From: "Milton Aupperle" <milton@outcastsoft.com>
Date: January 26, 2008 2:46:57 PM MST
To: Astro_IIDC@yahoogroups.com
Subject: Re: Astro IIDC, occultation, and time
Hi Ted;
Just to clarify the Time Stamp and ocmment on some of your questions.
Astro IIDC doesn't physically time stamp each frame with a visible
time stamp. The Time stamp is derived by :
1) Using the Mac System clock, get the current time when the first
frame arrives.
2) Measure the duration of each frame since the last frame and record
that information in the movie.
Since we know the duration of each frame in the movie and we know when
the first frame arrived, we can generate the arrival time of every
frame and that gets displayed in the movie if you turn on the Time
Code Track. All our Time Code tracks are recorded using UTC / GMT
time, not Local time os that it's consistent. We do the same thing for
the driver we wrote and sell for the DFG/1394-1 A/D uncompressed
FireWire converter too.
As to the half second accuracy with getting the current Time, I have
never really looked into that part of it, however you can get a
feeling for it sort of information in the console.log window.
For example, I just pulled up the System Preference application (this
is In Tiger), clicked on the Date Time Icon and it printed off this
information :
26 Jan 14:08:02 ntpdate[6390]: adjust time server 17.254.0.28 offset
-0.074806 sec
in the console log window as soon as the Date & Time window opened.
I'm assuming that if you did this right before you began recording,
the starting time would be a lot closer than 0.5 seconds off.
As to the sensitivity of cameras, the LUX sensitivities that companies
claim for their cameras are subjective and generally it's Apples and
Oranges when comparing them. They generally are not directly
comparable. I normally compare the manufacturers response curves for
the actual CCD to see what they rate for sensitivity.
I've used the Low Light Analog cameras (i.e. PC23C, PC-164C, PC182XS
etc.) in the past with a DFG/1394-1 A/D Uncompressed FireWire
converter. with varying degrees of success for astro imaging (I have
examples at the bottom of the pages for 2003 images at:
http://www.outcastsoft.com/AstroImages/AstroIndex.html ). The main
issues I had was that they need to be modded because the Auto Gain
feature causes all sorts of issues (excessive noise and flares) when
trying to image brighter targets and it needed to be disabled, so I
have switched over to non interlaced FireWire CCD Cameras which give
me a lot more control.
The Imaging Source use HAD CCDs so they are about 1/2 the sensitivity
as an EXHAD CCD - based on comparing the actual response curves that
Sony publishes for the CCD.
Point Grey Research manufactures cameras with Bigger CCD pixels (i.e.
Flea or Flea2 640x480 have 7.0 micon pixels, Scorpion are 9.9 micron),
and the bigger the pixel cell, the more sensitive it is. A 7 micron
pixel size has 400% more light gathering than a 3.5 micron CCD will
have. PGR also now offer Grasshopper EXHAD cameras now too, but they
are REALLY expensive though.
With Astro IIDC, a 7 micron or larger Mono CCD and using 2x2 binned or
4x4 binned modes on a C8 or larger aperture, you should be able to
pick up 10th or fainter mag objects at around 1/30th or 1/15th of a
second per frame, like I did in this stacked image:
http://www.outcastsoft.com/AstroImages/SaturnWMoons20070121_MJA.jpg
That was with a Flea Color 7 micron CCD Camera, and you lose about 30
to 50% of the light through the Color Filter Array than what a mono
camera will deliver. Also with mono cameras, don't use a infrared cut
filter and you can capture even more light, as all the mono cameras
see very deep into the IR range too.
Hope Some of this helps..
Milton Aupperle
--- In Astro_IIDC@yahoogroups.com, "Ted" <tjswift@...> wrote:
I'm interested in doing occultation timing, and just learned that
Astro IIDC can time-stamp each frame. This makes me wonder if it might
work for occultation timing, and make the whole setup much simpler and
cheaper (a digital video camera and a laptop, instead of a analog
video camera, GPS clock, time-stamp inserter, video to Firewire
converter, and laptop). My question is: Has anyone figured out how to
synchronize the Mac's clock to NIST or GMT closer than the ~half
second or so we can do over the Net? I'm confident an occultation's
duration could be measured to an accuracy related to the frame rate,
but the occultation's absolute start and stop time would be uncertain
by that ~half second or so.
I guess another question is: What's the most light-sensitive
Firewire or USB camera that works with Astro IIDC? I just looked up
the DMK 21AF04, and it says its sensitivity is 0.5 lx at 1/30s, with
gain at 20 dB (100x?). Several other cameras look like they're in the
same range. A lot of occultation trackers are using the Supercircuits
pc-164c analog camera, which reads down to 0.0003 lux (though I don't
know how good the signal-to-noise is there). The 164C can make out
stars down to about magnitude 9-11 with a moderate sized scope.
One hybrid possibility would be to use a low-light analog camera
like the PC-164C, a video-to-firewire converter, and Astro IIDC. Has
anyone tried this kind of thing? Thanks.
-Ted