May 27 2026 APP 2.0.0-beta45 has been released !
Fully Multi-Threaded LNC, many improvements for the registration engine, platform upgrade, and further tuning of internal memory consumption and memory release back to OS.
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Update on the 2.0.0 release & the full manual
We are getting close to the 2.0.0 stable release and the full manual. The manual will soon become available on the website and also in PDF format. Both versions will be identical and once released, will start to follow the APP release cycle and thus will stay up-to-date to the latest APP version.
Once 2.0.0 is released, the price for APP will increase. Owner's license holders will not need to pay an upgrade fee to use 2.0.0, neither do Renter's license holders.
Greetings,
I am a trial user for APP, a beginner in that matter. I mostly use APP to stack wide field Milky Way landscapes and the results are always pretty good. Recently I went to photograph in a Bortle 4 sky at zenith high in the mountains, but with light pollution remaining on the horizon due to the French Riviera. To make matters worse, high humidity in the valley made light pollution quite high above zenith. Which is why I tried to use the light pollution removal tool. I had some issues with it regarding noise but at the same I was stunned by its performance. I could not find relevant topic on the forum or in documentation so this is why I seek help here.
To describe briefly my workflow, I load unedited raw and integrate them, then stretch the result a bit and save the result. For some reason the stack will always be overexposed (fig.1) with basic preset, same goes with 30%bg 0,0%base (fig.2), I don't know if it's normal. Most of the time this workflow is sufficient, however with my most recent astro session, I dived into the light pollution removal tool. I followed online tutorials including the one from Mabula (tho the image in the tutorial was amazing and it was hard for me to compare it to my image) but I struggle with noise. I overstretch 30% and increase saturation. Each time I recalculate, I have to change white/black point and gamma, I don't know if it's normal as well. Typically I raise blackpoint and lower whitepoint (for contrast), reduce Gamma to 0,7-0,4 to get a good result (fig.3) and after that I save it.
After APP I usually work with Photoshop, where I noticed the noise level of the dark sky being quite high. I also noticed how good the algorithm is, it is the best light pollution tool I've used, because it manages to get rid of almost all the light pollution and recover low on horizon parts of the Milky Way that were completly drenched in light pollution. It is truly amazing but I don't know if I'm doing something wrong when using it, because noise increases in the dark areas.
This is why I would like to ask some questions : Do I push the algorithm too far in reducing the light pollution background ? Are there a maximum number of re-calculate steps before it could bring artifacts or noise ? Is it normal to expect noise after light pollution removal ? Is it normal to have overexposed images just after the stacking ? And after a re-calculate *? Is there a way to calculate the optimal parameters of BP WP and gamma ? Without using the tool this is the noise I get (fig.4), and with it I have this (fig.5). Lastly, here are the 2 results, without (fig.6) and with light pollution removal (fig.7). Any answers or feedback is very welcome.
* I guess not because in the tutorials the exposure seems to be alright all the times, but I wanted answers regarding that, because having to move the BP and the WP maybe is part of the problem.
I would really appreciate answers and tips regarding this, as the light pollution tool is a very solid argument to acquire the software. I praise the work over this tool and APP in general and I need to be certain about what it does and how to use it properly, I'm sure with the right steps it is almost perfect. I can also attach the tif files if it helps. I also hope the topic is in the right place.
Best regards,
Well I did not expect the attached files to be this big... I hope it's still readable.
Yann
Firstly - let me say you have made a good start with this, and you will only get better with time. My first widefield shot was nowhere near as good as yours.
I would like to offer some general comments, and then try to address some of your questions.
It was not 100% clear to me whether you are applying the light pollution reduction tool to stretched or unstretched data. It might not be clear, but the image stretch, blackpoint, saturation, etc do not change the data until it is saved. The stacked integration stays in its linear (unstretched) state until that save button gets pressed.
In brief, the workflow would be:
- integrate the data
- rgb combine if using a mono camera and filters (for a colour camera skip this step)
- go into the LPR tool and temporarily increase the stretch to show all the light pollution and nebulosity. Don't worry if the image looks bad at this point as the stretch only applies to the viewer - not the data. Place your boxes, recalculate, keep placing boxes and recalculating until you are happy with the gradient. Save the result - remember you are saving the linear unstretched data, but hopefully with the LP gradient removed.
- if needed, consider applying other tools - colour calibration, star reduction, etc
- at this point you can apply the final adjustments. Fine tune the stretch - get the black point where you want - adjust saturation, contrast, etc.
- now save the stretched, adjusted image
It might be helpful to understand that the LPR tool does not make adjustments while you are setting up your boxes and recalculating. You are just building a gradient model that is 'removed' from your image when you hit the save button in the LPR tool.
I'll try to respond to your specific questions.
Do I push the algorithm too far in reducing the light pollution background ? Are there a maximum number of re-calculate steps before it could bring artifacts or noise ?
No - you are just bulding a gradient model which is applied in one step, the number of recalculations has no effect.
Is it normal to expect noise after light pollution removal ?
Absolutely. The LPR tool attempts to remove one type of noise using a modelled gradient. It works very well, but there will always be noise left in the image. One way to minimise this is to take lots (perhaps hundreds) of exposures - preferably with dithering and calibration frames. There are also many tools available to remove noise from your final image. I often use Topaz Denoise for this, although it can be a bit too aggresive for astro work.
Is it normal to have overexposed images just after the stacking ? And after a re-calculate *?
APP has a good range of preset stretches that can get you roughly where you want and then you can fine tune the stretch parameters. As long as the images are not over exposed 'at the camera' then you should be able to find suitable stretch settings that don't overexpose the image. The images you have pasted in are not overexposed (except for the bright patch near the horizon). Light pollution can make images appear overexposed before the light pollution is removed.
Is there a way to calculate the optimal parameters of BP WP and gamma ?
No - these are largely aesthetic decisions that each astrophotographer makes.
regards
JC
Hi John, thank you very much for your very detailled tips and compliments. It is very helpful to have accurate feedback and explanations over APP and the LPR tool. It is also reassuring LPR tool is very trustworthy, now that I know more about it, I'll work my way into getting better results.
Regarding the stretch, I always leave the stretch box ticked, although I'm genuinely not sure it is streching or unstretching when clicking, but given the overexposure of the images on base preset, I would assume it is stretched, although you say "The stacked integration stays in its linear (unstretched) state until that save button gets pressed.", so I can't be sure. I will just say I use as less as possible utilities, I will work with a restricted set of operations and tools, that I mostly know from tutorials.
Thanks again, clear skies
Greetings,
As my trial licence is expiring, I would like to ask a last question regarding gradients : I'm working on a image with a good sky, except for the horizon, where light pollution is noticeable. There is a high contrast between the horizon and the rest of the sky which is dark. When I try to use the LP tool, it seems that it reduce the overall contrast in the image rather than cleaning it, but again I don't know much and I can totally imagine this is me issue. In a way, the gradient is reduced yes, but then it seems to me I cannot make the Milky Way pops from the background without darkening the Milky Way itself. Do people here have experience with strong yet very localized gradients ? Does LP tool solves them well with experience ?
I attached images to illustrate my problem.
Thanks,
Regards,
Yann
You could go a bit further with the LPR tool, but I personally don't mind a bit of extra light near the horizon as it can add contrast to make the horizon 'pop'. The best answer of course it to find a location with less light pollution if that is practical for you.
I don't know how much data you got (more is always better) but there is quite a lot of signal in your image. I took your pasted image and just boosted the stretch and the saturation and got this:
With access to the raw data you should be able to do better than my quick adjustment to your pasted image, but you can see there is a lot of colour in your image. You can even see the Rho Ophiuchi cloud complex in the lower right (near Antares).
If you want to get rid of the blur on the horizon you can use an edit mask with Photoshop or Gimp (which is free). Dylan O'Donnell did an old tutorial on how to do this.
He talks about using Nebulosity (an old astro program) but later he shows the masking technique in Photoshop. I have used this technique with Gimp and it woks pretty well.
JC
Hi John, again thank you very much for your feedback. This image is a stack of 20 subs 15s 6400iso f/2.8, short exposure because I don't have a motorized mount - yet. Tho for a larger stack I also struggled dealing with the gradient and it also "flattened the image".
Indeed this is a good stretch especially from a screenshot of the real photo ! I could give you the real tif file if it does not bother you, just so I can see what could it be in the hands of an experienced user. Also thanks for the tutorial you attached, I will take a loot at it.
Regards,
