[APML] Re: [Astrophotography] Auto Guiders

[Herm <hermperez@worldnet.att.net>]

Autoguiding is the only way to go for film photography. The 201XT is widely
hated by astrophotographers, but its not so bad if you can purchase one for
around $175 or so (they are not worth more), a used ST4 can be had for
around $400-$700 but its rare since they are jewels. I have owned and used
both units.

Be aware that the older 201 is unusable, make sure its an 201XT. What are
the problems with the 201XT autoguider?.. its not very sensitive, it issues
corrections in both axis simultaneously and it does not compute centroid
positions.

Sensitivity can be dealt by using longer exposures, probably safely up to
10 seconds or so.. while an ST4 will easily guide with 1 second exposures.
Longer exposures will allow your mount to drift too much. Since sensitivity
is low, sharp focus is extremely critical. 

Simultaneous corrections can be a problem with certain mounts.. usually not
an insurmountable problem if you do a careful polar alignment. 99% of your
corrections will be in the RA axis anyways. Losmandy mounts WILL work with
a 201XT as long as the x axis of the 201XT corresponds to RA.. the losmandy
will just ignore DEC corrections until no correction is issued on RA.
Obviously this is not a problem with Meade mounts.

The st4 will compute the centroid position of a blob of light.. thus it can
guide extremely accurately on a distorted star or out-of-focus blob (even
guide on the head of a comet). This feature is what really makes the ST4 so
well loved. To compensate for this, make sure the 201xt is precisely
focused to make the star as small as possible on the ccd chip.. avoid using
a 201XT with an off-axis guider and an sct..

SCT off-axis stars are distorted blobs (usually crescent moon shaped), the
201xt has difficulty locking on these.

I would recommend you use the 201xt with a refractor guidescope, preferably
80mm and up so it delivers sharp bright stars.. if you carefully make a
mark on the focuser you may never have to refocus it again for a 201xt. You
cant use a guidescope on an sct due to mirror flop issues, so avoid an
201xt for an sct.

Also note that the command flowchart tree that meade includes with the
201xt is wrong, there are correct ones avaialble in the web.


biggsnj@cs.com  wrote:

>  Does anyone use an autoguider for their long exposure photograhy? SBIG has discontinued their >ST-4 Autoguider, the only other stand alone unit I have found is Meade's Pictor 201XT. Has anyone >used the Meade unit? Would you recommend it? I'm on a budget.
>
>Nelson Biggs

Herm
Astropics http://home.att.net/~hermperez

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