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I can't match the informativeness of those numbers because all of my filter research has been done with the filter on the end of an eyepiece, stuck in a telescope, out under the stars. I can't find those numbers out in the bush in the dark. I have been doing these same "low tech" filter evaluations for a lot of years and they work for me.
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The key point here is observing planetaries
in light polluted skies.
There is absolutely no doubt that an OIII filter will outperform a UHC filter under these conditions.
Do your filter evaluations include light polluted skies? If not you cannot assume that relative filter performance is equivalent under different sky conditions.
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To quote respected US author Phil Harrington in edition 3 of "Star Ware" Para 2 Page 221, "Owners of medium and large aperture telescopes should also give consideration to an OIII filter"
Copied straight from the website of the Binocular and Telescope shop in Sydney:-
http://www.bintel.com.au/Filters.html
Click "Astronomik Bandpass Filters" then click "OIII filter"
Astronomik Profi OIII
Product information:
The Astronomik OIII filter is designed for visual observation of gas nebulas and planetary nebulas. Due to the extreme small band pass of the two OIII- lines, the filter will produce a relevant increase of contrast even under best observing conditions. At faint super nova remnants and faint planetary nebulas the Astronomik OIII filter often makes the gain, that decides if the object can be seen or not.
The best focal ratio for the use of this filter is 1:4.5 to 1:6. The field of application is from 1:3,75 up to 1:15. Loss of transmission and colour displacements, that appear at other filters, emerge with the Astronomik filters only at extreme light intensive focal ratio of 1:3 and bigger. So you can see widespread objects in the whole field of view and not only in the middle.
The OIII filter is to be used from a aperture of more than 6" (15cm) on. At smaller instruments there is not enough light available for a sensible use. Many experienced Deep- Sky- observers rather use the OIII filter at instruments with more than 10" (25cm) despite the smaller field of application than the more versatile UHC filter.
Directly from the Oceanside Photo and Telescope Website (OPTCORP) in the USA
http://www.optcorp.com/product.aspx?...-826-1622-1893
Astronomik OIII Filter for Visual Use - 2" Mounted
This Astronomik OIII Filter is made for visual observing of gas and planetary nebulae.
This model is mounted in a 2" metal cell.
With a FWHM of ca. 15nm, the Astronomik OIII Filter transmits at nearly 100% in the OIII line.
On larger telescopes (10" & over), this may very well be the filter of choice for deep-sky observing.
Astronomik OIII Filters are best on telescopes of at least 6" of aperture.
Astronomik 2" OIII Filter for Visual Use
From the start, the Astronomik OIII filter has been very specifically designed for the visual observation of gaseous and planetary nebulae. The extremely narrow pass band of the two OIII lines brings a substantial contrast gain to these lines even under best observation conditions. On faint super nova remnants and faint planetary nebulae the Astronomik OIII filter will often make the difference as to whether the object can be seen or not!
The optimal aperture ratio for the use of the filter is 1:3 to 1:15 with apertures of more than 6" (150mm). Transmission losses and chromatic distortions, which arise with other filters, occur with Astronomik OIII filters only when extremely bright aperture ratios of 1:3 and more come into play. The Astronomik OIII filters thus can expand the view of dispersed objects, generally speaking, to the whole visual field of view of the eyepiece and are not limited to only the center of the eyepiece.
The information on both the Bintel and Optcorp websites looks like it's a direct spiel from the filter manufacturer (Astronomiks in Germany).
Just wondering why, "if you're right and I'm wrong", the filter manufacturer agrees with me and further, why the largest astronomical dealer in Australia and one of the largest in the USA, post incorrect information on their website?
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The proof of the pudding is in the eating and my own experiences support the Lumicon site that exit pupil diameter is more important than aperture.
I have used a C5 SCT (5 inch) and a OIII filter. The objects are enhanced even though the background is dark. The primary role of aperture is to define the field in which the object resides.
For example if I wanted to observe the planetary NGC 2438 in the cluster M46 then aperture will clearly play a role in defining the field. If I was only interested in the planetary then aperture is less important.
With the same scope the Helix nebula observed in surburban Melbourne under magnitude 4.5 skies with an OIII filter is more conspicuous than without a filter under magnitude 6.0 skies (my current site).
People should not be discouraged from using OIII filters on small telescopes.
Regards
Steven