[APML]: Results of Low Vac Pre-Hyper

[Steve Bell <sb635@delphi.com>]

Last night I photographed M8 using the TP that I let sit in my un-heated
Model 300 chamber for 2 days with the best vacuum my Marvac R-10 pump
could pull. After the pre-hyper, I hypered the 4 cassettes in forming
gas at 30C for 3 days at 15 psi. I developed the shot this morning, and
I am cautiously pleased with the results. The base fog looks about the
same as if no pre-hyper was done. I exposed M8 for 50 mins through a
Lumicon Deep Sky filter. My previous shot, using TP with no pre-hyper,
was for 60 mins, also through the Deep Sky. The sky fog on the 50 min
pre-hyper shot was slightly less than the 60 min shot (you'd expect that
with 10 min less exposure), but it looks like I actually recorded more
detail, certainly at least as much as the 60 min. The overall resolution
is also better, but that's probably due to a bunch of things - mainly,
more than likely, better seeing than on the 60 min shot. This is only
one negative, but for now, I'm going to continue to do the no-heat
pre-hyper, even in the relatively low vac that the Marvac can pull.

I might also add that I used the Van Slyke Slider II off-axis unit, with
the tilt adjustable pick-off mirror. By tilting the mirror, I was
actually able to guide on a bright star right next to the main nebular
complex of M8. This is the third or fourth time I've used the Slider II,
and each time I've never needed to rotate the whole unit around, looking
for a guide star around the circumference at the field's edge. Each
time, I've been able to search along a line back towards the object,
which was centered in the field, and was able to find a suitable guide
star without needing the circular search. Like I said before, this unit
is great.

In a related note, I've decided to go back to using guidance exposures
of no greater than 1 sec. I have posted that sometimes I can use up to 5
sec exposures, but my system can only perform well for these relatively
long guidance exposure times when the OTA is pointed higher than about
50-60 degrees to the horizon. Lower, and I get worse guiding with these
longish times. So, it's back to 0.5 secs or 1 sec. And with the Slider
II, using its tiltable pick-off mirror, so far I've always been able to
find a bright enough guide star.

-- 
Clear skies,
Steve Bell
email: sb635@delphi.com
Astrophoto page: http://people.delphi.com/sb635