In between some saturn and Jupiter work this morning, I found some time to do a bit of lunar imaging.
The seeing was awful, as you can see from the areas where I didn't use the multipoint alignment (ie the blurry bits). This image was a 5-point align on the centre and 4 points around the rim.
Now I'll probably go back and do a 10-point align and try to get rid of the blurred parts...
Yes, and also you can get the sharpest focus ( red = longest wavelength).
You got me thinking about this one , Anthony.
The difference between an optical wavefront error of say 1/10 wave measured at 633nm and remeasured at 533nm is about 0.025 or 1/40 wave, which couldn't possibly make any difference to a photo. The barlow lens in your sustem is probably optimised for sharpest focus at 533nm . I wonder what is actually at play here to give you a better image with red filter?
Hmm. I noticed from the day I started doing RGB imaging that the red channel was by far the easiest to focus, and had the greatest focus tolerance when compared to green and blue (green has less tolerance, and blue has the least tolerance).
I measure the tolerance by shifting the focus position either side of best focus until the image starts to break down. I can measure this shift in 0.02mm steps.
I'd assumed (perhaps wrongly) that the focus tolerance was some function of the wavelength, and so green had 533/633 as much, i.e. 0.84 and blue was 450/633 or 0.71.
That seemed to fit empirically with what I see, but maybe there's some other explanation for it? Fingers crossed for clear skies at lostock, you can see this first hand.
Relative focus tolerance is a function of F#. An F4 has 1/4 the focus tolerance of an F8.
The airy disc diameter does scale linearly with wavelength though, which actually means slightly lower resolution in the red wavelength. I'm wondering whether this is more to do with the particles in the atmosphere, how it wants to scatter blue wavelengths much more readily than red ones.
what a ripper image.
