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Comparison of Alignment Tools - Methods

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Introduction

I have used CCDStack to combine my frames and to create the RGB combines for since I first began astrophotography. Typically I would use the "StarSnap" alignment tool. Later I became familiar with Mira through Adam Block's series of astrophotography videos. I found that the stars were sharper when the alignment was done in Mira Pro. Recently, CCDWare released a new version of CCDInspector that patches CCDStack if it is resident on the computer to add a new CCDIS method of alignment. This is quite slick and is certainly very convenient. Since there all these options of alignment tools with existing products, I decided that it would be a good idea to perform an objective comparison of the the results of combining with each of these tools.

Methods

The methods that I compared included:

  • the Maxim manual 2-star match
  • the CCDStack Star-snap method using 10 selected stars.
  • the new CCDStack CCDIS with precision matching selected
  • and Mira Pro where approximately 12 -15 stars were chosen.
  • One additional test was done on the globular cluster, M092, I doubled the image size before Mira alignment and then after combining the images reduced it by a factor of 0.5.

To standardize the measurement and allow objective comparison of the star FWHM, the program CCDInspector was used to measure all combined images. For interest a comparison was performed using the measurements by CCDInspector version 1.3.2 and 2.0.1.

Alignment was performed on two sets of images:

  • M027 data consisted of (44) 5-minute luminance exposures taken over several nights with some significant image shifting between nights. This was chosen to stress the alignment tools both in number of images and in size of misalignment. It was also done to see if the larger number of subframes would improve any gain in alignment advantage of one method over the other.
  • M092 data consisted of (12) 5-minute luminance exposures all taken in one session with dithering being the only source of misalignment. These images were selected to see what effect better starting alignment of the images would have and what effect smaller numbers of images would make on the results. Finally since this was a globular and since better alignment should benefit an image of stars greatly I decided to test another method that has been discussed on imaging forums. Aligning on double size dithered images (Drizzle Stacking) is a method for achieving better alignment. For this method Mira Pro was the alignment tool.

Process

  • All images were calibrated with the same master flat and dark frames in CCDStack using the same methods.
  • They were then debloomed with identical settings.
  • For the Drizzle stack test only, the images were doubled in CCDStack using the nearest neighbor method. Then all images were aligned with each of the methods described above.
  • Following alignment, the aligned subframes were combined in CCDStack using the same normalization method, the same STD sigma rejection with the same settings, and then a mean combine was performed.
  • The resulting mean images were then all loaded into both of the versions of CCDInspector identified above and measured.

 


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