Ripples in the lunar surface

[Relpet]

On Wednesday night. July 10th, I promised some visiting Parisians a picture of the moon after they had observed it with me that night.  I made a composite from about 15 images taken with a Canon 500D through my 12" f4 Dobsonian fitted with a 2x2"PowerMate.  The resulting image was pleasing enough to send the Parisians but seemed to contain more detail than my usual single shot.  The image on the screen was 15.5% of the original.  When I zoomed in to 100% I was astonished to see what on Earth might be sand dunes, or ripples left by the tide on a beach.  My first thought was ancient, solidified lava flows but if anyone can put me right from the cropped pictures attached I would be grateful.

[Stu]

Hi Peter,

Are you referring to what looks like contour lines in the image?

Sadly I suspect these are an image artefact, likely associated with limited bit depth so it shows as steps on the gradients from light to dark.

Happy to be corrected though!

Stu

[Ruud]

Yes, you probably have some processing artefacts here. 

How did you do the processing? Could you give it another try?

[carastro]

I was thinking the same thing.

Carole 

[symmetal]

Yes, that's an example of image posterization due to having an insufficient image bit depth. I assume you used the jpeg images from the camera which have a bit depth of only 8 bits rather than using the raw images from the camera which have a bit depth of 14 bits. An 8 bit mono image can only display 256 (2^8) distinct grey levels while a 14 bit image can display 16384 (2^14) distinct grey levels. A smooth brightness gradient will therefore show as a series of grey bands in the 8 bit image.

Stretching the image will make the effect more noticeable as the brightness difference between the bands will increase. Also, it appears the image has been sharpened, as the edges between the bands have been accentuated making them even more evident.

Alan

Edited July 13, 2019 by symmetal

[Relpet]

4 hours ago, symmetal said:

Yes, that's an example of image posterization due to having an insufficient image bit depth. I assume you used the jpeg images from the camera which have a bit depth of only 8 bits rather than using the raw images from the camera which have a bit depth of 14 bits. An 8 bit mono image can only display 256 (2^8) distinct grey levels while a 14 bit image can display 16384 (2^14) distinct grey levels. A smooth brightness gradient will therefore show as a series of grey bands in the 8 bit image.

Stretching the image will make the effect more noticeable as the brightness difference between the bands will increase. Also, it appears the image has been sharpened, as the edges between the bands have been accentuated making them even more evident.

Alan

That's such a clear explanation of what must, to more experienced imagers, seem an obvious error that it's even overcome my disappointment.  I've been told several times to shoot in RAW and I'm about to change the default setting this instant.  Thanks, Alan, and to all who replied.

[symmetal]

To clarify further Relpet your jpeg images will often be reported as 24 bit, which is 3 x 8 bit. This just means that the three colour channels RG and B are each represented with 8 bits. A 24 bit RGB image can display over 16 million different colours (2^24), so a coloured image normally doesn't show significant banding. A mono image (which the moon efectively is) has each of the RG and B components at the same value, so the actual bit depth is then effectively only 8 leading to your problem.

If you need to convert your raw images into a format that other programs can read (if they can't read the raw directly) ensure that you choose the output format to be 16 bit. This means 16 bits for each of the RGB components of each pixel so 48 bits in total. If your image is only mono then you can specify this in the conversion and it will create a single mono channel of 16 bit depth from the three colour channels, instead of three identical RGB channels of 16 bits, so your mono image file will only be 1/3 the size of the equivalent colour file.

Alan

[Relpet]

Thanks, Alan,

I need to do some more work with the manual.  I shot some RAW images of the moon this evening and found  each image was mirrored with a JPEG, which seems a bit unnecessary.  Pics weren't brilliant on first sight but will persevere on the next clearer night. Your lucid advice is greatly appreciated.

Cheers

Peter

[symmetal]

Glad to help Peter.  There should be an option in the camera menu as to whether you save a jpeg along with the raw and what size jpeg. Having the jpeg is useful when showing the folder contents if your OS can't recognize the raw format to provide a thumbnail. The accompanying jpeg is handy then.

Alan