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3nm vs. 5nm vs. 7-8.5nm pros and cons?

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#1 PGW Steve

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Posted 09 December 2011 - 11:01 PM

I'm going to be venturing into the CCD world as soon as SBIG starts shipping the STF cameras, I've ordered the Pro Plus package, which comes with what I believe are Baader NB filters which are in the 7-8.5nm range. I had inquired with Matt on here if there were options to upgrade to 'better' filters, myself meaning narrower band. He said not likely.

Now trying to read all I can on here and look at images and equipment lists to see what is being used for what, and it becomes very confusing. The ladies and gentlemen on here have some incredible skills when it comes to aquiring data, and processing it.

From what I can tell, NB imaging can be done in light polluted skies (my SQM ~20.4), and while the moon burns bright. This intrigues me as it means more potential photon gathering for some really high integration times.

So I'd like to ask the imagers here for some pros and cons as well as tips and trick of the various NB filters available.

Thanks in advance,

Steve

#2 microstar

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Posted 10 December 2011 - 12:04 AM

Steve,

I have a set of Astrodons: 5nm Ha and SII, and a 3nm OIII -- all 36mm unmounted. I have pretty dark skies, but like you have to deal with moonlight, with snow reflection through much of the best imaging season. Cons: expensive! Pros: I can image in full moonlight throughout the imaging season so basically any clear night is available. I previously used a set of Baader narrowbands. I could shoot Ha and SII with a moon (although I had to pay more attention to position of my target relative to the moon); shooting OIII with a moon was difficult (that's why I went for just the 3nm OIII -- it's the most susceptible to moonglow). Still you can make them work just fine: shoot Ha and SII with a moon and save your OIII for darker conditions. As someone said when I asked for advice "Get the best you won't loose sleep over".
...Keith

#3 Gus_Smedstad

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Posted 10 December 2011 - 01:19 PM

I think you'll find the Baader 7nm Ha filter to be fine. I've collected a fair amount of data under mag-3 skies (no idea how that translates to SQM) with the moon up and gotten near-noiseless results.

OIII is another matter. I've only had my Baader 8.5nm OIII filter out once, and that was under moonlight because I expected results similar to Ha. It was a wash of light with no detail, I felt I could have gotten better results with a simple light pollution filter. I obviously can't speak for narrower OIII filters.

I've only tried OIII once with a Baader 8.5nm filter, and I assumed that since I'd had such good success with Ha under lots of light pollution and moonlight that the moon wouldn't be an issue. It was, very much so.

#4 dawziecat

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Posted 10 December 2011 - 02:48 PM

I am "in transition" from a DSLR using a 6nm Ha EOS clip-in to an ST-8300M which I have ordered, but not yet received.

After my experience imaging with the 6nm Ha filter in moonlight, I opted for the 3nm Astrodon Ha filter. The OIII and SII will have to wait.

My decision was based on imaging with the 6nm in full moon conditions. I found that imaging with the full moon 60 degrees away was, generally speaking, worth the effort. Closer than that and "not so much."

I attach the image from my last session, just two nights ago. It is a stack of 33 four-minute subs of the SH2-157 area using the 6nm Ha filter with a Canon T3i DSLR, NOT a CCD camera. The moon was 97% illuminated and 61 degrees away from the target area. Ha only and not particularly lovingly processed either as I consider it a start only and plan on additional data being added, both Ha and RGB.

Folks will have to judge for themselves if it "worth it" to image with a full moon and at what distance from the target that moon can get before it's time to give it up with any given filter but, surely, narrower is better!

I have high hopes for the 3nm Astrodon but, while ordered, it isn't in my hands yet.

I suspect, even with the 3nm, my ardor for imaging in full moon conditions will wane with experience. Even with NB, darker is better!

Attached Files



#5 vpcirc

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Posted 10 December 2011 - 03:23 PM

Here's what Don told me at the Advanced Imaging conference. The greater the light pollution your dealing with the lower the number you want. I'm in the country with 4.5 skies, so I could use the 5nm. Had I been closer to town with more light pollution I would have coughed up the extra for the 3nm. I don't think you can get much better than the AstroDon E2's. I did my research and there's nothing close to para focal as they are. That does come at a price though.

#6 AJT12

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Posted 10 December 2011 - 09:19 PM

Steve,

I have a set of Astrodons: 5nm Ha and SII, and a 3nm OIII -- all 36mm unmounted. I have pretty dark skies, but like you have to deal with moonlight, with snow reflection through much of the best imaging season. Cons: expensive! Pros: I can image in full moonlight throughout the imaging season so basically any clear night is available. I previously used a set of Baader narrowbands. I could shoot Ha and SII with a moon (although I had to pay more attention to position of my target relative to the moon); shooting OIII with a moon was difficult (that's why I went for just the 3nm OIII -- it's the most susceptible to moonglow). Still you can make them work just fine: shoot Ha and SII with a moon and save your OIII for darker conditions. As someone said when I asked for advice "Get the best you won't loose sleep over".
...Keith

Please let me know where you found an Astrodon 36mm, 3nm, OIII filter? Don told me he had no plans of making one.

#7 microstar

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Posted 11 December 2011 - 12:31 AM

Please let me know where you found an Astrodon 36mm, 3nm, OIII filter? Don told me he had no plans of making one.


Don did a test run and I managed to snag one. However, I looked at the Astrodon site a week or so ago and I noticed that he had a notice about them.
...Keith

#8 reiner

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Posted 11 December 2011 - 03:15 AM

Just one thought about ultranarrow bandpasses:

How do these filters behave e.g. in an f/4 light cone? In have done spectroscopic measurements on emission line filters for visual use. If you tilt the filter in respect to the measuring beam, the passband shifts to shorter wavelenghts. This effect was the more pronounced the narrower the passband.

For the most of the visual filters, which have bandpasses of 9 to 12nm, this is not detrimental, as the band pass still covers the emission line. Here is a graph of a tilt series for one of these filters.

Posted Image

Note that in an f/4 light cone, the rays from the edge of the objective have a tilt angle of 7°. Here is a web page by Rob Brown, who did similar experiments with similar results.

Has anybody investigated this effect for these ultra narrow filters? I would expect that the faster the light cone, the more you cut down your effective aperture. But note: this is just a guess and not corroborated by experimental data. It would be great to have such data available.

#9 Konihlav

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Posted 11 December 2011 - 08:02 AM

AJT12: I managed to get from Don a narrow band set of 36mm round filters that were all 3nm some time ago for my friend who uses MII G2-8300 and is shooting at F/3.6 or F/4. The only drawback was the price - almost as high as 50mm round unmounted so do not expect the 36mm to be any cheaper.

reiner: I guess, you found this: http://blog.astrofot...avelpech/?p=207 (as I saw a visit hit from Germany).

#10 reiner

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Posted 11 December 2011 - 08:21 AM

Hi Pavel,

yes, I had found your page, as I was interested how this would work with these very narrow filters. As you had stated on your web site, your source is not narrow enough. What you measure is therefore a convolution of the characteristics of both filter and source.

What I found is that (for perpendicular incidence) the center of the band pass of the very narrow Astrodon filters is not precisely on the emission line, but shifted toward the red by 1.5 to 2nm.

http://ftp.sbig.com/...rrowband_g2.htm

This could at least partially compensate the spectral shift for by tilted rays. This might also explain, why the filters seem to function well also for fast optics in practice.

#11 Konihlav

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Posted 11 December 2011 - 11:04 AM

Hi Reiner, you are right and paid good attention to what I wrote :) the narrownest of the source is the potential problem of my testing. But anyway, the Astrodons work perfectly :)

#12 bill w

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Posted 11 December 2011 - 12:00 PM

narrower the better except Ha
would recommend against 3nm Ha.
with 4.5 skies and good transparency you can do very well with 7.5-8 nm on all but the brightest nights .
if you want to do planetaries go with OIII bandwidth = Ha.
if you want to shoot emission nebulae go with OIII narrower than Ha.
get 3 nm SII if you can afford it.

#13 PGW Steve

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Posted 11 December 2011 - 01:22 PM

Great discussion, thank you to all the participants. I found a lot of great info on the Astrodon site, but unfortunately it is down today. There was a chart on optic speed and angle of light entry. It also suggested that 3nm Ha blocks out NII light that is a very big contributor to detail in M27 and other planetaries.

Based on the posts so far, it seems like the only con is cost, nothing in the S/N or processing end of things. Pro's are better imaging under LP or moonlit skies.

The 36mm 3nm unmounted are listed on Astrodons site as available only from them as an AIC special or something, these are not listed on OPT's site, however the 5nm are.

Based on the big pro of imaging any time of the month that the skies are clear is a huge plus, it isn't always clear when it's a Friday or Saturday and the Moon is sleeping and the temp isn't -30C or the bugs are insane. More potential chances for data gathering sounds like a HUGE plus and something that I think offsets the huge cost that I'm willing to pay.

#14 vpcirc

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Posted 11 December 2011 - 08:00 PM

At AIC he only had one. The 3nm is a new product (if I understood him correctly) so most of the dealers probably don't have them yet. He sent a previous order I placed the same day I ordered. I also had a minor problem with a filter, he immediately sent a replacement no questions asked and trusted me to return the defective one, which I immediately did. Top notch service and fast, can't beat that. Great guy who teaches almost every year at AIC. I think he also hold some secret patents for optics he supplies to the government. That tells me he knows what he's doing....






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