How to create twinkling large stars using StarTools (mine are currently blown out)

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el_hop87
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Joined: Wed Mar 24, 2021 8:23 am

How to create twinkling large stars using StarTools (mine are currently blown out)

Post by el_hop87 »

Hi

I have still got a lot to learn with StarTools and was wondering if anyone could advise on how to create the "twinkle" on large stars in images please. I am editing my Horsehead Nebula shot but the large stars look blown out rather then sharp and the typical 4 spike twinkle that you see in some image.

Please see below link to my current file, and the original file to this post - if anyone could help with this particular issue as well as any other suggestions on how I could improve, that would be very much appreciated! :

- Image so far: https://www.dropbox.com/s/9vqn9babg1ff5 ... 3.jpg?dl=0
- Original stacked fits file: https://www.dropbox.com/s/oknlpjw92uv35 ... e.fts?dl=0


For info I used a Nikon 5200, SW AZ GTI, SW Evostar 72ED and IDAS P2 filter - 1.5 hours of data at 60' subs, stacked image (DSS) is lights, darks, flats and bias.

Many thanks in advance,

Elly
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Re: How to create twinkling large stars using StarTools (mine are currently blown out)

Post by admin »

Hi Elly,

I think the "twinkle" you refer to are diffraction spikes. They are an attribute if the optics and are typically caused by the spider vanes that hold the secondary mirror in place in Newtonians. In your sig, it says you are using a SW Evostar 72ED, which is a refactor (e.g. no mirrors). These do not show these sort of diffraction patterns.

Your stars look very well resolved/stretched. A faint halo/circle is visible around them (usually caused by some sort of secondary reflection).

If you really like diffraction spikes, you can induce them artificially in your images by adding some criss-cross wire across the opening of your scope.

And then there is the Synth module in StarTools. It will let you build your own virtual telescope and show you the effects the different components have on the stellar profiles. You can then use this virtual scope model to re-synthesise physically modelled stars in your image if you really wish. If you do this, take great care to match the the synthesised spikes with the stretch and "blur" of the image. You need to create a mask that contains the stars you wish to apply the spikes to. The Auto Mask typically works, but as a guide, only the brightest stars tend to show defined diffraction spikes.

It is good practice to let people know you added the spikes in post-processing when publishing your image.
Autosave(21)b4.jpg
Autosave(21)b4.jpg (97.22 KiB) Viewed 95 times
Autosave(21).jpg
Autosave(21).jpg (99.02 KiB) Viewed 95 times
Sidenote; looking at your dataset, it doesn't appear your flats are working very well. Lighting is pretty uneven, vignetting strong and I can see dust donuts smeared out into smudges. Try stacking with an outlier rejection algorithm (Kappa Sigma/Median), which will get rid of the satellite trail.

Hope this helps!

Ivo
Ivo Jager
StarTools creator and astronomy enthusiast
el_hop87
Posts: 8
Joined: Wed Mar 24, 2021 8:23 am

Re: How to create twinkling large stars using StarTools (mine are currently blown out)

Post by el_hop87 »

Hi,

Thank you very much for your reply and v useful advice! - I have since learnt about the spikes being from reflector scopes and not refractors like mine. Very good to know about the function in StarTools that helps with this, and thank you for your edit of the image! I am less keen on superficial additions, but again good to keep in mind!

I had a feeling the flats weren't great - I will look into this and follow your advice.

Thank you again, the support in this forum is brilliant.

Best wishes
Elly
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