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light pollution and exposure length


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So, have had my astro modded DSLR for about three weeks now and been enjoying the results of hitching it to my SW200. Started learning a bit about post-processing, too, but one capture question puzzles me. 

At ISO 800 with a 4 minute exposure, the signal is almost completely washed out by the light pollution.  Forget the tracking error (huge, I know) as the mount is unguided at the moment, but what would be the point in getting the equipment to guide for, say 3, 4, 5 minutes or more only to see the signal disappear into the pink fuzz ? (First image below of M81, M82.)

I must be missing something really obvious here but what to do, apart from move to a less light polluted place? 

Short subs have yielded results, (example of M31 below) but everywhere I read that longer is better for signal-to-noise ratio but my experience doesn't seem to match this and I wonder why?

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post-38153-0-87572700-1422912375_thumb.j

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do you have a light pollution filter if not i would suggest  that you get one ie; cls clip filter - a sky watcher lp filter - a baader lp filter - or a baader uhc+l filter that will help a heck of a lot.

here is an example with and without these where taken back in august 2012 when i first got my lp skywatcher LP filter and both are 30 second exposures with the same camera and scope and mount.

without LP filter 

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and with the skywatcher LP filter 

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as you cant tell there is a bit of a difference but you can get much better LP filters this is just a 2" skywatcher LP filter and it did its job.

hope this helps

chris

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LP will limit the exposure but a good LP filter will help.

A good LP filter will allow at least 2 > 3 times longer exposures.

You can use the camera histogram to get the correct exposure for your sky conditions.

If you look at the histogram for a typical exposure there is a peak, get that peak over 20% but no more thah 50%.

A good range is somewhere between 20 > 40%, do not expose to the right as you will have a hard time separating the faint stuff from the sky.

Here is a typical exposure from my Canon 60Da, use the luminosity histogram, the white one.......see the peak is at approx 25%.

This was a 10minute exposure with a Canon 200mm L lense at f4.5 ISO 1600, LP filter is the Astronomik CLS CCD clip  filter.

With no filter the sky would have been washed out orange colour with about 3 times shorter exposure.

post-30455-0-27031100-1422938102.jpg

Edited by wxsatuser
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Thanks for the thoughts! Sounds as though one of those Astronomik CLS ones might be on the shopping list soon. Then, of corse, a guide camera. Someone did say it could get expensive!

Just out of interest, how do you get the histogram up on the Canon when in bulb mode? I don't seem to be able to do that although am still getting to grips with all the cameras features.

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Thanks for the thoughts! Sounds as though one of those Astronomik CLS ones might be on the shopping list soon. Then, of corse, a guide camera. Someone did say it could get expensive!

Just out of interest, how do you get the histogram up on the Canon when in bulb mode? I don't seem to be able to do that although am still getting to grips with all the cameras features.

Press the "Display" button, located on the back of the camera on top LHS

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Thanks for the thoughts! Sounds as though one of those Astronomik CLS ones might be on the shopping list soon. Then, of corse, a guide camera. Someone did say it could get expensive!

Just out of interest, how do you get the histogram up on the Canon when in bulb mode? I don't seem to be able to do that although am still getting to grips with all the cameras features.

I have just been down this route   https://watchthisspaceman.wordpress.com/2014/10/20/its-all-about-the-wavelength/

The CLS filter works a treat but then, of course, don't be surprised when everything turns blue in the image - this is a consequence of how filter deals with the light pollution by 'removing' the related wavelengths.  Don't worry, the secret to restoring everything is in Deep Sky Stacker. Make sure the in the 'Stacking' settings, under 'Lights' you check the 'RGB Channels Background Calibration', located bottom left of the tab and everything will return to normal ex light pollution.  Of course it then needs to be stretched. I sometimes find this astroimaging all a bit of a palaver but really it's just what has to be dome to capture these marvelous night sky objects and, once you have it set up, is very easy and efffective. 

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Of course it all depends how good you want your images to be.

If you happy with your results and don't intend on making huge prints you can do as little as possible or as much as you like.

I'm not really an observer so imaging allows me to see things.

I'm not at the moment ever going to print, apart from showing the odd low res image on here.

Who knows in the future, if my imaging gets really good, it may be a different matter. :grin:

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Mike - that is some very interesting stuff...

DSLR imaging is more complicated than one would think.

To get really good images takes a lot of effort and like most photography ends in lots of scrapped images.

Two quotes by Ansel Adams sums it up.

Twelve significant photographs in any one year is a good crop.
and
You don't take a photograph, you make it.
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I done a test 2or 3 weeks ago, 300s @400 and 600s @ 100 a lot more data  in 300s @ 400 ,so I see no point in runing longer for less data.

In your tests, was there a way to measure the noise difference between the shots? You are amplifying the signal differently, but is the signal to noise ratio better in one or the other? More photons fall on the sensor in the longer sub, its just how the camera deals with them which is the issue I think.

Edited by MattJenko
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In your tests, was there a way to measure the noise difference between the shots? You are amplifying the signal differently, but is the signal to noise ratio better in one or the other? More photons fall on the sensor in the longer sub, its just how the camera deals with them which is the issue I think.

 Yes looking at the 2 subs the 600s less data was a lot better look cleaner all round.

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I have 3 LPS filters and the idas d-1 is by far the best and worth the money. It let's in more light than my other 2. You can look at my DSO images to see what I mean (My albums). From where the images were taken I can not see M31, just barely make out polaris and no milkyway. In the summer the Milky way is faint.

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I done a test 2or 3 weeks ago, 300s @400 and 600s @ 100 a lot more data  in 300s @ 400 ,so I see no point in runing longer for less data.

300s @ ISO400 would be about the same as 1200s at ISO100, so I would expect the ISO100 image to have about half of the signal as the ISO400 one.

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300s @ ISO400 would be about the same as 1200s at ISO100, so I would expect the ISO100 image to have about half of the signal as the ISO400 one.

Was looking at the 2 subs , sub1 300s 14,949kb / 600s 13,611kb  but odd 600s @ 100 have split the double in orion . so may be with all my LP this may be the way to go low ISO.

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