Strange Image

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johnpd
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Joined: Fri Jun 01, 2018 8:30 am

Strange Image

Post by johnpd »

I tried my first deep sky image with my new QHY5III365C camera. Some strange results. I used a 80mm refractor @ f/4.8 on an Alt-Az mount and took M81/M82. I stacked about 50 10 sec frames in RGB24 Color Space and applied darks. I am showing a slightly overexposed version to highlight the strange swirling effect in the background.You may have to zoom in to notice it. I am not sure what is causing this. It seems like the mount was tracking ok and Sharpcap was adjusting for the Alt-Az mode. Color is horrible. I tried White Balance on stars. I am using the latest Sharpcap 4.0 Beta. Any suggestions are appreciated.

JohnD
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Stack_50frames_510s-1_small.jpg
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admin
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Re: Strange Image

Post by admin »

Hi,

what you are seeing is the combination of two effects

1) field rotation - the field of view gradually rotates because you are using an Alt-Az mount

2) Hot (or at least warm) pixels from the camera sensor.

For every pixel that is consistently slightly brigher than average, a small section of curved line will build up during the stacking period as the place where that pixel is in the image is changed due to field rotation. The same will happen for darker than average pixels giving curved dark streaks, but they may be less obvious.

Normally I would expect the use of dark frames to reduce this problem to a very low level, but it can still persist if the dark frames do not properly match the light frames (any changed camera settings, significantly different temperature, etc). Certainly worth checking

In general, you would probably be better running in RAW16 mode for this sort of imaging - it's just possible that the limited number of image brightness levels (256 per channel) in RGB mode may be contributing to this.

cheers,

Robin
johnpd
Posts: 24
Joined: Fri Jun 01, 2018 8:30 am

Re: Strange Image

Post by johnpd »

Thanks Robin,

I actually tried a 16-bit version but did not have time to look at it. It turned out much better. No funny swirls and color was good and easy to fine tune. It had set up Sharpcap to create all static output as .TIFF and the resulting Settings.txt file confirms that. However when I chose the create the file from the stack as 16-bit, it apparently defaulted to .FITS. The dark frames I took ended up as TIFF instead of FITS. Don't know if that is a problem but they seemed to work. The FITS header had "COLORTYP" as "RGB" instead of what it was giving me before as "GBRG" (at least when I was using v3.2). So I might have confused things. I have attached the Settings file and FITS header. Anyhow it looks like if I want to stack I need to use 16-bit and learn how to really use the stacking.

JohnD
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Stack_16bits_32frames_337s.CameraSettings.txt
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M81_FITS_Header.txt
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M81_in_16-bit.jpg
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Re: Strange Image

Post by admin »

Hi John,

the stack output for colour cameras always is RGB - that's because the stacking process requires the image to be translated and/or rotated to fit it on top of the other images that have already been stacked. To do that translation/rotation you need to convert the RAW image (GBRG etc) into a full colour image - as a RAW image you can only offset in steps of +/- 2 pixels in each direction and you cannot rotate, which is simply not good enough for stacking.

Most places in the program where you cannot specify an output format end up using the 'Preferred Still Format' from the first page of the settings. The exception is saving the live stack as 16/32 bit - that goes tot FITS.

cheers,

Robin
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