SBIG 16803 - RBI

Discussion in 'STX and STXL' started by Niall MacNeill, Jun 24, 2020.

  1. Niall MacNeill

    Niall MacNeill Standard User

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    Hi Colin, Doug,

    As I await the water cooler for my camera, I continue to do trials to optimise my configuration and get set up to image with SGPro.

    I am happy to say that I have auto-focussing and PHD guiding working and have run a number of sequences successfully. As you know there is a little bit of setting up to get all the bits talking together.

    However, out of blue recently I suddenly started getting curved stripes across the images, which seemed to be worse with bright objects, or indeed when the Moon was out. I can do the trials when the Moon is bright because I am not intended to use the images. At first I thought these were reflections and something to do with the OTA. However, when I rotated the camera 90 degrees, the pattern stayed in the same place indicating it was within the camera.

    NGC 523605 29 20201x1-20.20C180.000secsLuminanceLight00000008.jpg

    Further investigation and indeed some help from an experienced astronomer friend told me that these are RBI. The pattern is entirely consistent with images I have seen where this problem is manifest.

    I am unclear as to why this suddenly started occurring, although I have a theory as elucidated below.

    In any case I need to resolve this before the water cooler arrives.

    SGPro has the ability to do an IR flash. I had set this to a 5 sec Flash and a Flush count of 1. However, in all honesty I didn't really know what I was doing and this was obviously ineffective. My friend subsequently suggested 0.5 sec Flash and a Flush count of 5.

    In hindsight it may have been my commissioning of SGPro that created the problem in the first place, if I was having a long Flash and insufficient to discharge it. What do you think?

    Screen Shot 2020-06-24 at 11.33.11 am.png

    Can I ask you for any advice on this matter, particularly the settings. It has also been recommended that when using RBI I should operate at a lower temperature to overcome the noise created and that the Darks have to be captured the same way. Please advise.

    Thanks,

    Niall
     
    Last edited: Jun 24, 2020
  2. Doug

    Doug Staff Member

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    That pattern is caused by RBI preflash.

    RBI problems happen when a bright star injects photoelectrons deep into the substrate, underneath the pixels. It then leaks out slowly and contaminates the subsequent exposure(s). So a faint image of the star appears on subsequent images.

    If you preflash you get maximum RBI absolutely everywhere across the chip. Then the camera flushes out what it can before it starts the exposure. You get those circular patterns, which are basically images of wafer polishing marks that were generated during sensor manufacturing. Then when you also RBI preflash for your dark frames, the patterns subtract out. RBI mitigation adds some extra photon shot noise, but any RBI effects from bright stars in your previous image are swamped out.

    If your controls say RBI is off, but it isn't, then some software component is out of date. Some years ago we made necessary changes to the RBI controls in the driver, and the firmware, drivers, and control software all needed to be updated to ensure everything works properly.

    You should make sure your software, drivers, and firmware are all up-to-date. Download the latest SBIG Driver Checker and use that to update drivers and if necessary the firmware. Also update your control software to the latest build.
     
  3. Niall MacNeill

    Niall MacNeill Standard User

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    Thanks Doug, that's what I figured.

    Are you saying that if I turn the pre-flash function off altogether in SGPro, the problem will go away?

    The is not my screen shot, it is from the SGPro website. As I said, when I set up SGPro, I put some settings in there....5 sec pre-flash and 1 flush.

    Or is it likely that I will run into problems with bright objects in the FOV in the future and will need to adopt the RBI pre-flash, with adequate flushes to ensure ghosts don't appear in subsequent images? Clearly if that is the case I will need to use exactly the same settings for the Darks.

    Thanks, Niall
     
  4. Doug

    Doug Staff Member

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    Your options are:

    1. Turn off RBI Pre-Flash

    2. Turn it on, but shoot your calibration frames with it on, also

    If you don’t take your light frames and calibration frames in the same mode, you will see the swirl marks.

    I recommend leaving it OFF except when imaging a field with extremely bright stars in it, and even then only using it if RBI is actually causing trouble. Otherwise you are adding noise for no advantage.
     
  5. Niall MacNeill

    Niall MacNeill Standard User

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    Thanks Doug, that is what I thought.

    How will I know how bright I can have the stars in the image? Will the circular arcs be manifest if they are too bright which will tell me I need to apply the Pre-Flash? OR will I see ghost star images if dithering?

    Would you say an image of Omega Centauri for example would require the RBI Pre-Flash.

    Thanks again,

    Niall 50mm Adaptor no spacer_NGC 513906 20 20201x1-19.13C120.000secsRedLight00000002.jpg
     
  6. Doug

    Doug Staff Member

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    RBI is only a problem if you over-saturate the sensor. Your Omega Centauri image does not saturate the sensor. Having Alpha Orionis in the field will definitely saturate it.
     
    Niall MacNeill likes this.
  7. Niall MacNeill

    Niall MacNeill Standard User

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    Thanks Doug, that is most helpful.
     

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