Brain tool

[atpatp]

Hi everyone, I have this question,
Does the analysis tool for the best "Brain" exposure also change the "offset" value?
If yes, where can I see the suggested value?
Thanhs

[admin]

Hi,

Yes, it will suggest a value based on ensuring that the histogram is properly separated zero point (left-hand side). In the brain suggestions window this is referred to as the 'black level' value, which is a synonym for offset or brightness (different camera manufacturers use different terminology).

See https://docs.sharpcap.co.uk/3.2/#The%20 ... n%20Window

Hope this helps, Robin

[atpatp]

So in the example shown it suggests an 'offset' value of 0, right?
Thanks

[admin]

Yes,

that screenshot was taken in an earlier version that would typically recommend a zero offset due to including the effect of the sky brightness in moving the histogram peak to the right. OK in theory, but didn't always work for dark frames with the same settings.

Robin

[atpatp]

Tanks

[Sven_Bortle5]

Hi there,

sorry for jumping into this topic, but I my question just starts off where the previous ends.

I have taken two tries right as the manual describes with my ZWO ASI 183MC (color) cam. Each time I did measure the sky brightness with the brain tool first, then I captured 1 or 2 sessions and stacked the images in DSS. After post-processing, I got the following results.

1st try: https://www.dropbox.com/s/l9oal9k61rqux ... l.jpg?dl=0

I used Max. Dynamic Range and applied the suggested values for Black Level, Gain (0) and Exposure 144s. In a second run I used the same settings, but increased the exposure to 300s. Total exposure app. 2 hours.

2nd try: https://www.dropbox.com/s/7efioz1n7x2e9 ... l.jpg?dl=0

Again I used Max. Dynamic Range and applied the suggested values for Black Level, Gain (0). I increased the exposure to 360s. Total exposure app. 1.5 hours.

The dynamic range is great - see the star colours inside the Iris Nebula. But, what I was trying to influence by increasing the exposure was seeing more of the weak nebulosity. Especially in the Iris Nebula, the dust is not really visible. Going up from 144s to 300s exposure on the Iris Nebula or to 360s on the Cygnus Wall, didn't really show an improvement.

How should I proceed in order to capture more of the weak nebulosity, same setting more images, same settings but filters, any other settings?

Best regards and thanks,
Sven

[admin]

Hi Sven,

if you want to see fainter nebulousity then you have two main choices –

* Reduce light pollution levels
* capture for a longer total time

SharpCap's smart histogram/brain functions will guide you to getting the best possible image given your current light pollution and the total amount of time that you intend to image for, but can't get past that threshold without changing one or other of the inputs.

Capturing for a longer total time is a fairly crude tool – if you capture six hours of images then you should be able to see things about twice as faint as if you capture 1.5 hours of images.

You can be a little more subtle with the idea of reducing light pollution – either you can move to a less polluted site for your imaging or you can introduce a light pollution filter. If you manage to reduce your light pollution by a factor of 4 (either different site or a filter) then that will have the same effect as increasing total exposure time by a factor of 4 (providing you have not reduced the brightness of the target - perhaps due to the filter not being perfect).

Hope this helps,
Robin

[Sven_Bortle5]

Hi Robin,

and thanks. I have been into astrophotography for 3.5 years by now. So I know all concepts and have most gadgets at hand. But what's new to me is Sharp Cap and the brain tool.

I started to use Sharp Cap because I'm on the scientific approach and wanted to reduce as many variables in using a cooled astro cam as possible. Sharp Cap and the brain tool allow me to measure the sensor and the actual darkness of the sky for instantly configuring my system (esp. the black level, gain and exposure) - that's way more precise than most other capturing software.

Great that you kind of confirmed my expectation.

I'll give minimal light pollution and max. exposure time a boost next months: 2 weeks in Bortle Class 2 region in Denmark for Elephants Trunk Nebula and maybe a second target. Really curious on the results.

Best regards,
Sven

[admin]

Hi Sven,

Hope that you get some clear skies for your imaging sessions - looking forward to seeing the results

Cheers, Robin

[Sven_Bortle5]

Well, it took three months until I had a couple of clear nights for doing another test on the brain tool and my settings. The weather in the Bortle Class 2 area in Denmark wasn't good. So I did this run back home in Germany.

In July I ran the two sessions on the Iris Nebula, using 144s and 300s subs (see below).

This time I checked the results of by sky measurement (the curves) in the Brain Tool and did setup everything according to a gain of 50.

According to the Brain Tool, the exposure of a single sub should now be 123s. I also took 90 darks and 100 flats.

I took 16.5 hours of total exposure (the best 85% of the images) into stacking.

Here's the result after processing: https://www.dropbox.com/s/0xl6hn6lmoby3 ... l.jpg?dl=0

The image is a lot better than the shorter ones with gain 0. You even get an idea of the dust in that area.

So here's my question on how to optimize the sky measuremnt in order to extract more faint details. This time I moved the scope to a "quite empty" area with no nebulosity, north-west of the Cepheus. This was pretty high up in the sky - away from the light polluted horizon. I selected a rectangle of app. 20% in a dark area of the preview-image for doing the measurement.

My expectation was that
a) the gain of 50 would reduce the shot noise of the cam (not sacrificing too much dynamic range) and
b) this way of measuring with the Brain Tool would deliver a good threshhold regarding the skyglow.

What do you think: Was this approach okay and is the result (after 16.5 hours of total exposure) a case of deminishing returns in a Bortle Class 5 area?
Is there a way to optimize the measurement (e.g. by selecting a smaller rectangle or the whole preview)?

Regards and thanks,
Sven