Brain tool

[admin]

Hi Sven,

Glad that you got some clear skies at last and that's certainly an excellent image which you must be very pleased with.

By moving up to a higher gain value you tend to reduce the read noise that the camera introduces at each exposure. The way that SharpCap calculates the exposure suggestions is to ensure that the shot noise – the random variation in the number of photons that arrive in each pixel in each frame – is big enough to be much larger than the read noise. This means that the read noise will have hardly any effect on the image quality of the final stacked image.

It's best to do the measurements on the darkest area of the frame that you intend to capture - moving to an even darker area of the sky will tend to give you longer exposure suggestions, but that won't necessarily improve the image quality of the final stacked image. On the other hand though you can always take longer subs than the ones SharpCap recommends as long as you are not hitting the tracking limits of your mount.

The optimum image quality for seeing faint detail would always come from a single very long exposure, which we know is impractical in reality. SharpCap aims to get within a small percentage of the image quality without pushing up the sub exposure length any more than is necessary. The following table shows the calculations for the signal-to-noise ratio in the hypothetical single long exposure case. The final image quality for a given target depends only on two things – the total exposure time and the background sky brightness that you have to cope with due to light pollution.

Capture.PNG (9.29 KiB) Viewed 822 times

You definitely end up in a situation of diminishing returns – two double your image quality you need to multiply the exposure by a factor of four. That means you will see a doubling of quality from one hour to 4 hours of total time and then another doubling from four hours to 16 hours and then another (if you have the patience) from 16 hours to 64 hours!

Hope this is useful, Robin

[Sven_Bortle5]

Hi Robin,

and thanks for your reply. To be honest... as long as I learn about how the parameters influence the picture, I don’t know when I’m supposed to be satisfied with an image.

If read noise is a less important parameter, I will now
a) go on a good dynamic range and the tracking limits of my mount
b) cap the the exposure-length of a single sub according to the saturation of the stars.

Your response regarding the selection of the measurement-area was really helpful. This underlines my observation that the area or size of the rectangle does not have a big effect on the quality of faint details in the final stack. I will only move away from the target from now on, if the target lies deep inside the Milky Way. In other cases I will select a smaller but dark area of my preview.

My next image will be the PoC. I think I‘ll go for the Heart Nebula, using the approach above and a dual-narrowband filter.

Best regards and thanks again,
Sven