A few more beginner shots... how bad is my light pollution?

naskies (Dave) · #1 27-09-2011, 11:35 PM

I had a fantastic evening last night: setting up my gear while watching the beautiful sunset, watching the Milky Way set (what do you call the half that has Scorpius it?), watching Jupiter rise, watching the other half of the Milky Way rise, finally watching the stars wink out under the beautiful sunrise, and then off to bed with the aircon cranked up for a great day of sleep

Here are a few photos that I took during the evening. I'm still aiming for quantity rather than quality at this stage (each photo was about 10 - 40 subs X 30 - 180 secs each) while everything is still new and novel.

I have a question for the experienced digital SLR shooters here... continued in the next post.

naskies (Dave) · #2 27-09-2011, 11:41 PM

Question: how bad is the level of light pollution I'm seeing here?

For example, I've attached three comparison photos showing a straight-out-of-camera sub against my stacked/stretched version. Are other people seeing anywhere near this level of background light?

I have a lot of trouble pulling out the details and colours of any non-ridiculously-bright nebulae, e.g. the horse head in the attached sub. I can see the horse head in the full resolution file if I squint and do a bit of stretching... but it's so close to the background colour that I doubt I can actually image it "properly".

For the record, I'm using a plain (unmodded) Canon 5DmkII, William Optics FLT-132 refractor on an NEQ-6 mount, and auto guiding with an Orion Awesome Autoguider package. I can easily see the outline of the Milky Way as it stretches from the Southern Cross past Scorpius/Sagittarius and beyond with the naked eye, and I can usually see the SMC/LMC with averted vision (and occasionally when looking directly at it).

Is it primarily due to my inexperience and/or lack of subs, or do I need darker skies?

Thanks for any input!

Osirisra (Ken) · #3 28-09-2011, 01:25 AM

Nice shots! If you can only just see the LMC/SMC these are impressive.

Yup, looks like a bit of light pollution in a few of the shots. Try taking shorter subs but more of them and try objects close to zenith. Filters might help too but nothing compares to dark skies if ya can get them.

naskies (Dave) · #4 28-09-2011, 05:45 PM

Thanks Ken!

In case any other beginners are interested, I came across this very interesting read about measuring the level of light pollution using a dSLR histogram, and how to optimise the number of subs vs exposure time per sub vs total exposure time.

http://www.pbase.com/samirkharusi/image/37608572

According to this method, it takes around a minute for my background sky at zenith to reach a mid-level histogram with f/4 at ISO 800... i.e. a fair bit of light pollution compared to the 12 minute+ figure for dark sites.

I might try imaging with hundreds of 30 sec subs at ISO 1600 to see how that goes.

Osirisra (Ken) · #5 29-09-2011, 12:54 PM

Nce find on that site, will have to check that out.

Try dropping the iso down to around 800 and shoot longer exposure if needed. 1600 is a bit high and will pic up the pollution very easily not to mention noise.

It take a lot of trial and error and each spot you point the scope will be different.

I took this sngle frame test shot last night in my polluted back yard. 2min exp at 6400 and was blown away how well it came out for a test shot. Taken with a 8" Newt, MPCC and 600D on EQ6 pro. Still haven't finished the proper stack of 15x1min subs @800 ISO yet. Would have taken many more but the neb dropped behind my shed :/

http://www.facebook.com/photo.php?fb...type=1&theater

NorthernLight (Max) · #6 01-10-2011, 10:50 PM

Hi Dave,

Great shots for such short exposures!
looks like we have about the same level of light pollution.
Since I moved to Auckland it got a lot better with light pollution for me. I can see the LMC and SMC in good nights with direct vision and the Milky Way almost always unless it´s too far down in Auckers skyglow (which is cute compared to Melbourne´s

).
However, I am still using the Astronomik CLS clip-in filter for my EOS and with it I can easily expose 5-10min (ISO 800 - depending on direction, elevation) whilst maintaining a histogram with the skyfog peak within the first third (left of center).
Keep the ISO down as it doesn´t create more image info (photon´s) it just amplifies the few you collect over time as well as the noise.
What focal lenght is your refractor? Your Jupiter looks awesome, I thought planets taken with SLR on prime wouldn´t be much of sight unless one deploys 5m focal lenght.

Cheers

naskies (Dave) · #7 05-10-2011, 04:14 PM

Thanks - I'll have to try more imaging time plus a light pollution filter.

My WO FLT-132 is 924 mm focal length... I must have lucked out with the seeing just for that shot, because the videos I took with a 5x PowerMate later that night were pretty average (worse than other times I've tried).

bmitchell82 (Brendan) · #8 05-10-2011, 04:38 PM

Just for a little bit of info the reason why you havn't caught the horse head isn't because it isn't "bright" (well it isn't really bright

) but to a DSLR you have a UV IR filter that cuts off the light coming out of Ha regions by a fair way so if you wanted to capture this target irrispective of dark skies or light polluted skies you will need to remove the filter or move to a Astro CCD that is sensitive to this light band.

This is a photo of the HH taken from the middle of South perth (you cannot see the LMC/SMC even if you try) with a Canon 350D with a filter modification on a skywatcher 8" Dobsonian. Just so you know it is possible!

http://s700.photobucket.com/albums/w...lames_web4.jpg

naskies (Dave) · #9 05-10-2011, 11:01 PM

Brendan, thanks - that's good to know

Nice work!

I've been very tempted to go the full hog with <= 6nm narrowband imaging and CCDs, since traveling to dark sites aren't a practical option for me (too much study, too much hobbies, not enough time, etc).

gregbradley · #10 06-10-2011, 07:45 PM

Good advice has been given here. Lower the ISO, expose to 1/3rd histogram, use a light pollution filter, Hutech one comes up as being most popular. Narrowband and CCD is another option. Lots of shorter exposures stacked. 30 seconds sounds too short though.

Allen (Allan?) Gould shoots in Brisbane light pollution and gets great results. I suggest you check out his site and setup as he constantly surprises me with his results from where he shoots.

Greg.

tempestwizz (Brian) · #11 08-10-2011, 11:18 AM

I would strongly recommend the use of a UHC filter. I use an Astronomik. Basically I can get twice the (signal) exposure time before the pollution (noise) reaches the same level.
UHC for me works best on nebulae. I find that if shooting globs it can make getting coloured stars difficult. But generally I rarely take it off shooting from my backyard in Canberra's south.
HTH
Brian

naskies (Dave) · #12 10-10-2011, 12:14 AM

Thanks for the tips, Greg & Brian... I'll look into it.