Astrophotography question

[schmety]

Hi there !

I'm using Affinity Photo also for editing astro photos.

I have a quesiton, how can i do "Star Removal" like i have here link, "how to do that in Photoshop" and how to do that in Affinity Photo ?

link : https://astrobackyard.com/star-removal-in-photoshop/

 

Thanks for any kind of information, 

Best, 

Matej

[James Ritson]

Hi @schmety, hope you're well. Good old Astrobackyard, been following Trevor since the start of this year as I've gotten into astrophotography!

I believe you can approximate the Photoshop workflow by using Select>Select Sampled Colour. Click on the middle of a star, then adjust Tolerance to select the rest (you might have to go up to 100%). Also, you may want to try switching the colour model from RGB to Intensity, as stars will typically be the brightest parts of the image.

Once you've made the selection by clicking Apply, go to Select>Grow/Shrink. Grow the selection by what is appropriate (I ended up with 16px for my image) and check the Circular option for nice round selection marquees around the stars.

Finally, go to Edit>Inpaint and the stars will be removed. This might take a couple of minutes to complete if you're working in 32-bit—which you should be! Not sure if you're aware but Affinity Photo can use practically all of its tools, filters, adjustments etc in 32-bit, so there's no need to tone stretch then flatten and convert to 16-bit. The only time I've needed to use 16-bit is when creating a synthetic flat frame to combat gradients/skyglow etc.

You can also use Select>Tonal Range>Highlights to make the initial selection, but you get less flexibility as there's no Tolerance option. Also, if you're in 32-bit Linear, what constitutes highlight detail will differ from 16-bit Nonlinear, so you may not find all the stars are successfully selected.

Hope that helps!

[schmety]

Hey James !

Thank you very much for this info and also that you're also doing astrophotography.

I'm working with Astro Pixel Processor application and i can save images also in 32-bit , so i'll try that option next time, so far i was saving just in 16-bit.

I'm going to shoot some more data this night for Flaming Star Nebula, i already have 30min of data, but that's not much, going to add at least 1 more hour of data, 

then i'll stack everything in APP and try with first edit in Affinity Photo and later i'm using Luminar 4 application (wish this one could perform better overall, somehow slow app) for final touch.

 

[James Ritson]

No problem, definitely give 32-bit a try! I use DeepSkyStacker—not sure if APP's export settings are similar but you'll want to use 32-bit float/rational, not integer.

A couple of other useful things I've found:

You can use a live Clarity filter (Layers>New Live Filter Layer>Sharpen>Clarity) to bring out structure in nebula objects etc. Add a live Clarity, then invert it (Layer>Invert) and you can paint the effect back into specific areas. If you duplicate the main Background pixel layer, you can apply the destructive version (Filters>Sharpen>Clarity), which has the benefit of being more aggressive since it doesn't have to render in real-time.

Synthetic flat frame creation: I've had to use this, not for lens vignetting as I shoot flats, but for light pollution and skyglow that creeps in when you do extreme tonal stretching. Unfortunately, Median filtering is too computationally expensive in 32-bit and cannot be used, so what I do is:

• Create a merged layer of my work so far (Layer>Merge Visible), copy it, then go to File>New from Clipboard.
• WIth this new document, I'll convert it to 16-bit (Document>Convert Format / ICC Profile), run Dust & Scratches to get rid of all the stars, then use the Inpainting brush to remove any traces of deep sky objects.
• Finally, I'll convert back to 32-bit, run Gaussian Blur at its max radius of 100px, then copy and paste the layer back into my 32-bit document.
• Once the layer is pasted in, set its blend mode to Subtract. The sky will be way too dark: to counteract this, add a Levels adjustment layer and clip it into your pasted layer (drag it over the layer's text/label). Now raise the black level (input, not output) until you're happy with the result.

Hope that helps!

[schmety]

Thank you very much for your detailed answer.

I have try to remove stars, it worked, but i think i was editing too aggressive and also possible that i need some more data for that nebula to get more details first.

I'm working on Flaming star nebula.

[Guest]

Hi Both

I have just worked through James's Astrophotography workflow - absolutely great.

For all that use moded DSLR's James noted in the video that he had residual artifacts from flat files when applied.

This is due to the BIAS information being left in the flat image.  As the temperature in the camera changes so does the read noise and BIAS hence nighttime image - cold camera, daytime warm camera.

As such you cannot subtract flat from light and be left with a good image - negative pixels come in as James describes.

Most DSLR's adjust for this automatically by taking a dark immediately after the initial exposure but this is generally disabled for astrophotography.  To test your camera take a 30 second image and it will take 60 seconds before it saves.  This is due to the camera taking the dark frame.  If it takes 30 seconds to save dark frame adjustment is turned off already.

The best way to resolve the issue is to use CFA scaling but is not always available.

You can scale the flat histogram to match the light histogram - hit and miss but I am sure James could cope with that.  Otherwise use BIAS frames - ie after an taking a shot (light and flat) take the fastest possible dark frame you can - say 1/4000 second.  This is near enough to be a bias frame but should be 0 seconds if possible.  Subtract each bias from each light and flat before flat adjustment and your flats will work much better.

For info that is why full blown astro imaging cameras are cooled and temperature controlled.  All files are taken at the same temperature so no issues.  They are also cooled so noise is kept to a minimum.

What you really need is

Light frames

Dark frames (at the same exposure and temperature as light)

BIAS Frames 0.00 seconds (at the same temperature as all other frames)

Flat frames (same temperature as lights)

And flat dark frames (at the same exposure and temperature as flat)

The whole process then becomes seamless.

Hope that helps