Re: Modelling - can of worms


Ladislav Nemec <nemecl@...>
 

Good summary of essential conditions for imaging. I just somewhat disagree
with the statement that 'modeling has nothing to do with polar alignment'.
It depends on the modeling algorithm. Gemini modeling compensates for polar
misalignment quite well - the question is, of course, whether modeling
should substitute for a good polar alignment. Not for permanent installation
but it may be handy for transient installation - building a model that
includes compensation for minor polar misalignment may be faster than very
precise polar alignment in the field.



Also, I still have to achieve significant reduction of tracking errors (some
of them non-periodic on my Losmandy G11 and thus not possible to correct by
PEC) that, at this time, are typically less than 8 arcsecs peak-peak. AP
mounts, perhaps, do not have these non-periodic errors - they are created
(general belief) by mechanical imprecisions.

-----Original Message-----
From: ap-gto@... [mailto:ap-gto@...] On Behalf Of
Rick Wiggins
Sent: Tuesday, September 02, 2008 10:37 AM
To: ap-gto@...
Subject: [ap-gto] Re: Modelling - can of worms



Hi,
I would like to chime-in based on extensive use of the AP mounts in
both observatory and portable setups (ap 400, 600, 900, 1200, &
Mach1.
A. First GPS: There is no real need for GPS. You can get location
coordinates from any map or look them up on the internet ahead of
time. Once programmed in (less than one minute to program during
setup even in the dark), they can be called up in less than 1 sec
during the startup routine. Also, the exact location is not critical
to imaging.
B. Modeling: Modeling is great for very precise visual or imaging
setup work. It is not necessary for imaging as long as the mount can
get you within the FOV of the camera. Doing plate solves is far more
accurate than modeling for centering an image. If you are using
something smaller than an ST-10 and focal lenght over 3000mm,
modeling may be a real benefit. Modeling has nothing to do with
polar alignment. I do not use modeling with my AP mounts when doing
fully remote imaging. I only use plate solves. With an AP1200 mount,
1200mm FL scope and ST-10 (app 30 x 45 arc mins), my target is
always better than 15 arc mins from center (prior to plate solve)
over an entire nights imaging session of multiple targets ranging
across the entire sky and including multiple meridian flips.
C. Polar alignment: The AP mounts align very well with the polar
scope. I typically get mine better then 10 arc mins with the polar
scope. That is good enough for 20 min subframes at 1200mm FL on a
24mm x 36mm chip or camera. If you need better alignemnt, Pole-
Align_Max, PEMPro, or drift alignment can get you below 30 arc secs
with a time investment of 30 to 60 minutes.
D. Tracking for imaging: Here is a list of factors using my AP
mounts in descending order of importance (factors affecting tracing
performance). These are based on monitoring of my tracking logs:

1. Polar alignment with the polar scope. I find this takes less than
5 minutes for portable use assuming mount has been prealigned with
compass or better during daytime.
2. Exception rigid mechanical setup (based on separate guidescope
and N/A in OAG or internal guiding) with no snagging cables. This is
a one time setup for each optical configuration if done correctly.
3. Very well adjusted (dialed-in) guider parameters (min/max
corrections, aggressiveness, etc.) Note: McMillain's paper is the
best guide I have seen. This is a one time setup for each optical
configuration.
4. PEMPro or similar PEC corrections. This will reduce AP mounts
from factory performance (I have found between 5 to 2 arc secs PE)
to below 1 arc sec PE. This is basically a one time setup, although
it should probably be checked yearly or if tracking changes.
5. PEMPro, PoleAlignMax, drift align, or similar polar alignemtn
refinement. This is done during each setup if required.

I hope this data provides some help in understanding imaging
capabilities regarding tracking and alignment with AP mounts.
Thanks, Rick

--- In ap-gto@yahoogroups. <mailto:ap-gto%40yahoogroups.com> com, Gerald
Sargent <sargentg@...> wrote:

In my initial submission starting the modelling topic, I should
perhaps
have noted that I am one of those who has to set up each time,
hence
the need for rapid polar alignment, and I mainly image, using
either an
SBIG with self guide or the STV for autoguiding, both of which
work well,
However both need that the mount be reasonably aligned. My
Gemini on a Losmandy G8 had an excellent routine for getting
aligned
using 3 stars, and it had an inbuilt GPS feature which enabled me
to
download time/location from an external GPS receiver. Both these
are
great time savers and one could be reasonably well aligned with the
mount knowing exactly where it was and what time it was in a matter
of a few minutes.
With my next mount, a Vixen Atlux, a really excellent mount I
had the SS2KPC controller which did not have the GPS facility which
meant a few minutes "lost", it did however have a very good polar
alignment facility using three stars and one could be very quickly
sufficiently well aligned for imaging.
One gets used to taking advantages of advanced routines,
image trying to use a car in a city without a self starter - the
crank
handle every time - well that is how I feel using the Mach1 with no
modelling or GPS facility, sure half an hour or so and I can get it
aligned but with a lot of "crank handle effort" that I have not
had to
do with my last two mounts. The Mach1 tracks superby once aligned,
and it is a perfect mount for imaging - but it does lack the "self
starter"
Gerald.

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