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Old 18-05-2016, 04:17 PM
ausastronomer (John Bambury)
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Quote:
Originally Posted by Don Pensack View Post
Photopic vision ranges from as low as 425nm to as high as 750nm.
But for most people at night, peak scotopic sensitivity is 507nm +/-50nm with response at 400 and 600 as low as 1 or 2%.
What changes this is when light is bright enough to start activating cones, and vision becomes mesopic (especially the L cones for red sensitivity).
Most nebulae, however, are not bright enough to activate the cones. I think M42 is and a fairly good handful of planetary nebulae, but generally, if an H-alpha filter is used on even as bright a nebula as M42, most people *barely* see any light except in the Regio Centrale area and even then it is exceedingly dim.
When people see red in nebula filters with a red response, the red they are seeing is often a peak about 625nm that is common in nebula filters that have a two-peaked response in the red. They see red shadows in the star images. In the nebula filters with a deeper response, say 650nm and below, people do not report any red in the star images and do not not reds being seen.

In general. But not necessarily in specific. There are individuals, usually those using very large scopes, who report a lot of red visibility in nebulae without using filters. For those individuals, having a red transmission at the H-alpha wavelength can be useful in picking up the reds. I am one of them. But very few other people have any scotopic sensitivity at that wavelength, so it is fair and reasonable to say that having a H-Beta transmission will help see nebulae better than H-alpha transmission.
In fact, the H-Beta wavelength is near the peak sensitivity for scotopic vision.

Every nebula that emits energy at H-alpha also emits energy at H-Beta, just at a substantially reduced level.

More information on the human eye response can be found here:
http://hyperphysics.phy-astr.gsu.edu...on/bright.html
It's an easier site to get to the data charts easily than most, which is why I list it.
Hi Don,

That all makes perfect sense. I would suggest that Andrew Murrell and I also have extended red wavelength visibility response. Andrew probably a bit more than me in fact. This would explain why we sometimes see some nebula and features in some nebula more clearly than a lot of other observers. It probably also explains why I see features in some targets with a UHC filter having a H-alpha bandpass, that I cannot see in UHC filters that don't have the High red end bandpass. Sometimes I think I just take it for granted that everyone else is seeing exactly what I see at the eyepiece and obviously this isn't always the case.

Cheers,
John B
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