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Thread: DSLR Sensor Temperature vs. ISO vs. Total Exposure Time

  1. #1
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    Default DSLR Sensor Temperature vs. ISO vs. Total Exposure Time



    Good Evening DSLR imagers,

    Being new to guiding and longer exposures with my dslr, I am curious how to best handle the summer heat here in South Mississippi.
    I have only shot a handful of guided targets so far, but this time of the year with cooler temperatures, it seems that my current settings are working okay. Once I am imaging high enough in the sky, 120 seconds at iso 800 get my histogram in the 1/4 to 1/3 range. Seems to work out well.
    Soon, temperatures will climb into the 70’s, 80’s, and 90’s in the evening.
    What I am wondering is, would it be better in the heat to lower the iso to say 400 and increase the exposure time to maybe 180 seconds to reduce heat generated noise?
    Also, if lowering the iso will help control heat, would I have to increase the total exposure time to capture an equivalent amount of photons?
    Does this make any sense? Or, should I just continue with my 120 second/iso 800 settings and shoot plenty of darks?

    Thank you for any information or opinions that you can share!

    Jim
    Scopes: Explore Scientific ED102 Triplet APO, Celestron Nexstar 130 SLT.
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    Default Re: DSLR Sensor Temperature vs. ISO vs. Total Exposure Time

    I would do the exact opposite: shorten the exposure time, and increase the ISO if necessary. The ISO setting does not affect the sensor at all: it is a computation performed by the camera after the exposure has been made. So a lower ISO will not reduce heating of the sensor. But a shorter exposure time will.

    More important is to take a break between frames. It is tempting to shoot frames as rapidly as possible to take advantage of limited sky time. But you are better off pausing for a minute between exposures to let the sensor cool off.
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    Default Re: DSLR Sensor Temperature vs. ISO vs. Total Exposure Time

    Thanks Kathy!

    Very helpful information and helps me understand better how iso works. That really helps me planning my summer imaging.

    I have been inserting a cool down pause after each image equal to one half of the exposure time. So far, that has been 60 seconds, so I am on the right path there.

    Thanks again Kathy for the very helpful guidance!

    Jim
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    Default Re: DSLR Sensor Temperature vs. ISO vs. Total Exposure Time

    I agree with Kathy that time between exposures is helpful in controlling sensor temperature to a point. If you haven't started dithering yet, this may be a good time to do that as well. It can be useful in reducing the effect of noise, and it builds in a cool-down time between subs. First you dither, then the guiding settles, then you can build in a cooldown period for everything to be running smoothly before beginning the next sub. If your backlash is as bad as mine, this can easily add a minute or more between subs. Just a thought.

    Bob
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    Default Re: DSLR Sensor Temperature vs. ISO vs. Total Exposure Time

    Hi Bob!

    Thanks for the valuable info!
    You mention dithering. I am pretty new to guiding, but I thought that dithering was when the guiding software (PHD2 in my case) moved the scope small amounts in between images to cancel pattern sensor noise. I really did not think about it, but I guess that it would take a bit of time to move the mount, but you mentioned guiding settling. I am just trying to understand if I am thinking about this correctly.
    I am currently using PHD2 v2.6.6 (new version). I have dithering set to Random, RA only (because of high DEC backlash), with the scale set to 1.0 under the Global tab (Brain). So, I am dithering and by setting a 60 second pause between images (Digicamcontrol), I am doing just as you mentioned? I assume that dithering is active. Is there any way besides the settings in PHD2 to confirm that dithering is working?
    I know, basic beginner questions. Just trying to learn and understand.

    Thanks,
    Jim
    Scopes: Explore Scientific ED102 Triplet APO, Celestron Nexstar 130 SLT.
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    Default Re: DSLR Sensor Temperature vs. ISO vs. Total Exposure Time

    Jim - yes, you are dithering. Beat me to the punch, I guess

    Bob
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    Default Re: DSLR Sensor Temperature vs. ISO vs. Total Exposure Time

    Quote Originally Posted by Juno16 View Post
    You mention dithering. I am pretty new to guiding, but I thought that dithering was when the guiding software (PHD2 in my case) moved the scope small amounts in between images to cancel pattern sensor noise. I really did not think about it, but I guess that it would take a bit of time to move the mount, but you mentioned guiding settling. I am just trying to understand if I am thinking about this correctly.
    Dithering itself only takes a fraction of a second. Typical dithering values are only a few pixels, and it doesn't take long to get there.

    The settling time is something built into your session control software. SGP has it; I don't know about others. After any slew, including a dither, SGP will wait until PHD reports that it has been within a certain number of pixels of its target for a certain number of seconds. Depending on your guiding parameters, this can take a while, which gives your camera time to cool off.

    I have my settling parameters set to 2 pixels for 0 seconds, because my camera has active cooling, and I just want to get on with the session.
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    Default Re: DSLR Sensor Temperature vs. ISO vs. Total Exposure Time

    Hi Kathy!

    Great information, thank you!

    I use digicamcontrol to run my imaging session. I have to, because digicamcontrol is the only software that I have found that supports my Nikon D3300 dslr.

    I have noticed a setting in digicamcontrol when I am using the BULB tool to control imaging. See the image below, the selection highlighted in yellow. I have no idea what it is for and cannot find anything on the internet yet about it.



    I have also found this tools selection for PHD in digicamcontrol.



    So, it looks to me that digicamcontrol can communicate with PHD, but I have not been able to find much documentation.
    Otherwise, digicamcontrol has been a great find for me.

    So, looks like I will continue to insert a pause step equal to one half of my exposure time (Time between shots). Seems to be working well so far.

    Thanks Kathy and Bob for all of the help with this!

    Jim
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    Default Re: DSLR Sensor Temperature vs. ISO vs. Total Exposure Time

    You might want to explore the question, "how much pause time is enough?" I don't have the answer to that, but just by picking up bits of information regarding this over the last couple years, the pause time isn't as long as it previously was, at least from the standpoint of a few imagers that have spoken about it. I use dithering as well and have lowered my pause time considerably by thinking the dithering process doesn't add to sensor heat and then back that up with 15 seconds of additional cool down time, which is also enough time for the set up to settle down. To me any opportunity to cut corners in this regard and still get the job done gives you more time to invest image time capturing photons instead of sitting there twiddling your thumbs when you might not have to.

    If you sit down and figure out how much actual imaging opportunities you have over time, I came to the realization it's not much and this small change helps you during the limited time you really have on any given night.
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