Confused about Average-integration

[MineralMike]

I'm a PI newbie, and have successfully processed a couple of images. I'm using a ZWO ASI1600MC one-shot color camera, and I know the proper workflow point to debayer the subs.

A couple of nights ago while testing a new autoguider setup, I imaged this: http://astronomy.mdodd.com/images/ngc6992-large.jpg  I got only 30 minutes total exposure - six 5-minute subs. The color is weak, and a knowledgeable friend told me that longer total exposure (more subs) would bump-up the color. He said 3-4 hours would be a lot better than 30 minutes.

This is where I'm puzzled. How can averaging more subs with the same exposure boost the color, when PI allows only average (or median) integration?

 I know longer subs would boost the color, but I don't understand how averaging 48 5-minute subs will give me color than averaging six 5-minute subs. In the past, I used CCDSoft's sum-combine, and it makes sense to me how that would increase signal. I just don't see now averaging more subs works.

Or did I miss a PI integration method that sums subframes?

Can someone help me understand, or tell me what I need to do to boost the color?

Thank you.

--- mike
http://astronomy.mdodd.com

[Niall Saunders]

Hi Mike,

From many years experience working solely with OSC images, and nearly always taking 5-min subs, I can tell you that I nearly always start off my post-processing phase (i.e. post-calibration, pre-final processing) with images that seem to have almost no saturation whatsoever.

It used to bug me, but I now know that the final colour (saturation) is going to materialise towards the very end of the post-processing phase.

The issue of saturation has very little, if anything, to do with the number of exposures that you take but is, instead, far more dependent on the length of exposure of each individual Light frame.

Think of it this way - let's say that you are taking 5-minute subs, and the very central pixel (when in RAW format) has an ADU value of +/- 500 counts of an average ADU value of 30,000 each time you take an exposure. The more exposures that you take, and subsequently integrate together, the closer you will eventually get to the perceived average value of 30,000.

Taking more 5-minute subs will not raise the final averaged value, in exactly the same way as taking less will not reduce the final averaged value. It is simple statistics - flip a coin 100 times and you would expect to see it appears as 'heads' 50% of the time. Flip it another 100 times and you would expect to see more or less the same result again. Flip it 100 MILLION times and . . . . (sorry, I was giving you time to try it for yourself !!), well you get where I'm going - it will still be 50% (probably even closer to 50% than when you only flipped 100 times).

Taking more subs is done purely to provide enough data to allow statistical analysis to reduce noise. Taking longer subs can help to increase saturation - but you still need just as many of these subs to reduce noise to similar levels. However, there are technical considerations that have to be taken into account if you want to actually collect photons for longer subs (mount capabilities, tracking and guiding accuracy, clear skies and light pollution, dew accumulation, frost build-up to name but a few).

Discuss this with your astro-colleague and feel free to come back with comments and questions.

And, of course, should anyone have issue with my thought on the matter, I am happy to take on board constructive criticism.

[pfile]

by the way, average and sum are the same operation. average is the sum divided by the number of subs, so it is just a scaled sum.

rob

[MineralMike]

It used to bug me, but I now know that the final colour (saturation) is going to materialise towards the very end of the post-processing phase.

The issue of saturation has very little, if anything, to do with the number of exposures that you take but is, instead, far more dependent on the length of exposure of each individual Light frame. (...)

Thank you, Niall. What you wrote is in line with what I thought - longer subs (not more subs of the same exposure) will increase the data. Tonight is predicted to be clear, so I will try some 8- or 10-minute subs and see what I get. I already have an 8-minute master dark, but I'll need to expose some 10-minute darks if I want to try 10-minute light subs.

As time goes on, I'll learn more about PI processing.

Thanks again.

--- Mike

[Niall Saunders]

Hi again Mike,

Before you get too concerned about your Darks, have a look at what you already have - especially if you have a TE-cooled imager, and you don't run it 'flat-out' (always a badd move, because if you can't get down to the temperature you are looking for, because ambuent temperatures are too high for example, then your statistical Calibration stage is going to be all over the place).

You might find that there is very little difference between 5 and 8 minute Darks, and this might make it easier for you to try some 10 miunte Lights without having to go to the trouble of taking 10 minute Darks as well.

You also want to seriously consider the implications of the time spent taking 10 minute Lights versus the same time taking twice as many 5 minute subs. It is an important decision for me as my local weather can change through three full seasons in the time it takes me to get ten 10 minute Lights !!

Your own personal experience will be important. Good luck 

[MineralMike]

Before you get too concerned about your Darks, have a look at what you already have - especially if you have a TE-cooled imager, and you don't run it 'flat-out' (always a badd move, because if you can't get down to the temperature you are looking for, because ambuent temperatures are too high for example, then your statistical Calibration stage is going to be all over the place).

My ASI1600MC Pro can reach and hold 0C with the cooler power hovering around 15%-20%. I am very pleased with this camera.

You also want to seriously consider the implications of the time spent taking 10 minute Lights versus the same time taking twice as many 5 minute subs. It is an important decision for me as my local weather can change through three full seasons in the time it takes me to get ten 10 minute Lights !!

Last night, 12 10-minute subs gave me more color (on this nebula, anyway) than did six 5-minute subs a couple of nights ago. Here is last night's result: http://astronomy.mdodd.com/nebulae-02.html

I like this one much better than the 5-minute result.

I'm toying with the idea of exposing 2-3 subs apiece through Ha and Oiii filters (the OSC camera records these colors well). This would add to the color when integrating the subs without increasing star brightness/size. My only concern is that there aren't enough stars in the NB subs for SkyX to focus. I don't know if my Astrodon NB filters are parfocal with the non-astrodon IR-block filter. I guess I'll just have to try it.

In your first reply, you wrote:

It used to bug me, but I now know that the final colour (saturation) is going to materialise towards the very end of the post-processing phase.

Can you tell me what additional processing steps you use? As a newbie, I go through calibration, cosmetic calibration, debayer, star alignment, and image integration. Then I open STF and histgram transformation on a copy of the integrated image.

Are there additional steps that could increase the color?

Thanks again.

--- Mike
http://astronomy.mdodd.com

[Niall Saunders]

Hi Mike,

I aim for around -15°C on my TEC, at around 40% full power on a cool-ish night. If it is winter, I cam hit that target with even less power, but always with some TEC power - which keeps the temperature stable.

You were right to at least try the longer subs - it may not always work, but you now have a baseline that you can work from. Personally, I hardly ever get good enough weather and seeing conditions to make 10-minute subs work, so I mostly stick with 5 minutes.

I don't know anything about any software package's ability 'to focus' - I can get away with almost no focal change requirements from one year to the next. I use a Bahtinov mask every so often, and it takes me about two minutes to confirm that focus is still under control. But, I hardly ever change my imaging system setup, so why should it change? (My thoughts are always based on 'why' the focus should change - I don't even get focal shift when ambient temperatures change but, then again, I do have the luxury of using a Carbon-Fibre tube).

As soon as I have my Calibrated and Integrated Master Light, I tend to follow along the lines of the following:-
1.) Crop/Rotate to eliminate artefacts along the edges of the image
2.) STF leading to HISTO stretch - but with no loss of any pixel data
3.) DBE to get rid of background colour gradients
4.) Elimination of both colour and luminosity noise - as two separate processes
5.) HDR processing to enhance contrast changes
6.) CURVES stretching to again enhance the image
7.) SATURATION to bring out the colours (multiple iterations of small steps)
8.) Final CROP to give the the image ratio and size that I am happiest with

Importantly, these are just a rough outline of some of the stages I expect to have to work through - every image is different, and each will call for a different approach.

Despite what you may read or learn about online - there are NO rules 

Just have fun

[MineralMike]

As soon as I have my Calibrated and Integrated Master Light, I tend to follow along the lines of the following....

Thanks, Niall. I have saved your steps for future reference when I climb that high on the learning curve.

--- Mike

[Niall Saunders]

Hi Mike,

Just remember that every image is different - and that my 'method' is only very loosely based on what I can remember (so, anything before breakfast today is . . . sorry, what were we talking about?)

Also, there are some fine, detailed, tutorials and videos out there upon which nearly all of my suggestions are plagiarised based. "Harry's Video Tutorials" are an excellent place to start - even though they might be based on older versions of the PixInsight GUI and Engine.

Hope this helps.