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Re: [Losmandy_users] Polar Alignment Questions


Jul 29, 2001

 


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#5174 Jul 29, 2001

I ought to know the answer to these questions by this time, and so it isembarrassing to have to expose my ignorance publicly.�� However, ifsomeone would explain this to me, I would be most appreciative.



Question 1:



My broad question is, what is the consequence of not being well polaraligned when using the Gemini to goto stars?



I understand that if I am not polar aligned, I will not trackproperly.�� But suppose I perform an initial-alignment, immediatelygoto a star of my interest, and guide manually for 10 minutes, keeping mystar in the center?�� Suppose now, 10 minutes later, I performanother goto.�� Will the Gemini find my new target star, or will mylack of polar-alignment keep it from knowing where the star is?



Another way of stating this question is, if I am not polar-aligned, do Ihave to initial-align the mount every time I want to find something withthe Gemini?





Question 2:



Is drift-alignment done with the tracking motor on or off?�� Andby the way, where is a good description of drift-alignment?�� Theonly one I know of is in the Celestron manual.





Question 3:



I have been led to believe that I can do either an initial alignmentor additional alignments on any bright star.�� However, the Geminihas just a few stars listed for this purpose on its menus.�� Can Ialign on stars which are not on those lists?





Mike Rudolph



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#5180 Jul 29, 2001

On 7/29/01 10:21 AM Michael Rudolph wrote:

> I ought to know the answer to these questions by this time, and so it is

> embarrassing to have to expose my ignorance publicly. However, if someone

> would explain this to me, I would be most appreciative.

>

> Question 1:

>

> My broad question is, what is the consequence of not being well polar

> aligned when using the Gemini to goto stars?

>

> I understand that if I am not polar aligned, I will not track

> properly. But suppose I perform an initial-alignment, immediately goto a

> star of my interest, and guide manually for 10 minutes, keeping my star in

> the center? Suppose now, 10 minutes later, I perform another goto. Will

> the Gemini find my new target star, or will my lack of polar-alignment keep

> it from knowing where the star is?



It will know where it thinks the star is, but it will be a little off --

depending on how far off your alignment is. Tracking on the star for 10 min.

Wont help any. The Gemini doesn't know you are tracking the star unless you

align on it again.

> Another way of stating this question is, if I am not polar-aligned, do I

> have to initial-align the mount every time I want to find something with

> the Gemini?



No, you don't have to initial align. You can do additional alignments, but

you don't have to start from the beginning. Additional alignments may help

the Gemini correct for poor polar alignment.

> Question 2:

>

> Is drift-alignment done with the tracking motor on or off? And by the way,

> where is a good description of drift-alignment? The only one I know of is

> in the Celestron manual.



Tracking on.



www.aa6g.org/Astronomy/Articles/drift_align.html

> Question 3:

>

> I have been led to believe that I can do either an initial alignment or

> additional alignments on any bright star. However, the Gemini has just a

> few stars listed for this purpose on its menus. Can I align on stars which

> are not on those lists?



I believe so. If you are using a PC (I use a Mac with Starry Night). I

select a star (any star) in Starry Night, click slew in the InfoGenie

plug-in. The mount (Gemini equipped MI-250 GOTO in my case) goes to where it

thinks the star is. Center it with the Gemini hand piece. Activate the menu

and select "PC Align". I haven't tried it (the PC Align is so easy), but I

believe you can use objects from other lists in the Gemini. Stars are best

because you know when they are centered. Hard to tell where the exact center

of a nebula is.



Regards,

Robin



Astro Accessories by Robin Casady

Stainless Steel Weights & Dovetail Saddles

www.CarmelCoast.com/Astro/sales.html



Casady & Greene, Inc.

www.casadyg.com



----------------------------

#5181 Jul 29, 2001

Robin Casady wrote:

> > Question 3:

> >

> > I have been led to believe that I can do either an initial alignment

> or

> > additional alignments on any bright star. However, the Gemini has

> just a

> > few stars listed for this purpose on its menus. Can I align on

> stars which

> > are not on those lists?

>

> I believe so. If you are using a PC (I use a Mac with Starry Night). I

>

> select a star (any star) in Starry Night, click slew in the InfoGenie

> plug-in. The mount (Gemini equipped MI-250 GOTO in my case) goes to

> where it

> thinks the star is. Center it with the Gemini hand piece. Activate the

> menu

> and select "PC Align". I haven't tried it (the PC Align is so easy),

> but I

> believe you can use objects from other lists in the Gemini. Stars are

> best

> because you know when they are centered. Hard to tell where the exact

> center

> of a nebula is.



The 20 or so stars that are in the initial align menu have been entered

into the database in a higher precision mode than the stars in the

general menu on the Gemini. If you read the Gemini manual, you'll find

that the initial align is best done with one of these high precision

stars. If I use these high precision stars to do the initial and

additional aligns (and I am well polar aligned through the drift

method), I can slew from one part of the sky to well over to the

opposite side of the meridian and have the object dead centered

consistently on an ST-7 chip working at a 32" focal length (19' x 29'

FOV.) Last time out it worked flawlessly (or maybe I'm finally learning

how to use it correctly g>).





--

Jim Girard

www.teleport.com/~argo







----------------------------

#5183 Jul 29, 2001

On 7/29/01 11:26 AM Jim Girard wrote:

> Robin Casady wrote:

>

>>> Question 3:

>>>

>>> I have been led to believe that I can do either an initial alignment

>> or

>>> additional alignments on any bright star. However, the Gemini has

>> just a

>>> few stars listed for this purpose on its menus. Can I align on

>> stars which

>>> are not on those lists?

>>

>> I believe so. If you are using a PC (I use a Mac with Starry Night). I

>>

>> select a star (any star) in Starry Night, click slew in the InfoGenie

>> plug-in. The mount (Gemini equipped MI-250 GOTO in my case) goes to

>> where it

>> thinks the star is. Center it with the Gemini hand piece. Activate the

>> menu

>> and select "PC Align". I haven't tried it (the PC Align is so easy),

>> but I

>> believe you can use objects from other lists in the Gemini. Stars are

>> best

>> because you know when they are centered. Hard to tell where the exact

>> center

>> of a nebula is.

>

> The 20 or so stars that are in the initial align menu have been entered

> into the database in a higher precision mode than the stars in the

> general menu on the Gemini. If you read the Gemini manual, you'll find

> that the initial align is best done with one of these high precision

> stars. If I use these high precision stars to do the initial and

> additional aligns (and I am well polar aligned through the drift

> method), I can slew from one part of the sky to well over to the

> opposite side of the meridian and have the object dead centered

> consistently on an ST-7 chip working at a 32" focal length (19' x 29'

> FOV.) Last time out it worked flawlessly (or maybe I'm finally learning

> how to use it correctly g>).



The Mountain Instruments manual on the Gemini says, "With the exception of

the Solar System objects, all coordinates are stored rounded to 20 arc-sec,

giving 10 arc-sec accuracy for the standard epoch 2000.0. The coordinates

are precessed to the equinox of the day when the object is selected.

Nutation is neglected. The apparent place is calculated, taking refraction

(for standard air pressure and temperature) into account. For the moon, the

topocentric coordinates are calculated."



Is that incorrect, has it changed with a new version, or am I

misunderstanding something?



Regards,

Robin



Astro Accessories by Robin Casady

Stainless Steel Weights & Dovetail Saddles

www.CarmelCoast.com/Astro/sales.html



Casady & Greene, Inc.

www.casadyg.com



----------------------------

#5184 Jul 29, 2001

Robin,

> > The 20 or so stars that are in the initial align menu have been entered

> > into the database in a higher precision mode than the stars in the

> > general menu on the Gemini. If you read the Gemini manual, you'll find

> > that the initial align is best done with one of these high precision

> > stars. If I use these high precision stars to do the initial and

> > additional aligns (and I am well polar aligned through the drift

> > method), I can slew from one part of the sky to well over to the

> > opposite side of the meridian and have the object dead centered

> > consistently on an ST-7 chip working at a 32" focal length (19' x 29'

> > FOV.) Last time out it worked flawlessly (or maybe I'm finally learning

> > how to use it correctly g>).

>

> The Mountain Instruments manual on the Gemini says, "With the exception of

> the Solar System objects, all coordinates are stored rounded to 20 arc-sec,

> giving 10 arc-sec accuracy for the standard epoch 2000.0. The coordinates

> are precessed to the equinox of the day when the object is selected.

> Nutation is neglected. The apparent place is calculated, taking refraction

> (for standard air pressure and temperature) into account. For the moon, the

> topocentric coordinates are calculated."

>

> Is that incorrect, has it changed with a new version, or am I

> misunderstanding something?



That's correct. After these calculations the Gemini knows the "Apparent Position"

of the object, to which an accurately polar aligned ideal telescope (without

any imperfections) should point. Since no telescope or alignment is perfect,

Gemini uses the data given by the previous (initial + additional) alignments

to calculate a model of (some, but the most important) imperfections to

calculate the "modelled" (telescope) position.



The corrections are recalculated just before executing a GoTo, sometimes even

twice, because it could happen that the positions shifts caused by the modelling

require a meridian flip.



After the GoTo operation, the Gemini (Level 1 or 2) is tracking only in RA.

That means, that the RA/DEC position the telescope is pointing to shifts

slightly with time passing by. You can see this while looking at the RA/DEC

display. The coordinates sent back to a PC are also corrected according to

the model (but not while slewing to save time).



Maybe once I'll find the time to introduce a "Closed Loop Mode" to adjust

tracking (RA + DEC) to keep the center of the FOV at a given coordinate.



The Alignment Objects are stored with an accuracy far better than an arc second.

For Catalog Objects, the position error can be up to 10 arc secs (epoch 2000.0).

Since I've introduced the Gaussian Least Square error correction already in

Level 1 (that means, after Additional alignments a calculation is done to

best fit the model parameters to all alignments) I think that the algorithm is

very forgiving to small positioning or systematical (rotated star diagonal, ...)

or coordinate errors, so alignments with catalog objects should work well.



Solar system object coordinates have to be calculated for the given date,

and, in case of the moon, also for the geographic coordinates. The calculations

have an accuracy of about one arc minute, somewhat worse than the "fixed" objects,

so they could influence accuracy, but only for a very discriminating observer.



Mmh, does this answer your question??



Regards,

Rene'







----------------------------

#5186 Jul 30, 2001

On 7/29/01 3:56 PM goerlich@... wrote:

> The Alignment Objects are stored with an accuracy far better than an arc

> second.

> For Catalog Objects, the position error can be up to 10 arc secs (epoch

> 2000.0).

> Since I've introduced the Gaussian Least Square error correction already in

> Level 1 (that means, after Additional alignments a calculation is done to

> best fit the model parameters to all alignments) I think that the algorithm is

> very forgiving to small positioning or systematical (rotated star diagonal,

> ...)

> or coordinate errors, so alignments with catalog objects should work well.



So, it really is more accurate to use the Alignment Objects than the catalog

objects, but not by a lot. I that correct?



Using the PC align would depend on the accuracy of the planetarium program,

and whether it calculates apparent location with the optical effects of the

atmosphere. Is that correct?



Regards,

Robin



Astro Accessories by Robin Casady

Stainless Steel Weights & Dovetail Saddles

www.CarmelCoast.com/Astro/sales.html



Casady & Greene, Inc.

www.casadyg.com



----------------------------

#5193 Jul 31, 2001

Robin,

> > The Alignment Objects are stored with an accuracy far better than an arc

> > second.

> > For Catalog Objects, the position error can be up to 10 arc secs (epoch

> > 2000.0).

> > Since I've introduced the Gaussian Least Square error correction already in

> > Level 1 (that means, after Additional alignments a calculation is done to

> > best fit the model parameters to all alignments) I think that the algorithm is

> > very forgiving to small positioning or systematical (rotated star diagonal,

> > ...)

> > or coordinate errors, so alignments with catalog objects should work well.

>

> So, it really is more accurate to use the Alignment Objects than the catalog

> objects, but not by a lot. I that correct?



Yes. Other error sources usually will exceed the 10 arcsecs.

> Using the PC align would depend on the accuracy of the planetarium program,

> and whether it calculates apparent location with the optical effects of the

> atmosphere. Is that correct?



Yes and No, all I need are the catalog coordinates for the current epoch

(i.e. precessed to the epoch of the day). Local effects like refraction and

the telescope model are applied by the Gemini. The planetarium program should

use the "High Precision" coordinate mode, that's the reason the Gemini starts

up in this mode in contrast to the LX200 because TheSky uses the mode the

telescope is in.



Regards,

Rene'



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