Mark IV

[droege@wwa.com]

I keep working away on the Mark IV.  I have looked a lot at what 
I see with the prototype and have made some decisions.

1)  I am not going for fast read out at this time.  I am not 
burning bridges and could go faster in the future by replacing
one PC board.  This design will take 40 seconds to read out.  
This will hold true whether there are 1 or 4 camera heads 
attached to the control box.  I figure that the first use of
the Mark IV will be for an all sky survey similar to the present
Mark III survey, but covering the whole sky from several 
locations.  For this, we might do a 500 second exposure.  This 
means a 90% or better duty cycle.  This is good enough for me. 
I would rather have the 16 bit ADC that I understand than some
of the new devices that are much faster.  The new devices are
bound to want to teach me something, and that is a slow and
painful process.  Besides, it is not so easy to have the ADC
and the data source separated.  I want to do this for practical
operation reasons.  Time helps a lot. 

2)  I have looked into ways to get blue response.  Not very 
encouraging.  The problem with the blue band is that the CCD has
about 1/2 the response of the other bands on the red side,
and near zero on the blue side.  This must be awful for calibration
to the standards?  What say Arne?  In any case, I would plan to
build quads, in the faint hope that we find a solution.  Lockheed
does not offer lumigen coating which would help in spite of what
the data book says.  One can get the chips thinned, but apparently
by sending them to someone in Texas.  One can guess what that
might cost.  My understanding is that you buy the chips and send
them with the hope that some survive the process.  We could run
a blue camera and just live with the fact that after we fix up 
the filter that it is many mag less sensitive from the rest of
the channels.  What say?

3)  I have been struggling with the packaging.  There are three
main worries.  Lightning, noise, and cost.  As one might expect
what is good for one is not good for the others.  One of my goals
has been to design the system so it could be cabled up with low
cost cables that you could buy at the computer store.  DB cables,
BNC cables and the like.  But it looks like it just can't be 
done and still be secure from lightning and also be low noise.
So I will make special camera cables.  Sigh!  I hate making 
cables.  Ah!  But this is a job that someone might volunteer
to do.  No amateur cable makers need apply.  Making good 
cables is one of the toughest jobs in the data business.  

4)  I am designing the Mark IV so that it is not possible to 
bin the data.  I know this is desirable for focusing.  The 40
second read out time will be a pain for focusing.  At least my
experience on the Mark III is that focusing is not at the top
of the problem list.  Sure we could gain from a better focus. 
At least my practical experience is that it is more important 
to get data every day one can than to get better data by 
spending more time fussing.  There is a trade off.  Right now
I think the best plan is to get pretty good data with many 
measurements.   The firmware actually allows two programs,
and one could be set up to read out a small area - say the
center - pretty fast.  So this could be a focusing solution. 

5)  I have backed off somewhat from when I am trying to get the
Mark IVs out the door.  I have been sick for over a months, and
an just starting to feel better.  So the old schedule is shot.  
I think I will watch what is coming out of the Mark III data and
try to adjust the Mark IV design for an optimum result.

Tom Droege