RE: Survey Strategy

[Tom Droege <tdroege2@earthlink.net>]

Rob and all,

I am (today) designing a fix for the Declination drive.  I expect it to be 
good enough to point to a fraction of a degree.  I know I have said this 
before, but this time I am really working on it.  The new version will not 
require clamping the dec, but it will still be possible and may be 
desirable to add to the rigidity of the system.  As the Japanese say "The 
nail that stands up gets hammered down."  So the biggest problem of the day 
gets a fix designed.  The dec drive prevents doing the survey as below, so 
I am fixing it.

Try this for the survey plan:

Break the sky up into 4x4 degree fields.  I would center the fields on even 
degrees.  This gives us a .2 degree overlap to handle positioning 
errors.  I think Arne has already done this but if it cannot be recovered, 
then it is easy enough to do again.  Someone just do it in some sensible way.

This requires about 1500 fields to cover -8 to +90.  About what I would 
attempt to cover from here.  You might want to extend this for those that 
can see more.

Assume that a Mark IV takes one 100 second exposure of the equator and five 
of someplace else.  This gets Landolt standards and gets everything that 
crosses the meridian.

Let's assume each Mark !V is assigned 600 fields.  This is 90 at the 
equator, and 510 elsewhere.  This then requires 3 Mark IVs to cover the 
whole sky.  I happen to have three ready to go on such an assignment.

The scheduler always gets an available equator field, then gets whatever 
else it can before another equator field becomes available.  It should 
always get the field which has gone longest without coverage on the list of 
fields that it can get.   In general, this can be done with only Dec. moves 
and small RA waits.

This way we always measure stars in the same position in a frame.  It is 
obvious from the analysis that I have done that this will get the best 
relative photometry.  Sorry it does not help the accuracy, but might allow 
later correction.  We shall see.  It does allow good detection of 
variability.  Note that we always want to measure as near the zenith as 
possible.  This will eliminate effects due to where we are pointing.  I.e. 
we always measure a field at the same declination and at nearly the same 
RA.  (To a few degrees.)

Arne suggests in his tech note that we mix 10, 100, and 1000 second 
exposures.  I would start with 100 second exposures.   Then do a 10 second 
survey to get the bright stars and last (somewhere else) do a 1000 second 
run.  I get 30 to 60 ADU per second sky background here so 1000 seconds 
saturates the detector.   I recently did a 250 second run with TOM2 and 
TOM3.  The I cameras have sky background that is half full scale.  There 
seems little improvement at the faint end over 100 second exposures, but 
possibly we do get the expected 0.5 mag improvement.

Running three systems here, I should get a few measurements of each field 
each month.  This should detect all the Miras between mag 8 and mag (near) 
13 in the Northern sky.  Also a bunch of other things but we might get a 
complete Mira survey in 3 years.  By definition a Mira has a magnitude 
variation of a minimum of 2.5 and a period between 80 and 1000 days.  OK, 
this will not be enough data for absolute classification, but will make a 
good hunting list.

Once this 3 year run was done, I would break off two of the TOMs and use 
them to make 3 hour runs of the frames to look for short period 
variables.  If I build no more telescopes this would cover the sky with a 
short period search in another 4 years while the one unit would keep 
updating the sky for longer and longer periods and random variation.

The plan is to build several Mark Vs to help with the short period search 
and possibly track fields for 4-5 hours.  If we stick to the selected 
fields and use standard analysis, then we will probably be able to 
calibrate between sites.  This can either speed up the coverage, or get 
longer coverage of selected short period fields by a cooperative program.

What do you all think?

Tom Droege

At 01:45 PM 10/4/02 -0600, you wrote:

>Yup.  This is exactly what I'm doing.  I'm also keeping track of the last
>time a picture was taken so I can move during the DARK exposure
>(transitioning from object to dark) without executing a clearing scan.
>
>I may work on moving RA and DEC at the same time to increase the possible
>distances traversed during the transfer.  Otherwise, moving DEC on ROB takes
>most of the available time (45.5 (steps/second) / 120 (steps/degree) * (45s
>- 20s clamp open/close) = 5.7 degrees).  I need to open the DEC clamp on ROB
>400 steps to not bind with the brake, providing a much smoother DEC
>movement.
>Moving both mechs at the same time will just entail breaking down the DEC
>move into multiple moves which coincide with the desired RA move timing.
>Easy ;-)
>
>Unless I am mistaken though (in reference to Arne's tech note), the minimum
>exposure is around 20-30 seconds, the time it takes to download the data
>from the memory card.
>
> > -----Original Message-----
> > From: Tom Droege [mailto:tdroege2@earthlink.net]
> > Sent: October 04, 2002 1:18 PM
> > To: Creager, Robert S; tass@listserv.wwa.com
> > Subject: RE: Survey Strategy
> >
> > 1) close the shutter
> > 2) start the scan
> > 3) look to see what you want to do next
> > 4) move the telescope to the new position
> > 5) start the RA drive going to get rid of backlash
> > 6) test for the block done
> > 7) note the time and open the shutter to start taking the next image
> > 8) go to 1)
> >
> > Note that with this sequence, or something like it, one is dead for
> > exactly
> > 46 seconds between frames.  You should always be able to get ready for the
> > next frame in this time.  I do not think that you will be able to see any
> > effects from moving motors, etc., in the data, but one should check.