I'm just trying out CCDI, and really liking it. But in comparing FWHM of file images measured by the Monitor vs. the Anaysis function, I find the latter consistently yields average figures of 0.5 to 0.6 arc sec larger than the Monitor. (That is, 2.90" vs 3.50")

Also Monitor yields aspect ratios easily 10x reality, while Analysis seems only 2-3x off the mark.

I'm using raw OSC bayer images (Monitor won't read color debayered.) Scope is a 12" astrograph f/3.6 with an image scale of 1.2". Collimation reads 4.1) I can provide screen shots of the two test runs on the same images.

Suggestions?

Tnx, Bob

CCDI_Test.doc (457 KB, 21 downloads) Screenshot.doc

Hi Bob,

The main problem is most likely the Bayer matrix. When images are debayered by CCDI this results in a small loss of resolution, and that's reflected in 0.5-0.6 arcsecs larger FWHM.

The aspect ratio value will not be accurate at all when using Bayer-matrix images in FWHM Monitor, since the light isn't distributed equally in the neighboring pixels, making star shape appear very distorted.

What makes you say that Analysis produces aspect ratio 2-3x off the mark? I assume you have the image type set to Bayer-matrix in CCDI. If so, the aspect ratio should be accurate.

The other reason why CCDI Analysis can result in higher FWHM values (and also higher aspect ratio numbers) than the FWHM monitor is that it takes into account the full image, with as many stars as can be detected all the way out to the corners.

FWHM monitor, for expediency sake so as not to tie down the camera by excessive processing, analyzes just the middle of the image (about 2/3 of it). It will also stop the analysis after detecting a sufficient number of stars.

As the result, FWHM monitor might ignore some of the larger stars in the corners, with higher FWHM due to field curvature, and higher aspect ratios due to coma and astigmatism. When these stars are included by CCDI Analysis tool, they will tend to increase the reported FWHM and aspect ratio numbers.

Regards,

-Paul

Paul,

Thanks for this. I'll ignore the Bayer-biased aspect rations in Monitor. I estimated Analysis was measuring aspect at 2-3x reality, as CCDI averaged 11-12% whereas using MaxIm to look at (many) individual stars in both raw Bayer and RGB images I couldn't find any with that high a reading; my sample consistently ran 5-6%. And at 400x view, they are all nicely symmetrical.

This is a highly corrected astrograph, so curvature is low (measured at 12%) with no coma or astigmatism.

In using Analyis to measure FWHM in my sample of 21 raw and RGB-converted images, I find very close and consistent agreement - variation of only 0.01-02", which is trivial. Image type was set to bayer matrix for both.

Last question: If I want to measure FWHM in the color-converted OSC images, should I set CCDI to "monochrome" or "bayer matrix" (though it's now removed?) Or is CCDI not really meant to assess RGB images?

Great application. Thanks again,

Bob

Hi Bob,

Remember that 12% elongation of a star with FWHM of 3 arcseconds represents a real difference of only 0.3 pixels at your image scale. I don't think you'll notice this elongation, even at 400x magnification Your astrograph sounds excellent, but even the most highly corrected optic is not free of some aberrations, especially as you look further away from the optical center.

To measure FWHM of a color-converted image, set CCDI to monochrome (even if the image you are trying to measure is RGB). CCDI can and does measure RGB images, but for best results, I would suggest measuring an unconverted OSC image with CCDI set to Bayer Matrix sensor type.

Regards,

-Paul

Thanks again, Paul. CCDI is as intuitive as it is powerful. Terrific tool.