Best LPR filter for imaging around here ?

[RobertM]

Hi All,

Has anyone been able to test the results of different brands of LPR filters ?  I'm wondering what would be the best and which size to get (yes it does matter!).  I currently don't use an LPR filter, which although giving me the shortest exposure, does mean that there is a lot of fog which needs to be 'tweeked out' with post processing.

It seems the best out there are the Astronomik and Hutech IDAS.  The Astronomik is by far the cheapest so seems the obvious choice but if you only ever need one then why not get the best, which ever that is.

Reviews seems to come out in favour of the Astronomik for extreme LP (Mag 3.5-4.5) and IDAS for moderate to light (4.5-5.5).  Orpington seems to be, at least where I live, around the border between the two.

Any comments on either of these or others most appreciated.

Thanks Robert

[Rocket Pooch]

Hi Robert,

The current best filter is the Hutech IDAS, however, it will let thorugh some light pollution, baader uhc-s is good (i have this) and so is the astronomik CLS (got this as well), the astronomic is very severe and you will find it hard to do lrgb because the R will be just about gone, have a look on google for the spectral responce for each and you will see the graphs an the emission lines of light pollution.

The Baader 2" is about £70 http://www.company7.com/baader/options/uhc-s.html
The Astronomik CLS is £50 1.25" http://www.astronomik.com/english/eng_cls.html
The Hutec 1.25" I think is about £100?  http://www.sciencecenter.net/hutech/idas/lps.htm have a look at the colour cart here it kinda explains the lrgb issue.

Chris

[RobertM]

Thanks Chris.  I've had a look and it really is the lesser of the evils here.  If I go for the IDAS then I'll have to double the exposure time but it seems to be the best to use with the LRGB Type 2's.  Also looking at the NoNaD filter which looks interesting and cheaper (1.25" £40, 2" £47) though only for LPS it does seem better balanced (bit light on green) :

http://www.nezumi.demon.co.uk/nonad/nonad.htm

The alternative is to get the S/N up and post process which is more work.

Still investigating...

[Rocket Pooch]

Hi,

I have one of them as well if you want to try it?  http://www.baader-planetarium.de/download/filter_uebersicht_en.pdf

Chris

[RobertM]

Yes please !

When are you around next ?  I have a terrible cold and sort throat at the moment so it would be best left till the W/E at the earliest.

Have finished regreasing the G11 and it seems a lot smoother, I just hope that translates into a a smoother PE curve.  Next step polar alignment then consistent 10-20 minute guided images with luck !

[Mike]

One you haven't mentioned is the Lumicon LP filter (known as the Deep Sky Filter) that I use. It blocks all high & low pressure mercury and sodium vapor lamp light, neon lights and airglow, while transmitting the rest of the visible spectrum. The filter is tested and inscribed with the percentage of light transmittance of the H-Alpha, H-Beta and OIII emission lines. I've found it to be very good.

Take a look at this - http://www.astronomyratings.com/product/products.php?cid=59

[Ian]

at what point does LP rejection become narrowband imaging?

I've got an astronomics UHC, but that really is a more of a combined O-III and H-beta narrowband filter rather purely LP rejection...

[Mike]

I guess anything that cuts out the LP can be classed as an LP filter. The true LP filters strictly ONLY cut out the part of thr spectrum that the usual culprits radiate in.

I guess it's a bit like the "When does a cup become a mug?" argument that has been going on in The Metro recently.

[JohnP]

I do all my imaging in Bromley with Astronomik CLS - I find it very good at suppressing the local light pollution but still letting through reasonable signal such that exposure times don't get too long - If you check out my webpage I think most of the newer images (last few years) are with the CLS - I also use it with my DSLR when imaging from Bromley - I have both the 1 1/4 inch & 2-inch filters. It does tend to add a colour cast but this is easily corrected at the processing stage. I would highly recommend this.

Cheers,  John

[Rocket Pooch]

Hi,

Its not adding a colour cast its removing a colour.

Chris

[JohnP]

Chris, What I meant is that it upsets the colour balance.... sorry if my terminology was not correct...  Anyway, isn't the result of removing a colour to add a colour cast.....???? All I know is that it's easy to rebalance the colours at the processing stage of the RGB so it does let through enough of each colour - I guess if you wanted to be technically correct you could always expose the specific colour for a longer time to compensate for the effect of the filter but at my level/ skill of imaging I don't think this is necessary yet.

John

[RobertM]

Excellent, thanks for all your views it's exactly what I was looking for.  I've analysed the bandpass charts of all these and this is my conclusion with use with the Astronomik LRGB type IIc set :

Baader Neodymium filter: Gives very good throughput of Blue and Red (90%+) with extensive cutoff in the Green (est 50%).  Complete cutoff of LP Sodium and partial/good HP Sodium/Mercury vapour.

IDAS: Heavy cutoff of RGB but it's even which is probably why it gives a better colour balance.  Complete cutoff of LP/HP Sodium and very good Mercury vapour.

Astronomik CLS: Very similar to the IDAS but with more Green and less Blue and Red.  Exceptional cutoff of LP/HP Sodium and Mercury vapour.

Baader UHC: Not very good Green or Blue transmission (peak is right in between the type IIc Green/Blue peaks) but good red transmission.  Even more severe LP cutoff than the CLS.

Lumicon Deep Sky: This might as well been branded Astronomik CLS as it has exactly the same transmission curve give or take a knats.

For a one off filter I think it would be the Baader Neodymium filter for covering all types of LP very well, is there any particular reason why it's not marketed as a CCD filter ?

This seems obvious to me...  why not...

    Use the blue filter on it's own and cut down the exposure time
    Use the Astronomik CLS (or Lumicon) with the Green.
    Use the Baader Neodymium filter for the Red (cut down the exposure time) and also for the Luminance.

The best of all worlds or is this not accepted practice (could be I'm missing something fundamental here!)...



Yes I know it's tacky and cobbled together ...

Thanks
Robert

[JohnP]

Robert - That's a very thorough post & some good work. I guess the only issue with imaging the way that you propose is to ensure that you get the balance of exposures correct for all the different filter combinations you are talking about. Also - you may find that you will be spending half the night taking dark frames as well (due to different exposure lengths). I must admit my imaging is not that critical - when I do colour I find it best to do LRGB - i.e. spend as much time as I can collecting the L data using my CLS filter (if shooting from Bromley) - I then normally just grab a handful (approx 1/4 or 1/3) of R,G,B (also with the CLS in place - because that's the way I have my filter wheel setup) - I take everything with the same length subs so I only have to complete the imaging sessions using one set of darks. Any colours casts etc. due to the CLS letting through less of certain colours I just correct in PS. It seems to work OK.

John

[Ian]

I wonder if you're going to use a stack of different LPR filters together with colour ones whether or not it's easier to just go straight to single bandpass narrowband filters...

I think Chris and Paul have both had success using Si, Ha and OIII filters for the RGB exposures.

[RobertM]

Yes, Narrow band is one way forward but thats generally used by people who have an exceptional amount of LP and I'm not sure that I'm in that category.  Narrow band = less photons = longer exposures, but maybe I'm making too much of this...

Dark frames - have found no need to use those yet, I'm just using Bias and Flats.  The H9 doesn't seem to generate much internal noise though I do have some light leak that I'm in the process of sorting out.