Quote:

---

It will be another week before I am at a dark site to test the H-Alpha view of M42.
However, I have invenstigated the spectrum of M42, and it has more energy at that frequency than at any other. The second strongest emission is at 500.7nm (O-III). All other emissions are too low a level to have any relevance in this discussion.
I will also tell you what the SQM reading is during the observation, as this may be relevant for dark adaptation.
I will test first with O-III having zero red transmission and report the colors I see, then with H-Alpha to tell you what of the nebula is visible. I suspect the central Huygenian region to be faintly visible. Other than that, it remains to be seen.
However, I would point out one caveat: a typical H-alpha filter transmits about 90% at that frequency. That ten percent could represent a threshold brightness factor that cannot be eliminated, so NOT seeing a part of the nebula with the Ha filter doesn't PROVE that a reddish color is not sensed WITHOUT the filter in place.
However, since the Huygenian region is so bright, I suspect there is reduced rod sensitivity when viewing the nebula, and increased cone sensitivity. Unfortunately, my test will not tell me much about the threshold of red sensitivity, but it will allow me to see if illusory colors are visible. If I still see faint reds and pinks with the contemporary O-III filter in place, that will prove to me that the colors are illusory.
This will be fun, regardless of what it doesn't and does prove. I typically observe with others, so I'll see if others would be interested in participating.
Since exit pupil would have an effect, I will first stick to a large exit pupil (over 5mm) and then go smaller to see if certain aspects disappear at higher magnifications (perhaps crossing a threshold in surface brightness?).

---

Thank you Don. Try to skecth the regions that you see with both filters. If there is a region that you see in H-alpha and that is black in OIII that region is likely to stimulate ONLY L-cones. According to the state of the art a hig L-cone to rod stimulation ration is what is needed to see redn (fig1 center top).
Exit pupil do have effect. The surface brightness of the apparent impage at the scope depends on the amount of collected light (the diameter squared) divided by the amount of apparent area (the magnification squared). This is the exit pupil. RThe maximum exit pupil ensures the hishest surface brightness (close to the originalo minus the lesses).
Please note if you see colors with the OIII and without and scketh where you see them (to be compared with where you see H-alpha and where you see OIII).
If possible check also other nebulae with the H-alpha (M27 would be interesting but now its gone.. so try M97 and some other at your choice).

PS do you have maps of the energy emission in H-alpa and OIII? How strongest is H-alpha?

PPS we also will probably be at a dark site (SQM ~21.4) next weekend. There will be a couple of 16" (one is mine, and maybe a 20"). Unfortunately we have no H-aplha filter (we do not look the Sun), however I have seen that I can combine my UHC with the red leakage (94% transmission at 654 nm) with a wratten 23A (I know, that is a workaround but better than nothing). With the W23A only I have seen the Hyugenian region red. I will look how much of the nebula is seen oth with the combination of the filters and with the only W23A.

Edited by Mauro Da Lio (12/02/07 03:13 PM)

Post Extras