Re: Mark IV mount

[Tom Droege <droege@fnal.gov>]

Chris and all,

I am building a complete system.  Much like buying an LX-200 from the
store.  This is pretty much like I did with the Mark III.  For the most
part I sent everything needed except a computer, a pump, and a garbage
can.  This helped get almost everyone on line promptly.   

At 12:04 PM 5/19/98 -0700, you wrote:
>Just a few more loosely related ideas...
>
>I have made a couple of suggestions like Arne's but Tom seems to have 
>settled most recently on the German equatorial mount.  I was a little 
>surprised.  I agree with Arne that one mount per four scopes seems
>simpler.  Half as many motors, control cards, power supplies, roll
>off roofs, and on and on.  A lot however depends on the ratio of
>"fours" to "twos".  If most systems are "twos" I see Tom's point.

>From correspondence I have had recently, it looks like 5 or 10 to 1. 
The B and R will just be needed to calibrate the northern and southern
sky.

>
>I don't think keeping two mounts pointed at the same place is so hard.
>First off it need not be exact.  Secondly, I assume the Mk IV system
>will be able to reduce frames in real-time.  If so we will know to
>within an arc second where a camera was pointing after the frame is
>reduced.  This is better then running back to the limit stop and counting
>steps.  We could even build a mount flexture model but I doubt we'll
>need it.

I agree.  I have to build a focus scheme.  This involves allignment to 
a milliradian or better.  Once I work out how to allign the camera and the
telescope, aligning two telescopes is duck soup.

>
>What are the color bands for the "fours"?  I thought a back illuminated
>CCD would be required to do more then tree colors.  Has this problem
>been solved?

It would be B, V, R, and I.  There are two possibilities for the B.  We
get the CCD lumogen coated.  The B would then be about a mag less sensitive
than the V.  A second possibility is that someone that has a thinned 
chip will dig it out of their safe and give it to us.  This will be a 
gain in sensitivity for the Blue over the V most likely.  

>
>It seems that having a mount that can only hold two scopes means you
>would never want a "triple".  If the mount could hold four scopes
>a triple would become a reasonable option again.

Yep, just the R does not seem to add much.

>
>I think the simplest possible mount is a long tube
>or beam aligned with the Earth's axis  The center of the beam is bored
>through for a short Dec axis.  This short dec axis connects two optical
>tubes or two pairs of optical tubes.  I think (?) this is called an
>English mount but we use two scopes rather then a scope and a counter
>weight. 

Possibly I am building an English mount.  I mostly try to work from first
principles and design something that fits all the needs.  As Ross says,
the devil is in the details.  We are only a week or two away from a 
drawing, then you can see what I have in mind.

>
>A lot also depends on the style of construction.  If I where building
>the system I would design it to use construction techniques I could do
>myself.  I'd likely use thin sheet metal and rivets, aircraft style
>or maybe wood using furniture or boat building technique.  A machinist,
>I am sure would find a way to put his lathe or mill to use.

Me, I am an electronic type.  I first try to build things out of printed
circuit boards.  After that I revert to pretty simple mechanical design.
The present mount is being designed out of 1/2 x 3" and 1/2 x 2" 6061-
Txxxx (can't remember the temper) extrusions. Lots of cap screws.  As 
Chris says, one designs around experience.  

>
>Another question is, "What is the cost of a commercial mount?".  Back
>when the talk was about a barn door mount "build it yourself" was
>clearly cheaper but is a scratch built German equatorial still cheaper
>then an off the self mount?  I don't know.  It could also be that the
>people at each of the proposed Mk IV sites would be able to provide a 
>mount and pier.  

It is not the mount, but all the other stuff that determines the design.
Where to put the power supplies.  Where to run the cables, where to mount 
all the controls, limit switches, position sensors, etc...  Because I
am building an integrated design, I can keep changing things until I find
a design with a place for everything.  For example, the lens tube mounts 
are spaced to match the printed circuit board length.  The printed circuit
boards are just mounted to the lens tube frame.  Now I don't have to build
a special electronic enclosure.  Furthermore, the cable lengths are now 
about as short as they can get.  I am suspending the mount between two 
plates.  This allows setting up for latitude just by a slightly different
hole pattern.  It also give me a place to mount the power supplies.  What
is being designed is a completely automated telescope.  Just look at one
and see the stuff.  For example, there is a thirty-two channel, 16 bit data
system built in just to keep track of all the environmental stuff and the 
well being of the electronics.  

The present design just requires a flat space and a 3'x 3' x 4' cover.  It 
could be a spot under a roll off roof, or a spot on a slab, or you can build 
your own (as I am) mini roll off roof. But I am designing my mini roll off
to also include a flat field scheme.   

>
>There is some advantage in site built vs. "supplied by Tom".  One of
>them is that things like concrete and block become reasonable
>construction materials.  It may be that this kind of "site preparation"
>is within the means of each site

I plan to design a "house" as part of the whole design.  I will build one
and mount it on my roof deck.  I will provide plans to anyone who wants to
build a similar design.  It will probably be built from 1/4" plywood, with
a 1/2 or 3/4" base.  But the house is optional.  There are a bunch of 
stepper motor and limit switch channels available for those that want to
do their own thing.  This will be part of getting people involved.  I will
ship the camera when I see a picture of it's new house.  ;^)

Tom Droege