ASI294MC BINNING

[matija]

Hello!

 

I have a 10" RC on the way with 2000mm of focal length. I also have an ASI294MC Pro camera. My resolution with this setup is 0.47"/pix which is not good at all for my average seeing. I was debating getting a reducer, but that would ruin my nice FOV, so I decided to ask around about binning on this particular OSC camera.

 

The MM variant of the 294 has a mode that when binned 1 has some very small pixels and at BIN2 it works normally. The MC variant is not like that. Its working normally at bin1.

 

I wonder if binning 2 will actually work and let me image the night sky under poor seeing conditions.

 

Thanks!

[vlaiv]

With CMOS sensors like that in ASI294MC - you don't have to decide right away.

In fact, it is better to capture at full resolution even if you plan on binning later.

In any case - you can decide on bin factor later after you have gathered the data. Simplest way would be to integrate image and then bin at linear stage before you start processing.

Do note that you won't get almost anything if you bin x2 data from OSC that has been debayered with interpolation methods (and not super pixel), so it is best if you bin x3 or higher.

Good sampling rate can be inferred from the data itself. Measure average FWHM in your resulting linear stack and aim for value that is close to FWHM/1.6.

My personal preference is to be slightly lower than higher sampling than this value if you can't get it right.

Here is an example.

If you bin x3 you will have effective 0.47"/px * 3 = 1.41"/px

and if you bin x4 you will have effective 1.88"/px

Now imagine that your average FWHM is around 2.6"  - this means that you are best sampled at around 2.6" / 1.6 = 1.625"/px

Which one of the two would you choose?

Well, if you have large amount of data and good SNR and you can sharpen your data extensively - then it makes sense to go for bin x3 and 1.41"/px - but in all other cases - I'd say go for bin x4 and 1.88"/px

 

[matija]

@vlaiv So binning my 294MC (BIN3) won't ruin my colors and data as some said? I find that 1.41"/pix is quite perfect for my usual conditions. So when imaging, I select bin3, and that is it?

Thanks!

[vlaiv]

2 minutes ago, matija said:

@vlaiv So binning my 294MC (BIN3) won't ruin my colors and data as some said? I find that 1.41"/pix is quite perfect for my usual conditions. So when imaging, I select bin3, and that is it?

Thanks!

Binning x3 won't ruin your color data if you bin at later stage. If you bin at capture time - then it depends on how the binning is implemented in firmware - and yes, it can create mono data and loose color, so it is best not to do it at capture time.

When imaging - don't select bin x3. Image like you normally would - bin x1 - or "normal".

Then after you calibrate your data and debayer it and you stack your subs and get final output - then before you start processing - perform x3 software bin on that data.

Depending on software that you use - binning will be called differently.

In PixInsight for example - it is called integer resample - and you select average option. If your software does not have option to bin resulting data - and you don't want to pay for software that does - use ImageJ. It is java based (will work with any OS) and open source software for scientific image manipulation.

It can load fits / tiff files and perform binning (Image / Transform / Bin - again choose average method and x3 as bin factor).

 

[matija]

@vlaivThat sounds great. Ill capture data in BIN1 and then perform that after stacking. Now the real question is(sorry if you already answered in more technical terms because I'm a little slow and don't quite get all the details of AP:)), will it fix the effects of bad seeing?

[vlaiv]

1 minute ago, matija said:

@vlaivThat sounds great. Ill capture data in BIN1 and then perform that after stacking. Now the real question is(sorry if you already answered in more technical terms because I'm a little slow and don't quite get all the details of AP:)), will it fix the effects of bad seeing?

Ah, ok.

Here is what you need to understand. There is no "fixing" effects of bad seeing.

Best you can hope is to sharpen up back some of that softness and blurriness produced by poor seeing or poor mount performance (or that two combined). How much you can sharpen - depends on how good your signal is. In theory, one should be able to sharpen all the way to telescope aperture limit - if they had perfect signal and no noise.

Problem is - we have noise (and often quite a lot of it) - and when we sharpen - we only make that noise worse.

Binning does not directly address poor seeing as such. With poor seeing you get situation where you have too fine sampling (too much pixels) for level of detail that is available because seeing blurred things. This is called over sampling, and over sampling is bad because you loose SNR and gain nothing as there is no detail.

Binning addresses this SNR loss by making sampling rate more adequate for the level of detail that you've captured.

Amount of blur in the image is directly related to star profile in the image. It is in fact PSF of blur (point spread function) as star is point like object - and star profile shows how light from a single point spread into "blob" of light. This is why we can use FWHM of star profile to assess how much blur there is in the image. That is why we have sampling rate = FWHM / 1.6 (there is some complex math behind that expression).

Even when you sample at that rate - you won't get perfectly sharp image. There is still room for sharpening, and in order for that sharpening to work - you need good SNR.

That is why I say - I'd rather bin x4 than x3 - even if it seems like under sampling in above case - better SNR will allow you to sharpen up image in processing more.