Using GoTo Telescopes to Observe Asteroidal Occultations
by Walter Morgan, with additional remarks by David Dunham
Some concerns by Guy Nason and Tony George have now been added
New: 2004 October 4; updated 2005 April 1
From: wvm1313 [mailto:wvm13@aol.com]
Sent: Saturday, October 02, 2004 9:02 AM
To: IOTAoccultations@yahoogroups.com
Subject: [IOTAoccultations] Re: GOTO problems
My switch to a GoTo scope about three years ago has greatly increased
my ability to positively locate faint target stars. As with most
things, there was a learning curve, so perhaps others can benefit
from my experience, described here for an 8-inch SCT with f/3.3 focal
reducer and video-recording. Note: I do not use a laptop.
1. Finder scope. The manufacturer's straight-through finder with
six-point adjustment (which would not stay in alignment) was replaced
with an Orion right-angle finder with two-point adjustment.
- I prefer the similar Orion straight-through finder (so you can
more easily see where it's pointed), but I bought a couple of
the right-angle ones for the recent Palisana occultation since
that event occurred within 10 deg. of the zenith. If you can afford
only one finder, then the right-angle one is probably the one to
get since you can use it easily at all altitudes of the star above
the horizon (the Orion finder has an erecting prism so you don't
have a reversed image). I heartily recommend the two-point
adjustment system; I have them on my newer, smaller telescopes but
have old six-point finders on my two C-8's; I intend to replace
those the next time I make an order. Then with the same dove-tail
mounts on all of my telescopes, I can easily switch from straight-
through to the right-angle finders when I need them. I suppose
other dealers other than Orion sell these finders as well, so check
with your usual source.
2. Custom finder chart. I now use Starry Night Pro to create a
chart about 2 by 3 degrees. Key features: no stars fainter than the
target are included; magnitudes are labeled on stars brighter than
10, a rectangle representing my video FOV is superimposed. A copy is
printed to use in the field.
- Even for faint targets, I would include stars at least half a
mag. fainter than the target. It's probably better to always set
the mag. limit close to what your experience shows will be visible
with your system under good conditions since even for bright stars,
you want to see the faint ones nearby to verify the identification.
Steve Preston's 2-deg. charts work pretty well for me (so I don't
go to the trouble of making my own charts; just use his, knowing
the approximate size of my video field of view on those, and use
the wide-field and 5-deg. charts for use with the finder scope).
- When I have to find other targets for running remote stations,
I copy charts from the Millennium (for the main scope) and Herald-
Bobroff C-charts (for the finder scope) atlases, but I plan to write
some software to produce my own charts for that using the Tycho-2
catalog, to show the time line along the target star's declination
and also generate a list of conjunction times in RA with stars
near the target's declination.
3. User defined objects. I will enter into the telescope memory the
coordinates for a group of two or three stars that are magnitude 9 or
brighter, and within about one-half degree of the target. For
Herculina, the group chosen was named H1. Coordinates of the target
are also entered; for Herculina that "object" was named H2.
- Before using your GoTo telescope to find occultation targets,
make sure it works well for finding things that you know. Solve
any mechanical or power problems before trying the telescope for
an asteroidal occultation [see the concerns raised by Guy Nason and
Tony George below]. My 10" GoTo telescope just doesn't work as
advertised, probably due to a problem with the clutches; until I can
get those fixed, I can only use it as a non-GoTo instrument (it is
equatorially mounted) and have successfully used it for lunar
occultations. I've been a little more successful with my 4" GoTo
altaz telescope, but it seems to need a more robust power supply
than the multiple AA batteries that it's designed to use, for
reliable use for more than half an hour, especially in colder
weather.
4. After successful alignment, I GoTo any bright object not used in
the alignment process, to confirm that the alignment worked. (On
rare occasions a "successful" alignment has actually been in error,
and it is necessary to repeat the alignment procedure. That is most
likely to happen in light-polluted situations, when the alignment
stars cannot be located with the naked eye.) Up to this point scope
use has been visual. The video system is connected while the bright
object is centered, making the necessary refocusing (~ one turn CCW)
a simple process.
- I wouldn't (and don't) bother with a visual eyepiece and the need
to take time to refocus for the video; I just start with the video.
I first collimate the finder with the main scope (video) by using the
Moon (if it's up) or a distant tower light or street light, and
focus the video. Then I use the finder to find a star of mag. 3 to
5, focus on it, and then look for fainter stars in the field to
further refine the video focus. Once the finder is collimated, it
can be used to put the alignment targets into the video field, and
you can center them there.
5. GoTo the vicinity (H1 in this case). Slight manual slewing is
usually necessary to show the entire group. When that is displayed,
there is no doubt that I am in the correct vicinity.
6. Star-hop to the target. This involves slewing only a small
distance, in a direction known from the finder chart.
7. GoTo the target (H2 in this case). The target and/or some nearby
stars should remain in view, positively confirming that the process
has been blunder-free.
8. Start the video recorder, and repeat steps 5 and 6, leaving a
record confirming absolutely that the correct target was used. This
step is most likely to be significant if I report a miss that seems
inconsistent with other observations.
The fact that I prefer to use my scope in alt-az mode is an extra
reason to locate objects via the GoTo approach. Star-hopping is best
done with an equatorial mount, but placing an equatorial wedge on my
mount makes the system flimsy - and more cumbersome, too.
- Certainly, star hopping is best with an equatorial mount, which
allows use of setting circles, allowing you to find fields like
your "H1", or nearby 7th or 8th-mag. stars that you can see in your
finder scope, offsetting from very bright stars, which is almost
like a GoTo system, but not quite as accurate. But if your GoTo
system really works virtually all of the time, the advantage you
site for the alt-az mode is true, and makes a more compact setup
for travel. David
Walt Morgan
with comments added by
David Dunham, e-mail dunham@erols.com, 2004 October 4
Phone home 301-474-4722; office 240-228-5609; car 301-526-5590
_______________________________________________________________
The process and advice given above usually works well. But
some problems that are sometimes encountered with the GOTO process,
mainly the mechanics (and electronics) of the process, are
described below by Tony George and Guy Nason. When using GOTO
telescopes, it's useful to be aware that the problems below might
occur, and allow time and/or take actions to ameliorate them in
advance. Walter Morgan's techniques should ameliorate some of the
problems described below.
From: "TriAstro"
To:
Sent: Thursday, March 31, 2005 9:50 AM
Subject: RE: [IOTAoccultations] GOTO MInuses
Tony George wrote:
1. When the alignment doesn't work and you are left wondering where
the heck your telescope is pointed.
2. When the alignment does work, but you can't identify the star
field no matter how many times you GOTO the star.
3. When your wires get all tangled up during your GOTO slewing.
4. When your power connector momentarily loses contact and your
GOTO alignment is lost just minutes before the occultation (now I
use a right-angle connector and no longer have this problem).
5. When your illuminated reticle batteries poop out leaving you in
the dark and unable to line up your alignment stars (now I just
unscrew the LED module and shine the Handbox LED light down the
hole!).
The list can go on, depending on how many years you have been chasing
occultations with a GOTO scope...:-)
Cheers!
Tony George
Umatilla, Oregon
________ adding to the above list:
From: Guy Nason [gnason@rogers.com]
Sent: Friday, April 01, 2005 1:25 AM
Subject: Re: [IOTAoccultations] GOTO MInuses
I'd add:
6. When the ambient temperature is sub-freezing and the LCD crawl
display on the controller is smeared to the point of illegibility.
I've given up on using my Baby NexStar mount in the winter time.
Way too frustrating. - Note - taping a chemical hand warmer to the
back of the controller helps keep it alive in cold weather.
7. Nason's Second Law: "The likelihood of Murphy's Law scuttling an
enterprise is directly proportional to the amount of electronic
technology in the system." In other words: "K.I.S.S."
But by far the most frustrating is Tony's #4: power interruption
caused by the lousy coaxial plug that's endemic in telescopic
hardware these days. In all fairness, though, this applies to all
motor-driven mounts, not just Go-To's. One project on my To Do list
is to convert the power-in connectors on all my mounts to RCA's or,
even better, XLR-3's.
(XLR-3 connectors are common in professional audio equipment and
include a positive latching mechanism to prevent inadvertent
disconnections. But they're a bit bulky, so I'll have to see if
there's room in the NexStar housing.) [a larger discussion of
connectors is ongoing on IOTAoccultations; I'll report on that here
later - D. Dunham].
Guy Nason
Toronto, ON