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old_frankland
professor emeritus
Reged: 03/28/05
Posts: 525
Loc: San Francisco Bay Area
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Acquiring a Ha filter specifically for imaging prominences, is staying with a .7 or .8 angstrom system better than going to a double stack, ~.5 angstrom system? Some of my reading suggests the wider filter better allows for doppler shifts due to motion of the prominences. Would this mean that the narrower filter will not record portions of the prominence?
Cut me some slack, now. I'm a greenhorn when it comes to Ha.
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Cheers,
Jim
Livermore CA
http://www.lafterhall.com/astro.html
3" APO, 4" ED refractor, 9" SCT, GM8
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brianb11213
Carpal Tunnel
Reged: 02/25/09
Posts: 2119
Loc: 55.215N 6.554W
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The way I understand it, the narrower the bandpass, the lower the brightness of the proms and the bigger the contrast with the disc. Doesn't make imaging impossible but you might miss proms because they're doppler shifted enough to make them too faint to be obvious. You should be able to get around the problem by shifting the tuning & rescanning but it makes life tougher.
My 0.7A single stack works quite well but it is noticeable that some proms brighten when the etalon tuning is changed from the "normal" setting.
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Fish
sage
Reged: 10/13/07
Posts: 470
Loc: Norridgewock, ME
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Jim,
Generally, prominences are best viewed and imaged with a filter bandpass of about 1.5 Angstroms. The downside is this is far too wide to see any surface detail; energy from the photosphere below washes it out.
On the other hand, surface detail tends to improve with filters that have a lower bandpass, such as 0.5 A or less. Of course, these tend to reduce but not eliminate the brightness - and detail - of prominences.
A reasonable compromise then is something in between these extremes, say 0.7 - 1.0 A. This allows for good performance on both phenomona with the same hardware. That is the chief reason many commercially available etalons have bandwidths in that range: to please as many customers who want a good "bang-for-the-buck".
So, if you are interestred ONLY in prominences, look at the wider bandpass products. DayStar and others have these units but are not as popular as others because they sacrifice surface features for great prominences.
Regards, Marc
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Goseck Observatory
Kunming 152 f/5.9 & Baader Wedge
Orion ED80 & LS75FHa2/B1200
Meade 102ED & LS18CaKMDd2
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