Quote:
Originally Posted by dvj
I can't help but think that a 3nm bandpass is not optimal for an f/3 system. Are you not seeing significant narrow-bandwidth shift at f/3? Although 3nm does isolate NII which I guess is your mission.
j
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John my mission is to get the highest signal to noise in a light polluted sky. I am only 16km from Melbourne. So 3nm bandwidth is the way to go.
It is a myth that 3nm NB is not optimal with fast optics of F3 say. It is based on a simplistic formula of frequency shift with angle. See Astrodons FAQ's on NB filters.
Here
http://www.astrodon.com/Orphan/astro...arrowband/#h13
I only see about a 18% attenuation at the edges of the PL16803 with RGB and with NB. Nearly all of this is due to vignetting not frequency shift. In fact I use my NII flats for all filters LRGB and OIII, SII and Continuum. I can get away with this because I have no dust in the system. It is a pain to make flats with LRGB as the system is so fast that stars are visible in even a few seconds of twilight flats when the flat is at the linear part of the sensor.
Light pollution gradients give me far more trouble than any variation that is introduced by using a NII flat with LRGB.
Another consideration with NII is that it is only emitted by monatomic N near hot blue UV emitting stars. The vast ancient Gum nebula behind the Vela SNR seems to only emit HA. I hope to use this fact to separate the two.
I will get a 5nm HA filter as it gives twice the signal for most NII/HA emitting objects as most HA images are really 50:50 NII:HA.
My real aim with this system was to attempt to image very dim stuff out of reach of slower systems at relatively wide fields and to make large mosaics. To start making images en masse of many bright objects in a record time would be like shooting fish in a barrel. Bright objects should only be used to test the system.
Bert
