CC_START_EXPOSURE takes 0.7 sec on ST8XE

Discussion in 'General' started by Vello, Feb 10, 2015.

  1. Vello

    Vello Standard User

    Joined:
    Feb 10, 2015
    Messages:
    4
    I'm trying to add a "focus mode" to my custom SBIG camera control software, which has otherwise been working successfully for many years. I am testing with an SBIG ST8-XE detector and the SBIG Universal Driver Library Ver 4.68 Build 4, dated Dec 23, 2010.

    The limiting factor for taking multiple integrations in quick succession seems to be the CC_START_EXPOSURE command which, regardless of the duration of the integration, requires 0.70 +/- 0.06 s to start an exposure. In an attempt to improve performance, I explicitly open the shutter first and specify the "leave shutter alone" option when starting each exposure, and also set the flags START_SKIP_VDD and START_MOTOR_ALWAYS_ON. I have also set the SetDriverControlParams parameter DCP_HIGH_THROUGHPUT flag to TRUE prior to commencing the sequence of integrations.

    Are there any other options I can use to decrease the time taken to start an exposure?
     
  2. Jan Soldan

    Jan Soldan Cyanogen Customer

    Joined:
    Oct 11, 2014
    Messages:
    239
    Location:
    Czech Republic
    This seems to be a long dead time, although the StartExposure driver function is very complex.
    For the ST-7/8 this involved:
    1) Telling the microcontroller to blink the LED
    2) Clearing the CCD
    3) Measuring the bias level
    4) Telling the microcontroller to start the exposure.

    I suppose, you are talking about "focus mode" of your Imaging CCD. For an _external_ tracking CCD,
    there is 500 ms delay inside the driver waiting for the shutter to move. There is also 120 ms delay
    for an imaging ccd's shutter, but the camera has to have an electronic shutter which is not the case of ST-8.

    In any case, you will have to close the shutter at the end of exposure, before ccd readout, because not
    interline or frame-transfer ccd type.

    I am sorry, I cannot give you a better answer now, because I have no ST-8 XE camera to do the similar
    measurements. I will try to borrow one.

    Jan
     
  3. Vello

    Vello Standard User

    Joined:
    Feb 10, 2015
    Messages:
    4
    Hi Jan,

    Thanks for your reply. Yes, I am using (a small sub-frame of) the imaging CCD.

    As mentioned earlier, the shutter is left "as is" by setting the openShutter parameter to 0, to avoid delays waiting for the mechanical shutter to move. I shall try turning the LED off to see if that helps, but I suspect the majority of the dead time is elsewhere.

    Vello
     
  4. Jan Soldan

    Jan Soldan Cyanogen Customer

    Joined:
    Oct 11, 2014
    Messages:
    239
    Location:
    Czech Republic
    Vello,
    yes, I understand. I did similar measurements some time ago related to ST-i and other cameras, so have to look inside the
    source code how to activate it via logger. I also remember that, I was not able to go below ~50 ms on STi camera between
    start/end of StartExposure function, while it was only a few ms for STF-8050 camera, for example.
    So, what I want to say is, that it also depends on the firmware inside the camera, although 0.7 sec. is pretty much, of course.
    Jan
     
  5. Doug

    Doug Staff Member

    Joined:
    Sep 25, 2014
    Messages:
    6,870
    I haven't looked into the ST8 camera firmware myself, but CCD sensors do need to be flushed prior to accumulating photons. Usually there's a slow flush while the camera is idle, and a faster full frame flush (often several) just before taking an exposure. So it's likely that the delay is due to flushing the CCD in preparation for the exposure.
     
  6. Jan Soldan

    Jan Soldan Cyanogen Customer

    Joined:
    Oct 11, 2014
    Messages:
    239
    Location:
    Czech Republic
    Doug,
    yes, the driver does at least one ccd flush for Imaging ccd and two for tracking ccd. It also measures
    video signal to calculate ccd bias.
    Jan
     
  7. Vello

    Vello Standard User

    Joined:
    Feb 10, 2015
    Messages:
    4
    Curiously, and despite the extra flush, starting an exposure on the tracking chip (a TI TC237) does not exhibit this long delay. In fact, it returns with zero delay (measured at millisec resolution).
    BTW, these obsevations are reproducible with CCDOPS by alternately setting the imaging/tracking CCD as the active CCD, and specifying a 0.01 s integration time and Turbo mode.

    It would be nice to know where the delay is and what, if anything, I can do to reduce it. However, I can use the tracking CCD as a workaround.

    Thanks for your interest and help,
    Vello
     
  8. Doug

    Doug Staff Member

    Joined:
    Sep 25, 2014
    Messages:
    6,870
    The tracking CCD does not exhibit as much delay because it is a frame transfer sensor and has a charge dump function. The main sensor is a full-frame sensor and it has to be flushed by clocking out the entire array.
     
  9. Vello

    Vello Standard User

    Joined:
    Feb 10, 2015
    Messages:
    4
    Thanks Doug. That explains it.

    And with that, I'm entirely satisfied. Using the tracking CCD had plenty of merit - it's fast and of higher resolution that the imaging chip, so I'll use it instead.

    Vello
     

Share This Page