Stacking ultra wide

wjiang

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Now, I don't have a guided tracking mount. Nevertheless, I've had quite a bit of success with stacking longer focal length shots in DSS, e.g. this one from the PL25:
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I don't have a fast wide angle, however, and my research online seemed to suggest that trying to stack wide angle shots was a much more difficult process.
Well, I went ahead and tried it anyway, with my Samyang 7.5mm f/3.5 FE, out on my street in light polluted suburbia no less.

Here's the result:
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That was a 4x stack of f/3.5, 15s, ISO800 light frames (with automatic in-camera dark frame subtraction enabled). 15s is about the longest where each individual frame does not show star trailing.

How did I avoid foreground blur or star trailing? The trick was to do the stack in 2 steps:
  1. Stack with star alignment enabled, output in 32-bit float TIFF as star frame.
  2. Stack with star alignment disabled (but with other settings identical), output in 32-bit float TIFF as foreground frame.
  3. Import both as layers in Photoshop.
  4. Blend foreground and star frames and flatten.
  5. Continue with rest of PP as usual (down converting from 32-bit as necessary).

I'm pretty pleased with the result - I was surprised that a deep sky object, the LMC, showed up. The slightly weird haloing on the horizon wasn't actually a result of the blending, that was there originally as a result of the light pollution. I will have to try again in a darker sky next time.
 

Jazz80

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Great photos! Thanks for sharing the process details.

Can I ask:
- where the first photo was shot, i.e. suburbia, rural, dark sky?
- what was your process for the first photo you shot with the PL25? Same as the one described for your second photo?

TIA
 

OzRay

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I've been waiting at least a month or so for clear skies and free time, and none have come together so far. I was up in our High Country last weekend in perfect conditions to do a variety of astrophotography and all we had were cloudy night skies.
 

wjiang

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Great photos! Thanks for sharing the process details.

Can I ask:
- where the first photo was shot, i.e. suburbia, rural, dark sky?
- what was your process for the first photo you shot with the PL25? Same as the one described for your second photo?

TIA
The first was shot in almost the same place in suburbia, but because of the narrow FoV the light pollution isn't really obvious after PP.

It's just a simple stack of 7x light frames at f/1.4, 4s, ISO800 (DFS enabled). With automatic star alignment enabled (as described for the star frame above) it means you don't get star trails, but you will have to crop a bit off the edges as they won't overlap after the transformation.
 

Jazz80

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Apr 27, 2014
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Thanks for explaining. [emoji2]

I'll have to give it a try. Your shot looks
awesome.
 

PhilS

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very impressive photos wjiang, the PL25 one is great i love the colour rendition. The stars are sharp across the field of view, great lens.

I notice the mu43 lenses don't have the same vignette issue as do the FF lenses so I rarely do flats. Why do you do on camera darks, just curious?. I do darks after the lights and do the calibration in DSS. I also bring the raws in to PS and remove the CA which can be quite obvious on bright stars, save as TIFFs, same for the darks and go from there.

So you save the data out of DSS as a 32 bit tiff and convert to 16 bit in PS?

Also what part of the sky is it ?
 

wjiang

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very impressive photos wjiang, the PL25 one is great i love the colour rendition. The stars are sharp across the field of view, great lens.

I notice the mu43 lenses don't have the same vignette issue as do the FF lenses so I rarely do flats. Why do you do on camera darks, just curious?. I do darks after the lights and do the calibration in DSS. I also bring the raws in to PS and remove the CA which can be quite obvious on bright stars, save as TIFFs, same for the darks and go from there.

So you save the data out of DSS as a 32 bit tiff and convert to 16 bit in PS?

Also what part of the sky is it ?
Thanks! You clearly have more experience at this than I do. TBH I've only just gotten into stacking using DSS - previously I just used single frames straight out of the camera, with in-camera DFS enabled. Stacking such frames was the natural progression (no chance that I'll screw the darks up). So really, I haven't graduated into doing darks, flats, calibration etc properly yet :wink:. Now that I'm getting the hang of this, I probably will try it next time, maybe to see if the results are significantly better.

The reason I save 32-bit in DSS and down convert in PS is that I hate the post-stack adjustment controls in DSS - I prefer to just throw the data at PS rather than trying to massage it in DSS first. I honestly haven't noticed any C/A problems with the lenses I use (PL25, O45, 7.5FE), but I'll keep your suggestion in mind.

Remembering that I'm in the Southern Hemisphere...
Image 1 is looking at the galactic core between Scorpius on the left and Sagittarius on the right. I think the star cluster towards the left of the image is M7.
Image 2 is... pretty wide. Orion is on the left, and Crux is on the far right, somewhat behind the very tall tree. The two brightest stars in the centre are Sirius (just above the light glow) and Canopus (higher, to the right), and point to the LMC in Dorado.
 

atnbirdie

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If you're shooting with a 7.5mm lens, you should be able to get longer exposures without trailing. Experiment a bit with your times. Of course, it will also depend on where you are aiming with shots of the ecliptic most likely to give trailing before those closer to the pole.

If you can't afford something like the Orion Sky Tracker (which is easy to use and works wonderfully) consider making a barn door tracker if you have DIY skills. Google it with astrophotography and you'll get sources for plans and parts.
 

wjiang

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If you're shooting with a 7.5mm lens, you should be able to get longer exposures without trailing. Experiment a bit with your times. Of course, it will also depend on where you are aiming with shots of the ecliptic most likely to give trailing before those closer to the pole.
I did actually. I think with a fisheye it's kind of unavoidable to get stars from all over the sky (including the ecliptic), and 20s was already giving lots of sausage shaped stars.
 

Petrochemist

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Great shots!!!
I really must give DSS a go - I downloaded it ages ago, then basically forgot it amoungst the many, many cloudy nights...
 

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