Resolved Image Quality Differences

Can someone explain what causes the differences in the two images that were taken one after another with my STXL16200 ? FYI I was trying to move Jupiter to the edge of the field so that I could try to focus the guide chip in the FW8G-STXL.

 

Thanks Doug. That did it.
As a follow-up question, I experience the same issue frequently when I am focusing in the continuous mode. One time the images will be clean and crisp, and the next will be like the one above. Is it possible to manually set the screen stretch in the continuous focus mode so that of the exposures are using the same screen stretch settings ?

 

OK I want to follow up on this a little bit more because I am trying to understand why I see so much variation in the noise levels from image to image. For the experiment below I used my STi camera, but I have seen similar behavior with my STXL16200 and my ST2000-XM.

I set up the STi camera inside with a cover so that no light could reach the chip, and I started started running it in the continuous focus mode with the manual screen stretch settings shown in the screen shots (These were just arbitrarily set). I was using a 1 second exposure with a simple Auto-dark for the focusing images.

I would expect that under the conditions I described, each image would have nearly the exact same level of noise, but as you can see, there is quite a lot of variation from image to image. Is there another setting that maybe I should change ?

FYI when I run using the auto screen stretch I still see variation, but it is not so extreme.

One other thing that I have noticed is that the images with the high noise levels seem to occur more often when I am taking short ( 1 - 2 second) exposures.

 

OK Here is the same experiment with the STXL16200. See the screen shots below for details, but in summary, it does not appear that I am getting the same image to image noise level variation with the STXL16200 as I was getting with the STi. What can we conclude from that ?

I am using basically the same the same setup (indoors, camera covered) as before but I am running the cooler on the STXL16200.


Here is the first image using manual screen stretch. This is not a sub-frame. It is the entire image shrunk down to about 14%. I watched the successive images over a period of several minutes and saw no discernable variation in the noise levels when running with the manual screen stretch.


Next I switched back to Auto Screen Stretch. Here is the first image. On the subsequent images I would see minor variations in the numeric values for the minimum and maximum, but there was little discernable difference in the noise levels in the subsequent images.


I tried one other experiment. I turned off the cooler and waited several minutes for the camera to warm back up. Here is the first image using auto screen stretch:


After several minutes worth of consecutive exposures there was a significant jump in the minimum and maximum stretch settings. After a few more images, the settings went back to same order of magnitude as they were when I started this focusing run.


As a last experiment I changed to manual stretch. As with the earlier trial, I did not see any discernable change in the noise level of the subsequent exposures when I was running with the manual screen stretch.