Subject: Refresher on offsets

From: Russet McMillan

Submitted: Fri, 31 Jan 2003 15:43:06 -0700 (MST)

Message number: 654 (previous: 653, next: 655 up: Index)

Hi folks,

We've had multiple cases in the last few weeks where some time
was lost because observers and/or observing specialists were
unwittingly requesting the wrong kind of offset.  This occurred with
both GRIM and DIS in use, so it's a matter of concern for most

We have several different kinds of offsets available so that they can
serve as a powerful and flexible observing tool.  However, powerful
tools can cause problems when used carelessly, so here's a quick

All offsets as requested from Remark are in arcseconds, regardless of
type of offset or position on the sky (i.e., offsets in RA include
compensation for cos(dec)).  [Note: the above statement is not
strictly true for very large offsets >10 arcminutes, but those are
almost never used in normal observing, so I'm ignoring that issue.]

Object offsets are always in RA and Dec space; they will move the
telescope east or north regardless of instrument rotation or telescope
altitude/azimuth.  Positive RA offsets move the telescope east and the
stars west; positive Dec offsets move the telescope north and the
stars south.

Instrument offsets are always along the rows and columns of the
detector, regardless of instrument rotation or telescope
altitude/azimuth.  An X offset always moves along rows, although it
may be to the right or left depending on which instrument is in use.
A Y offset always moves along columns, although it may be either up or
down depending on the instrument.  [Exception: with DIS, an X offset
moves the target along rows on the slitviewer, but along columns on
the detectors.]

Absolute offsets are the cumulative offset from the original slew
position.  Requesting a 20 arcsecond absolute offset, then another 20
arcsecond absolute offset, will put the telescope 20 arcseconds from
the original slew position; that second offset did nothing.  In most
cases, a 0, 0 absolute offset is equivalent to reslewing (but
reslewing is better in some subtle ways that I won't explain just now).

Relative offsets move the telescope the specified amount from its
current position.  Requesting a 20 arcsecond relative offset, then
another 20 arcsecond relative offset, will put the telescope 40
arcseconds from the starting position.  A 0, 0 relative offset does

Slewing normally clears out all of the kinds of offsets that can be
requested from Remark.  The two kinds of sticky offsets (Guide and
Calib) that persist during slews should be adjusted only by the
observing specialist.  The exception is if the Keep Boresight box is
checked in the main slew control window of Remark; if that box is
checked, Instrument offsets will also persist during slews.

Combining different kinds of offsets is trickier, but here are some
general guidelines: switching from absolute to relative offsets
usually produces intuitive behavior, while switching from relative to
absolute usually produces surprises.  Object offsets and Instrument
offsets can be combined, but they will behave more intuitively if they
are both relative.  If more than one user is making offsets, everybody
should be using relative offsets.  Combining offsets of any kind with
changes in rotation in the order offset+rotate+offset is generally a
bad idea, but it can be successful if the offsets are relative;
reslewing at the time of changing rotation is usually a better idea
(that means pressing the Slew button to change rotations, rather than
the Set button).

When you get non-intuitive behavior from offsets, the first thing to
do is check that you have the type of offset you think you have.  The
next thing to do is reslew, making sure that Keep Boresight is not
checked by accident.  Many observers think that reslewing means
starting over and losing whatever work they've already done, but it
usually simplifies behavior enormously and saves time.

Here are a couple of instrument specifics: 

1) DIS slitview recommendations are always relative instrument
offsets.  They can be used with absolute instrument offsets by adding
the recommendation to the previously accumulated offset.

2) GRIM dither scripts (and SPIcam dither scripts, for that matter)
can use either relative or absolute, instrument or object offsets; but
the script should always end with a 0, 0 absolute offset or a reslew.
If the script is aborted halfway through for any reason, it's wisest
to reslew and start over unless your script was deliberately written
with such eventualities in mind.

3) When taking sky flats by hand, use relative offsets.  That way you
can enter the number of arcseconds once, and just press the Offset
button each time you want to move.  This is not necessary when using
the SPIcam "mornflat" and "eveflat" scripts, which take care of all
the offseting automatically.

4) For the echelle, just use shift+click.  You can use it for guiding
as well as centering, with full frames or subframes, and it saves you
from worrying about everything else in this message.  The only
troubles will arise if you do shift+click on an image that is not up
to date, if your offsets compete with autoguider offsets, or if two
people are making offsets at once (a very bad idea).

I hope this helps avoid some confusion!


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