Polaris as alignment star?

[gregl]

On this thread https://theskysearchers.com/viewtopic.php?t=31820, in post #16, AstroBee says not to use Polaris as an alignment star. But on another forum there is a thread where many folks do use it with success. It looks like the difference is that for some reason Mead scopes aren’t set up for that, and you wouldn’t want to do that with an eq mount. But many folks have said that Celestron altaz mounts do work fine with Polaris as one of the alignment stars.

So the question is: Is there a reason a Celestron altaz mount should not use Polaris, and if not, why not. I’m not challenging anyone; just trying to learn. THANKS!

[Graeme1858]

My Celestron CGX mount is an Equatorial mount, not an altaz mount so I can't answer your question directly but I can open the debate and tell you why I wouldn't use Polaris on a CGX.

When you first switch the mount on it slews to the Home Position, which is the North Celestial Pole and stops on the switches in the RA and Dec axes. The accuracy of the position is a function of the accuracy of the Polar Alignment. So the mount already knows where Polaris is because the Location, Date and Time have already been entered. Then if you were to do a two star Alignment, say Schedar and Altair, to the East, the mount now has three points of reference. It then asks if you want to add additional calibration stars, to the West.

So using Polaris as an alignment star would be pointless (I'm sure there's a pun there somewhere!) because it is already included in the Pointing Model. I can't remember for sure but I wouldn't be surprised if Polaris was not on the list of stars listed as alignment star candidates on the Hand Controller.

I would agree with Greg, if you use Polaris, then you're doing a one star alignment.

My 2p.

Graeme

[StarBru]

Here is a .PDF file mentioning a polar alignment by iterating on one star and Polaris, but it also mentions it cannot be used on a telescope that has been initialized on two stars in order to compensate for polar alignment errors such as the Celestron NextStar!

I remember years ago following a similar method from an alignment tip on Weazner's Mighty ETX site to align my ETX-90 with good results, but I cannot find the exact method at this time. If I run across it, I will post it here. I must mention, as also mentioned within the link below, that the Meade LX200 and the Meade ETX has a one star alignment method.

[Polar Alignment by Iterating on One Star and Polaris https://www.covingtoninnovations.com/as ... rating.pdf]

[KathyNS]

So the question is: Is there a reason a Celestron altaz mount should not use Polaris, and if not, why not. I’m not challenging anyone; just trying to learn. THANKS!

The key is that you are referring to an alt-az mount. There is no reason at all not to use Polaris as an alignment star for an alt-az mount, though you will need other star(s) as well.

On the other hand, you should not use it for the goto alignment of an EQ mount, because it will give the targeting software absolutely no useful information.

[AstroBee]

I've only ever used EQ mounts and always understood that Polaris should not be used. So maybe I wrongly assumed the same was true for ALT AZ mounts.
I still don't think even with an Alt Az mount that it should be used. When the mount is first initialized and goes to the "home" position, isn't it pointing towards Polaris in this position? Correct me if I'm wrong again, please.

[KathyNS]

I've only ever used EQ mounts and always understood that Polaris should not be used. So maybe I wrongly assumed the same was true for ALT AZ mounts.
I still don't think even with an Alt Az mount that it should be used. When the mount is first initialized and goes to the "home" position, isn't it pointing towards Polaris in this position? Correct me if I'm wrong again, please.

An alt-az mount has no knowledge of its orientation until you start to align it, unlike an EQ mount which can assume it is polar aligned. It can only point towards Polaris after you tell it what direction that is. And, depending on the mount's firmware, the best way to do that may well be to use Polaris as an alignment star.

[Ylem]

I've only ever used EQ mounts and always understood that Polaris should not be used. So maybe I wrongly assumed the same was true for ALT AZ mounts.
I still don't think even with an Alt Az mount that it should be used. When the mount is first initialized and goes to the "home" position, isn't it pointing towards Polaris in this position? Correct me if I'm wrong again, please.

On my Alt-Az SE, (old firmware) it doesn't home or initialize, it literally just goes Duh

I have used Polaris along with a southern star and I get good results.

Of course we must keep in mind; Polaris is that star behind the tree lol

[gregl]

Thank you all for your comments. After reading the above and thinking about this, it seems that the answer is that it depends on the mount. In the case of an eq mount, even a manual one has some sort of beginning orientation. And a computerized one would have a better idea after the time, date, and location were entered into the software. I can also imagine that some alt-az mounts with GPS could have a fairly decent orientation.

But considering the Celestron SE mount, there is no GPS and no “home position.” The only data it has is what you enter. There is no way for it to orient itself to anything at all without user input. You begin with the date, time, and location. So at this point it knows where it is on the planet and where the celestial objects are going to be in relation to that data.

But it has no info as to the orientation of the mount. In fact, the alt mechanism includes a slip clutch, so even if you were to somehow preserve a previous alignment, there is a possibility that the alt could have been moved during setup or take-down. (And mine most definitely is.) And the az orientation could be anything depending on how you set the tripod and attach the mount and, once again, the software has no info about this.

So it would seem to me that aligning on Polaris orients the mount to the alt and az of one star. Then the second star provides two more pieces of data, and you thus have a two-star alignment.

So I think Kathy is correct in this case. Yes?

[StarBru]

Kathy wrote: "An alt-az mount has no knowledge of its orientation until you start to align it, unlike an EQ mount which can assume it is polar aligned. It can only point towards Polaris after you tell it what direction that is. And, depending on the mount's firmware, the best way to do that may well be to use Polaris as an alignment star."

Words of wisdom! Kathy is right on target with this answer. I vaguely remember using Polaris to help with my alignment using my ETX-90 in alt/az mode years ago, and I recall having a good alignment as mentioned earlier.

[JayTee]

As usual, Kathy is spot on.

I have 4 Celestron mounts. 3 of them (CPC, SLT, GT) are Alt-Az. Not one of them has a clue where they are pointing when power is first applied because there is no "Home" position. The only thing that matters in a goto alignment on Celestron Alt-Az mounts is that the stars are sufficiently spaced far apart in the sky. So go ahead and use Polaris as an alignment star (if the HC lists it) just don't use Kochab as your next alignment star!

[gregl]

So go ahead and use Polaris as an alignment star (if the HC lists it) just don't use Kochab as your next alignment star!

And I think the best alignment would include something that is not only significantly away on the azmuth, but also on the altitude. Since our observing site has a significant light dome to the west we're usually looking eastward so I try to find something in the southeast at somewhere between 90 degrees and about 140 degrees from Polaris and at least 20 or 30 degrees different altitude.

[JayTee]

so I try to find something in the southeast at somewhere between 90 degrees and about 140 degrees from Polaris and at least 20 or 30 degrees different altitude.

Sounds good to me.

[John Baars]

An alt-az mount has no knowledge of its orientation until you start to align it, unlike an EQ mount which can assume it is polar aligned. It can only point towards Polaris after you tell it what direction that is. And, depending on the mount's firmware, the best way to do that may well be to use Polaris as an alignment star.

Kathy is spot on.



An ALT-AZ goto mount needs ( apart from time, longitude and lattitude ) two kinds of information.
1- How am I oriented in Altitude?
2- How am I oriented in Azimuth?

1- Polaris is just one- of- the- guys to give that info about Altitude. But so are Wega, Deneb in the summer on the Northern Hemisphere.
2- Another star is needed for the info about the Azimuth. Preferrably a star low in the West or the East. The wider apart from the first alignment start, the better.
Some goto systems want three or even four alignment stars. I presume for various reasons (precision, lacking GPS-data) for it should not be essential.

For the next session it depends on how much info the goto system remembers or collects from GPS-data. (In those cases three, two, one or even no star ( GPS oriented mounts) may be needed)

For instance.
My first Alt-Az goto system, 15 years ago, wanted the whole circus completed. Every single session. I had to start from a certain start position, enter all data like date, time, lattitude, longitude and three pre-assigned alignment stars. My current Alt- Az goto system just asks (after one initial fourstar alignment for the very first start) for one alignment star and uses cell-pone data to complete.

On a modern ALT-AZ mount Polaris can be used, but is not essential. Use it at free will. One star high-up, and one low West/ East however are essential.
Most Northern EQ mounts assume they are already aligned on the true North. But those are quite a different story.

[Mike Q]

When i set up the XX16G i always start with Polaris, why.... Because it just sits there nicely so i can get it perfectly centered then i head off to something in the southern sky. With the iOptron alt az, its gps driven and after you center the first object and set it you are off the the races. You can do multi star alignments if you want to but you dont have to.

[Richard]

Yes used polaris some times on my SLT go to mount when in Czech Rep , made no difference to any star
Here in RSA wish I had a similar star would make set up for my manual eq mounts so much easy