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Imaging Or Processing Problem (Advice/Guidance Appreciated)

imaging dslr beginner astrophotography
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3 replies to this topic

#1 AG_Alex2097

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Posted 25 September 2020 - 02:04 PM

Hey everyone!

Long time lurker, first time poster here as I'm seeking for advice after some of my results.

A couple weeks back, I had go at imaging the Veil Nebula, but am unhappy with the results and want to improve, the problem I'm facing however, is that I'm not quite sure what's causing the sub par results, which is where I'd love to hear your input!

 

 

I'm using the following equipment:
Astrotrac TT320X-AG (No Auto Guiding)

Astrotrac TH3010 Counterhead Bar (properly balanced)

Astrotrac Wedge

Astrotrac Polar Alignment Scope (Not the best, but learned to work with it and manage to get 4mins)

Sirui R-5214X Tripod

 

Olympus OM-D E-M5 Mk II Unmodded (Definitely not the best, but it's what I have and I've seen others with great results)

Olympus 35-100mm f/2 Pro ED Zuiko Digital (With 4/3 to µ4/3 adapter)

Olympus Zuiko Digital EC-20 Teleconverter (2.0x)

 

Accessories:
Anti Dew Strip Lens Heater

Pluto Trigger (Bluetooth intervalometer)

 

Imaging Steps:

After setting up all gear, use a Bahtinov mask to achieve perfect focus (using a bright star)

Get polar alignment as good as possible, testing it by taking varying length exposures and checking if there's any star trailing (Got 4 minutes for this particular image)

Find and frame my target and start shooting away

Once the Astrotrac runs out, I stop and shoot my dark frames, same exact settings (Used to do in camera dark frame subtraction, but that severely reduces my imaging time, so started doing this now (first time))

 

Post Processing:

Open all the raw files with Camera Raw and apply vignetting correction, slight amount of noise reduction (10 – 20), Highlight reduction (-100 – -90), slight exposure boost (0.2 – 1) and save them to TIFF for Deep Sky Stacker
Add the lights in Deep Sky Stacker and register them, I usually to go for a star detection rate of 1000 on my best light
Ensure that bad images are filtered oud (clouds, light pollution, wind, ...), manually checking the lights to make sure the scores are accurate

Add the darks as well

Stack the images (Lights with Kappa-Sigma Clipping (Default Settings), Darks with average)
Stack it and save the resulting stack as TIFF for further processing

In SiriL apply photometric color correct and occasionally background extraction to combat light pollution 
Save the image for further processing in Photoshop where I apply a curves filter to stretch the image
Further adjustments to colour and such, not super relevant to all this

 

Now, as you can see in the following image, it's not the best, and I would like to know, I've also attached a single light frame (after Camera Raw Corrections)

Stacked & processed image (9 total): 

ProcessedPhotoshop_Small.jpg

 

Full size: https://drive.google...iew?usp=sharing

 

Single light frame: 

OLY93410_Small.jpg

 

Full Size: https://drive.google...iew?usp=sharing

Each frame is 240s, ISO640, 200mm f/4.0

 

And here's the raws if you want to have a look yourself: https://drive.google...Zui?usp=sharing

These are already filtered down to the best lights (Out of 30, many had feint clouds or shake due to heavy wind that night)

 

Note: Please ignore the left side of the image, I think I was experiencing tilt here (or my lens is decentered :/)

 

I understand there's not many light frames here to work with, but for a 4 minute exposures, I would've expected better

It's as if there's way too many stars in comparison to other images I've seen of the veil nebula

I also understand that my equipment is not the best out there, but I'd like to do with what I have, at most maybe introducing a STC Clip In Filter

 

So if anyone has input on what could be the problem here, I'd highly appreciate feedback.
I don't know if more lights would do it, because I would think it would just amplify all those tiny stars even more

 

I do think I'll try stopping down further next time, but doubt that will make the difference

 

Thanks in advance!

Alex

Attached Thumbnails

  • ProcessedPhotoshop_Small.jpg
  • OLY93410_Small.jpg

Edited by AG_Alex2097, 25 September 2020 - 04:32 PM.


#2 BQ Octantis

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Posted 25 September 2020 - 05:58 PM

Hi Alex,

 

Welcome to the forums!

 

I took a look at your subs. There are a couple of problems with them:

 

1. Vignetting. You need flats. Just a single flat produces several orders of magnitude better vignetting control over other vignetting removal methods. Here is a single sub with my best one-sub workflow. Without the flat, there is a blob of star wash at the center that can't be separated easily from the nebula.

 

(Click for full size.)

OLY93410.jpg

 

2. Stretched stars at the corners. This is a feature of zoom lenses—always too many compromises in the design. And stopping down with the lens aperture produces star spikes. Star spikes are fine if you like them, but they're very difficult to remove if you don't. So I use a filter step-down ring as an aperture mask to stop down the lens to get round(er) stars out to the corners. I get substantial improvement going from f/2.8 to ~f/3.84 (< 2 stops). This is a bit of a challenge for your aperture—it’s got a whopping 77mm filter for just a 100mm FL, and you have to get down in the ~35mm range. If it were my aperture, I'd get a 77-to-52mm ring and a 52-to-37mm ring. This would stop the lens down to f/2.7. If that didn't work, I'd try a 52-to-30mm ring (which would put the lens at f/3.3).

 

As to your workflow, ACR stretches the lights non-linearly, so the darks subtraction in DSS is no longer optimal. For the best results, you need to subtract your darks and apply your flat(s) before stretching or adjusting anything. And ideally, you should be stretching those after you stack them.

 

If you haven't read Charles Bracken's book on calibration, I highly recommend it.

 

Lastly, for any target deep into the plane of the Milky Way, you'll need to use a star reduction technique. There are several methods; if you make it that far, we can discuss them…

 

Cheers,

 

BQ


Edited by BQ Octantis, 25 September 2020 - 06:01 PM.

  • AG_Alex2097 and limeyx like this

#3 AG_Alex2097

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Posted 25 September 2020 - 08:09 PM

Oh wauw, that's quite impressive to see out of a single sub!!

 

I was aware of the bad vignetting but was hoping camera raw adjustments would counteract it, I've not played around with all the different types of other frames to take as I saw others doing it without them, but I guess that really depends on the lens huh, I'll try this out next time!!

 

Unfortunately I only have zooms for my telephoto focal lengths, which I'll probably stick to for now until I maybe get a better hang of it (I've also got the 90-250 f/2.8, but always worry the focal length will slip as it rotates on my mount, ruining the whole shoot, it also makes it more difficult to capture over multiple nights as focal length will never match unless I use the outer ends of the focal length range)
Although I did see about that filter step down ring trick before, I think star spikes is the least of my worries (for now), but I'll definitely keep it in mind for the future once I start getting better results overall, I want to work on improving nebulosity first and work my way up from there smile.gif

 

For the post processing workflow, I learned most of what I know from reading through Clarkvision's guides on post processing (https://clarkvision....cessing.basics/) who seems to have quite the know how on things, it was my main resource when learning astro, along with just researching on the internet. He makes his ACR adjustments before stacking, so that somewhat surprises me, but at the same time, I kind of also get your point as I'm guessing it reduces the dynamic range towards the darker end, creating bias during stretch, hmmm, interesting, I will have to revise my workflow for that and try as you say next time! Exciting! laugh.gif

 

As for the book, like I mentioned I've been sticking to mostly online resources for now and didn't know which astro photography books would be suitable, I'll see about picking up a copy, thanks for the tip!

Will look into star reduction as well, once I get to that part! 

 

Thank you so much for all the pointers!! I'll do my best! ^^


Edited by AG_Alex2097, 25 September 2020 - 08:11 PM.


#4 BQ Octantis

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Posted 25 September 2020 - 10:10 PM

While I am a disciple of Bracken, Roger Clark's RawTherapee workflow works wonders for limited data (which I used on your single sub). I now use RawTherapee instead of ACR to pre-process my subs in linear space before stacking. With that workflow, I can get good results at 100-170% scaling.

 

Cheers,

 

BQ




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