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M51, Struggling

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15 hours ago, JSeaman said:

Well we have differing views about black point, I know it's widely popular to avoid dark backgrounds but I like it so that bit stays

Don4l - thanks that's useful to know, I am just starting out in nb so finding my feet.

The focus was set with a Bahtinov and I can indeed measure FWHM, although I've never used the numbers, what does that give over the mask?

PA is something I worked on last time and I considered flexure too and have tied the guide scope down harder

I might well play with the Ha short stack you mentioned and see how I get on - thanks for your thoughts



If you have a reliable way of measuring FWHM, or HFD, then you will always be able to see how are doing as the night goes on.  I used to only find out the next day that my subs were rubbish.

I use CCDCiel to capture the images from the camera.  As each image is downloaded, CCDCiel measures the HFD and FWHM of the stars in the image.  If the HFD is 2.8, or below, then all is fine.  If the HFD is 2.9, or above, then I need to refocus.  If the HFD is 3.1, then I know that I will be able to see that the stars are big and round.

Don't ask me what HFD, or FWHM are.  The only answer that I can give is that they tell you something about the size of the stars, and the smaller the number, the better.

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:) Thanks, I did look up the two measures and it looks like I'm in the good area FWHM are <2 and focus is showing as perfect on the Bahtinov

I had about 20 minutes of clear sky last night so did a DEC backlash tighten and it did seem to guide OK but still had an anomaly at the start of the guide log. When we get another clear sky I'll try again

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FWHM, Full Width (at) Half Maximum HFD Half Flux Diameter. I think.

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