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.

[Ian]

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. 

I don't think Paul has that problem, but it is a different sort of imaging. I'm not sure that stacking filters as you've suggested is not going to stop more light than a good set of narrowbands. I would expect the total cost to be lower too.

But I'd do what John says, at least to start with. I think the images he posts lend a lot of credibility to his words...

[RobertM]

Yes you're probably right, but I'll try that filter that Chris has kindly offered to let me try first.

On the Starlight message group I saw that someone has suggested using G2V stars to balance - maybe this needs further investigation.  It would certainly seem sense to balance the RGB exposures against a known stellar source from each image and would take a lot of guesswork out of the process.  Maxim allows white balance to ba achieved by clicking on either the background or a stellar source and it does seem to work very well if you choose the right star.   Whether it's a practical proposition identifying that 'right star' is another matter...

[Ian]

I tend to set colour balance against other peoples results... Not rigorous, or scientific but I'm still trying to make pretty pictures rather than photometrically accurate representations.

That'll come, but I have enough fun just getting the faint fuzzy on the ccd and keeping it there...

[JohnP]

Ian - Exactly - I normally do final colour balance by searching for similar images on the web.... John

[Rocket Pooch]

wow what a thread,

i'm with mike on the cup and mug issue here

Robert I would not stack the filters, there not parfocal and you will be messing about with focus all the time

I also agree with john, if the bandwith with the CLS cuts partial wavelength just expose more or balance the weaker signal when processing, but as ian points out you will never get a true lrgb with any of these filters because something goes missing.

personally i just fiddle with the colour balance until it looks nice?

for real LRGB we all have to hike to the south a few miles

your truly, board from canada

but the sushi is excellent

[RobertM]

The stacking filters idea was just just an idea, in any case my Atik is barely wide enough for the filters I have let alone stacking.  I take your point about colour balancing though but there are just so many things to tweek, fiddle and fudge anway.  Wouldn't it be nice if someone out there made RGB filters with just the right cutoffs for an LP site, the Astronomiks do rather take the ideal world approach which is excellent for dark skies.  Now there's a thought ... dark skies ... I see the next DSC is on 7th Dec, I might actually go if I can get my act together.

Chris, if you haven't already then try soft shell crab dipped in Terayaki sauce, now that is yummie ...

[JohnP]

Robert - did you ever get a filter in the end - if so which one? Cheers,  John

[RobertM]

Hi John,

Yes, I got the Baader Neodymium, my choice was based on it letting more target signal through (according to the graphs).  It was cheap (£18 second hand) but seems to be quite good, both the M81 and M101 images were taken with it.  I've yet to see how it mixes with colour channels but it seems to do what it says on the tin

Hope that helps
Robert

[JohnP]

I have one of those as well (bought it ages ago - only 1 1/4 inch though) - I must try it again sometime - I guess this is the 'moon & skyglow' filter..???

John

[RobertM]

Yes, it's the Moon and Skyglow filter.  My SXV-H9 chip is only 2/3" size so the 1.25 filter is not a problem for me.

[JohnP]

SXV-H9 chip is only 2/3"

I would die for a chip that big - the Atik 16ic is 1/4 inch...

Thks for your help.... John

[MarkS]

Are you sure its 2/3" ?

The Starlight Express homepage says about the SXV-H9:
"Large, high resolution 'Exview' CCD chip, with 1,447,680 x 6.45uM pixels in a 9 x 6.7mm array "

In the end the only way you're going to find out which filter is best around here is to do a side-by-side comparison.  And it will vary from area to area depending on what type of streetlighting (or floodlighting!) is used in your neighbourhood. Yes, something like the Baader UHC-S or the Astronomik CLS is very severe in what it removes (and increases the necessary exposrue time) but it might still give a better overall result.

I'd certainly like to give a Neodymium a try versus an Astronomik CLS versus no filter and see what happens.

[RobertM]

Yep, I'm sure it's 2/3" - 8.8 x 6.6 mm are the standard dimensions for that size according to DPreview.

I agree about the side by side comparison for the conditions but that was too expensive an option for me.  Some do manage to work their way through filters to decide what's best but I decided to take an educated guess and it seems ok for now.  I see something that produces better results for where I am then I'll consider it.

I'd be more than happy to loan you my filter Mark but I only have one and never know when I'll need it   Chris did mention something about having one he doesn't use thought so it might be worth a PM.

[Rocket Pooch]

I have:

Neodium 1.25"
Astronomik CLS 1.25"
Baader UHC-S 2"

The CLS is the best where I live but the other two are less severe.

If you want to borrow one let me know.

Chris