I spent several hours recently timing image downloads from
the 416XT. There are plenty of variables:

serial / SCSI / serial-usb / extended SCSI
pview / MaxIm DL
binned / unbinned / focus mode
short / long exposures

Some questions I was trying to ask were:
- How fast is serial compared to SCSI?
- How does MaxIm DL compare to pview?
- Does the USB converter slow serial down?
- Does SCSI extension slow SCSI down?

All full frame timings are from instant of pressing "start
exposure" to image visible. (Including A/D and all other
overhead). SCSI was extended with a pair of Paralan
SD10B's to 60 feet. Serial to USB conversion was with an
"Inside Out Networks" Edgeport/4.

The highlights were:

- Serial is not too bad. Unbinned full frame overhead
(total time - exposure time) is 50s to 100s. For some
odd reason, the overhead goes up as exposure time
increases. Focus mode is fine.
- Out of box SCSI can be slower! than serial (see binned
60s in raw data). A SCSI_DELAY workaround is required for
taking advantage of SCSI. With workaround, unbinned full
frame overhead is a consistent 13s for unbinned. Without
workaround, overhead can be 29s for binned, or < 0 (the
all zeros bug) for long exposures.
- pview is slightly faster than MaxIm DL for serial, due no
doubt to the custom serial code in pview
- pview won't work with the USB converter, due no doubt to
the custom serial code in pview. For multi-serial port
setups, the built in COM port will have to be for pview
data acquisition. USB conversion works for all else.
- Serial to USB conversion does slow things down a bit
(10%-20%). (But I add 4 serial ports ...) So even MaxIm DL
should use the built in COM port. TheSky and others can
use the USB ports.
- SCSI extension does not slow things down. (A cool 60 ft!).
- MaxIm DL focus mode is great, even if it is the slowest
configuration for focus. 3.5s for pview SCSI, 4.5s for
pview serial, 6.6s for MaxIm DL. It does seem to be
possible to switch back and forth between pview SCSI for
imaging, and MaxIm DL serial for focusing if I shut down
each SW package carefully in-between. (no camera or
computer reboots were required.

For a complete look at all the data see <http://surf.to/Brewster>.
Go to "Observing and Engineering Runs", then to the last
documented run (5/30/99).

Jon B

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