I'm losing the battle with Light Pollution

[broadsword]

Hi all,

I've been imaging for a few months now with a kit that I know is far from ideal for imaging (local focal length (C11), unguided, unmodded camera, free 8 bit GIMP s/w), but I'm not expecting miracles and I just want to know I'm getting about as good as I could expect. I've have had pretty good results with relatively short exposure stuff (double stars, open and globualr clusters, small bright nebulae like the Ring and Cat's Eye), but I'm really struggling with the big but faint stuff, ie. galaxies and large nebulae.

The problem seems to be a losing battle with LP, although my semi-rural location shouldn't be the worst for this. Basically, I find that anything less than ISO 1600 and 2 mins exposure (which seems to be the limit for the unguided mount) gives very little data, even with around 20 frames. Therefore, I've been relying on 20x2min at ISO 1600, where I can get some object detail but it ends up lost in the orange glow - the two seem to increase in direct proportion! I use darks but not yet flats (that's the next task) so I also get bad vignetting with the 6.3 FR. See the example below for the Bubble Nebula, exposures as above stacked in DSS with initial RGB alignment, saturation increase and small stretch done in that programme.

By the time I process in GIMP and get vignetting and background colour sorted out the galaxy image becomes horribly noisy, probably due to the LP being of the same Pixel intensity as a lot of the detail...I've seen much better images on the net gained with similar kit to mine, so I'm very keen to see if the answer lies in more skill at processing, whether flats will help a lot, or ultimately whether I need to move house!

Toying with getting an 80mm refractor for these large items, will the problem be as bad?

Any help much appreciated.

[rfdesigner]

Basics:

1. you are downloading RAW files I assume?

2. Never process in 8 bits. I use IRIS (free) and there are others such as DSS which I believe are also free which work in at least 16 bits. Ideally 32bit floating point is best.

(this is your real problem)

3. ISO1600 is really too fast. A good balance is to be had at ISO800 or ISO400. What you are overcomming by going for high ISOs is the camera readout noise. But there is not a whole lot of difference in readout noise between ISO800 and ISO1600. So you get two ISO1600 frames or one ISO800 frame, which means you get 4x 0.7 noise (1600), 2x 0.8 noise (800) or 1x 1.0 noise (400)

4. You need to take "flats"

hope this helps

clear skys

Derek.

[Digz]

Your shot looks promising. My advise would be to start tweaking one thing at a time, but before that keep doing what your doing and research and master flats. These should go a good way to reducing the vignetting that your shot shows. I'm always amazed at how effective flats are.

Once you've done that you can then focus on the next thing.

[StarlightKnight]

Neil

Your main problem is vignetting and has nothing to do with light pollution (light pollution tends to give you a linear gradient across or up the screen dependent on where the light source is)

The darkening in the corners of the photos is a factor of the size of chip to the diameter of the lens or main mirror

The larger the chip for a given diameter of scope the worse the vignetting will get

The solutions

1. Increase the diameter of your scope - usually cost is a major problem !!

2. Reduce the size of chip you use or crop the image - cheap option but not if you want to get detailed images - (the more pixels the better is what you are normally aiming for !)

3. Use Flats - definitely the best option

It takes a bit of practice but is definitely the way to reduce vignetting (and dust bunnies)

There are a number of ways of producing flats - I actually use a camera flash

http://stargazerslounge.com/topic/137682-another-flat-tip/

and it works well for me but I don't know of anyone else who does

Most people seem to use either sky flats, light boxes or electroluminescent screens

hth

Don

[StarlightKnight]

Neil

I hope you don't mind but I'm done a little bit of processing on your image to show you what else you can do with software

I used Photoshop Elements to produce this image - as the initial image was only a jpg the end result was never going to be perfect !!

Don

[broadsword]

Basics:

1. you are downloading RAW files I assume?

2. Never process in 8 bits. I use IRIS (free) and there are others such as DSS which I believe are also free which work in at least 16 bits. Ideally 32bit floating point is best.

(this is your real problem)

3. ISO1600 is really too fast. A good balance is to be had at ISO800 or ISO400. What you are overcomming by going for high ISOs is the camera readout noise. But there is not a whole lot of difference in readout noise between ISO800 and ISO1600. So you get two ISO1600 frames or one ISO800 frame, which means you get 4x 0.7 noise (1600), 2x 0.8 noise (800) or 1x 1.0 noise (400)

4. You need to take "flats"

hope this helps

clear skys

Derek.

Thanks Derek.

To answer your points:

1. Yes, the original frame were all downloaded as RAW files.

2. I used GIMP as I thought (like Photoshop) layer-based software was the way to post-process. Do you think the 8 bit format is reducing quality in a major way?

3. As I said originally, I don't seem to get much image signal versus the background at less than ISO 1600 when limited to 2 mins. Do you think I should push the mount and go for 3 or 4 min at the slower speed?

4. Yes, planning to try flats next ime out, using a plasma sreen and white powerpoint slide indoors!

Thanks,

Neil

[rfdesigner]

I had a play with your jpg you posted.

The bubble is certainly there.. but there's a lot of digital noise from the JPEG image.

I used a sky background removal tool in IRIS that simply makes a local estimate of the localised skyglow. It uses a mask so I could remove the effect of the stars and the slight nebula that was showing. This would all work so much better on uncompressed 16 or 32 bit data.

Derek

[rfdesigner]

Thanks Derek.

To answer your points:

2. I used GIMP as I thought (like Photoshop) layer-based software was the way to post-process. Do you think the 8 bit format is reducing quality in a major way?

certainly. until you've fully compressed your image don't touch 8 bit, and even then I'd stay with 16bit minimum until publishing (like here on SGL). The point is, it puts all your bits into your stars and leaves half a dozen levels left over for the background.. which is where all your signal is, so you end up with a signal to noise ratio of almost nothing.

3. As I said originally, I don't seem to get much image signal versus the background at less than ISO 1600 when limited to 2 mins. Do you think I should push the mount and go for 3 or 4 min at the slower speed?

4. Yes, planning to try flats next ime out, using a plasma sreen and white powerpoint slide indoors!

Thanks,

Neil

The trick is to get the skyglow to swamp your readout noise, that's all, without much skyglow you'll need longer subs. If you want better final SNR then you need to minimise the overall noise and you need more imaging time. As a rule your skyglow on any one sub needs to be something like 10x the square of your readout noise. Readout noise at ISO400 is around 6e on a canon 40/50D, which means you need ~360e skyglow, or about 500ADUs. What it would be on a 1100 I don't know, but it won't be vastly different. If that means longer subs then so be it. Do watch out though longer subs required better polar alignement, if your polar alignment isn't good enough then you can't do long subs.

Right now I'd stick at 1600 and get the processing and flats sorted. Once you've got those reasonable then you can worry about longer subs and optimising your noise.

Derek

[Josh Wilson]

First up, that isn't really LP. It's just vignette and can be fixed with a couple of flats.

On a side note, I'd suggest going around to your neighbours and ask them to just turn of their security lights and/or all other outside lights. Do this for maybe 1-2 blocks around your house and you'll be amazed at the difference it makes.

Josh

[broadsword]

Thanks everyone for some great advice and for the attempts to get more out of the image. Out of interest, below is the best I could do with the image I showed at the start and using Gimp.

I'm going to move to flats next time out, as I said above. However, I'm most surprised overall at Derek's comments that the 8 bit processing is killing the quality. I got a bit lost in the discssion on image compression (I'm still rather new to all this), but in a nutshell, what is me best sequence of events? After stacking in DSS and doing some processing with the DSS tools, what then? Could I get any recommendation for packaging, and a sequecne of what to do?

Also, any way of getting images onto SGL without converting to JPEG? I don't have a website to link to.

Thanks again

[Josh Wilson]

You could make a Flickr account and put the link in your images. You also give the highest res possible.

[Cloudwatcher]

I thought I'd have a go at tracking down a bit more of the 'red stuff' in the background on a cropped separate layer and then recombine........... there's quite a bit to be found but I'm not very good with nebulae,never sure where they finish. Anyhow,heres the result which I think shows up the bubble a little better although it still looks as if it has burst.

[broadsword]

Hi CLoudwatcher, wow, that's really amazing in terms of the extra nebula detail you got there. Could you maybe run me quickly through how you did it?

[Cloudwatcher]

NGC7635Bubblejpeg 1.psd

Hi CLoudwatcher, wow, that's really amazing in terms of the extra nebula detail you got there. Could you maybe run me quickly through how you did it?

It's fairly easy to do but rather difficult to explain........perhaps I should keep history notes.

I'll try to give you a brief rundown from memory with added pictures!

As I recall,the bubble was isolated on the original image and the background tweaked with the levels and exposure tools (pic.1).

Colour Balance,Hue/Saturation was then applied shifted towards the red to bring up the background nebula......all a bit hit and miss (pic.2).

Next,a layer was made of the area to be cropped and with the background isolated,worked on with the Dodge,Burn and sharpen tools.......again,all hit and miss especially as I'm not very good on nebulae (pic.3)

Finally,some blue photo filters were applied (warming and cooling) and the image cropped (pic.4). Phew!

I hope that is of some help but the best advise I can give is to just play around with all the tools available till a result is achieved that you like.

[Cloudwatcher]

NGC7635Bubblejpeg 1.psd

Hi CLoudwatcher, wow, that's really amazing in terms of the extra nebula detail you got there. Could you maybe run me quickly through how you did it?

It's fairly easy to do but rather difficult to explain........perhaps I should keep history notes.

I'll try to give you a brief rundown from memory with added pictures!

As I recall,the bubble was isolated on the original image and the background tweaked with the levels and exposure tools (pic.1).

Colour Balance,Hue/Saturation was then applied shifted towards the red to bring up the background nebula......all a bit hit and miss (pic.2).

Next,a layer was made of the area to be cropped and with the background isolated,worked on with the Dodge,Burn and sharpen tools.......again,all hit and miss especially as I'm not very good on nebulae (pic.3)

Finally,some blue photo filters were applied (warming and cooling) and the image cropped (pic.4). Phew!

I hope that is of some help but the best advise I can give is to just play around with all the tools available till a result is achieved that you like.

NGC7635Bubblejpeg 2.psd

NGC7635Bubblejpeg 3.psd

[Cloudwatcher]

For some reason I can't seem to get pic.1 to load......... will try again.........there you go........sorry about that.

[Cloudwatcher]

Well,what a rotten couple of posts! Blame it on Christmas!

Here's a combi image which I hope is easier to follow.

[broadsword]

Hi CLoudwatcher, this is really good, as it gives me the incentive to focus on learning more about the processing side. Sorry to be thick, though, but is this Photoshop you're describing? As I mentioned earler I'm slummining it on GIMP. It looks like 8bit processing is really limiting things....

[Cloudwatcher]

Ah! Yes,I'm afraid I did use Photoshop but looking at the cropped image you posted,which is rather good,I reckon that you should be able to pull out a bit more detail using GIMP......give it a go.

[broadsword]

I'll give it a go! I'm going to put another stacked image up on the processing board, this time of a galaxy, for anyone to have a go at as I was so impressed the last time...