Hi all - Never posted here (or anywhere) before, and hopefully this isn't addressed in another thread somewhere.  Maybe this isn't even the right place to post a question? 

 I have a Nikon D850 which I am running with KStars on RP4 with Stellarmate, and processing with PixInsight.  I can capture light and dark frames fine, and when captured by Ekos/KStars and opened in PI, the size shows up as 8256x5504, which is the sensor size listed in the camera specs, and what I have been telling Ekos.  Bias and flats, however, are a pain because it seems Ekos doesn't like fast exposures.  I've seen that discussed elsewhere.  Whether or not it captures a picture with shutter speed less than 1" seems random.  Maybe it will make it through 0, or 1, or 5 before aborting with the "Exposure failed to save image... Unspecified error" message, so it has to be babysat and frequently restarted.  Not sure if anyone has sorted that out, but I haven't found it yet.  So, it seems it should be easy to just take a bunch of quick flats and biases manually and download the pics from the camera, but here's another problem:  When I do that, the files show up in PixInsight as 8288x5520 pixels.  The specs for the camera list 8256x5504, and that is the size they have always shown up as in any other photo program, such as Lightroom.  I've read in some D850 reviews that the sensor size is in fact 8288x5520 but is slightly cropped by the camera somewhere deep in its firmware to 8256x5504, so PI isn't in error reporting that as the size.  I can download pictures from the camera as RAW to Lightroom, confirm the 8256x5504 size, then open them in PI and it shows 8288x5520.  So then I figured I'd just tell Ekos the sensor size was 8288x5520, but it won't let me. It maxes out 8256x5504.  I am therefore unable to combine lights /darks taken via Ekos with bias/flats taken manually.  Help.

Hi Nathan, the Nikon driver internally used gphoto2. What does gphoto2 give you?

Do you use raw for all pictures (lights, darks, flats)?

Thanks

Do you have Force Bulb = OFF set? On my Z7, keeping this off, lets me choose the built in options, e.g. 1/8000 (for bias frames) etc.
I don't recall at the moment, but I may have to enter that 1/8000 as a decimal value e.g. 0.000125