Dark capture with PNG

[Jean-Francois]

Hello Robin,

I'm doing some "library" of dark with my QHY miniCam8 camera.

The camera was first in Live View mode and I did the dark capture up to 60s.
But for 120s, I changed to Still mode. I did not remark that SharpCap changed the output format from FITS to PNG.

Are the PNG image in SharpCap full equivalent to the FITS image ?
I ask while the FITS does not have "unsigned" type and the image is first scaled (shift).

But, that is not the topic ... I remark an image shift between the FITS and the PNG. Both images have a size of 3856x2180 pixels.
When I reopen the FITS (60s) and the PNG (120s), a small shift of the hot pixels is visible on the image.
I was first looking at the middle of the image, so I though that it was the up-down read direction. But it is not, when I look at the top-left corner.

Here both images reopened in SharpCap:

Capture_Dark_FITS-PNG_1.jpg (278.24 KiB) Viewed 105 times

Capture_Dark_FITS-PNG_2.jpg (302.99 KiB) Viewed 105 times

Note the capture was done with last 4.2 beta. I reopened the image with 4.1.

Regards,
Jean-Francois

[admin]

Hi,

SharpCap can save and load PNG images at the full 16 bit bit depth, so no pixel data information is lost if you use PNG instead of FITS. Of course PNG is missing out on all the extra header metadata that is stored in a FITS file, but I don't think that is particularly relevant here. On the plus side, PNG is faster to read/write and gives smaller files. Note that a *lot* of software will read a 16 bit PNG file and automatically convert it down to 8 bit as part of the reading process, so if you open 16 bit PNG files in any other software do check that you still have 16 bit after opening them.

On the subject of the pixel shift, that's not something I would expect to be related to PNG/FITS file formats - I would expect that to be down to some camera setting difference or SDK change between the two versions. I don't think that there are any significant differences in the QHY camera code between 4.1 and 4.2, but it could be possible that the 'overscan' handling setting is different between the two versions of SharpCap as they have independent settings.

cheers,

Robin

[Jean-Francois]

Hello Robin,

Thanks to mention that a lot of software open a 16 bit PNG only in 8 bit per pixel. It is sometime very annoying.
I know this problem for several years ... since I started the SHG imaging and processing of the INTI software PNG images output.

Concerning my images with the shift of the hot pixels ... maybe small misunderstanding:
I captured both images (PNG and FITS) with the same version of SharpCap (4.2 beta).
I opened both images with the SharpCap 4.1 version (the reason, I was continuing the capture of darks with the 4.2 beta).

I installed now the last beta 4.2.15246 and repeated the images check ... If I open the past images, I have the same pixel shift.

Capture_Dark_FITS-PNG_3.jpg (115.73 KiB) Viewed 10 times

But, if I recapture new dark single images with the last beta, no shift visible. Not in SharpCap or if I open the images in Prism.
I checked FITS, PNG and TIFF ... it seems all right now. Why not yesterday ???
OK, one point still to investigate (but not yet) ... yesterday it was the master dark generated by SharpCap, not a single image capture.

Concerning the "overscan" ... the QHYminiCam8 camera has no overscan ... it has 3856x2180 resolution on Live View and Still modes.


Regards,
Jean-Francois