Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
06-21-2022, 11:52 AM   #1
Pentaxian




Join Date: Feb 2015
Photos: Gallery | Albums
Posts: 10,445
Star trials capture & PP work flow

This year as the spring / summer nights get more friendly, I've been out again for some mid-night shots. My idea was to compare some of the K1 in-camera composite processing, to new composite editing functionalities in Silkypix Pro 10 show cased for fireworks images, but I wanted to try with star trials instead.
So I captured a series of 30 seconds , ISO 800, M mode, exposures with the K1, using interval composite stacked in camera, and also firing a hundred exposures with the same settings for processing raw files in SPD 10. In total I shot several sequences, or over 400 images.

[A] Star trials with K1 OOC composite:
- it's nice because I get a final raw file, simple processing, adjust WB to taste, and levels / tone curve / pull shadows. Done.
- what's not so nice is all the stray tracks from commercial aircrafts crossing the camera field of view at the time of exposure
- what's also not so nice is image quality: low contrast around star trials
- foreground subject pulled from dark is crippled by noise (pulling a few stops from ISO 800). Also, dark frame subtraction (slow shutter speed noise red.) is not possible for continuous star trials, or else it'd create dashed star trials.

[B] Out of camera processing, processing the same exposure sequence, e.g 120 raw captures (Manual mode, 30 seconds, ISO800) in two different way:
- stack all DNG as average, output the average as a 16bits TIFF or DNG => star trials disappear , but the whole image is smooth , noise free, except the foreground subject still has the white dots noise
- stack all DNG as "bright" , retains all the brightest pixels of the image stack.
- layering overlay of start trials from the "bright" composite stack output, onto average stack output, so that to get nice bright star trials over smooth night sky background and foreground.

[A] is great for quickly checking out how the result looks like, straight out of camera. [B] workflow give nicer image quality , easier to sharpen thanks to the much lower noise.

Still, [B] workflow has room for improvement because the average of stack retains the white dots that appear similar on the same pixels from one exposure to the next.

I imagined I could stack add combine M mode exposures in camera (addition mode) to reduce foreground noise, but the white dots soared, no surprise, yes because dark current noise is not random, becomes bigger each time we add one more frame on top of the others.

To remove the white dots, I tested LENR enabled (dark frame removal) on 30 seconds exposures. LENR removes white dots, but pink/magenda cast appear when pulling shadows, ah ah.

I came to the conclusion that I'd need to use two drive modes and capture two sequences of images , one sequence for the star trials and one sequence for the foreground:
- a Bulb mode long exposure (like 4 minutes or more... I tested up to 16 minutes, at ISO100) with LENR, eliminates white dots on the foreground subject
- a series of M mode, 30 sec. , ISO800 exposures (e.g. 120 exposures for 1 hour long total star trials) in continuous burst mode, LENR disabled (of course), extract the star trials from the sky, overlay star trials layer over the Bulb mode long exposure layer.

Disadvantage: I need to switch the camera from B to M, then if the camera or tripod moves, the foreground / sky / trails overlay don't quite match on top of each other I need to line up layers in GIMP.

What's your workflow? Any other method for best possible image quality (low noise + sharp details) ?


Last edited by biz-engineer; 06-21-2022 at 12:05 PM.
06-21-2022, 03:38 PM   #2
Pentaxian




Join Date: Jan 2018
Location: Jersey C.I.
Photos: Albums
Posts: 1,977
You don't mention which lens or aperture you're working at.
I've found a good wide-aperture lens helps keep the ISO (and hence the noise) reduced … a bad wide-aperture lens can have it's own problems with needing to be stopped down to reduce coma etc., so no real gain!
06-21-2022, 08:55 PM   #3
Pentaxian




Join Date: Feb 2015
Photos: Gallery | Albums
Posts: 10,445
Original Poster
QuoteOriginally posted by kypfer Quote
You don't mention which lens or aperture you're working at.
Lens aperture, f4 , f5.6, f8, due to depth of field, which is another issue if we want to have both the foreground subject and the stars in focus. Of course I can shoot the sky at f2.8 (dfa24-70 or dfa15-30 wide open) but then the foreground won't be sharp.
In my series of exposures for the star trials, I used f4, but even though the stack average reduced noise, the foreground subject isn't as sharp as in the single bulb mode exposure at f8 for the foreground.
06-22-2022, 08:26 AM   #4
Site Supporter
Site Supporter
MossyRocks's Avatar

Join Date: Nov 2017
Location: Minnesota
Photos: Gallery | Albums
Posts: 2,618
Your B work flow is fairly similar to mine. I don't' put the camera in interval shooting mode but fast continuous but that is the biggest difference. I also don't' use the LENR and instead shoot a pile darks at the end while packing up to create a master dark. That maximizes the shots taken during the session and provides a better correction frame to subtract.

Another thing to consider is taking some flat frames to correct vignetting.

For the foreground I would suggest taking a bulb mode exposure of it in focus and then blend it in as its own layer. Usually when I am doing night landscapes I am using my 12/2.8 so there anything beyond 10 feed is at infinity from the lenses perspective so I don't need to adjust focus. Then just blend the foreground in as its own layer.

06-22-2022, 11:15 AM   #5
Pentaxian




Join Date: Feb 2015
Photos: Gallery | Albums
Posts: 10,445
Original Poster
QuoteOriginally posted by MossyRocks Quote
shoot a pile darks at the end while packing up to create a master dark. That maximizes the shots taken during the session and provides a better correction frame to subtract.
Thanks for the advice. What do you mean by "pile darks", is one more exposure at the same shutter speed and ISO settings with lens cap on?
How do you subtract the dark frame from other frames, would it work to exports all files as TIFF 16bits and do the layer works with those TIFF files?
06-22-2022, 03:07 PM   #6
Site Supporter
Site Supporter
MossyRocks's Avatar

Join Date: Nov 2017
Location: Minnesota
Photos: Gallery | Albums
Posts: 2,618
That should be "a pile of darks" sometimes my fingers don't keep up with my mind when typing and I miss a word.

When doing astro shooting dark frames are "black" images taken with the same exposure length, ISO, and temperature as the light shots (images that contain actual images you want to do something with). Most astro stacking programs will take a pile of dark frames and automatically combine them (averaged to make a master dark) and subtract the master dark from the light frames. I am pretty sure RawTherapee supports dark frames and I believe will create a master dark for you. I haven't used this feature in it but have seen options that look like it is supported. By creating a master dark from a pile of shots you drive down the true random noise that is recorded in each one producing a better representation of the actual systematic error that you are trying subtract out of each light frame. By lowering the true random noise in each dark frame you minimize the destruction of actual data in each of the light frames. A single dark frame (LENR) has a lot of true random noise so you may delete too much or not delete enough. Ideally it would be a wash in the final image but since the true random nose is still large it may dominate over the systematic error. Because of this the improvement of using LENR is not great. Also using the collect dark frames at the end while packing up results in being able to collect more light frames for a given time out photographing. Lets look at a realistic example where you are out for 2 hours and spend 15 minutes setting up and 15 minutes tearing down leaving 90 minutes for shooting.

Using LENR and 30s shots results in you collecting 90 light images that have some amount of systematic error removed. This give you 45 minutes of total exposure
Instead taking a pile of darks while packing up results in collecting 180 light images with all of the systematic error. This gives you 90 minutes of total exposure. You also have 30 shots of the back your lens cap

In the second case you have manage to collect 2x the light data which means you have an entire stop worth of additional exposure. That extra stop of exposure means you have 1 stop less true random noise (1/sqrt(2)) in the final image. This increase alone likely is better than what you got with using LENR. At this point there is still the ability to use the dark frames and with 30 of them the true random noise gets driven down almost 5 whole stops in the master dark. When you subtract that out of each of the light frames you do a very good job of removing the systematic error which is what dark frame subtraction (LENR or master dark) tries to do

Unfortunately the only way to deal with true random noise (shot noise from the quantum nature of photons, true random thermal noise with the sensor, and any number of other true random events) is by collecting more light frames which the second method involving taking dark frames does.

If you are on windows (or keep a windows VM on another OS) and going for star trails you might want to play around with the program sequator. I think it can to star trails as well as other stacking options. It also supports dark frames and flat frames (these correct vignetting as well as spots on the sensor) and if you provide them it knows what to do as mathematically they are very simple to handle.

You can look up how to shoot flat frames online but basically you need to focus the lens to infinity, set it to the same ISO and f-stop that you used for shooting the light frame. When I shoot them I put the camera in Av mode with an exposure bias of +0.7. then stretch a clean white t-shirt that is folded in half (4 layers of white cloth to really diffuse the light) over the front of the lens and point at a uniformly lit portion of the sky (have the sun at your back or do it on a cloudy day). I shoot a buffer's worth of flat frames and call it good.

There are other corrective frame types but flat frames are the most beneficial and dark frames the next but there is a pretty big difference between the value of them. The others also have about as much difference in value as you keep going down so I don't bother with bias or flat-dark frames.
06-23-2022, 03:11 AM   #7
Pentaxian




Join Date: Feb 2015
Photos: Gallery | Albums
Posts: 10,445
Original Poster
QuoteOriginally posted by MossyRocks Quote
I am pretty sure RawTherapee supports dark frames and I believe will create a master dark for you.
Yes, correct, I could take a look at RawTherapee and it can indeed load multiple dark frames. Now I understand, averaging a stack of dark frames is a good idea. I previously captured 8 dark frames (I let the camera keep firing with the lens cap on) after I had captured enough star trial frames. and yes I could notice some of the bright spots do not appear consistently on every frame. I read your whole commentary, bookmark it so I can refer back to it when I process my files. Many thanks for taking the time to respond.

Reply

Bookmarks
  • Submit Thread to Facebook Facebook
  • Submit Thread to Twitter Twitter
  • Submit Thread to Digg Digg
Tags - Make this thread easier to find by adding keywords to it!
camera, dots, exposures, foreground, image, mode, noise, photography, photoshop, stack, star, trials
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
Thematic Post & Discuss Your Astro Tracer Star Trails & Star Shots wtlwdwgn Mini-Challenges, Games, and Photo Stories 48 02-22-2021 01:47 PM
Raw Therapee Pixel shift work flow GUB Digital Processing, Software, and Printing 11 07-15-2019 02:17 PM
Abstract Trials By Loch & Castle Kerrowdown Post Your Photos! 27 09-15-2018 01:28 PM
Capture in PEF and convert to DNG, or capture in DNG? pete_pf Photographic Technique 9 05-28-2011 11:24 PM
B&W foggy PP trials kurt Post Your Photos! 16 02-15-2009 12:13 PM



All times are GMT -7. The time now is 01:46 AM. | See also: NikonForums.com, CanonForums.com part of our network of photo forums!
  • Red (Default)
  • Green
  • Gray
  • Dark
  • Dark Yellow
  • Dark Blue
  • Old Red
  • Old Green
  • Old Gray
  • Dial-Up Style
Hello! It's great to see you back on the forum! Have you considered joining the community?
register
Creating a FREE ACCOUNT takes under a minute, removes ads, and lets you post! [Dismiss]
Top