| |
Main
Date: 09 Nov 2006 13:30:38
From: HeyJP
Subject: Polar Alignment using Digital Setting Circles
|
I have a 10 year old 8" Celestron SCT to which I added Digital Setting
Circles (DSC). Even though the DSC controller with compensate for a
non-aligned polar axis, when I do astro-photography it is important to
get very good axis alignment first.
The most commonly used technique for accurate polar alignment is the
"drift method" which is detailed on many, many astro websites. However
it takes FOREVER and is VERY tedious.
I have been trying a technique using the Digital Setting Circles to
rapidly iterate in on proper alignment. I haven't gone through the
math or the geometry, I've just been doing something that intuitively
seems reasonable. And, it seems to work. Has anyone else done this?
Is there a small refinement that would make my technique iterate in
quicker? Here's what I do:
(1) First I set my scope up with a rough alignment aimed at the North
Star. Then I pick my two stars that I will use for the technique: the
North Star and another bright star that is at least 90 degrees away,
such as Spica in the summer. I'll use Spica as my example
(2) I turn my scope so that Spica (or whatever Star#2 is) is centered
in my eyepiece. I then tell my DSC controller that this is Spica and
to do a 1 star alignment. The DSC reads Spica's coordinates: 13h 26m
and -11.2 degrees.
(3) I then spin the scope until the DSC reads the coordinates for
Polaris (The North Star) which is 2h 39.4m and 89.3 degrees. If the
scope was already perfectly aligned, the North star would be in the
eyepiece. But since its not aligned yet, Polaris is close, but not in
the field.
(4) I then rotate the tripod base and adjust the wedge angle until I
have cut the difference in half from where it "thought" Polaris should
be and where it "actually is".
(5) Next I repeat from step 2. Turn the scope until Spica is in the
eyepiece. Reset the setting circles to read Spica's coordinates. Then
spin the scope until the circles read Polaris and 1/2 correct the mount
towards Polaris' actual location. The trick is: point at Spica,
adjust the circles. Point at where Polaris should be, adjust the mount
towards Polaris. Repeat.
(6) After 3 or 4 iterations of this technique, my scope seems to get
pretty well aligned. When I spin back to Polaris' theoretical
position, very quickly it get's closer and closer to the actual.
It seems to work, so it must be valid. And it sure as heck is faster
than the drift method. This takes me 10-15 minutes max from when I do
the rough scope setup. And I'm doing something the whole time instead
of sitting around waiting for drift.
Now, I haven't really worked out what is going on, nor done the math.
It just heuristically seems right and it does work. If anybody figures
this out more precisely, are there things that would make this
technique better?? For example, what is the optimal delta from Polaris
for the 2nd star? Is making a 50%-of-the-error adjustment in the mount
the optimal when pointing towards Polaris? Or should it actually be
150% correction?
Any thoughts?
Jim
|
|
| |
Date: 10 Nov 2006 05:07:38
From: Mark Gingrich
Subject: Re: Polar Alignment using Digital Setting Circles
|
"HeyJP" wrote:
> I have been trying a technique using the Digital Setting Circles to
> rapidly iterate in on proper alignment. I haven't gone through the
> math or the geometry, I've just been doing something that intuitively
> seems reasonable. And, it seems to work. Has anyone else done this?
You've rediscovered a technique -- prosaically dubbed "Method B" --
that was described in the following:
Truman P. Kohman. "Polar Alignment of Portable Equatorial
Telescopes." _Sky and Telescope_, February 1976, pp. 135-139.
Though Kohman suggests modifying your Step 4 so that you end up with
Polaris centered in the field of view. I don't know if this assures
the most rapid convergence (i.e., fewest iterations); nor does Kohman
mention anything about optimal strategies in his brief, non-technical
article.
--
~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
Mark Gingrich grinch@rahul.net San Leandro, California
|
| | |
Date: 10 Nov 2006 06:13:30
From: Loser
Subject: Re: Polar Alignment using Digital Setting Circles
|
"Mark Gingrich" <grinch@rahul.net > wrote in message
news:ej11eq$or4$1@blue.rahul.net...
> "HeyJP" wrote:
>
>> I have been trying a technique using the Digital Setting Circles to
>> rapidly iterate in on proper alignment. I haven't gone through the
>> math or the geometry, I've just been doing something that intuitively
>> seems reasonable. And, it seems to work. Has anyone else done this?
>
>
> You've rediscovered a technique -- prosaically dubbed "Method B" --
> that was described in the following:
>
> Truman P. Kohman. "Polar Alignment of Portable Equatorial
> Telescopes." _Sky and Telescope_, February 1976, pp. 135-139.
>
> Though Kohman suggests modifying your Step 4 so that you end up with
> Polaris centered in the field of view. I don't know if this assures
> the most rapid convergence (i.e., fewest iterations); nor does Kohman
> mention anything about optimal strategies in his brief, non-technical
> article.
The bottom line here is YOU CAN"T KEEP SCREWING WITH YOUR
TRIPOD...there comes a point where 1/2 the distance takes twice as long.
Diminishing returns.
|
| | |
Date: 09 Nov 2006 21:01:08
From: Brian Tung
Subject: Re: Polar Alignment using Digital Setting Circles
|
Mark Gingrich wrote:
> You've rediscovered a technique -- prosaically dubbed "Method B" --
> that was described in the following:
>
> Truman P. Kohman. "Polar Alignment of Portable Equatorial
> Telescopes." _Sky and Telescope_, February 1976, pp. 135-139.
>
> Though Kohman suggests modifying your Step 4 so that you end up with
> Polaris centered in the field of view. I don't know if this assures
> the most rapid convergence (i.e., fewest iterations); nor does Kohman
> mention anything about optimal strategies in his brief, non-technical
> article.
When are we going to hear about hook and crook? I'm really curious as
to how it works.
--
Brian Tung <brian@isi.edu >
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.html
|
| | | |
Date: 10 Nov 2006 20:33:50
From: Mark Gingrich
Subject: Re: Polar Alignment using Digital Setting Circles
|
Brian Tung asked:
> When are we going to hear about hook and crook? I'm really curious as
> to how it works.
I'm going public with H&C when my test users are happy with it.
At present, half are of the opinion that it's too easy to make
mistakes, despite the time savings over the drift method. And
frankly, they're right.
--
~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
Mark Gingrich grinch@rahul.net San Leandro, California
|
| | | | |
Date: 10 Nov 2006 16:28:58
From: Brian Tung
Subject: Re: Polar Alignment using Digital Setting Circles
|
Mark Gingrich wrote:
> I'm going public with H&C when my test users are happy with it.
> At present, half are of the opinion that it's too easy to make
> mistakes, despite the time savings over the drift method. And
> frankly, they're right.
Is there any chance I can take a look at it, if I sign an NDA?
--
Brian Tung <brian@isi.edu >
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.html
|
| |
Date: 09 Nov 2006 23:04:07
From: AM
Subject: Re: Polar Alignment using Digital Setting Circles
|
HeyJP wrote:
Hi
I've done your method in the past, but it is
NOT the same as a drift alignment. Somewhere
between 50% and 100 % is a good amount to
move the tripod/wedge when doing this. A
100% move at the start will be off, but will
give you a good idea how far off you really are.
I gaurentee there will be nights when this can
and will take a while.
All this said, I preffer a true polar alignment.
Started at the onset of dusk, you can be ready
when darkness really arrives. Takes me at most
around 45 minutes for a good drift alignment, and
half again as fast if it is at the same local club
site that I haunt...
You practice drift alignment enough, and it becomes
insticive. Very easy, and simple.
--
AM
http://sctuser.home.comcast.net
CentOS 4.3 Fluxbox 0.9.13
|
| |
Date: 09 Nov 2006 17:02:29
From: HeyJP
Subject: Re: Polar Alignment using Digital Setting Circles
|
AstroApp. wrote:
>
> 1. Do you know the resolution of the encoders used for the DSC's?
> The optical encoders inside my C-11 have a resolution of 1.2
> arcminutes which is no great shakes.
>
> 2. How long an exposure can you make, using this method of polar
> alignment, before any field rotation shows up in the picture?
>
AstroApp,
1. I'll have to check my encoders. I no longer remember. They were
after-market and kinda nice, but I'll have to check.
2. I've done some 20-minute exposures without problem. I haven't
really pushed it since. I do know that for several hours, I get great
results when using the DSCs as target object is nearly always in the
field of a mid-powered eyepiece when I crank it around. Gotta love
that.
Good seeing,
Jim
|
| |
Date: 09 Nov 2006 16:58:05
From: HeyJP
Subject: Re: Polar Alignment using Digital Setting Circles
|
Hey Guys,
Thanks for the responses.
I have done 20 minute exposures with no noticeable rotation since using
this technique. But I haven't been as active in the last few months.
I will play with the 100% and 150% corrections to gather "empirical"
data. But, when I get out, I get lazy and just do what seems to work
and get the eye glued onto the eyepiece as soon as possible.
What I have noticed is that when using this technique, for a several
hour observing session, dialing in objects with the DGC is pretty
awesomely accurate. I get disappointed if the object is more than 25%
away from the center of a medium power lens. ;-)
Thanks for your inputs, again!!
Jim
|
| |
Date: 10 Nov 2006 13:31:26
From: Llanzlan Klazmon the 15th
Subject: Re: Polar Alignment using Digital Setting Circles
|
"HeyJP" <jp.pollock@gmail.com > wrote in news:1163107837.170830.275790
@k70g2000cwa.googlegroups.com:
<SNIP >
> Now, I haven't really worked out what is going on, nor done the math.
> It just heuristically seems right and it does work. If anybody figures
> this out more precisely, are there things that would make this
> technique better?? For example, what is the optimal delta from Polaris
> for the 2nd star? Is making a 50%-of-the-error adjustment in the mount
> the optimal when pointing towards Polaris? Or should it actually be
> 150% correction?
>
> Any thoughts?
I guess you could try 100%, 150% and see which method gets the fastest
convergence. Nothing like an empirical test!
Klazmon
>
> Jim
>
|
| |
Date: 09 Nov 2006 21:54:12
From: AstroApp.
Subject: Re: Polar Alignment using Digital Setting Circles
|
On 9 Nov 2006 13:30:38 -0800, "HeyJP" <jp.pollock@gmail.com > wrote:
>I have a 10 year old 8" Celestron SCT to which I added Digital Setting
>Circles...
>(6) After 3 or 4 iterations of this technique, my scope seems to get
>pretty well aligned. When I spin back to Polaris' theoretical
>position, very quickly it get's closer and closer to the actual.
>
>It seems to work, so it must be valid.
>Any thoughts?
Two things come immediately to mind.
1. Do you know the resolution of the encoders used for the DSC's?
The optical encoders inside my C-11 have a resolution of 1.2
arcminutes which is no great shakes.
2. How long an exposure can you make, using this method of polar
alignment, before any field rotation shows up in the picture?
AstroApp
|
| | |
Date: 09 Nov 2006 21:55:40
From: AstroApp.
Subject: Re: Polar Alignment using Digital Setting Circles
|
Once you have done your method of polar alignment, can you check a
drift alignment and see how close it is? Maybe you can save this step
for last and get a really fine result.
AstroApp
|
| | | |
Date: 10 Nov 2006 13:27:05
From: Llanzlan Klazmon the 15th
Subject: Re: Polar Alignment using Digital Setting Circles
|
AstroApp. <AstroApp@BLOCKEDearthllink.net > wrote in
news:ks87l2tdt4cmg1ib6mp375090ikts85n4o@4ax.com:
> Once you have done your method of polar alignment, can you check a
> drift alignment and see how close it is? Maybe you can save this step
> for last and get a really fine result.
>
> AstroApp
>
>
That was my thought as well. Do a drift alignment at the end to do any fine
tuning. Other than that the OP's method sounds like a good idea to speed
things up.
Kalzmon.
|
| |
Date: 10 Nov 2006 09:44:23
From: HeyJP
Subject: Re: Polar Alignment using Digital Setting Circles
|
Loser wrote:
>
> The bottom line here is YOU CAN"T KEEP SCREWING WITH YOUR
> TRIPOD...there comes a point where 1/2 the distance takes twice as long.
>
> Diminishing returns.
Dear Loser,
After reading the Covington/Univ of Georgia paper mentioned in this
thread, it is much easier to understand. Making a 50-75% correction of
the mount at the Polaris end only is also shifting the orientation at
the Star #2 end. So a 50% correction can actually result in MUCH more
than a 50% more accurate alignment. In fact, as Covington points out,
a 100% correction in certain situations can result in NO solution.
It is theoretically possible that if your scope is close to correct at
the start and you pick the "right" 2nd star, with one 50% correction
you might nail it to a very small part of a degree. This is not
necessarilty intuitively felt.
Thanks for everybody's inputs and thoughts.
Jim
|
| |
Date: 10 Nov 2006 09:34:02
From: HeyJP
Subject: Re: Polar Alignment using Digital Setting Circles
|
Rick Evans wrote:
>
> Michael A. Covington has addressed the theory of this method
> in an online paper.
>
> http://www.covingtoninnovations.com/astro/iterating.pdf
>
> I direct your attention to Case D which
> specifically asked the question "What if your alignment star
> is Arcturus or Spica?"
>
Rick,
Perfect! This is EXACTLY what I was hoping to find. Heuristically it
seemed that making 50% corrections should be the way to go, but there
are too many axes for my puny mind to easily grasp this. The
Covington/Univ of Georgia article above explains it. If you do 100%
corrections, there are certain situations (spica and arcturus as Star
#2 specific) that it never converges to a solution. However if you
make 50-75% corrections at the Polaris end, even they work.
Excellent. Thank you.
|
| | |
Date: 11 Nov 2006 03:13:13
From: Loser
Subject: Re: Polar Alignment using Digital Setting Circles
|
"HeyJP" <jp.pollock@gmail.com > wrote in message
news:1163180042.796367.323140@h54g2000cwb.googlegroups.com...
>
> Rick Evans wrote:
>>
>> Michael A. Covington has addressed the theory of this method
>> in an online paper.
>>
>> http://www.covingtoninnovations.com/astro/iterating.pdf
>>
>> I direct your attention to Case D which
>> specifically asked the question "What if your alignment star
>> is Arcturus or Spica?"
>>
>
> Rick,
>
> Perfect! This is EXACTLY what I was hoping to find. Heuristically it
> seemed that making 50% corrections should be the way to go, but there
> are too many axes for my puny mind to easily grasp this. The
> Covington/Univ of Georgia article above explains it. If you do 100%
> corrections, there are certain situations (spica and arcturus as Star
> #2 specific) that it never converges to a solution. However if you
> make 50-75% corrections at the Polaris end, even they work.
Oh GOD..ahhhhhh...
|
| |
Date: 10 Nov 2006 14:22:07
From: Chris L Peterson
Subject: Re: Polar Alignment using Digital Setting Circles
|
On 9 Nov 2006 13:30:38 -0800, "HeyJP" <jp.pollock@gmail.com > wrote:
>I have a 10 year old 8" Celestron SCT to which I added Digital Setting
>Circles (DSC). Even though the DSC controller with compensate for a
>non-aligned polar axis, when I do astro-photography it is important to
>get very good axis alignment first.
I don't know if it's an option with this particular scope, but a good
polar alignment scope will allow you to align accurately enough to make
10-20 minute exposures without significant field rotation. (You say
"photography", but I'm assuming you are using a digital camera of some
kind, not film).
IMO, a better option than what you propose, and also better than drift
alignment, is to take advantage of your camera. There are tools such as
TPoint which will directly give you your alignment error very quickly.
One good free option is PoleAlignMax, which will let you tune in your
alignment with ease. Finally, there are a variety of camera-based drift
techniques that people have developed. These are conceptually similar to
traditional drift alignment, but quite a bit faster because the drift
times can be significantly shorter.
_________________________________________________
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
|
| |
Date: 10 Nov 2006 12:11:54
From: Rick Evans
Subject: Re: Polar Alignment using Digital Setting Circles
|
"HeyJP" <jp.pollock@gmail.com > wrote in message
news:1163107837.170830.275790@k70g2000cwa.googlegroups.com...
<snip >
>(1) First I set my scope up with a rough alignment aimed at the North
> Star. Then I pick my two stars that I will use for the technique: the
> North Star and another bright star that is at least 90 degrees away,
> such as Spica in the summer. I'll use Spica as my example
>
> (2) I turn my scope so that Spica (or whatever Star#2 is) is centered
> in my eyepiece. I then tell my DSC controller that this is Spica and
> to do a 1 star alignment. The DSC reads Spica's coordinates: 13h 26m
> and -11.2 degrees.
<snip >
> Any thoughts?
Michael A. Covington has addressed the theory of this method
in an online paper.
http://www.covingtoninnovations.com/astro/iterating.pdf
I direct your attention to Case D which
specifically asked the question "What if your alignment star
is Arcturus or Spica?"
--
Rick Evans
----------------------------------------------------------------
Lat +42° 11' 07"
Lon -71° 04' 35"
----------------------------------------------------------------
http://www.chempensoftware.com
----------------------------------------------------------------
http://mysite.verizon.net/hiltonevans33050/astroimaging/astroimaging.htm
|
| |
Date: 09 Nov 2006 22:56:59
From: HeyJP
Subject: Re: Polar Alignment using Digital Setting Circles
|
Loser wrote:
>
> The bottom line here is YOU CAN"T KEEP SCREWING WITH YOUR
> TRIPOD...there comes a point where 1/2 the distance takes twice as long.
>
> Diminishing returns.
>
The few times I've tried this, after 3 or 4 iterations (which takes
about 10 minutes), I'm within part of a degree. Polaris is in my
mid-power eyepiece field of view. Again, I haven't tried to figure the
math of what this technique does, but I tend to like one of the earlier
suggestions of doing a couple or three fast passes of this, then finish
with the drift method. You might cut a couple of drift iterations out
which take a long time.
The only reason I would do this is to cut time, not get frustrated ;-)
|
| |
Date: 09 Nov 2006 22:48:21
From: HeyJP
Subject: Re: Polar Alignment using Digital Setting Circles
|
Mark Gingrich wrote:
>
> You've rediscovered a technique -- prosaically dubbed "Method B" --
> that was described in the following:
>
> Truman P. Kohman. "Polar Alignment of Portable Equatorial
> Telescopes." _Sky and Telescope_, February 1976, pp. 135-139.
>
Mark,
Excellent. Thanks for noting this. I'm freakin' impressed that you
can yank a 4 page article from a 30 year old pre-internet magazine iss.
Wow.
I have googled the heck out of this assuming that if it was half valid,
it would be written up SOMEWHERE. But couldn't find it. That's why I
posted it... to dig something out of the woodwork and get some advice.
Thanks!
Jim
|
| | |
Date: 10 Nov 2006 20:41:09
From: Mark Gingrich
Subject: Re: Polar Alignment using Digital Setting Circles
|
"HeyJP" wrote:
> Excellent. Thanks for noting this. I'm freakin' impressed that you
> can yank a 4 page article from a 30 year old pre-internet magazine iss.
Actually, my memory isn't that good. It just so happens that I've
a special interest in polar alignment, a consequence of my tinkering
with a yet-unpublished method of my own, called "hook and crook." I
began collecting articles on polar alignment several years ago to see
if anything similar to the H&C method had been tried. (And indeed
someone came close -- though his scheme is a tad more elaborate, and
his algorithm contains a subtle mathematical blunder that hinders its
efficiency. This might explain why it never caught on!)
So I only had to reach into my file folder on polar alignment to
retrieve Kohman's 30-year-old article.
My file folder is obscenely fat, by the way, stuffed with photocopies
of papers from magazines, journals, and books, some dating back to the
late 1800s. I can confidently declare that the variety of ways to
align a telescope mount with the pole are more numerous than recipes
for meat loaf. ;)
--
~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
Mark Gingrich grinch@rahul.net San Leandro, California
|
| |
Date: 10 Nov 2006 06:46:00
From: Terry B
Subject: Re: Polar Alignment using Digital Setting Circles
|
"HeyJP" <jp.pollock@gmail.com > wrote in message
news:1163107837.170830.275790@k70g2000cwa.googlegroups.com...
> I have a 10 year old 8" Celestron SCT to which I added Digital Setting
> Circles (DSC). Even though the DSC controller with compensate for a
> for the 2nd star? Is making a 50%-of-the-error adjustment in the mount
> the optimal when pointing towards Polaris? Or should it actually be
> 150% correction?
>
> Any thoughts?
>
> Jim
This method is very similar to what is described in the instruction manual
using a Vixen SS2000.
I have used it but find that aiming the scope south (in my case) accurately
with the southern celestial pole in the centre of my field is good enough
for my purpose.
Terry B
|
| |
Date: 14 Nov 2006 04:35:48
From: George Normandin
Subject: Re: Polar Alignment using Digital Setting Circles
|
"HeyJP" < > wrote
...
> The most commonly used technique for accurate polar alignment is the
> "drift method" which is detailed on many, many astro websites. However
> it takes FOREVER and is VERY tedious.
>
> I have been trying a technique using the Digital Setting Circles to
> rapidly iterate in on proper alignment........
I've seen your method described by Roland C as a method to roughly align AP
mounts during the day time using 1st mag stars.
Optical Guidance Systems now sends out a sheet with each mount ( I use an
OGS 100 GEM) that describes this method but notes that it is most accurate
for setting altitude. For zeroing in Az OGS recommends slewing to a star
near 0 Degrees Dec, sync'ing on that, then slewing to another star higher in
altitude, also with near 0 Dec, and adjusting only the Az to center the
second star. Personally, I just use a polar scope and then 'drift' to tweak
the alignment of my MI-250 mount. If you do CCD with short exposures the
image processing software will adjust out any remaining image rotation. I
once had the nose of my camera come lose and each of ten 60 second exposures
was very rotated from the prior exposure. IRIS was able to match up the ten
exposures rotating them thru over 30 degrees to match up.
George N
|
|
