Which ISO-value to exposure time?

I use a CANON EOS M100(a) for astrophotography. I asked myself the question which ISO-value to exposure time combination could be a good way to go.

ISO value to exposure time: Until yesterday i had a technical limitation with my CANON EOS M100(a). I was not able to expose longer than 180s per frame. This was not really a dealbreaker, since i had enough to do with increasing my guiding and get dithering working. Over the past few months i used the 180s exposures and lowered the ISO value from 1600 to 400.

Finally, I am now able to expose longer than 600 seconds per frame. But at ISO 400 i would over expose most of the deep-sky-objects, or not?

The longer the better?

In the past I have read that several times, at least when it came to increasing signal to noise ratio. It is no clever idea to go with 600s exposures as long as I am not the master of guiding. But i think it is really ok – at least i could try.

The test ISO value to exposure time

I did a really basic testing, since i sat in my room in the roof and tried to extend the exposure time beyond the 180s wall for my EOS in EKOS.
After i found the solution, i had to test more. Since I did a focus test earlier that day with an electric pylon in the distance, the test target was picked quickly.

At the beginning it was still to bright to do longer exposure times. Therefore i darkened the room, blinded the window and did my exposures. I could still see the pylon, not sharp but good enough to do some noise testing as you can see on the picture.

This picture was taken later that evening with realistic exposure times:

ISO value to exposure time
KLICK to expand! Here a picture with 4 realistic ISO-exposure time combinations. Increasing noise from left to right.

Meanwhile my room had a temperature of about 26°C and the camera chip was working the whole day. This explains all the color-dots on the single frames. Since i wanted an answer about the ISO to exposure time question, i focused on the noise and not on the dots and did a simple test:

  • ISO 100 – 600s
  • ISO 200 – 300s
  • ISO 400 – 150s
  • ISO 800 – 75s

The exposure time of 150s with ISO 400 is more or less the combination i used until now for my photos. In the future the combination will be a different one.

The results and how it will change my astrophotography

I can see very clearly, that the signal to noise ratio gets better longer exposure times. This result will have an effect on my astrophotography. It is not possible to compensate a shorter exposure time with a higher ISO value. I think i ll go with 360s or 600s and plan with ISO 400 or 800, depending on the object.
I hope, that i got my guiding as much under control as i think. 🙂

2 replies on “Which ISO-value to exposure time?”

HI Thomas.

I have seen your post in indilib forum about your M100 shutting down with exposure longer than 300s. I have also seen that you thought you found a solution with setting mirror lock to 10 which would avoid the problem… and from your facebook post that you have not completely solved the problem to this day… I have been investigating the problem myself since I have a M100 also and use Ekos to control it. I have been in touch with Jasem who developed Stellarmate and he has remoted in my RPI to try to solve the problem by modifying the gphoto drivers without any luck… My main issue is that when using mirror lock the camera sensor does not get illuminated… BUT I have just found out that if you switch the capture format from FITS to Native in the Ekos capture window the sensor does get illuminated! go figure.. so in short mirror lock set to 10 + Native capture = works all the time! I don’t know where you are at with this issue yourself or if you have found any workaround this, but I thought I would share this with you.

Hello Steeve,
thanks for your answer. I am impressed that you have seen both, idlib forum and facebook, and then came here to tell me. Thanks very much for that. It is exactly as you wrote in your comparison. With mirror lock the camera is really busy for the exposure value time but there seems to come no light to the sensor. The thing i am struggling at now is, that i am sure i always use nature capture anyway and never used fits. maybe its another attribute that helped you?
I am very pleased, that even Jasem did not find a simple solution. One thing i still got in mind is, if its possible to trigger a script after each mirror lock to run an ordinary gphoto script. This would help me. since i could still combine dithering and long exposures.

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