Re: [M]: M: pictor problems
| Subject: | Re: [M]: M: pictor problems |
| From: | Greg Hartke |
| Date: | Fri May 05 12:54:20 2000 |
On Fri, 5 May 2000 antares@oz-online.net wrote:
> Hey everyone, Last night I set up to use the 201 to autoguide my
> 10"@6.3 through my ST-80. The focal length of the ST-80 w/barlow is
> half the imaging scope.
I would be very concerned using a barlow in this way; its just asking for
flexure to take place. The probability of this configuration actually
working would be greatly enhanced if you drilled and tapped 2 more
mounting screws in the ring on the guidescope and again on the barlow.
This way the screws are all spaced 120 degrees apart around the mounting
ring. I drilled and tapped for 6-32 knurled screws in a diagonal for use
on my guidescope and again on the guidescope itself, then I clamp em down
plenty tight when I insert the 201XT. This way, I have 3 holding screws at
each junction - it works for me.
(snip)
> 1- the field of view of the guidescope w/barlow is essentialy the same
> field as the imaging chip (416xt). I could never really get a star in
> tat field that the 201 would pick up on (I was imaging certain
> galaxies). I would then off-set the guidescoe a small amount to center
> on a bright star. I believe this is alright as long as the two scopes
> are fairly close to alignment.
Of course. SOP.
> 2- The hardest task was getting the 201 to lock onto a star. I was
> able to make a parfocal eyepiece the other night. Even with (what I
> consider) a fairly bright guidestar I had extreme difficulty getting
> the 201 to see it - even with varying Ets. I did remember to take
> darkfields for each different setting.
I question whether youre 201XT is actually well focused. This is not the
easiest task in the world. My 201XT locks onto and guides on stars very
easily; Ive successfully guided on stars that would really be too faint
for me to guide on manually. (BTW, I never use an exposure time of greater
than 10 seconds on the 201XT.) Any star brighter than about magnitude 2 is
too bright to use for testing the guiding - it saturates the CCD. For
example, stars of approx mag 2 to 3 or so can be easily used for guiding
with an exposure time of 0.1 seconds. These are suitable for guide tests
and I always use a star of about this magnitude for calibrating the 201XT.
BTW, Im using an 80 mm f/11.4 refractor as a guidescope so collecting
area is identical even if focal length is not (although its close with
your barlow). 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.
Ive never been able to consistently improve the focus I obtain using the
autoguider at the same focal position as the 9 mm. 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.
> 3- When I did have a guidestar I would notice corrections like 10,
> 01,20,11,00 - fine. then suddenly for one iteration I would get
> something like 51. Sometimes I would simply lose the star (--).
Interesting. Several possibilities based on your symptoms:
I assume youve successfully completed the calibration procedure and have
not changed the position of the autoguider nor the optical path after the
calibration procedure. I find that calibration proceeds with the highest
probability of success when I calibrate using stars that are successfully
captured with a 0.1 second exposure time. Its very irritating to be
unable to calibrate properly and it didnt take me long to find that if I
use a star bright enough for these very short exposures, it works
perfectly, virtually every time.
Does your scope exhibit any retrograde motion, most likely in declination?
My scope (also a 10" f/6.3 LX-200) originally exhibited a bit 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 accessed
Dr. Greiners web site for the appropriate information and pulled the dec
drive, just generally tightening and tweaking as per instructions, and the
problem went away. Check it on your scope at high power using, say, a 9 mm
EP. If you see any retrograde motion, your autoguider aint gonna be
happy.
Train your PEC. Its very easy to do - just follow the directions in the
owners manual which are quite clear and explicit. Go thru one training
and one update and Ill bet youll see a dramatic improvement in the small
time-scale RA tracking. (There may be residual long time-scale drift but
this is easily compenated in guiding.) This could conceivably be part of
your problem: You may suddenly hit a section of the worm that has a very
high error. I remember seeing fun spots like that when I was manually
guiding. If the rates are too high and the integrating exposure too long,
the star image could be driven right out of the small area of the chip
thats being used for the differencing in the guiding algorithm.
(snip)
> By the way, I agree with the veterans of this group who have spoken at
> legth about the difficulties of guidescopes. Thank you for the very
> informative articles. Jeffrey A. Edmonds 1LT AR
> http://www.oz-online.net
Guidescopes are quite useable but require a lot of attention to detail.
Everything has to be tight, tight, tight. As I mentioned, I triply lock
all tubes in the optical path, I capture the cables to the autoguider so
they cant put even the slightest force on the guidescope, I tightly lock
the main mirror of the scope, and I tighten the screw on the Losmandy
mounting rings to insure the guidescope is solidly held. I have 60 to 90
minutes exposures taken this way at the prime focus of the 10" that say it
can be done. I prefer this to fighting with an OAG. Actually, if guide
stars were easier to find and the off-axis star images were better, I
might prefer an OAG. Everyone has their own preferences - there is no one
right way: The trick is to find what works for you.
Greg Hartke
Sykesville, MD
