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Problems with long exposures and pollution...


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Hi everyone. I finally got a capable mount for my equipment and i got my guider to work with the mount. I can finally take long exposures :D

So finally after months of getting my equipment to be compatible with each other, now i have this problem...

Here is a single 120sec exposure of M109. I know its not the easiest object to work with as a beginner, but this is just an example of what happens to my exposures...

Im using the equipment thats in my signature... Eventhough i have my CLS clip on my camera, the exposures come out bright! I tried a 4min exposure to end the session the other night, and it came out even brighter than this! I could barely distinguish The two or three brightest stars...

Is this normal for a severely light polluted sky like Los Angeles? Or is the clip not doing its job correctly? Should i consider a different filter like SkyGlow?

Any help will be appreciated... Thanks :rolleyes:

post-24989-133877562124_thumb.jpg

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Yea something must be wrong... At first I thought that the telescope was getting dew on the primary, but all the subs look the same from beginning to end of my 2hr-ish sessions...

I'm gonna head over to my local dealer to see if they have a different skyglow filter I can try out :D

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Looks strange as its blue, almost looks like you have the white balance set to indoor. You could try doing some tests with the camera, no filter and a standard lens, that way you should be able to establish at what exposure point the local sky (and strange colored lighting) wipes out your images.

Edited by nightvision
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Well the visible light that the filter lets through is blue.. So what I was thinking is that it's filtering something out, but not the skyglow :rolleyes:

The best exposure I can do that's still a little dark is one minute.

I already ordered the Baader Neodimium as I've seen alot of users like Lukebl get great results with 5min exposures on an unmodded dslr in light polluted skies :D

I'm crossing my fingers now for the first test night either tomorrow night or wednesday...

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I'm "lucky" in still having the old fashioned low pressure sodium lights which give an orange glow which is easily removed as its pretty narrowband...

I have Just been looking at some LA Light Pollution pics pics - some from 100miles away in the Mojave desert... :D

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Being new to all this I can't help on filters etc. But I would perhaps try the following first to try and get to the bottom of the colour cast.

This is presuming you have a wide angle canon lens.

Put camera on mount (no tracking) or tripod without the filter and take a wide angle shot at 120secs, auto white balance in RAW (at a guess f5.6, ISO 800), try and get the same histogram.

Then do the same again with the filter in place, this should give you an idea what the filter is then doing.

If it's still blue, then if at all possible drive out of the city and do the same again. At least from this you'll get and idea if the filter does anything or there is a problem with your camera.

Then do the same with the 80mm, all a bit of a pain but it should help start to narrow down the problem... I think. :)

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Thanks CW and Mike :)

Yea in every frame there is a small hint of M109. The problem is that when I stack my 2 hours of data, the spiral arms of the galaxy BARELY start showing... I think that for two hours of data I should at least get some nice structure. In order to get the arms to start showing, I have to push the levels and curves on CS5 to the limit and then the image just gets noisy and unappealing :p

I'm sure that all this glow is taking alot out of my stacked data :p

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The good thing is that i already have my baader skyglow filter. And I just recovered from a slight leg injury. I should be able to take some test shots tonight finally and post them here for you guys. Hopefully there is a noticeable difference :)

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Well here is an embarrassing 2hr stack of 2min subs like the one above. As you can see, ive pushed the levels and curves in CS5 pretty far. i know that i have alot to learn about processing, but im pretty sure that for 2hrs of data, there should be more here... :p

Cant wait to test the Baader Skyglow on something brighter ...

Thanks for all the help :)

post-24989-133877564438_thumb.jpg

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first, I don't agree with you that you have 'pushed' the data a lot. There was quite a bit more to go. Pushing it more shows a lot of noise that seems to me to be co-related noise. That which you get when all your subs are too short on exposure time. Either way it can be tamed although it is better not to have to do it. I stretched it a bit more, one Levels and one Curves and then applied a fine blur to 'spread' the data a bit as it was posterising horribly. Then Neat Image to reduce the noise and finally another iteration of Curves just to darken the background and hide the noise a bit more. I only cropped it to save space.

There is no substitute for exposure!

Dennis

PS: far more noise showed in this after posting than was visible in either PS or Windows picture viewer.

post-15519-133877564483_thumb.jpg

Edited by roundycat
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Isolating and making a separate enlarged layer of the centre section and working it over with the Dodge tool will start to show up detail.

My effort looks rather like a lop-sided "Bat Signal" but hey,it's worth a try.:)

post-13495-133877564545_thumb.jpg

Edited by Cloudwatcher
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Wow! I have alot of processing to learn! :)

Thanks for all the help guys.

So last night I did the test shots with the baader skyglow and it was even worse! :p

I guess the moon contributed as well, but it was still pretty bad. One thing I did notice was that if I put the iso down to 400 it doesn't come out as bright.

Will I lose data for going down in iso? Will that mean that I have to just add more and more data time to compensate???

(i will post my shots from last night a little later today)

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It is something of a fallacy that higher ISO on a digital camera will work wonders with exposure. How much light there is going down the telescope is what it is all about. Raising the ISO raises the gain in the ADC and that in turn will reduce the bit depth so you will be unable to stretch what data you have quite so much.

There is no short cut - more exposure wins every time.

Dennis

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If it helps, I use ISO 800 and a makeshift dew shield for my 6" Newt. ISO 1600 is way too noisy, and ISO 400 generally too weak. 800 is the best compromise for my 1000D and might be a good starting point for you too.

The dew shield is just a few inches long, and attaches to the end of my Newt via some velcro strips. It's not pretty, but it works!

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