Can a CLS filter negate the need for dark skies?

[OneCosmos]

Now, even before reading this I know you are all immediately answering the question in the negative - and, in truth so am I, but.... read on.

My observatory is in Nundah, an inner city suburb for sure and as light polluted as any other in Brisbane. Actually, the milky way is easily visible and the naked eye views aren't anywhere near as bad as from my home town in England which has a population of only about 30,000!

The observatory is really taking shape now and I am having enormous success making imaging as effortless as possible - which means automation. My view is that anything that is a chore will act as a deterrent against bothering. I am happy to report it is now about as effortless as it can ever be.

I still do, of course have to deal with the light polluted skies. I tried imaging without a CLS filter and results were rubbish. I acquired a CLS and the transformation was astounding.

I won't elaborate too much here in words, I'll let the pictures do the talking. Take a look at the screen grab from PixInsight attached. Each of these images are simply RAW FIT files. They are single 10 minute subs. The one on the left is M42 taken without a CLS filter and under it NGC253, also without a filter. Hopeless.

Next comes Eta Carina with the CLS filter on a decent night, later in the evening with no moon. The next one, for comparison is Eta Carina earlier in the evening (more civil light pollution) and a really bright moon high up in the West. Finally, I have included M20 taken from Astrofest - a pretty dark sky by most people's standards.

I have tried to make sure that all of these subs can be compared the same so the left sliders are moved to the extreme left edge and the middle slider is moved to the right to the same degree - you can see this in the screen transfer function also shown in the screen capture.

I am finding it really hard to accept the empirical evidence that Nundah with as CLS is better than a dark site without, so someone, please enlighten me as to what I'm missing.

The rationale is that if the vast majority of the light pollution is Sodium or Mercury then it really doesn't exist as far as the camera is concerned, leaving little else.

I have also included a processed version of the Eta Carina, which is comprised of just 12 x 10 min subs, flats, darks and bias frames subtracted. All processed exclusively in PI. Poor processing aside (I need to get better at that), the result looks as good to me as anything I might get from Astroest!

Comments welcome.

Chris

[glend]

Suggest you find out where Brisbane City Council is going with street lights. I stayed in a short stay rental that backed onto the busway near the M3, and the entire backyard was orange from the sodium vapour lights. While in theory it is filterable by a CLS, you'd be shooting blind. If, like many councils, they are moving to broad spectrum LEDs to save money then your CLS is going to be less useful as time goes by. Narrowband imaging is where most city slickers are headed, but you will need a good mono camera and filter set, and be ok with shooting blind. Nothng beats a dark site.

[rustigsmed]

the CLS increases the signal to noise ratio vs not using it at the same location. it almost acts like a (mid range) narrowband filter for all the important wavelengths.

at a dark site using the CLS will also increase the signal to noise ratio but by a much lesser difference.

lesser filters (wider and/or with less transmission %) may not to as good a job as the cls.

it is really hard to compare your images, so many variables still remain esp things like the elevation when shot, moon, the dimness of the target, perhaps even background illumination from a field of laptops.

but I agree the cls does a great job - not such a great job on star colour balance though ... as not using it. from home I went from 70second exposure max to 3 minutes before background sky glow started causing havoc (when using my dslr).

[OneCosmos]

Absolutely agree with everything said here and all bets are definitely off for the future as lighting policy is likely to change. My post wasn't really asking is it better to use a CLS in an urban sky than not - because the answer to that is obvious it was more noting the empirical evidence that, at the moment at least, it would appear that images taken from home with the filter are just as good as one taken from Astrofest without the filter?

Chris

[OneCosmos]

Agreed, and yes the M20 at AF was from a different location to the others but my point was that AF should have been massively better but didn't seem to be and in fact what shocked me most was that the ones from Nundah seemed to be better! My post was really one of incredulity :)

All the shots from Nundah were taken from the exact same location of course and the two Eta Carina's were there as a comparison of results with and without the moon and other early evening civil illuminations.

[rustigsmed]

maybe give m20 a shot from home when its at similar conditions. it is a lot fainter than m42 and eta and that could be potentially make the difference look larger than it infact is.. or maybe not!

[OneCosmos]

May have missed the boat for M20 this year now, but I did consider that some of the difference could indeed be the magnitude of the subjects, but all of them are bright nebulae.

I should also add, if it wasn't obvious that I am using a one shot colour camera. I have long coveted a mono instead, but the results with the CLS make me wonder because a mono is great for narrowband but useless for RGB in the city and you can't have RGB through the CLS and narrowband not without masses of effort - a definite disincentive to bother!

[RickS]

Chris,

Congrats on your obs and automation! I'm looking forward to seeing your LRGB images with the CLS filter. I've done a little bit of LRGB from home but without any sort of LP filter and the results have been very ordinary.

Cheers,
Rick.

[OneCosmos]

Thanks, although I don't really have LRGB as I still have a OSC camera :). The Eta Carina attached to my first post in this thread is the first proper image from the observatory in my garden.

[RobF]

Things will get a lot harder processing-wise on fainter objects too of course with the stronger background skyglow, even with a filter. Apart from signal to noise dropping, you're much more likely to be fighting gradients across the field that makes colour balance a lot harder too.

Agree there are still lots of objects you can have a lot of fun with from backyard suburbia however.

[alocky]

As already mentioned, getting star colour right becomes very difficult, particularly yellow stars. I gave up using the cls filter for that reason, but I might revisit it and start using seperate exposures for bright stars and use the cls for faint detail. Interested to see how you go.
Cheers
Andrew.

[OneCosmos]

Actually, I have just remembered an important point that will definitely be playing a part here. At some point during setting up the equipment in the observatory I must have inadvertently changed the gain and offset on the QHY12 and all the raw images were horribly clipped.

So I measured the bias frames properly and did some experimenting to set a value for the gain that produces what I thought were pleasing results. I suspect though that the settings I am now using are much more appropriate than whatever I had set at Astrofest. It is possible that for a couple years, since owning the camera it has been under performing until now.

It will be interesting to do this comparison again when I do have some data once more from a dark location - which will probably be Astrofest.

Chris