> Can anyone recommend what scopes are good for using as a guidescope
> for use with the Pictor 201XT autoguider and how the guidescope can be
> mounted onto an LX50-10". Thanks!

Questions of this sort come up from time to time so I cobbled together
detailed comments from disparate posts I've made about guidescope and
201XT use with the Meade SCTs with the idea of trying to put as much of
the requisite information as possible in one place. When I eventually get
around to developing a web page of my own, I plan to write a section on
its use anyway. Most of this material is conceivably superfluous (I put it
together from pieces of a few other posts) but there may be one or two
hints that you and others might find useful. My apologies to others who
have already seen pieces of this post in the recent past.

First of course, it's important for the user to make sure they have
sufficient focal length for the guider. I've heard complaints from people
who are using guidescopes with focal lengths obviously too short. They
then blame the guider for their own mistake. Silly.

The SBIG ST-4 has guiding algorithms superior to those of the 201XT and
may be capable of subpixel guiding resolution but as far as I know that's
not true of the Meade unit. (Then again, the 201XT costs less than half as
much as the ST-4 so I'm not complaining. I think the 201XT is an absolute
bargain.) For this reason, it's necessary to pay attention to the
resolution necessary for adequate guiding. The usual rule of thumb says
the guidescope should have a focal length of at least half the effective
focal length of the main instrument but this is unnecessarily vague: The
requisite focal length can be quantified quite easily.

No matter what the focal length of the scope being used for photography,
the seeing (particularly when considering long exposure photos) will
prevent the user from getting star images smaller than a couple of
arcseconds or so hence any guider resolution greater than this is actually
wasted because it won't be possible to see finer detail than this in our
photos. We need only calculate the focal length required for a guidescope
to have an accuracy of at least 1 arcsecond per pixel:

To a good approximation, the image scale at prime focus is

I = 1/F

in units of radians per unit length where F is the focal length of the
scope. (The length units here are whatever units you use to express F.) In
degrees per unit length, this is

I = (180 degrees)/(pi*F) ,

and in arcseconds per unit length, it's

I = (2.063 x 10**5 arcsecond)/F .

The pixel size (S) of the 201XT is 10 microns or

S = 10**-5 m = 10**-2 mm.

We then want an image scale of

I = (1 arcsecond) / S
= (1 arcsecond) / (10**-2 mm)
= 100 arcsecond/mm.

Thus:

100 arcsecond/mm = (2.063 x 10**5 arcsecond)/F .

Solving for F:

F = 2.063 * 10**3 mm.

Thus any guide scope with a focal length of around 2000 mm yields a
resolution of very close to 1 arcsecond per pixel on the 201XT. As
previously mentioned, I expect we could do handily with a resolution of
1.5 to 2 arcseconds/pixel which would reduce the focal length requirements
to, say, 1000 to 1500 mm focal length. My 80 mm guidescope has F = 912 mm
which gives a resolution of 2.26 arseconds/pixel for the 201XT. I don't
think I'd want to go with resolution any worse than this with my 10"
f/6.3. (I have some nice round stars on 90 minute exposures that indicate
that the focal length of my guidescope is adequate.)

So now we know the minimum focal length necessary for adequate guiding. I
would suggest the user choose nothing less than an 80 mm aperture for a
guidescope. The 201XT will be able to guide on any star you can see in the
guidescope and larger apertures will, of course, equate to more stars from
which to choose. I use an 80 mm guidescope and wish I had more aperture.
Then again, I *don't* wish I had more weight to put on the mount!

After mounting the guidescope above the main OTA, it will be obvious that
it must be counterbalanced. I (and most others) recommend the flexibility
of a 2-D counterweight set for the Meade SCTs. Scott Losmandy (of
Hollywood General Machining) makes a version that works very well and is
used by many of us.

Focusing the 201XT is not the easiest task in the world and of course it's
imperative to have good focus to insure the guider properly locks onto the
guide star. My 201XT locks onto and guides on stars very easily; I've
successfully guided on stars that would really be too faint for me to
guide on manually. (BTW, I use an 80 mm f/11.4 Celestron refractor as a
guidescope and never use an exposure time of greater than 10 seconds on
the 201XT and that only for the absolutely faintest stars under rather
dewy conditions. Without dew, the longest exposure time I ever need is 5
seconds for the faintest star I can see in the guidescope.) Note that a
guide star that's too bright can't be used - it saturates the CCD. For my
guidescope, any star brighter than about magnitude 2 is too bright to use
for guiding. With my equipment, stars of approximately magnitude 2 to 3 or
so can be easily used for guiding with an exposure time of 0.1 seconds and
I always use a star of approximately this brightness to calibrate the
201XT. (More on calibration below.) Anyway, I focus by cheating: I have a
Meade 9 mm illuminated reticle EP that is well known to be very close to
parfocal with the 201XT. I've never been able to consistently improve the
focus I obtain using the autoguider at the same focal position as the 9 mm
reticle EP. Since I seem to be able to guide on stars which are quite
faint (and for exposures in excess of an hour), I would suggest that my
focus seems to be perfectly adequate. I understand the 201XT does not have
the most complex algorithms on the market so I would expect that it
requires star images that are well focused.

As mentioned above, I always calibrate the 201XT motion using a moderately
bright star. Experience has shown that the calibration procedure works
best this way: I found it fails with alarming regularity on faint stars
and succeeds with virtually uniform success with stars bright enough to
use a 0.1 sec exposure with a brightness reading in the 60 to 90 range.
Because of this, I index the 201XT in the guidescope so that I rarely have
to recalibrate - I can calibrate and go through an entire roll of film
over many nights without having to recalibrate. Again, I cheat. <G>
Against the advice of others, I mount the 201XT in a diagonal on the
guidescope. (I find this makes it much easier to find and center guide
stars when the guider is swapped out with the reticle EP. This can,
however, increase the likelihood of differential flexure. I'll discuss
this below.) Whenever I place the 201XT in the diagonal, I always make
sure that the 201XT is aligned parallel to a portion of the backplate of
the diagonal. In this way, the guider is effectively indexed so that I can
always replace it in exactly the same orientation obviating the need to
continuously recalibrate. This is very sneaky and works great.

A serious problem for any astrophotographer using a guidescope is
differential flexure. I have several techniques to maintain rigidity.
First, I have the guidescope focuser locked for rigidity. Secondly, I've
drilled and tapped the diagonal *and* the mounting ring on the guidescope
for 2 extra 6-32 knurled screws (that I got from mcmaster.com) at each
coupling so that I don't have a single set screw holding any tube in the
optical path in place. Instead, all tubes are tightly held in place by a
trio of set screws set 120 degrees apart to effectively eliminate any
chance of flexure in the system. I do the same thing for all couplings in
the camera optical path on the main scope. It was traumatic to drill and
tap my 2" TV diagonal, but necessary if I wanted good results! I would
expect this to be unnecessary with the 2" AP diagonal which uses a brass
locking ring.

Another source of potential flexure in the guiding system comes from
stresses induced by the power and signal cables running to the guider. I
capture the cables and run them from the control panel, along the mount,
down the side of the guidescope, and to the guider using ratcheting
adjustable quick clips that I found in the electrical section of the
hardware store. These clips have adhesive pads on the bottom allowing me
to stick them where I need them. I used duct tape before I found these but
of course the clips are much neater.

In addition, much consideration must be given to the mounting of the
guidescope to insure there is no flexure there. Many recommend (as do I)
Losmandy components here. The dovetail and rings attach very rigidly on
the main scope OTA. When I'm shooting, I make darn sure I tighten the
rings very smartly on the guidescope without worrying about marring the
finish on the OTA. The proper mantra for using a guidescope is tight,
tighter, tightest! Don't use rings that are very large compared to the
size of your guide scope OTA. I use 108 mm rings for my 80 mm guidescope.

For mass-market SCT drivers, the major source of potential differential
flexure comes from the moving primary mirror. Meade SCTs of 10" and
greater aperture actually have provision for locking the main mirror in
place which can, with care, eliminate this major source of flexure in the
optical path. You literally cannot use a guidescope for exposures over 15
or 20 minutes with an SCT (at least a Meade SCT) without the ability to
lock the primary rigidly in place.

If you go to Chris Vedeler's web site at

URL = <http://www.isomedia.com/homes/cvedeler/>,

you'll find that he has a nice discussion with accompanying pictures that
should easily allow you to properly put together the bits you need to lock
the mirror down. The basic parts are a 6" long 1/4"x20 bolt with some
nylon spacers and a wing nut. Chris also details how to spring load the
focuser for a better feel. This isn't suitable for astrophotography but
works nicely for casual observing. For my own use while photographing
without an aftermarket focuser, I found it very clumsy to use the wing nut
etc. when trying to tweak the focus with a Spectra SureSharp. (Check
focus, loosen wing nut, change focus a tiny amount, retighten wing nut
while trying to hold the head of the bold with fingers. Check focus, find
it still isn't right, repeat ad nauseum.) I bought a couple of 1/4"x20
knurled nuts from McMaster-Carr. (mcmaster.com, part no. 91833A134, $3.27
each, although maybe you can find them locally. I tried but couldn't.) One
I permanently attached to the top of the 6" long 1/4"x20 bolt using JB
Weld and the other takes the place of the wing nut I once used that is
tightened against the nylon spacers and locks the mirror in place. This
makes it *much* easier to lock the mirror consistently.

All of this allows me to take prime focus exposures up to 90 minutes with
a pretty reasonable probability of success. I'd guess I get approx 60-70%
(say, 2 out of 3) success rate at the moment with exposures this long.
That's actually pretty good with this equipment - SCTs are notorious for
making guidescope operation extremely difficult with long exposures. BTW,
the likelihood of a good exposure does indeed improve with shorter
durations. I'm still refining my techniques and equipment and expect to
get my success rate still higher: I have reason to expect it to be at
least 80% or so for these long exposures by the time I'm done.

I actually find the 201XT to be very easy to use. I had no real trouble
right out of the box - my only real challenge has been to systematically
refine my techniques to eliminate sources of flexure now that longer
exposures are very practical. Once done, the above techniques (locking the
primary mirror in place, triply locking in place all connections between
tubes in the optical paths, and capturing cables to remove strain) will
serve to eliminate most of the rest.

If you have trouble remembering the mode tree for the 201XT, I would
suggest that there are at least a couple of mode diagrams on the web that
are easier to use than that in the 201XT instruction manual. One that
comes to mind is that of Guido Pasi on his web site at

URL = <http://www.geocities.com/CapeCanaveral/Runway/4144/>.

I printed this out and placed it in a clear plastic vinyl slip cover for
protection. With a little experience you'll probably never need it again
(I didn't after about 2 sessions) but it's still handy to have around.
Guido also has some other interesting info on the 201XT on his website
that you might like to read. It's worth noting that he's working with an
OAG (not a Lumicon GEG, either) on his LX-200 and is doing very well with
his 201XT. Go figure.

Using a guidescope with an SCT is definitely a challenge but it can be
done. If the weather ever improves here, I'm going to try some shots using
an AP barlow in the optical path to increase the image scale when shooting
certain galaxies. I'll probably add a barlow to the guidescope, too, to
effectively get guider resolution of around 1 arc second per pixel. I'm
curious to see if this can be made to work because of the additional
potential flexure problems with the barlows in the paths. With everything
triply locked in place, I think it has a fair chance of success. I'll
start with some bright planetaries like M57 etc. which record nicely in
only 30 minutes or so. (I'm shooting with Elite Chrome 200 pushed 1 stop
and a 48 mm Tokai LPS filter.)

Another common mass-market SCT problem: My scope (a 10" f/6.3 LX-200)
originally exhibited a bit of retrograde motion in declination before I
went to the 201XT. I was able to work around it when I was guiding
manually (I always drift align so the dec corrections are small and rare
anyway) but I knew an autoguider would never be happy about it. I pulled
the dec drive, just generally tightening and tweaking and redistributing
grease, and the problem went away.

My impression is that a lot of problems with the 201XT come from some
users inability to focus and calibrate consistently. If you can beg,
borrow, or steal a Meade 9 mm reticle EP, the focus problem would be
solved in one swell foop (sic). If you can borrow the Meade 9mm, you can
make a parfocal EP with one in your possession and you're good to go.
(Heck, buy one. You'll need an illuminated reticle EP anyway.) The
inconsistency with the calibration procedure was a real irritation for a
while 'til I found a way to hugely reduce the necessary frequency of the
procedure and to do it so that I need only calibrate on nice bright stars.
Notice how I solved both of these problems: I cheated. Fie for shame!

As an aside, any astrophotographer will tell you that one of the key
elements of a good shot is good focus. This is very difficult to achieve
and absolutely critical. I use a Spectra SureSharp knife-edge focuser but
this is no longer available. My opinion: Knife-edge is the best way to go.
I suggest looking at Richard Shell's new Stiletto Mark 2 knife-edge
focuser at <http://www.stellar-international.com/>.

This is most of the experience I've gained with developing good techniques
for the use of a guidescope with a Meade SCT. OAGs work well but have
their own difficulties. Some OAG devotees will tell you it's not possible
to use guidescopes with mass market SCTs to take long exposures. Wrong.
It's demonstrably practical. Everyone assumes that trailed stars in guided
long exposures with mass market SCTs comes from uncontrolled movement of
the primary mirror and so they say they can't do anything about it. In my
experience, properly locking the primary is important but it's not the
only piece of the puzzle. It's also absolutely vital to eliminate
potential flexure in all of the couplings in both optical paths and
eliminate strain on the cables to the guider. Only in this way is it
possible to get good results for long exposure photography with an SCT and
guidescope.

Bottom line: You can't be lazy and expect good results. You have to work
for 'em. It's very hard to get good results in astrophotography without
working very hard to learn the craft. The results, however, can be very
rewarding. Just don't expect great results too quickly. On the other hand,
my first shots were 10 to 20 minute manually guided exposures which were
almost uniformly good but I was a very experienced amateur astronomer
before starting to use my LX-200 for photography. It got *really* hard
when I started trying longer exposures.

Greg Hartke
Sykesville, MD

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