Gain and exposition time ASI294 MC Pro

Ernesto221

I start using this camera with a TEC 130 APO triplet. I took confidence with this set but there are some points on astrofotografy experience that need to be fixed. One of this is Camera Gain and exposure time.
Considering that my first target is DSO, is there any quick guide, istruction, tutorial that can explain me clearly how to define those 2 parameters. I found some treat and discussions on internet, but all are too technical and difficult to understand. Any Suggestion ?
Thanks

w7ay

Ernesto221 I found some treat and discussions on internet, but all are too technical and difficult to understand.

It is going to be difficult to explain if you already find the simple stuff on the internet already difficult to understand.

In the very simplest terms, if you are used to normal photography with a DSLR, you can think of camera "gain" as equivalent to the DSLR ISO number. And exposure time is the same and exposure time of DSLR.

So you choose high gain with the same consideration of using high ISO on DSLR.

More gain will increase the "brightness" of your image. Longer exposure time will also increase the brightness. The difference between the two (just like the ISO vs exposure time case) is the noise. Roughly speaking, to reduce the total noise and get the largest dynamic range, you typically want to use lower gain and longer exposures. But if you cannot use longer exposures, then you compromise by using higher gain (and live with the extra noise) -- same as you would increase ISO if you have no other choice.

Why don't you just try it in real life. Take an image of a bright galazy for gain = 0. Then take an image for a large gain, and reducing the exposure time to get an image that is about equally bright. Then compare the noise between the two images.

Now do the same thing with a dim nebula. Take images at different gains, again changing exposure time to get the same "brightness." Compare the noise.

That should start your learning process.

If you want to learn more, start reading up on the web what an electrophoton is, how it is converted to numbers by an analog-to-digital converter (ADC), how an analog gain is placed between the sensor and the ADC, what the camera "read noise" and "dark current noise" are, and how dark current noise is a function of exposure time. Once you understand those things, you will know how to balance between more gain versus longer exposure time.

Chen

P.S. keep in mind that the number that ZWO calls "gain" is actually a logarithmic scale. Each unit of ZWO "gain" corresponds to 0.1 decibels of analog gain. A ZWO "gain" of 0 is an analog voltage gain of 1, ZWO "gain" of 100 correspond to an analog voltage gain of 3.16, while a ZWO gain of "200" is an analog voltage gain of 10.

denababy

Hi I have that same camera but on an Explore Scientific ed80apo, my gain is usually 140-50 and depending on the seeing conditions, my exposure usually is three minutes (180 seconds).
Both can change depending on the dso your looking at and of course seeing conditions. I also have light pollution filters in the image train, which filter will depend on your local light pollution, I have a street light bout ten feet away from my scope, I’ve got no where else to setup. The filter/s I use are L’enhance or Altair Astro Triband.

Ernesto221

Hello Chen,
Many thanks for your clear explanation. I know the concept of ISO e Time explosure, what I'm looking for is a sort of table or excell file or formula to calculate the gain and exp time values dependable to the object you want to shoot.
I know that the best (but it thake too time) way to find those value is the experience, but I can't belive that such info hasn't been fixed on a document or a SW program. As you know there are so many DSO object in the sky with different Mag. than everyone have different seeing and different light condictions. So it could be very helpfull a compilable excell file with all this informations and capable to output a suggested gain and total exp time for all manin NGC objects. just an indication a sort of starting point value setup to start first shoots. M42 for istance is a very bright nebula than explosure time so short. In this case you can test different Gain value in one night then achive the conclusion in short time, but what about week object that need hours of acquisition ? How many night you have to spend to find the best value setup between Gain and Exp Time ? This is what I mean.
Best regards

w7ay

Ernesto221 I'm looking for is a sort of table or excell file or formula to calculate the gain and exp time values dependable to the object you want to shoot.

No one can give best exposure values to you since it depends on your sky background. If you are in a Bortle 8 area, for example, the optimum exposure time will be different from someone in a Bortle 6 area.

The choice of gain versus exposure time is also very different for your cooling temperature. When cooled to lower temperatures, for example, you can use a longer exposure time (vs using higher gain) before dark current noise dominates over read noise.

For maximum dynamic range, simply use the minimum gain and exposure that will still bring your sky background brightness over the quantization noise of the analog-to-digital converter. Once the sky background exposure is about 10 dB over the quantization noise, there is no longer any need to increase either the exposure time or the gain, since you can postprocess with digital gain.

(The 10 dB number basically causes the quantization noise of the ADC have less than 0.1 dB affect on the signal to noise ratio. Any extra exposure will not give you perceptably better SNR, but will decrease the dynamic range.)

The reason to use the sky background noise is that if your target is below the sky background brightness, no amount of exposure is going to bring the target out of the sky background -- that is where you need to go to dark areas to capture dimmer targets -- where you can use longer exposures profitably.) And if the target is brighter than the sky background, it will also satisfy the "10 dB above quantization noise." Even in a Bortle 5-6 area I am in, I find 180 second type exposures on a modern cooled CMOS camera to already be ample for wide band work.

If you are using narrowband filters, do the same measurements again. You will find that the sky will be much dimmer, and thus has the effect of moving to a lower Bortle area (i.e., be able to use much longer exposures profitably). But again, it will depend on exactly what filter you using and if your sky is abnormally bright inside the passband of the filter.

Chen

Ernesto221

In other words.......experience is the anwer to all my questions......