Flat2 Image above).   

These images were then Flat Reduced using Smoothed Mean Flats created from Medium Combined Night Sky Images from same part of the sky to correct for Vignetting Effect

The following section demonstrates the method by looking at the Image Reduction & Processing of Image of  M61 Galaxy.

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M61 Image Reduction & Processing  

The following example shows the image reduction and processing of M61 Galaxy (61 light years distance from Earth).  

Firstly the generation of the  Night Sky Flat with the Vignetting Component :

M61 - Single Raw Image (one of seven)		5 Frames Used for Creating The Night Flat (one centered on M61 and four surrounding positions)
Master Flat from Session S79 60 sec exposure  (average combine) 2x2 binning, C Filter 2004-12-29 03:34h UT (#81263) (Note : Raw Frame is not rotated ,  ie North is at bottom)		CCD Frames of  slightly different celestrial  positions around M61 (ie they don't directly overlay) Each frame is reduced with dark frames & a  flat frame containing just the Intra-Pixel Variation/ Dust Shadow Component (ie no Vignetting  Correction is incorporated - this is what  we are trying to determine) 5 x 60sec exposures  2004-12-29 (#81269-73)
Night Sky Flat  (Rough Vignetting Component)		Smoothed Night Sky Flat  (Vignetting Component)
Medium Combined Image of  5 individual frames (top right) Image shows significant noise and residual  traces of stars		Smoothed Mean Processing of Rough Flat (left) Noise and residual star details have largely  been removed with processing

And secondly the application of the Night Sky Flat with vignetting component to generate final processed picture.

M61 - Processed Image without  Vignetting Component Correction		M61 - Processed Image with  Vignetting Component Correction
CCD Image 7 x 60s exposure (average combine), C Filter 2004-12-29 03:38 h UT (#781263-69)		CCD Image as left, but with Additional Flat  Reduction using Smoothed Night Sky Flat
For demonstration : M61 - processing with  Unsmoothed Night Sky Flat (Whilst Vignetting Effects have been removed, the image  contains significantly more noise, than the above Image.  Also anomalies have been introduced due to the presence  of residual star traces in the Unsmoothed Flat )		For demonstration : M61 - processing with  Unsmoothed Night Sky Flat (Again whilst Vignetting Effects have been removed,  the lower S/N in this image is clearly noticeable  when compared to using the Smoothed Night  Sky Flat - see above)
CCD Image as above, but with Additional Flat  Reduction using UnSmoothed Night Sky Flat		CCD Image as left, but with different  background & range setting.

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