"Rick Johnson" wrote in message
ster.com...
Richard Crisp wrote:
I have been attempting to use the '155 f/7 with the PL39K for three weeks
now. The faster focal ratio, f/7 versus the f/9 of the '180edt, means
nearly
a 2x improvement in light gathering rate with a modest increase in image
scale: 1.15 versus 0.87 "/pixel. Finally the weather cooperated on
Saturday
night.
I took 4.75 hours of exposure on Sat night using 15 minute subexposures.
the full 39 megapixel resolution on the linked page below is
interesting:
there are a lot of stars in there!
http://www.narrowbandimaging.com/m8_...pl39K_page.htm
Nice resolution on the central "star" of M20. Well resolved.
Glad someone sees sky. At least lately the clouds have given up some
rain. Well shy of what's needed but it helps. Still two solid months
of clouds and only 2" of rain is not a good trade.
I'm a bit lost on the image scale. 155mm at f/7 is 1085mm focal length
while f/9 at an aperture of 180mm is 1620 or a 49% increase. Increasing
.87 by 49% gives a 1.3" pixel. Does the flattener make the difference?
You are right about image scale Rick. I went by memory thinking my focal
length was 1215mm. It is 1085 as you said.
the image scale is 1.27"/pixel instead of the 1.15 I stated above.
so moving from the '180 f/9 at 1620 to the '155 f/7 at 1085mm gives me
1.27"/pixel instead of 0.87"/pixel
I did a lot last week: Sunday I flew to Taipei and had a week's worth of biz
meetings, flew home friday evening, drove to the ranch upon landing, set up
the scope and imaged. I got crap for data that night but found the warts and
fixed them for sat night. I guess in the confusion I just did not remember
my focal length correctly.. thanks for double-checking!
here is the view from my hotel room.... that's Taipei 101, the tallest
building in the world (at least today it is).