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Old 27-03-2016, 07:16 PM
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luka
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Mounting of a special filter

Hi

I may be able to get hold of a special H alpha filter. Originally this was not an astronomical filter and has 50.0mm diameter and transmission FWHM of 0.5nm, i.e. it is extremely narrowband. I believe it is about 6-7mm thick (to be confirmed). It does not have any threads for mounting, just a metal ring on the outer diameter.

Any ideas how to mount such filter? Currently I use ED80 + Hotech SCA flattener + T-ring + DSLR (IR modified).

Also would FWHM of 0.5nn be a problem? I can imagine it would completely kill the light pollution :-)

Thanks
Luka
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Old 27-03-2016, 07:39 PM
glend (Glen)
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It will may also significantly lengthen sub exposure times. Typical Ha filters are about 7nm, extreme narrow gets down to 3nm, wider are around 12nm. Is the filter multi-coated? The thickness will likely affect light path and at least change the focal point.
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Old 27-03-2016, 10:00 PM
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Thank you for a reply Glen.

You may be right about the longer exposure time. I don't have the actual transmission numbers for this filter (yet) but the narrower they are the less transmission they have and more expensive they get. I would be surprised if this one lets more than 50% at its peak.

Multicoating - not sure but I believe so.

Focusing is probably not an issue as there is lots of room either way in my current setup.

I was more concerned about finding a way to physically mount it. I can borrow it to test it but I have to solve the mounting problem first...

I was looking at 2" filter wheels but I can't justify spending $200-$300 for a test... and the filter thickness may be an issue.
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Old 27-03-2016, 11:03 PM
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OzEclipse (Joe Cali)
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Luka
0.5nm is more typical of a solar Ha prominence filter

Joe
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Old 28-03-2016, 10:15 PM
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csb (Craig)
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It would be risky business to use that filter for solar observing.

Also for solar Ha telescope viewing must have an ERF (Energy Rejection Filter).
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Old 28-03-2016, 11:20 PM
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The filter was used in some lab for laser experiments. I would not use it for solar, too risky as already mentioned.
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