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2 nights ago after capturing all my lights I stopped the sequence and set up a new sequence to capture my dark frames. I clicked the "dark" box so it would set up a "dark" folder (which it did). I put the front cap on the telescope and did the AutoRun sequence for the darks. It appeared to work and there are no error messages in the log.After stacking I found weird things happening. When I checked the dark frames I discovered that they all had star data in them! The cap was on, so the images must have been data from a previous light capture. The weird thing is that every picture is time stamped correctly during the time that I was taking the darks. To be clear, in the ASI Autorun directory there are two folders labeled Light and Dark. The light folder has all the correct lights data, and the Dark folder has the correct number of dark frames, all time-stamped correctly. However there is a star field in each frame!Does anyone have any idea what is going on? I have an ASI AIR Mini Thanks! Neil |
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| I have the plus but haven't had anything like this happening before. Might be helpful if you posted some examples on here so that we could see what we're digging into. |
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| This happened to me once but never again. Fortunately I was able to rename the files and still use them as Darks. |
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This is a longshot hunch but is your 'scope cover made of plastic or metal? I used to have poor darks with mono cameras because my black plastic cover was passing infrared light, and unlike OSC cameras, the mono cameras' internal window usually do does not block infrared. But the images were blotchy, not starry, since my cover has a fairly rough molded surface texture that diffused the IR light. Since then when taking darks, for smaller covers like on guiders I apply aluminum HVAC duct tape to them, and for the larger covers I use a solid metal shutter in my 'scope's filter drawer. Nice clean darks ever since. If your dark frame stars are sharply focused and well resolved, just as if there were no cover, what I've described is unlikely to be what's happening in your setup. CS Jim |
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Just for kicks, do a plate solve on the "Darks". It might offer a clue as to whether it is Jim's explanation. If it correlates to where the telescope was pointed at the time you did the darks. Also, for this to have worked, the mount would have to be tracking or else a dark of any length would be steaked. Alan |
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Jim Lindelien: . Not a bad suggestion, Jim, thanks. My cap is metal and the stars in my dark frame are sharp. I think have ruled out user error and I am pretty sure that this is a software glitch in the ASI AIR. |
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Alan Brunelle: . Good suggestion, Alan. I plate solved one of my darks and got data from a time during one of my lights, even though the time stamp on the FIT file was during the dark capture time frame. So now I'm pretty sure it is a software error arising from the ASI AIR. Neil |
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Agreed, something along the image handling chain from the camera's buffer RAM through the driver's etc. likely hung onto a prior frozen image. You can confirm this by zooming into two or more successive darks, looking at exactly the same spot in them and seeing if the pattern of the noise floor is identical, as it should not be if they are in fact separate exposures. Edit: of course one expects some fixed pattern to be discernable, but here I am referring to varying thermal electron noise patterns that accumulate during the exposure time of darks. CS |
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Interestingly, tonight in the ASI AIR I turned off the "copy image onto the DSLR SD card" checkbox and redid the darks. They worked perfectly! So now I am wondering if there is a speed or sync issue related to writing to the DSLR SD card. I don't need the data from the SD card (I use it as a backup in case writing to the ASI AIR internal memory fails). So if this technique continues to work I will stay with this! Neil |
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We need more information. Here are my questions: 1) What camera are you using? 2) Is it using a front illuminated or back thinned sensor? 3) Were the star images in every single dark frame or just a few at the beginning of the run? 4) Did the star field in the dark frame look like the star field from your last light image--mostly just showing the bright stars? 5) How cold was the sensor? 6) How much time was there between the last light field and the first dark image? Your answers will determine if you might be seeing residual bulk imaging (RBI). This is an effect where photo-electrons become trapped in the bulk silicon where they slowly leak away in subsequent exposures. Lower temperatures cause the leak rate to be fairly slow allowing the residual image to contaminate a larger number of subsequent images before the RBI charge completely dissipates. This effect is much larger for front illuminated sensors than for back illuminated sensors. It is also much more noticeable for very bright stars than faint stars simply because a bright star knocks loose a lot more photo-electrons. Many folks believe that RBI only occurs in CCD cameras but it can still affect CMOS sensors. Cameras with larger pixels are more susceptible to RBI but the effect can still happen with small pixels. So, if you are running your camera as cold as it will go and the star pattern matches your last light image and it fades fairly rapidly throughout your dark stack, you may be seeing RBI. If none of that applies, then it's something else. John |
