23 replies to this topic

[jimsmith]

How accurate does your Polar Alignment need to be for an unguided equatorial mount?

 

I’ve never been able to find a resource anywhere that says how inaccurate your P.A. can be without you noticing a problem with your images. I don’t mind doing polar alignments but I would rather spend two minutes getting it to within 15 arcminutes, than spend twenty minutes getting it within 1 arcminute if I can’t see any difference in my photos.

 

I would think the answer depends on the image scale, exposure time and perhaps the declination of your target.

 

Can anyone point me in the right direction?

[Monel76]

2 or 3 arcseconds are enough for me. But I use auto guide.
It also depends a lot on the focal length and mount.

[Tapio]

Like Monel76 wrote wrote it depends on focal length and exposure.
Less than 10' rs enough for wide field shots. Longer focal length and longer need autoguiding.

[Spaceman 56]

I would rather spend two minutes getting it to within 15 arcminutes, than spend twenty minutes getting it within 1 arcminute if I can’t see any difference in my photos.

Ha Ha... 

 

problem is that at 15 minutes you probably will see a difference in your images, unless you shoot very short exposures, and your focal length is short.

 

some people settle on about 5 arc minutes, and guide the drift error away. apparently quite successfully. 

 

if you get the PA error down to 1 minute (or less) the Drift will also be less, and the guiding does not have to work as hard.

 

experts will surely know more.  

[kathyastro]

I used to get within about 3 or 4 arcminutes using a polar scope, and that was good enough for 60-second unguided subs at 926mm focal length.

 

I still use a polar scope, so my alignment accuracy is similar, but now my scope is guided, and I can shoot 900s subs reliably.

[scanner97]

OP, if you use something like SharpCap or NINA, you can get it under 5 arc minutes in 5 minutes or less.  This should be totally fine for almost any situation.  If you're doing it some other way and getting 15' in 2 minutes, the question is whether you're happy with the data you're getting.  With an inexpensive mount, the mount mechanics (and f.l.) will have a bigger impact on how long you can go unguided than whether your PA is 5', 10', or 15'.

Edited by scanner97, 24 May 2025 - 07:43 AM.

[pyrasanth]

When I used SharpCap to refine my PA it gave an initial evaluation of fair. I then dialled it in to give excellent within 5 minutes of tweaking.

 

The results were quite visible in the after guiding results. Long stacks of subs show very little rotation and my stars are rounder and guiding does not need to work as hard.

 

The effort to get to excellent PA is well worth the journey irrespective of going guided or unguided.

[BrianThePrimate]

The answer will always be the more accurate the better. Astrophotography is a game of mitigating variables. Since there are a ton of variables, it is one less variable you got to contend with to get the best images possible. 

Edited by BrianThePrimate, 24 May 2025 - 08:32 AM.

[jimsmith]

"The answer will always be the more accurate the better." ...at any cost?

[BrianThePrimate]

"The answer will always be the more accurate the better." ...at any cost?

Nope. Unless you count time setting up which there is enough time between the stars coming out and astronomical darkness to dial in polar alignment assuming one is setting up their rig and taking it down every night.

Edited by BrianThePrimate, 24 May 2025 - 09:25 AM.

[PirateMike]

How accurate does your Polar Alignment need to be for an unguided equatorial mount?

 

I’ve never been able to find a resource anywhere that says how inaccurate your P.A. can be without you noticing a problem with your images. I don’t mind doing polar alignments but I would rather spend two minutes getting it to within 15 arcminutes, than spend twenty minutes getting it within 1 arcminute if I can’t see any difference in my photos.

As I often say, "you need to test things out to fully understand if the return is worth the investment", but...

 

When it comes to imaging unguided, probably the biggest issue is how closely your mount can move as compared to a perfect mount in a perfect world. If your mount has issues moving smoothly and accurately, then there probably isn't much one could do to fix the issue, unless you like taking the mount apart and fixing the underlining issues... or risk your luck and buy another one.

 

The second biggest issue to attaining perfect unguided subs is probably polar alignment, but luckily a lot can be done to correct any misalignment. I assume that the 80-20 rule is valid here.

 

The next most common issue is seeing. Not much you can do about that except to only image on nights of great seeing, but that is a little unrealistic.

 

These three three things are probably what negatively effects your unguided efforts the most, and it appears that polar alignment is the only reasonable thing that can be done to improve your results.

 

 

I do have two different model mounts, and even with excellent polar alignment and the same guiding setup (I know, you're talking unguided), one mount can reliably guide at ~0.42" RMS error all night, while the other can only muster up ~0.55" RMS. Mounts count, and if you have a great one, well your a lucky person.

 

But please re-read my first line and follow it if you really want to know the answer to your question. 

 

 

 

Pirate Mike

 

.

Edited by PirateMike, 24 May 2025 - 09:34 AM.

[Old AZ Guy]

I’m new to polar alignments, using it for my Seestar S50 which I’ve only had for a couple of months. Based on my observations so far, I think at least part of the need for accuracy depends on what part of the sky you’re looking at. Objects near the celestial equator whiz by your field of view a lot faster than objects near the celestial poles. So I think the need for alignment accuracy increases with your object of interest’s proximity to the celestial equator.

[rgsalinger]

The PHD2 manual states that if you are within 5 arc minutes of the refracted pole you'll be just fine, if you are guiding.

 

If you are attempting to take unguided images, then you will generally need to get closer. The larger your guider image scale (guide pixels/arc second), then the less you will notice any elongations and/or field rotations due to PA error.

 

It's not at all obvious or proven that the numbers produced by apps like Sharpcap and/NINA are accurate to better than 1 arc minute. So, what I do is get down close to one arc minute and then leave it. Finally, bear in mind that none of these methods eliminate drift, they just minimize it.

 

So, if you are having fun zeroing out that PAE number then enjoy. If you'd prefer to spend less time on the task, under a couple of arc minutes will do just fine.  

[c2m2t]

Hi Jim!

I can appreciate the issue, but what concerns me is the notion that polar aligning is such a dirty word(s)!!  Given the phone apps available that will provide the position of Polaris for many alignment reticules found in polar scopes...polaris's position will be adjusted for time within the app. If your neck is made of rubber, than positioning your head to view through the polar scope will be no problem. For those of us with aging arthritic necks, I have fabricated an attachment for my polar scope that provides a very comfortable neck position. The key materials are a short length of 1.25" diameter PVC conduit, three nylon screws and a suitable plastic cap from a vitamin bottle to which you cut out the opening to suit a right-angled viewer for a dslr. 

 

With a very little bit of practise. getting polar aligned should take less than 5 minutes...less than a minute when you get the process down. 

 

Polar aligning is easy...believe me!!

 

Cheers, Chris.

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[PIEJr]

You need to be aware that your Polar alignment is a mechanical aiming of your telescope. That is its function and why we do it.

A Starting point.

 

So the more accurate you are with your Polar Aligning, the happier you will find you are later on.

 

Sloppy starts equal sloppy results.

 

When I subscribed to Sharpcap, I decided to do a little test one night. Purely for my own interest...

I did 10+ Polar Alignments with Sharpcap. Each one wanted for some adjustment. As far as I could tell, after doing things to a T, I wound up back where I started.

I just wanted to see what would happen. But minor tweaks each time was what I learned.

I've since learned that seeing variations also affect those results. And we really can't do much about that.

 

Now when doing my modeling (Star Alignment) after as good of a PA as I can, I've always err'd toward more Stars is better.

I reasoned I was giving my mount reference stars, you are aimed here. *

Just something I liked to do, call me an over achiever. I'm retired, I have lots of time.

I was explaining that to someone on the Gemini 2 forum on the WWW. And Rene` Grolsh, the creator of the Gemini 2 system for Losmandy Mounts posted he felt 10 or more stars were better than less stars.

So in my typical outhouse luck, I'd been doing good all along. With my AVX mount and now my Losmandy Mount.

 

The more you put into accuracy with your equipment, the more you can get out of it.

The less you put into it, the greater your frustration level.

I'd rather know I did my best.

YMMV

[starslicer]

Unguided? Pretty accurate. If you have Sharpcap or Nina it can do a pretty good alignment, it will ask you to put rotate your telescope along the RA then it will figure out how you need to adjust the mount. If you can't see it like me, Sharpcap does a pretty good drift alignment but it's a bit more complicated.

[rgsalinger]

You need to be aware that your Polar alignment is a mechanical aiming of your telescope. That is its function and why we do it.

 

To be really clear, it is the MOUNT that you are aligning, not the telescope. You want the MOUNT to track celestial objects accurately. And, because that rate varies at the target moves up/down guiding will be useful where you have small image scales. 

[F.Meiresonne]

I use sharpcap to polar align. I usually try t get to 1 arcminute or under. That is good enough for me. 

 

If all works well , sharpcap is easy to use and it goes fast, and i don't have to hurt my neck and legs to use it, which was the case with my polar scope.

[freestar8n]

This comes up periodically, and the simple answer is that once you are below a few arc-minutes it simply won't have any impact to try to improve it more - whether you are guiding or not.

 

The higher level answer is - there is no such thing as "perfect polar alignment" because no single polar alignment will avoid dec. drift across the sky.  There are two main types of polar alignment - refracted or "true" (i.e. non-refracted) - and they are different.   So if you are "perfect" in terms of one, you know you are off in terms of the other.

 

The irony regarding unguided tracking is that instead of it being the one that requires perfect alignment - in reality it is the one that requires intentional misalignment - if you want to avoid dec. drift.  You can align the mount "perfectly" (by which of the two main ways?  It doesn't matter) - and then track a star somewhere in the sky - and you will see it drift in dec. due to refraction.  To prevent that drift you will need to adjust the mount alt/az until that drift is cancelled.  That way you will intentionally misalign the mount so it tracks well in that part of the sky.

 

If you then go somewhere else in the sky you will need to misalign in a different way.

 

A "perfect" polar alignment just gets you close overall in the sky - and being a few arc-min off won't make much difference.  Guiding or not.

 

It's just one of the things that people used to need to worry about to get it close because polar alignment was hard to do well.  But nowadays cameras and software make it easy to get very close, and in a short time.   So just get it close and don't worry about it.  Spend more time on guiding, focus, and collimation - where improvements really do matter.

 

Frank

[jimsmith]

Thanks for all of the answers and suggestions.

 

Recently I have been using my Seestar S50 in equatorial mode. I use a Peterson Engineering mount on my fixed pier. P.A. can be changed quite easily and only needs to be done once because the mount and pier are permanent fixtures. However, with this set-up, it is hard to be very accurate in the P.A. adjustments. The Seestar P.A. alignment routine only reports to an accuracy of one tenth of a degree.

 

I love my Seestar but even I would admit that its optics and tracking are not the world’s best so errors due to slightly inaccurate P.A. probably go unnoticed. At least that’s what I think at the moment. But I am curious as to what sort of errors poor P.A. might cause.

I have tried to work this out using some half-baked assumptions and dubious reasoning so please feel free to tell me where I’m wrong.

 

The Seestar alignment routine says you should be polar aligned to within 1˚. Its maximum exposure time is 60 seconds.

 

Let us first consider a 24 hour exposure on a mount with perfectly timed tracking but with P.A. off by 1˚. You centre a star on the sensor. You start the exposure and the star drifts away from the centre. I’m not sure what shape the track of the star on the sensor will be but I think it might be close to a line (perhaps a narrow ellipse) aligned North to South. After 24 hours the star should be back in the centre of the sensor. The line will be 2˚ long.

 

The speed at which the star traverses the line might vary but I will ignore that as I have no idea how to work it out!

 

Now let’s suppose we expose the for 12 hours. The tracking error will have displaced the star by a maximum of 2˚. Assuming that the tracking error increases and decreases steadily over time, that means that every hour the star will move (2x60x60)/12 arcseconds across the sensor…that’s 600 arcseconds.

 

So, during a 60 second exposure the target will move ~10 arcseconds across the frame. For a Seestar S50 that’s about 4 pixels.

 

So I think, by similar reasoning, if you polar align to within half a degree your target will only drift by about 1 pixel for a 30 second exposure. Would that be noticeable? Unless you significantly cropped the image, I doubt it. Tracking inaccuracies and vibrations due to wind etc. will probably cause greater problems.

 

So I’m going to stick to 30 second exposures and P.A. alignment within 0.2˚ for now. I reckon that’s good enough.

 

 

 

[scanner97]

But I am curious as to what sort of errors poor P.A. might cause.  I have tried to work this out using some half-baked assumptions and dubious reasoning so please feel free to tell me where I’m wrong.

 

There are many factors, as others have shared.  Some folks enjoy the hobby most when they are making certain numbers as good as they can, whether it's PA, guide RMS, or whatever.  Nothing wrong with that; the hobby is different things to different folks.  Other folks are more focused on what their final image looks like.  But even there, we're not all the same.  Some of us prefer to look at individual pixels and see how "round" the stars are.  Or maybe we're hooked on minimizing star size.  Some of us dislike having to crop more than a few pixels at the edge of the image and others will happily crop quite a bit, feeling that it can improve the composition.

 

If you get images that you enjoy, keep doing what you're doing.  

[CalifDan]

I'm not sure how close is close enough but my stars are round, and I'm satisfied with the images.  I usually try to align pretty close and using NINA 3 position alignment get within 20 arc-seconds on each axis and usually even closer.  That can take more than 5 minutes of effort.  What I do not understand is that when I then run Guiding Assistant in PHD2 it reports a Polar Alignment error of over 6 minutes.  I'm not convinced that the NINA 3-point alignment is giving me good results.  I'm a long way from a PHD2 expert, but it looks like it is working pretty hard to keep things centered.

[Phil Sherman]

My simple answer to the unguided polar alignment question is: it depends.

 

Take a 60 second exposure to determine the exact direction of drift in the image. Shorten your exposure until the brighter stars aren't ovals. The better the polar alignment, the longer the usable exposure at that particular target's declination.

[matt_astro_tx]

Others have covered it well.  I use the ASIAir and polar align each night in under 5 minutes.  I aim for a total error of 1'.  Sometimes I get lucky.