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Old 27-08-2013, 08:45 AM
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gregbradley
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3nm Ha versus 5nm Ha

I damaged a 5nm Ha some time ago and am in the process of servicing my gear and fixing up little defects that have built up.

So I am about to replace the filter.

I am using an F7 to F5.6 refractor or CDK17 at F6.8 or F4.5 is the switch to 3nm worth the extra cost?

Same with O111.

Greg.
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Old 27-08-2013, 09:15 AM
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I like the additional contrast and fine detail that I get from the 3nm Ha and OIII filters. They also deal better with a bit of moonshine.

Both times I bought 5nm filters (50mm round and then 50mm square) I ended up upgrading to 3nm.

Cheers,
Rick.
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Old 27-08-2013, 09:21 AM
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Thanks Rick.

How much extra exposure time do you find you need to match the brightness over a 5nm?

Greg.
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Old 27-08-2013, 09:36 AM
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Greg,

I don't find there is really a need to expose for longer. You still get the same amount of true Ha signal through either filter. The only difference is that the 3nm filter rejects more spurious stuff.

Cheers,
Rick.
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Old 27-08-2013, 09:52 AM
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The 3 nm also rejects the NII signal too, but it's picked up by the 5 nm. The NII signal can be substantial depending upon the object (e.g. some of Bert's NII images).

I emailed Don Goldman a while back before ordering my 3 nm Ha filter. He recommended either (1) 5 nm Ha & SII + 3 nm OIII, or (2) all in 3 nm. Scenario 1 was recommended if cost was an issue, or if picking up NII with the Ha was important. Scenario 2 was recommended for imaging under heavy light pollution. OIII is more susceptible to light pollution than Ha or SII (especially moonlight), so he recommended getting the 3 nm regardless of the other two.
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Old 27-08-2013, 10:03 AM
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Thanks Rick and Dave. My 5nm Ha still works it just has some corner damage that can cause problems in moonlight. So 3nm would be good for getting extra filaments and detail. Don went over the fact O111 lets through extra light in moonlight as O111 is in the moonlight as well.

Greg.
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Old 27-08-2013, 10:15 AM
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Quote:
Originally Posted by naskies View Post
The 3 nm also rejects the NII signal too, but it's picked up by the 5 nm. The NII signal can be substantial depending upon the object (e.g. some of Bert's NII images).
Good point, Dave. For objects with a lot of NII you'd need longer exposures with a 3nm Ha.
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