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Old 22-09-2014, 12:05 PM
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pluto (Hugh)
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Ha 7nm vs 5nm

Managed to get a single sub last night before the clouds rolled in.
Anyway this is a comparison between a couple of 30 min Ha subs of the Helix.

The one on the left is with my new Astrodon 5nm filter and it was taken when the Helix was only about 45-50 degrees high, also the last couple of minutes would have had cloud covering.

The one on the right was taken with my Baader 7nm filter when the Helix was near zenith.

Both have just an auto stretch applied.

With the extreme light pollution here the difference is quite pronounced!
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Old 22-09-2014, 03:17 PM
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Octane (Humayun)
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I was wondering what kind of difference it would make.

Seems like, quite a lot.

Oh, well. Back to saving several hundred dollars... again.

H
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Old 22-09-2014, 04:05 PM
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Yep, basically the narrower you go, the better the performance under light polluted skies (with commensurate cost!), and assuming your f/ratio is not too fast. I got myself a set of Astronomik 13 nm filters, because I'm going to be shooting from super dark skies, and so a bit of 'leakage' into the non-target wavelengths is fine (desirable in some ways, due to improved signal). They work okay in LP skies too, but not as well in that instances as the narrower bands. Horses for courses...
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Old 22-09-2014, 04:24 PM
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pluto (Hugh)
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Yeah I don't think the difference would be anywhere near as pronounced under less light polluted skies, let alone dark skies.
While I like the tighter stars they'll give, especially on the 3nm SII and OIII, I wouldn't have spent the extra money if I wasn't trying to image from the middle of Sydney.
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Old 22-09-2014, 04:33 PM
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The 5nm is a good performer. I have been using them for several years now. The 3nm can show a bit more detail but it takes a lot more exposure time so no free lunches.

Greg.
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Old 22-09-2014, 04:43 PM
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pluto (Hugh)
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Quote:
Originally Posted by gregbradley View Post
The 5nm is a good performer. I have been using them for several years now. The 3nm can show a bit more detail but it takes a lot more exposure time so no free lunches.

Greg.
I haven't tested myself but according to the Astrodon site the 3nm should transmit the same amount of light at the target wavelength as the 5nm filters. So exposure times should be about the same. Obviously this doesn't apply to the 3nm Ha as it misses out on a significant amount of NII light which the 5nm doesn't.
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Old 22-09-2014, 05:08 PM
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Thanks for sharing Hugh, pretty big difference.
anyone know if adding an LP filter in front of a 7nm Ha, O3 and SII would help at all?

Pics attached are as expected, same 7nm Baader Ha from an LP site and a dark site. single exposure.

Cheers
Alistair
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Old 22-09-2014, 05:13 PM
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A LP filter won't help Ha, OIII or SII -- it has a broader acceptance band than the NB filters, but (typically) overlaps with them.
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Old 22-09-2014, 07:20 PM
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Quote:
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A LP filter won't help Ha, OIII or SII -- it has a broader acceptance band than the NB filters, but (typically) overlaps with them.
ditto
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Old 29-09-2014, 07:30 AM
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Quote:
Originally Posted by pluto View Post
I haven't tested myself but according to the Astrodon site the 3nm should transmit the same amount of light at the target wavelength as the 5nm filters. So exposure times should be about the same. Obviously this doesn't apply to the 3nm Ha as it misses out on a significant amount of NII light which the 5nm doesn't.
Ah right, I was referring to the Ha. Not sure about the others, perhaps no difference. Some here use all 3 and would know. I do read posts about extra exposure time needed with 3nm Ha filters. Its been a consideration for me about getting one. Probably will anyway.

Greg.
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Old 29-09-2014, 09:25 PM
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Thanks for the comparos chaps, very useful. Always wondered the diff. No substitute for dark skies!
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