Quote:
Originally Posted by ngcles
Hi Suzy, Steve, John & All,
Re: the opposing views of Steve and John on the use of OIII -v- UHC in small apertures. My experience leads me to agree with John's posts on this issue 100% and I really can't add anything useful to them. (I seem lately to be endorsing what Bamo advises here on IIS quite a bit, but here again, I believe he is right on the money.  )
For visual use in 'scopes <25cm go for the UHC. Same goes for the very narrow band-pass H-Beta filters -- you need significant aperture at the right exit pupil for them to work well in my experience. If you've got 10"+, buy the UHC first and the others later if you feel you need them. I've got 1 1/4" OIII and UHC, 2" UHC & H-Beta. |
Anyone care to place a bet
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The primary objective of any LPR filter is enhancement. A LPR filter is only as good as the brightness/darkness of the sky background.
The brightness of the sky background is related to the exit pupil
not the aperture. The larger the exit pupil the brighter the background.
Exit pupil = aperture/magnification.
Let's suppose I have a 250mm aperture scope at 50X magnification.
The exit pupil is 250mm/50 =5.0mm.
A Lumicon OIII is designed to operate in the exit pupil range 2-5mm in light polluted skies.
Doubling the aperture will not improve image enhancement, it will be in fact be a lot worse as the exit pupil = 500mm/50 =10mm.
The background is now so bright that image contrast is lost.
I need to double the magnification as well. Since increasing magnification
spreads out the photon flux from an extended object, I'm not gaining anything by increasing aperture.
As I mentioned in a previous post, using a larger aperture will result in more stars in the background.
Regards
Steven