DSO Imaging in Light Polluted Skies - Searching for a magic solution

[craigg]

Hi All,

I live in a moderately light polluted area, and have reached the limits (I think) of what I can image before getting so much orange pollution it obscures faint details.  So far I've found the likes of M42, various galaxies, clusters, etc reasonably OK, but at ISO 800 and exposure times > 2mins I get a lot of pollution.  Trying to image fainter nebulosity (e.g. flame/horsehead) I think i need longer exposures but post-processing seems to be failing to pull out details.  This could be poor processing on my part, but I suspect I either need to spend many hours out in a remote location somewhere, or I was wondering about LPS filters.  Some of them claim to be very effective, but are not cheap.  So, before I put my hand in my pocket, does anyone have any recommendations?  Are there particular filters that are better than others, or are the benefits not worth the cost?

FYI - I have a SW 200P, HEQ5 Pro and Canon EOS600D.

Thanks in advance,

Craig

[nmoushon]

I used a Canon CLS clip in flip. It gives a blue tint to your images but this can be over come by taking new flats, which will also have a blue tint to them instead of being white. Stack them together and they will get rid of the blue and you'll have normal looking images. Depending on how bad your LP really is will depend on how well these work. Dropping the ISO to 400 and with the CLS should get you about 5min exposures at least. Again depends on how bad the LP is. But it will help.

[Ant]

I had a 2" Skywatcher light pollution filter which helped me hugely Very similar in results to the Baadar Moon/Skyglow filter.

Also quite cheap

With the aid of a step ring you can also attach it to the filter thread on a zoom lens to help with some the wide angle shots

Ant

[wxsatuser]

Astronomik CLS clip filter or one of the Hutechs are most likely the best.

Not cheap but they do the business.

They should give about 2 to 3 times longer exposures.

I have moderate LP and can do 10minutes at f4 and ISO 1600, on good nights.

I normally get anywhere between 5 and 10 minutes at ISO 1600, which is my prefered ISO with the 60Da.

[craigg]

Thanks for the prompt responses.  Ant, thanks for the SW filter suggestion, and yes they are a lot cheaper.  I might give one of those a try - I was looking at the IDAS LPS filters, only because I've seen a few images from folks using them recently, but they are 10x the price!!  The same applies to the CLS filters, although I will have to remove it for wide-field using the DSLR lens (it's one of these that sticks into the place where the clip filter goes.

[DaveS]

Another vote for the Astronomik CLS, I find it cuts out more LP than the Hutech and is a bit cheaper.

Else bite the bullet and go CCD and Narrow Band, it's about the only sure-fire way of beating LP, and moonlight too.

[x6gas]

Another vote for the Astronomik CLS, I find it cuts out more LP than the Hutech and is a bit cheaper.

Else bite the bullet and go CCD and Narrow Band, it's about the only sure-fire way of beating LP, and moonlight too.

Yes that would be my suggestion too.  Let's face it, it's a very expensive step unless you can find a bargain on the second hand market (and even then...) but moving to a mono CCD and being able to capture narrowband seemed to quadruple suitable nights for DSO imaging and the results are better too...

[DaveS]

Especially moonlight.

How many clear nights coincide with the moon getting in the way? A disproportionate number I would say, as would most on here. With a DSLR, even using a LP filter you could find your imaging a write-off. NB gives you a weapon to fight the moonlight.

Another thought, H alpha and [sII] could well fall together in the red channel of a DSLR, assuming anything gets past the IR filter. NB will tease these apart, [NII] as well if you can afford 3 nm Astrodons ( )

[Tim]

Hi Craig, I am moving your post to a section where the replies will be more readily found in future.

I have bad LP too. Thought this image might cheer you?

http://stargazerslounge.com/topic/235409-light-pollution-me-too-x-10-at-the-moment/

Cheers

Tim

[craigg]

Thanks for the replies folks.  I agree, cooled CCD & Narrowband is the way I would like to go, but bearing in mind this gear is going to cost me at least as much as the rest of my kit already has, it's likely to be a while (any money saving tips gratefully received!!), I'm looking to see whether I can increase my DSO opportunities (to your point DaveS) at reasonable cost.  So I guess the question comes down to this:- the cost of an Astronomik CLS filter, for example, is around £130, is it worth it or would I be better off putting the money towards a more sophisticated imaging setup?

If FLO will take returns without quibble, then maybe the best option is to try it out and see whether it makes  a significant difference to DSLR imaging as an interim.  

[DaveS]

I think you'll find it makes a huge difference to your DSLR imaging, and I'd hang on to it even if you go the CCD route, as you can use it as a pre-filter in a LRGB array. You can even leave it in the train as it will have little, if any detrimental effect on the NB, and may even clear up some of the out-of-band blocking.

[Robp]

When I imaged with a DSLR I used a Skywatcher light pollution filter and it made a big difference.

My back garden is surrounded by light pollution and without the filter I was limited to 180 second exposures before it became to much.

With the Skywatcher filter I could comfortably achieve 300 second exposures. For a small price I doubled my exposure times.

I don't doubt that the more expensive options would be even better, but you get what you pay for. It does also depend on what type of light pollution you suffer from so if you could try one either from a fellow astronomer, or buy one that you could return if it didn't work very well then that would be advisable.

As a comparison I recently went the CCD and narrowband route and I now image at 900 second exposures. I'm sure I can push it further as the limiting factor now is clear sky time and guiding/wind not light pollution.

[dph1nm]

Just some things to note. If your sub length is limited by sky pollution then you are almost certainly already at the optimum sub length (you are sky noise limited by definition)! Longer subs will not help, even if you could magically do them - only more total exposure.

LP filters cut light from both the sky and the object - ideally they cut the sky more than the object, thus giving the potential of higher signal-to-noise. However, the acid test is whether you get better signal-to-noise in the same sub exposure time as without a filter, not whether you can do longer subs. This may not always be the case - it depends on how much LP you have to start with, the spectrum of the object, and the filter response. So if you use a filter when there is little light pollution you will almost certainly be worse off and end up needing a longer total exposure to get the same result!

NIgelM

[craigg]

Having read a bit and spoken to some people who live in (approximately) the same area, I've dived in and bought a IDAS P2 2" filter which I'll screw into the bottom of the coma corrector.  Just waiting for the skies to clear and I'll report back on the results.  Thanks for all the help so far, much appreciated!