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Perplexed about resolution of lunar images in ImPPG


Iainp

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Hi all, I'm normally over on the Solar imaging forum but following the breakdown of my AVX mount (be prepared for a long wait and an expensive bill, said the nice man in the shop,) I'm having a go at lunar imaging. The camera is a Point Grey Blackfly GigE (EV76 Mono) and the scope is a 25 year old 10" Meade SCT.  Following helpful advice on this forum I recently purchased a ProPlanet IR 742 filter, which is an absolute game-changer in my limited experience, giving far better results than unfiltered.

Anyway, here's my confusion.  I'm processing initially in ImPPG, as it's what I'm used to for solar work, and I love the easy-to-use Deconvolution. After that I use Astral Image and/or GIMP for final adjustments.  First image below is an example from yesterday morning.  Second image is a screenshot of a close up crop. I was hoping to be able to spot lots of the very small craters and small details, but the resolution isn't high enough, and the image is highly pixelated at this scale. Then I had a closer look at the unsaved image in ImPPG. Much more detailed! (3rd image). The problem is that there's no way to save this image, apart from as I did here, saving a screenshot.  What's happening?! It seems a screenshot of the ImPPG screen gives a much better image than a saved Tiff of the image! Some mistake surely! 

Thanks, Iain

 

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I suspect this is still the same image (2nd and 3rd). To get the 3rd screenshot, you probably applied some magnification in ImPPG, which by default performs a smooth cubic interpolation. On the other hand, GIMP shows a "nearest neighbor" (pixelated) magnification. Try using Image/Scale Image... in GIMP and choose Interpolation: Cubic or Sinc. The result will probably look similar to the 3rd picture.

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17 minutes ago, GreatAttractor said:

you probably applied some magnification in ImPPG

Yes,  that's exactly right, I zoomed right in then did the screen shot! That's very illuminating, thank you so much. I'll have a closer look and reply properly soon .

Regards, Iain 

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Hi, I've had a chance to play around with this, and the results are...remarkable!  I changed the setting in GIMP and Astra Image Plus, and what a difference. Choosing 'Image'  'Resize'  'B-Spline' in Astra, then resizing the image 250% from 872px x 544, allowed a very close crop (indicated in blue, below) which could then be further enhanced in Wavelets and Micro Contrast. The result is a usable closely-cropped image over 1000px wide. This (second image) shows tiny crater not even visible in the original unresized version. Nice! Many thanks for this.

 One further question:  This is a fantastic solution for cropping small areas of a larger image, but most of the time I just want to zoom in to check the effect of sharpening etc on small details, eg small craters.   I assume that I'll always be working on the 'pixelated'  version when post processing in Astra or GIMP unless I resize the image, because the Zoom feature doesn't actually change the image in any way. Does that mean I should always resize  the image in Astra or GIMP, then apply post processing, eg wavelet sharpening, then reduce it afterwards? 

Iain 

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49 minutes ago, Iainp said:

Does that mean I should always resize  the image in Astra or GIMP, then apply post processing, eg wavelet sharpening

Be careful here: GIMP 2.8 supports only 8 bits per channel (experimental 2.9 builds support more), so resizing with it and then post-processing (e.g. with Registax or ImPPG) may be suboptimal.

Anyway, remember that the smoothly enlarged image contains exactly the same data as the original. I'd think that upscaling, sharpening and downscaling isn't going to show any more details (but I could be wrong). I'd go with this only if the image was intended to be left upscaled for final presentation.

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Thanks, this is all very helpful. I try to stick to Astra for the main post-processing, and use GIMP mostly for adding colour (for solar) and adjustments in Curves, Levels, etc, which I prefer to Astra, probably just because I'm used to GIMP. 

21 minutes ago, GreatAttractor said:

I'd think that upscaling, sharpening and downscaling isn't going to show any more details

Yes, I see what you mean here, but isn't it easier to make decisions about the amount of sharpening etc to add, using the smoothed image? I was struggling to make fine-grained adjustments in the 'rough' original, where some of the very small craters look like just a few pixels,  but found it very easy in the smoothed version, as they actually look like craters, and it was easier to judge how much sharpening to add, even though I 'lose' that smoothed detail when I size down again.  Or perhaps it's better to apply post processing at the same scale as the final image is intended to be? 

Cheers,

Iain 

 

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3 hours ago, Iainp said:

Yes, I see what you mean here, but isn't it easier to make decisions about the amount of sharpening etc to add, using the smoothed image?

Well, sure, hence the zoom function in ImPPG. I agree, if you want to fine-tune processing in another program which requires actual image resizing for a smooth upscaled view, you resize.

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