Date: Fri, 7 May 1999 09:16:11 -0700
From: Russell E Owen
Subject: Re: 3.5-m obslog 990506
>The star stayed on the [Echelle] boresite (which is now very well
>aligned with the slit) without any guiding, for over two hours.  Note
>that star drift will be greater if the star is offset onto the slit
>with instrument coordinates (instrument coords move the boresite).  So
>good pointing is essential.
In case it's not clear, I'd like to say a few words about this.
This effect (use of boresight offsets causing drift) is intrinsic to
use of non-rotating instruments such as the Echelle. The trick is to
first tweak up local pointing error (e.g. using TRACK/PTERR), then
use arc offsets or object offsets to bring your object into the slit.
This is discussed in more detail in the TCC User's Guide.
Congratulations on getting usable pointing on NA1 with the NA2
telescope model. It is my hope and expectation that we can always do
this. However, to do a good job we'll need the position of the
tertiary (for best collimation) for each instrument port at a variety
of altitudes. (I am assuming, perhaps rashly, that the secondary's
optimal position will not vary with instrument. The TCC is capable of
moving both in an instrument-specific matter.)
-- Russell