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Old 16-07-2014, 06:13 AM
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codemonkey (Lee)
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Polar alignment for very long exposures

Since starting out in astrophotography I've been limited to exposures of <= 30 seconds, so this has never been an issue for me, but I've been wondering... how do you align your mount so precisely for exposures that are 20-30mins long?

I've been drift aligning prior to taking my exposures, and I currently wait no longer than 5 minutes for drift to appear before calling it done. If, however, your exposure length is 20m, 30m or longer, is it as simple as waiting for say double that amount of time for drift to occur? That seems like it would take an absurd amount of time to assess the success of any correction made to alignment, so I'm thinking there must be another way.
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Old 16-07-2014, 06:32 AM
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You won't get more than a minute or two at most unless you are using a guider. Yes some people with incredibly expensive mounts can do longer unguided but I'll bet the the still use a guider anyway just In case.

You'll need a guide camera and a a guide scope if you want to progress. Keep an on the ice trades there are always cheap second hand stuff that comes up.

Cheers
Sandy
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Old 16-07-2014, 06:38 AM
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codemonkey (Lee)
Lee "Wormsy" Borsboom

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Thanks Sandy :-)

My understanding is that guiding will protect you from periodic errors in mount tracking, but not from bad alignment, which will present itself as field rotation around the guide star.

So if that's correct and guiding doesn't help, how do we get alignment so precise that we can expose for 20-30mins?
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Old 16-07-2014, 06:48 AM
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I set up every night to do 30 minute exposures and use alignmaster to get my polar alignment good enough.
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Old 16-07-2014, 07:00 AM
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PRejto (Peter)
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I don't know what equipment you are using (mount!) but polar alignment is always a compromise. There really isn't such a thing as "perfect alignment" unless you are speaking of a small area of the sky for a relatively small period of time. Refraction changes with altitude and with temperature and humidity so it is impossible (as best I understand!) to align perfectly and have no issues with tracking.

However, it is possible to use programs such as TheSkyX/T-Point to achieve different polar alignments that are very close for what you want to achieve. In addition it is possible to "model" the mount/scope combination so that flexure and things like non-perpendicularity of the scope to the DEC axis (and many others) are then compensated for during tracking. With high end mounts like a Paramount or AP (and others) it is possible to go quite long unguided if everything is exactly right (not easy!!). Most imagers probably guide just to compensate for any residual error.

Anyway, to answer your question as specifically as I can, I think that if you are inside of an arc-min in both altitude and azimuth, I doubt that you will be troubled by rotational effects. What you might want to do is drift align in the area of the sky you intend to image. You will find that a drift alignment at dec=0 will be different than drifting elsewhere. The benefit of drift alignment at DEC=0 is that motion in RA is maximized. As you move closer and closer to the pole the velocity of the tracking arc decreases (not the mount motor, but the actual speed the star appears to move across the sky). Anyway, sorry if I sound like I'm lecturing!!

Peter
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Old 16-07-2014, 07:44 AM
glend (Glen)
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Look I have no problem with gettting down to a few arc seconds in aligning my NEQ6Pro now that I have version 3.35 of the controller software (which provides alignment error display and correction routines. I also run the PAE zone correction routine on objects that I am going to image. This works great for me. I also have a guider but don't use it much. However, your question about 20-30 minute subs has me thinking - why? It's not much trouble to run shorter subs and stack them and not encounter any problems. It's the cumulative signal data that is important. And for people that use a simple DSLR setup, long exposures create potential problems with the camera heating up, unreliable signal noise profiles, and possible power supply problems if relying on the camera battery and not external power.
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Old 16-07-2014, 10:01 AM
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Amaranthus (Barry)
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Quote:
It's the cumulative signal data that is important.
That's not quite true: http://www.dens-astropics.org.uk/page%2035.htm

But I agree that in most cases, 30 min subs are overkill and 10-15 min is fine even for faint DSOs (and shorter for bright cores etc.)
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Old 16-07-2014, 10:11 AM
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This is an interesting thread to me, so apologies to OP for any hijacking

I guess the question I have is...where is the sweet spot? The answer, I suspect, is dependent on the equipment, and the target

My last trip out I got my alignment down to 10-12 arc seconds in both axes and I was getting 20 second subs at f/l 2000mm (C8) with round stars, 30 seconds I could see elongation. I'm sure it's diminishing returns without guiding, but I'm sure with a little more effort I could get longer exposures. I'll have to play with some darks to see just where my noise sweet spot is with my 1100D.
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Old 16-07-2014, 12:26 PM
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Quote:
Originally Posted by Amaranthus View Post
That's not quite true: http://www.dens-astropics.org.uk/page%2035.htm

But I agree that in most cases, 30 min subs are overkill and 10-15 min is fine even for faint DSOs (and shorter for bright cores etc.)
Thanks for the link to the article Barry - great info
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Old 16-07-2014, 02:57 PM
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I routinely do 30 minute subs and I do it to get my background ADU up over 3000. I do this with my 102TSA and for my RC12. My thinking here is that the signal strength is more important.

It is not true that doing shorter subs is all you need. It all depends on what your objectives are for a given imaging project. If you are using tight narrow band filters you will need to go very long to get the signal up. Mathematically you can work out how long you need to go for a given aperture and sensor combination. Ray has had several threads on this and those threads are informative. Though I still prefer to go deep myself.

So back the original question. I use Tpoint to get my PA very tight. However I have seen Pete use Alignmaster and it is very fast from what I have seen. Drift aligning is good too but if you want to go deep then it might take an hour or so to get it good enough. Again it all depends on your objectives. This hobby has varying levels of precision requirements and it is up to you to decide what level you want achieve.
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Old 16-07-2014, 03:20 PM
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You can speed up DA significantly by using a software-assisted solution. I've found the new automated drift alignment in PHD2 works really well (is fast and quickly gets one down to <10" or lower), and is just part of the logical 'workflow' since I'm using this for guiding anyway.

I've heard that using plate solving in AstroTortilla can also allow for precise and fast alignments - but I'm yet to try that out thanks to the %$#@! clouds over the last two months!
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Old 16-07-2014, 04:23 PM
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Eden (Brett)
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PEMPro is another useful tool for nailing down a good PA.

http://www.ccdware.com/products/pempro/
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Old 16-07-2014, 04:40 PM
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PEMPro is pricey though! PHD2 has the advantage of being free, and I think it does exactly the same thing... (well, you then use EQMOD to do the PEC training, provided you have a Synta mount)
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Old 16-07-2014, 05:38 PM
gary
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Quote:
Originally Posted by PRejto View Post
I don't know what equipment you are using (mount!) but polar alignment is always a compromise. There really isn't such a thing as "perfect alignment" unless you are speaking of a small area of the sky for a relatively small period of time. Refraction changes with altitude and with temperature and humidity so it is impossible (as best I understand!) to align perfectly and have no issues with tracking.
This is absolutely correct.

Quote:
Originally Posted by codemonkey
Since starting out in astrophotography I've been limited to exposures of <= 30 seconds, so this has never been an issue for me, but I've been wondering... how do you align your mount so precisely for exposures that are 20-30mins long?
There is no such thing as a perfect polar alignment and equatorial mounts
are really just engineering compromises.

Even if one were to align an equatorial mount's polar axis perfectly
with the celestial pole, field rotation will still occur.

How much and to what extent is trigonometrically relatively complex
and is a function of what area of the sky you are imaging and the
time you are imaging for.

I have talked extensively on this subject at venues such as the
IceInSpace Astrocamp in a presentation entitled "The Myth of the
Perfect Polar Alignment".

What comes as a surprise to some amateurs is that the problem
of field rotation due to refraction for long exposure times was first
extensively studied by professional astronomers, such as Arthur A.
Rambaut, as far back as 1893.

The photographic plates in those days were not very sensitive and long
exposure times were the norm.

Astronomers realized that not only field rotation was upsetting their
long exposures on equatorial mounts but that the first differential, that
is the tracking rates, would have to be dynamic as well.

In a theoretical sense, guiding cannot compensate for the field
rotation due to refraction either. One would still need to either
have the camera revolving on a third axis -a derotator - or would one
would have to dynamically change the mount's alt axis as well.

Conveniently the field rotation caused by refraction is in the
opposite sense to that caused by elevation.

Arthur R. Hinks in the Monthly Notices to the Royal Society in 1898
derived a nice table that showed for various declinations and
hour angles in which you propose to image suitable offsets by which
to raise the scope's polar axis.

A good compromise is to attempt to align the mount's polar
axis not with the true pole but the refracted pole.

The effect of refraction is to "lift" the apparent position of an object
so the refracted pole is always above the true pole.

So for example at 25 degrees south latitude there in Qld at 100m above
sea level, the refracted pole is about 117 arcseconds above the true pole.

So you should be aligning the polar axis 117 arcseconds above the true
pole.

If you were to go up a mountain to 1500m, the refracted pole at the
same latitude is around 102 arcseconds.

So you can appreciate from these numbers alone it tends to make a
nonsense of claims by many amateurs that their mount is aligned
with the pole to within some small number of arcseconds.

The question then is, which pole? True or refracted?
And which part of the sky are they imaging in and how long for?

It also says that one can waste an inordinate amount of time chasing
something that does not exist. There is no such thing as a perfect
polar alignment.

By the way, the drift test is by no means the gold standard for aligning
a mount. It simply gives a rough compromise between the two areas
of the sky which you performed the drift test on.

Best Regards

Gary Kopff
Managing Director
Wildcard Innovations Pty. Ltd.
20 Kilmory Place, Mount Kuring-Gai
NSW. 2080. Australia
Phone +61-2-9457-9049
Fax +61-2-9457-9593
sales@wildcard-innovations.com.au
http://www.wildcard-innovations.com.au

Last edited by gary; 16-07-2014 at 06:16 PM.
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Old 19-07-2014, 04:12 PM
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lazjen (Chris)
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Quote:
Originally Posted by Amaranthus View Post
I've heard that using plate solving in AstroTortilla can also allow for precise and fast alignments - but I'm yet to try that out thanks to the %$#@! clouds over the last two months!
This is the method I'm using since I can use it under both Windows and Linux. The major limitation for me is determining the correct amount to adjust the mount given the error reported. I foresee a time in the future where these mounts will have electronic control over Alt/Az and will do the adjustment for you, just as we have routines for electronic focusing, etc.
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Old 19-07-2014, 05:17 PM
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Expensive mounts still need autoguiding to get round stars. Its somewhat of a myth to think you can get an expensive mount, align it and you can do 20 minute unguided round star exposures. Certainly not routinely even if someone can produce an image showing round stars which has been done up to 10 minutes or so. I have been able to get 10minutes sometimes with software assistance. But it would be errratic and not reliable.

There are elements of flexure, gear inaccuracies, imperfect balance etc etc.

The current most accurate mounts I believe use both high quality encoders and autoguiding as in high level AstroPhysics mounts.

Setting up every night and doing a fresh alignment can be painstaking.

One trick I used if you have a permanent pier was to do an accurate drift alignment and then I used a torch through the polar alignment scope in my Tak mount. It made a small spot on the ceiling of my observatory and I marked that with a piece of electrical tape.

Next time I setup the mount I simply adjusted it until that torch spot was on the tape and I was good to go.

A similar thing would be to have markers on pavement or pavers where you setup your mount outside then have a laser on the side of the mount shot a light to your fence or similar and leave a marker there at the correct spot. That would take a minutes to setup each time.

A good polar alignment is vital for even 10 minute exposures. It would be one of the first things to be sure of if you were having trouble with getting round stars despite using an autoguider.

Greg.
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Old 19-07-2014, 06:01 PM
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Polar alighnment for very long exposures

Lee, very interesting thread.
A few months ago I also searched for 'perfect' polar alignment with my equipment which compared with others on this site you could only describe as basic (EQ5 pier mounted)
The best I could achieve was 2 min exposures unassisted, but one thing I did experiment with to achieve the best result I could, was to put a 0-100 ohm potentiometer inline from regulated voltage suppy to motor control.
yup..not conventional but it helped. Went from 30 sec to 2 minutes.


John
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Old 19-07-2014, 11:03 PM
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Lee "Wormsy" Borsboom

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Wow, thanks everyone. So much to read! I'll have to go over this a few times I think.
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Old 20-07-2014, 08:23 AM
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codemonkey (Lee)
Lee "Wormsy" Borsboom

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Quote:
Originally Posted by johnnyjetski View Post
Lee, very interesting thread.
A few months ago I also searched for 'perfect' polar alignment with my equipment which compared with others on this site you could only describe as basic (EQ5 pier mounted)
The best I could achieve was 2 min exposures unassisted, but one thing I did experiment with to achieve the best result I could, was to put a 0-100 ohm potentiometer inline from regulated voltage suppy to motor control.
yup..not conventional but it helped. Went from 30 sec to 2 minutes.


John
That is interesting, John. I'd heard that if your power source starts to run low you can get more tracking issues, so that does make some sense in that you need a stable source of power.

Thanks again everyone for your advice, I really appreciate it. As soon as I have the budget I'll be getting an OAG (likely) or a guide scope (less likely), and from the sounds of it, that combined with reasonable alignment should do the trick.

The alignmaster software looks very promising, so I might give that a shot too, now that I'm stuck with Windows for imaging anyway (Nebulosity gives me horrible amp glow, Artemis CCD doesn't).

I was out last night and able, for the first time, to take exposures over 30 seconds. I was able to get a perfectly tracked 2min exposure with good alignment (I've done better, but not by much) and no guiding, but the ratio of keepers would have been terrible. For the moment I'm limited by guiding regardless of alignment so that's the first thing on my list.
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