RE: [APML] Exposure with lumicon HA filter

[John Mirtle <spampit@shaw.ca>]

George,
	When shooting H-a with tech pan I never gave a hoot about how many
stars. The whole idea was to capture emission nebula that wasn't even
visible without the filter. With a Lumicon filter, I never did shoot long
enough to get very dense backgrounds. Even at f2.8, from a dark site, max
exposure time can run into several hours. With the Schmidt at f1.5 I used a
Wratten 92, similar to the Lumicon H-a. The longest I could shoot was from a
mountain top in BC and 12 minutes was the longest unfiltered shot possible
on most nights. With a filter, this could be extended out to 60, 70 at the
zenith on really good nights. So, looking at sky fog levels, you can
increase exposure 5 to 6 times from a dark site with no moon. Under light
polluted skies this could change. How much of your light pollution is due to
say, red neon lights?? As for how long to expose to capture the same number
of stars, I have no clue. :-)

John Mirtle
Calgary, Ab. Canada

-----Original Message-----
From: astro-photo-bounces@seds.org [mailto:astro-photo-bounces@seds.org] On
Behalf Of George Anderson
Sent: Friday, August 06, 2004 11:08 PM
To: APML
Subject: [APML] Exposure with lumicon HA filter

I am looking to get a rough idea as to what (if possible) the filter factor
is for the HA filter on starlight.
For example if I were to shoot a region (widefield) and be able to expose
for ten minutes unfiltered before background (light polution) fog became a
problem, how long would I have to expose with the HA filter to capture the
same number of stars? This is assuming the same B&W film with good spectral
response to 656nm.
I realize that mileage may vary, but I am looking for a ballpark figure to
quantify some results.

George Anderson
Montreal Canada

Clear skies and good health
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