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Which method of alignment is most accurate?

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#1 Bruce N

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Posted 10 July 2013 - 09:54 PM

Sky Align, Auto Two-Star Align, Two-Star Align or One-Star Align?

#2 Sorny

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Posted 10 July 2013 - 11:07 PM

Sky Align, Auto Two-Star Align and Two-Star Align are all the same accuracy wise.

Most of us use Auto two-star because it is fast.

The accuracy improves if you choose stars between 40-70 degrees in elevation and at least 90 degrees apart in azimuth. I like to use Polaris as one of the stars since it is easy to find.

#3 Digital Don

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Posted 11 July 2013 - 02:28 AM

I like to use Polaris as one of the stars since it is easy to find.


Me too! I use Polaris as the second alignment star.

Not only is it easy to find, but since it "doesn't move", I know I'll get at least one of my alignment stars 'perfect'.

Don:usa:

#4 Bob Griffiths

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Posted 11 July 2013 - 07:39 AM

AS noted All 3 methods identical in accuracy and all three (including Sky-Align) only use two stars... the third star you use with Sky-Align is NOT an alignment star...its only a conformation star used by the scope to check that it guessed correctly the names of the other two stars...

Most of us do in fact use auto or manual 2 star methods as well as use Polaris as one of those two stars...

Bob G

#5 Larry F

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Posted 11 July 2013 - 10:09 AM

The accuracy depends on which stars you use. You can download Best Pairs II which will tell you which combination gives the highest accuracy at any given date, time and location (note: it's specifically designed for Meade LX-200 but I think it provides accurate data for Celestron, just no interface), but you really don't need it. The proper way to maximize the accuracy of alignment it is to pick two widely spaced objects initially and then replace the alignment stars a couple of times as you observe. This refines the model of the sky in the HC's computer and gets the go-to's to be extremely accurate. The replacement process is detailed in the manual.

This past Saturday, I did a Solar System Align on Saturn early in the evening, then went over to Vega, centered it, replaced "Unassigned", then slewed back to Arcturus, replaced Saturn, then over to Dubhe and replaced Vega. After that the object of every goto was in the field of my Mallincam, which is a good bit less than half a degree even with aggressive focal reduction. When you slew to an object that you select from the computer, you can use it to replace one of the two alignment stars/objects. It doesn't matter if it's a star, Messier object, NGC object or whatever. Sometimes you can simply replace an object with itself after you slew back to it from somewhere else. The scope's computer knows the object's exact location. Doing the replacement makes the computer model of the sky conform to the scope's mechanical position. There's no downside to doing this a few times a night, and it takes seconds.

#6 HeyJP

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Posted 12 July 2013 - 08:40 PM

If my wife would agree to cut down one big tree, I could use Polaris every time too. But, <sigh>, not going to happen. So I use a different pair of stars, in the general southern direction, changing them every month or so.

I did find an excellent iPhone/iPad app called "Alt Az Align" that automatically picks a prioritized set of optimal alignment pairs for any location and current date/time. You can even pick from several star-lists including the Celestron alignment set. If you pick the latter, it finds and ranks the best pairs from the ones available to your Celestron controller.

Best,

Jim in Boulder

#7 cn register 5

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Posted 13 July 2013 - 07:20 AM

I'd have to demolish the house to see Polaris.

I have good views to the South so choose a star to the SW and a star to the SE, currently Arcturus and Altair.

I don't think that choosing the best stars matters as much as doing a good job of centring them consistently.

Chris

#8 Bruce N

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Posted 13 July 2013 - 01:56 PM

Thank you all. Similar to Jim, Polaris is behind trees unless I move out into the street. I am new to the cpc and just awaiting a clear night (I live near the beach in central California and we often get foggy nights in the summer).

I have also found my cpc 800 is just too big to take with me for family vacations- there is no room for it in the car along with three or four suitcases and three or four people- so sad. I'll have to take my old (smaller Mak-Cas) scope on our August trip to Yosemite.

#9 HeyJP

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Posted 13 July 2013 - 04:04 PM

Bruce,

You could always leave a couple of kids at home. Teaches them young that life choices are tough.

Jim

#10 Digital Don

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Posted 13 July 2013 - 08:13 PM

Not to worry, guys!

If we are talking about Alt-Az alignment, Polaris isn't really 'better' than any other star. It's only easier to center since it essentially doesn't move. If you're precise, you can get sufficient accuracy using any alignment stars.

Best Pair II is actually pretty neat software, however I rarely use it. Last night I used Altair and Polaris as alignment stars for my CPC 1100/MallinCam VSS+/MFR-5. I observed for about 3 hours. During that time, every object I sent the scope to was in the field. Not necessarily centered, but well inside the field. I consider that quite acceptable.

Here a a couple of items to consider when performing any aligment method. First, once you start the alignment process, do not refocus. Doing so may slightly shift the optical center of the field of view and adversly affect the accuracy of your alignment. Second, do not rotate the diagonal during alignment for the exact same reason.

Don:usa:

#11 Bruce N

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Posted 13 July 2013 - 09:32 PM

Good point, Jim. They are grown kids, flying back to CA for a family vacation. But your suggestion is a good one. After all, what is most important, double stars, galaxies, nebulae, teaching young adults life lessons or a family experience? Seems clear to me, but I will not ask my wife her opinion on this one.

#12 cn register 5

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Posted 14 July 2013 - 04:14 AM

By the way, while Polaris is fine for an AltAz mount it's a terrible choice for an equatorial mount because it's so close to the hour angle axis.

I know this discussion is about AltAz mounts but I think it's worth mentioning.

Chris

#13 Digital Don

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Posted 14 July 2013 - 03:39 PM

By the way, while Polaris is fine for an AltAz mount it's a terrible choice for an equatorial mount because it's so close to the hour angle axis.

Chris


Correct! And for similar reasons, one should never use a star at or near the Zenith for Alt-Az alignment.

Don

#14 iam1ru12

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Posted 15 July 2013 - 11:03 AM

.....I have also found my cpc 800 is just too big to take with me for family vacations- there is no room for it in the car along with three or four suitcases and three or four people- so sad. I'll have to take my old (smaller Mak-Cas) scope on our August trip to Yosemite.


Either time for a bigger car or strap them to the roof. :grin:

#15 rookie

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Posted 15 July 2013 - 10:28 PM

Most often, accuracy depends on the user and how well the alignment stars are centered in the field of view. In other words, if your goto is off, you can do better. There's lots of ways to improve the process.

#16 Doc Bob

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Posted 18 July 2013 - 07:13 PM

. . . auto-2-star alignment. Very accurate and fast! But you have to try all of the other methods - to learn what works best for you!!

Good viewing,
Bob

#17 Paul McC

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Posted 18 July 2013 - 10:46 PM

Whichever one you are most comfortable using.

I prefer the one that works for me without fail, every time:

Auto-Two Star. I always use Polaris as star one, because it's always in my line of sight in my backyard. I then choose a second star as far as possible from Polaris that's not hidden by buildings or trees.

Pointing accuracy can be made very solid by using a high power ep and defocusing the stars into a larger donut that is easily centered/aligned in the FOV, if you want higher accuracy.

Keep in mind though that there are awesome tools in that little hand controller to make pointing to very dim fuzzies even easier.

Further, if you use a laptop (or cell phone) with a program that drives your scope around the sky, you can do some pretty sloppy initial aligning and firm it up later. For instance, if I know I'm going to mostly be viewing in a tiny parcel of sky for some time, I'll have the program (say, Cartes du Ciel, for instance) take me to a bright star, and then I'll sync on that star. This does a highly accurate location lock on this small area that gets most of the objects in this area dead center every time. Once I'm ready to leave that area, I just unsync and the scope goes back to the original alignment. Try it.

For the impossible targets that I'm second guessing and that my laptop isn't finding, there's always the Precision pointing method (can't remember the name) in the Hand Controller that allows you to center a bright nearby star and then takes you over to the challenging dim fuzzy.

Cheers,
Paul

#18 Sorny

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Posted 19 July 2013 - 02:42 AM


For the impossible targets that I'm second guessing and that my laptop isn't finding, there's always the Precision pointing method (can't remember the name) in the Hand Controller that allows you to center a bright nearby star and then takes you over to the challenging dim fuzzy.

Cheers,
Paul


You're thinking of precise goto. It is very nice for the stuff you aren't sure of.

I like to use a reticle eyepiece during alignment, and if I'm going to do a precise goto...






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