No Subject

[Tom Droege <droege@wwa.com>]

Dear David,

The first thing you should know is that I have little experience
with all of this, and almost no experience as an observer behind
a lens.  Everything so far has been automated CCDs.  So I have
no optical experience, or any knowledge of Airy disks (vaguely 
know what they are) or the like.  So I am eager for comments by
those with knowledge or experience.  Having said this, I have
been corresponding with some pretty clever people the last 
several years, and am sort of holding my own.  

At 01:50 PM 1/19/98 -0800, you wrote:
>I ask your patience with my questions as I'm new to the TASS
>program.
>
>Some observations:
>
>1) A >= 100 mm dia optic will be needed to get to 16 mag with 
>
>a CCD.

That is what we are designing.  The big problem is the sky background
which tends to make sensitivity increase with d instead of d^2

>
>2) With a 2kx2k CCD with 15 micron pixels, you'll get 4+ 
>
>degrees on a side with an F/4 system (now I understand the 
>
>400 mm fl).

Yep, I grudgingly increase the focal length and lose sky coverage.
It is a question of field crowding.  One does the best one can.  The 
usual thing is to give up and go to long focal lengths and live with the
small field.  I am after large field science.  I will probably run a system
at home with 50 mm lenses and pointing at different angles to cover 
a lot of the sky (70 x 70 degrees).  Look for GRB.  No one knows what
they might look like if you are looking at one when it happens.  Should
take about a month to get one with such a set up.  

>
>3) A 500 nm Airy disc will be greatly undersampled at this 
>
>F/#. There is a conflict between field of view size and 
>
>astrometry, and fov wins. 

Yep,  everyone does work with small fields.  Note that this is not a 
problem with the present optics on the Mark IIIs.  I found some $19
135mm f/2.8 lenses.  We are not bothered by a spot size that is too
small.

>
>Are these simple observations consistent with your thoughts? 
>
> 

Pretty much.  Nice observations.

>Questions:
>
>1) What is your ideal goal for the system? 
>

I really don't have a specific goal.  This is outrageous, I know.  If you
are going to a funding agency to ask for money, you better have a firm
plan.  And you better stick to it.  Or you won't get any more money.  Lets
see,  try going to your funding agency and saying "I want some money.  I
am going to build a few telescopes and just give them away to people I
contact on the internet, and hope that some science comes out of it."
Well, so far its working pretty good.  ;^)

OK, I started out as a comet finding machine.  I was going to mount all the
cameras in one spot and search the sky in lanes.  Then professionals started
contacting me and saying "why don't you measure variable stars while you
are at it".  Then I realized the work the software would take.  So then I gave
away the cameras to those that looked like they might work on software
development.  Meanwhile, Bohdan Paczynski of Princeton bought filters.

>a) 3 sets of optics looking at adjacent regions of the sky, 
>
>each with it's own bvri filter wheel or
>
>b) 4 co-aligned optics each with its own filter ( b or v or 
>
>..)looking at the same region of sky simultaneously?
>
>Is there another possibility I haven't touched on?

The plan is a little more devious than that.   It is 4 cameras with B, V,
R, and I
filters all co-aligned (more or less).  But there is no way I can afford blue 
sensitive CCDs.  So I plan to build quad mounts with cameras mounted in
the V, R, and I positions.  The idea is to shame the community into "finding"
the blue sensitive CCDs  They are there in desk drawers and safes.  Everyone
buys more than is needed for a particular project. 

Note that there is little or no data in existance which has been taken with 
multiple telescopes looking at the same region of the sky.  I am taking such
data now.   OK, one does not expect to get anything.  But someone should 
look at least once.

>
>
>2) How many of the mark iv's do you want in operation, total?
>

The general concept is to do the sky survey with lots of cameras in not so
good
locations.  This is because it is expensive to hire full time observers to
live at 
big name observatories.  I hope to line up a bunch of interested
individuals and
astronomy clubs to run the survey.  They will not be at great locations for
observing,
but good spots for people.  

So the answer is as many as I can find the money to build.  I am about to
retire and
have saved some money, and plan to spend it before I die.   I figure about
10 triplet
systems.  More if I get some help.  But I am not looking for money.  I
figure I can either
do it of get money to do it.  Not both.  You can see which option I picked.  

>
>Comments:
>
>1) If you can point the optics around the sky automatically,
>then you can trade the optically complex first possibility
>for the simpler second to make up for total system fov. This
>takes more time to cover a region of sky, of course.

Yes, the mounts are planned to cover the full range in declination, but only
a limited range in RA.  This allows covering the whole sky that transits you
meridian.   A simple "barn door" mount.  

>
>2) In my view, it's best to keep it simple mechanically and 
>
>operationally - one filter, one optical design for it. No 
>
>moving parts of a filter wheel, no potential focus problems. 
>

Well, I am doing the best that I can.   But the features keep 
creeping in.  ;^(

>You've got mount problems to handle, but that's no mystery.
>
>3)  The more mark iv's you make, the more you can justify a 
>
>different design tailored to each filter (which can be much 
>
>cheaper in glass).

Elliot seems to have found a good design.  We shall see.

>
>Practical comments:
>
>1) I've had great luck with imaging quality of off-the-shelf 
>
>Hoya filter glass. The peak-to-valley  wavefront aberration 
>
>across a 1.5" dia of several 2mm thick filter glass samples 
>
>was 1/20 wave (HeNe). I'm not joking. I had it measured 
>
>interferometrically and confirmed in practice. There are Hoya 
>
>and Schott glass equivalents.

OK, this I know nothing about.  I am going with the filters specified
by Michael Richmond and Bohdan Paczynski.  Arne Henden seems
to like them and was able to calibrate our filter measurements.  So
I will go with the experts.   They all seem to be in contact with the 
people whose names are attached to the filters used.  At least Kron
is working on the Sloan at Fermilab, where Michael Richmond also
was working until recently.  OK, I am not trying to name drop, just
to say I am following advice from those who have the appearance
of experts.

>
>2) If you go with an independent optical design for each 
>
>filter bandpass,
>it may be possible to tailor some parts of the overall design 
>
>to some off-the-shelf elements saving a lot.  Or it may be 
>
>possible to design to standard test plates used by mfgrs in 
>
>making optics. This might help. Of course, performance/cost 
>
>has to be the final criterion. I'm sure Elliot is well aware 
>of this.

He seems to have a design where he uses the same set of lenses
and just positions them for the filter bandpass.  He calls this a zoom
bandpass and seems to be quite pleased with the idea.

Tom Droege