Odd results in a stacked Ha run

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Odd results in a stacked Ha run

Postby Radcliffe » Sat Oct 20, 2018 1:38 pm

Hi all,

I stacked a run of 10 x 1 min Ha exposures of the Ring Nebula and got quite an odd result. Hopefully a .jpg image is enclosed with this post. I used DSS in the normal way and got good results with LRGB exposures from the same session. Does anyone know what causes Ha to produce this result and more importantly, does anyone know what I should have done ? My other efforts have turned out very well. I used my normal stacked bias and dark files and the same reference file for all LRGBHa stacking.

kind regards

Brian
Attachments
Ha Stack.jpg
Odd result here
Ha Stack.jpg (190.02 KiB) Viewed 174 times
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Re: Odd results in a stacked Ha run

Postby admin » Sun Oct 21, 2018 10:50 am

It's a work of art itself! :)

That is quite bizarre. I've never seen anything quite like this.
Are you using the FITS format by any chance? Does opening the dataset take longer than usual?

The artifacts are reminiscent of StarTools behavior when it encounters large portions of "Not-a-Number" (NaN) or out-of-bounds pixel values (such as + or minus infinity, or values that lie outside the MAX and MIN values specified in the FITS header). In these case, StarTools "makes up" plausible values, based on valid neighboring pixels.

Other cases in which this may happen is if large areas are unusually smooth (e.g. all have the same value). StarTools in that case also assumes something is wrong with the pixels; in real celestial signal there is always a measure of Poisson noise variations. Are you sure the dataset you imported was of sufficient bit-depth (e.g. 16-bit or higher)?

The whole reason why StarTools "makes up" pixels it likes, is so that these anomalous pixels will not interfere with StarTools' various modules that rely on artifact-free data; the pixels it makes up are more "agreeable" to these modules than leaving the artifacts in. Leaving them in, might cause ringing artifacts otherwise.

All that said, I can't be entirely sure that this is a StarTools issue, but it seems somehwat similar to the edge cases outlined above.
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Re: Odd results in a stacked Ha run

Postby Radcliffe » Mon Oct 22, 2018 8:36 am

Hi Ivo,

I too doubt it is an ST issue and as I am still very much a beginner, making the odd mistake now and again. I am finding my way in this fascinating hobby. I'm glad you like it and you can have it for the Startools Christmas calendar. I agree it looks mathematical so maybe the algorithms are been sent off on a wild goose chase by something that I have done.

Yes, the originals are .fits taken with a Trius H-694 and are unbinned. The images take some time to load into ST. My Ha always have a very low star count so there are great voids of no data which points towards the NaN theory. Everything else is working fine. I have to say that your software is in the PFM category...... pure flippin' magic !

As an aside, are there plans to allow Ha, O3 and other filtered images to be loaded as per LRGB and standard schemes to be selected e.g. the Hubble suite ?

All the best from the UK.

Brian Fraser
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Re: Odd results in a stacked Ha run

Postby admin » Tue Oct 23, 2018 4:17 am

I'm inclined to say it's ST getting confused by an issue with the dataset. If it helps, feel free to share it with me, if you wish.
I'm glad you like it and you can have it for the Startools Christmas calendar.

:lol:
I have to say that your software is in the PFM category...... pure flippin' magic !

Thank you - it means a lot!
As an aside, are there plans to allow Ha, O3 and other filtered images to be loaded as per LRGB and standard schemes to be selected e.g. the Hubble suite ?

Indeed, you can use the LRGB module to load SII, Ha, and OIII as R, G and B respectively to achieve the famous hubble palette.

Hope this helps,
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Re: Odd results in a stacked Ha run

Postby Radcliffe » Tue Oct 23, 2018 8:41 am

Every day is a day in the classroom !

Is that L plus the narrowbands or just the three narrowband filters on their own ?

:thumbsup:
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Re: Odd results in a stacked Ha run

Postby admin » Tue Oct 23, 2018 11:48 pm

Radcliffe wrote:Every day is a day in the classroom !

The learning never stops - even for me. That's the great thing about this field!
Is that L plus the narrowbands or just the three narrowband filters on their own ?

:thumbsup:

That's just the three bands on their own.
You could create a weighted luminance (L) composite from the 3 bands if you wanted though.
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