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Dealing with Light Pollution

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#1 Deetrix

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Posted 02 December 2013 - 11:49 AM

I set everything up last week with intentions of testing out my new auto-guider and focal reducer. It was one of those times that a bunch of stuff went wrong. I kept getting a later start than I wanted and I even forgot to charge the DSLR battery one night. The biggest take away was that with the new focal reducer, I'll be limited to about 30 sec exposures due to light pollution. That's kinda disappointing considering the recent purchase of an auto guider. I guess a light pollution filter is in my future. Ever have one of those sessions where everything goes wrong?

#2 Seanem44

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Posted 02 December 2013 - 11:58 AM

Keep checking classified for filters. I got mine from there. It's an Astronomic CLS. I can go for about 4-5 minutes now in my white zone.

I feel you though, it becomes a comedy of errors. I've had many of those nights.

#3 Deetrix

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Posted 02 December 2013 - 01:46 PM

Thanks, Sean. I'll keep my eyes peeled.

#4 Tom and Beth

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Posted 02 December 2013 - 02:28 PM

Yep. The equipment gremlin has taken a few bites out of many. They say that builds perseverance and character. :foreheadslap:

#5 spectre

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Posted 03 December 2013 - 07:46 PM

I also have a CLS filter and it definitely cuts the LP, but.... and I don't mean to hijack this thread. Sean, how do you deal with the blue cast in your images?
Thanks
John

#6 Seanem44

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Posted 03 December 2013 - 08:22 PM

DSS does a good job of correcting the white balance. Good, not great. But there is an option in it to autocorrect.

#7 jgraham

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Posted 03 December 2013 - 09:05 PM

Oh yeah, been there, done that. When things go wrong I at least try to come away having learned something new (or not to do).

I use either an IDAS LPS2 or an Orion Imaging Skyglow Filter with mine (rec zone skies). However, here is a trick that you might try... After the sky is fully dark (or as dark as it i going to get) take a image of a fairly blank section of sky in the general direction that you're going to be observing. I usually take one 60 second exposure at ISO 1600. Save ths image to you camera's memory card. Now use this image to set a custom white balance. Voila! No more light pollution. I do this all the time even ith my light pollution fllters since my camera is modified. I even did this once while using a clear 'filter' to take photometric data and it worked very well. I'm almost tempted to stop using my light pollution filters, but since I have them anyway...

#8 srosenfraz

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Posted 03 December 2013 - 09:35 PM

The biggest take away was that with the new focal reducer, I'll be limited to about 30 sec exposures due to light pollution. That's kinda disappointing considering the recent purchase of an auto guider.



You probably should try cutting down your ISO - that'll allow you longer subs. You'll find fairly small differences between doing 240 x 30 second subs (2 hours) at ISO 1600 versus 30 x 4 minute subs (2 hours) at ISO 200. The advantage is you'll have 30 subs to deal with versus 240.

#9 ScenicCityPhoto

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Posted 04 December 2013 - 12:58 AM

Oh yeah, been there, done that. When things go wrong I at least try to come away having learned something new (or not to do).

I use either an IDAS LPS2 or an Orion Imaging Skyglow Filter with mine (rec zone skies). However, here is a trick that you might try... After the sky is fully dark (or as dark as it i going to get) take a image of a fairly blank section of sky in the general direction that you're going to be observing. I usually take one 60 second exposure at ISO 1600. Save ths image to you camera's memory card. Now use this image to set a custom white balance. Voila! No more light pollution. I do this all the time even ith my light pollution fllters since my camera is modified. I even did this once while using a clear 'filter' to take photometric data and it worked very well. I'm almost tempted to stop using my light pollution filters, but since I have them anyway...


Professional photographer for years and it's never occurred to me to do that for astrophotography although I do that on every real estate shoot I do. LoL. Can't wait to try that in my light polluted area.

Thanks for the tip!

#10 spectre

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Posted 04 December 2013 - 05:45 AM

Thanks, Sean, I'll try that. John, I was inder the impression that a CWB does not affect RAW capture. Am I misunderstanding the process?
Thanks

#11 Deetrix

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Posted 04 December 2013 - 11:58 AM

I changed the title of this discussion to reflect where its going. If anyone applies the custom WB settings, I'd love to see your reslut.

#12 pfile

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Posted 04 December 2013 - 03:12 PM

custom white balance is a quick and dirty way to get there, but the sad truth is that all astrophotographs will need proper background neutralization and color correction in postprocessing.

if you've been doing it long enough you won't bother with a custom WB anymore, since the above steps replace the need for a custom WB.

rob

#13 jgraham

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Posted 04 December 2013 - 05:44 PM

Yep, even with a custom white balance I still use the automatic color balance function in Nebulosity to fine tune it. This usually works very well. (I'm red-green color blind, so getting the color balance right is always a challenge for me.)

With Raw capture you kinda get the best of both worlds. The custom white balance does not effect the RAW data, but the white balance data is stored in the file header. The image that is displayed in real-time by BYEOS, Digital Photo Professional, or your camera's view screen will have the white balance applied at it looks pretty good, but you still have the source RAW data. You can applied the custom white balance to your source images by simply using Digital Photo Professional to convert your RAW images to TIFFs. I find it most useful when I'm using my camera for real-time camera-assisted observing.

Data corrections can only do so much, which is why I still use my light pollution filters most of the time, but using the sky itself to set the custom white balance is an interesting option. It also offers an option for areas that are seeing more LED lighting which cannot be effectively filtered with the current light pollution filters.

Neat stuff.

#14 Falcon-

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Posted 04 December 2013 - 06:03 PM

Thanks, Sean, I'll try that. John, I was inder the impression that a CWB does not affect RAW capture. Am I misunderstanding the process?
Thanks


This depends on the software. Some software will accept camera white balance, some software totally ignores it, some software gives you the option of using the camera's white balance or not.

Nebulosity for example does NOT use the camera's white balance (it has a preference with a couple of "typical" white balance settings to select from).

With PixInsight I have it set to not use any white balance (not camera, not auto at import) and just do the white balance after stacking is done. I have had more consistent results that way. Of course I also only have a medium level of light pollution here, not sure how much of a difference that makes.

#15 PongosDad

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Posted 04 December 2013 - 06:27 PM

with intentions of testing out my new auto-guider


Paul,
Funny that I'm in such a similar position. Finally set up my auto-guider, and realized that a filter would be necessary to shoot from my back yard - in a yellow zone up in the mountains west of Denver with street lights nearby.
Decided on the Baader Semi-APO filter as my other concern was violet halos from my achro.
Here's a before and after during first light with the filter. They are un-altered 4 minute subs at ISO 800. Needless to say, I was pleased.
One thing I have noticed since using the filter is dealing with all the blue in post processing. Which makes me curious....

DSS does a good job of correcting the white balance. Good, not great. But there is an option in it to autocorrect.

Sean,
Could you point me to where that option is in DSS?

Attached Files



#16 Tonk

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Posted 04 December 2013 - 07:26 PM

I was inder the impression that a CWB does not affect RAW capture


Correct. However the setting/custom WB data is in the exif data and downstream software (such as canons own) may use this setting during the debayering step when synthesisting the final colours.

On the other hand astro software such as Images Plus/ DSS can be set to ignore the camera WB setting. In Images Plus you can also set an entirely different WB as to that set on the camera - which is great if you forgot to set your camera.

Personally I process my initial colour balance using gradientXterminator - it has a colour balancing option that works very well. After that I can fine tune in photoshop. At no time do I use the camera WB setting - its irrelevant for my workflow.

#17 pfile

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Posted 04 December 2013 - 11:33 PM

It also offers an option for areas that are seeing more LED lighting which cannot be effectively filtered with the current light pollution filters.


man, i know. these new LEDs are going to be our undoing. there are already several of these on the main street near my house. they are super bright and super white.

rob

#18 Samir Kharusi

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Posted 05 December 2013 - 12:07 AM

You have the "correct" color of sky for a dark site. I suspect there may be a lot of misunderstandings in the above thread. Let us assume that the "correct" color rendering of an astrophoto is what human vision would see using photopic (bright light) vision. This would make the color of the background sky toffee, like in your example above (for proof click here) but if there is a bright Moon (basically reflected sunlight but dimmer) then the sky will come out like a daytime blue. I have yet to come across anyone that likes a toffee sky in his astrophotos. So everyone fiddles the background sky color. These days most people make it grey but use the fancy term "neutralizing" the background. But obviously if you have made toffee to equal grey, then all your stars will be far too blue since you have reduced red by quite a large amount! So then you reset the star colors to white by using an eyedropper tool or pretending to use Custom white balance, etc. Basically do note: you need a white balance for the background sky (very dim areas in your image) that removes a lot of Red to get rid of a toffee sky, but you do not want to do the same for the rest of the image. In brief, astro images have very bizarre White Balances that are anything but "balanced". When the camera is modded then the "balances" are even more skewed. And then we increase Saturation to the max we can tolerate in chromatic noise... Using Custom White Balance on a white sheet of white paper in daylight yields a reasonable starting point, but there is no way you will stop there unless you want to end up with a toffee sky. After you have made your sky grey by "neutralizing" it, a simple end step is to white balance on a star halo for the mid-tones and bright areas. You can then yank up your saturation to absurd levels yet still retain whitish stars. Of course, this has only a very remote connection to human photopic vision... An example of absurd levels of color Saturation after fixing the White Balance on the mid-galaxy grey patches, and then toning the background sky to a greenish blue using a modded camera:
Posted Image

and an example that is closer to human vision using an unmodded camera but increased Saturation anyway (increasing Saturation brings out the child in most of us):
Posted Image

and finally here is a frame used in stacking the latter image, minimal manipulation using Daylight White Balance in an unmodded camera:
Posted Image

Advice to beginners: do not worry too much about White Balance. There is no right and wrong way. The "correct" balance (= human photopic vision) does not yield pretty astro images. You end up with the last one above :tonofbricks: A good start is to set Custom White Balance on a white sheet of paper in daylight, but this is no more than a rough starting point (hence no point in yearning for a photographic grey card). However much manipulation we impose in subsequent processing we tend to want to end up with whitish stars, but I preferred golden ones in my second image above :roflmao: This was taken in 2004 when we were all still at the toddler stages of learning to walk in DSLR astrophotography...

PS to the OP on light pollution. Please click here. For longer subs just lower your ISO, but preferably not lower than ISO 200 otherwise you may be increasing the Read Noise too much (depends on actual camera model).

#19 Wmacky

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Posted 05 December 2013 - 09:25 AM

Yep, even with a custom white balance I still use the automatic color balance function in Nebulosity to fine tune it. This usually works very well. (I'm red-green color blind, so getting the color balance right is always a challenge for me.)

With Raw capture you kinda get the best of both worlds. The custom white balance does not effect the RAW data, but the white balance data is stored in the file header. The image that is displayed in real-time by BYEOS, Digital Photo Professional, or your camera's view screen will have the white balance applied at it looks pretty good, but you still have the source RAW data. You can applied the custom white balance to your source images by simply using Digital Photo Professional to convert your RAW images to TIFFs. I find it most useful when I'm using my camera for real-time camera-assisted observing.

Data corrections can only do so much, which is why I still use my light pollution filters most of the time, but using the sky itself to set the custom white balance is an interesting option. It also offers an option for areas that are seeing more LED lighting which cannot be effectively filtered with the current light pollution filters.

Neat stuff.


I thought I was the only one on the forum with RED-Green color blindness, a condition that left me unsure of this hooby. I don't want to Hijack the thread, So please look for a new thread.

#20 PongosDad

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Posted 05 December 2013 - 10:43 AM

Samir,
Thanks for taking the time to explain.
It sure is nice tackling these learning curves with help along the way!
Jim






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