#1  
Old 22-12-2008, 10:19 AM
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cookie8 (Vincent)
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Drift alignment or polar axis scope?

Drift alignment of GEM before astrophotography can be time consuming. Would it be easier by just line up sigma octan in your polar axis scope? These scopes have markings on them & they should be reasonably accurate I presume especially the more expansive mounts am I correct?
Experienced photographers, what's your answer?
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Old 22-12-2008, 10:22 AM
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iceman (Mike)
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I think the polar axis is only going to be good enough for visual work or short exposure astrophotography.

For long exposures, drift alignment is the only way to ensure you get it as accurate as you can.
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Old 22-12-2008, 11:07 AM
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Matty P (Matt)
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I always drift align for atleast 30-45 minutes before imaging. This ensures that you have an accurate alignment for imaging.

I've actually never tried to find Octans in my polar scope as I have found drift aligning much easier. But doing that will will certainly speed up your aligning process.

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Old 22-12-2008, 04:11 PM
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I used the polar scope in my Eq6 ONCE!.

Mind you, the alignment was good... I was planetary imaging at 6.2~6.5m focal lenght, and holding jupiter in the tiny tiny field of view for 35~40 minutes without my input.... however if I plan on imaging deep sky, even with guiding I drift align. Unless you're using a TOP TOP quality mount, the polar scope is not going to be accurate enough to get the scope properly aligned. The Takahashi mounts seem to have perfect polar scopes in them, as do the Astrophysics mounts (from what I've been told)

I've taken to drift aligning in a big way... Before I did it, i could do 30 second exposures before drift ruined images... After drift aligning, I was able to run 4 minute exposures without guiding with nice round stars...

Even WITH guiding, your results are always better if your polar alignment is perfect, and with any of the cheap, mass produced mounts, the polar scope is not adequate enough... Drift alignment is easy, and a skill everyone with a GEM hoping to do photography should learn...

Alex.
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Old 22-12-2008, 06:49 PM
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I have a Tak EM-200 mount and use the polar scope quite often, especially when the weather looks suspect to change { which has been the case for ages it seems, in Perth at least}. It usually takes me no longer then 5 minutes to have the mount aligned...... the Tak routine is simple, effective and fast.... the hardest part is familiarizing yourself with the sig-oct asterism . In practice I've used a SBIG St2000 camera up to 15 mins and a DSLR 10mins with a ST-4 guiding using the polar scope for alignment. Its a great feature on the Tak mount which helped my decision to buy one with the hope it would help with set up times. Last night was a good example, I had everything to go within 15 minutes with clear skies but as per the norm lately cloud rolled in as I was going after the Rosette/Orion nebs with a few 5 min subs taken It's a lot less disheartening when set up times are fast and less effort is required
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Old 22-12-2008, 07:32 PM
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I guess Drift aligning for photography is what Collmination is to newt owners... sure you can get away without it... But with it the results speak for themself.!
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Old 22-12-2008, 08:22 PM
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Before I setup my permanent observatory I always used the polar scope but them aligned the main scope so it aimed south and then centered the SCP asterism in the eye piece of the scope. I then imaged (admittedly with guiding.) I did the same thing when I set up my observatory but then drift aligned. It was reasonably good anyway. Depends how much time you have and your ability to guide. If you are way out with the aiming of your scope then you will get field rotation despite guideing so the accuracy of the polar scope has to be reasonable.
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Old 22-12-2008, 10:55 PM
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Sorry i hope you didn't mind, i thought the photo was great but the curves and colour balance needed some lovin and i was a tiny bit bored
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Old 23-12-2008, 02:25 PM
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Quote:
Originally Posted by bmitchell82 View Post
Sorry i hope you didn't mind, i thought the photo was great but the curves and colour balance needed some lovin and i was a tiny bit bored
I felt the same way.
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Old 23-12-2008, 03:33 PM
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cookie8 (Vincent)
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Thanks guys for sharing your experience on polar alignment. I guess the summary is:
Polar scope if you have a top of the range mount and in a hurry
Drift align otherwise.
Got it.
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  #11  
Old 23-12-2008, 04:29 PM
gary
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Quote:
Originally Posted by AlexN View Post
Even WITH guiding, your results are always better if your polar alignment is perfect ...
Hi Alex,

Thanks for the post.

I thought it timely for just a quick "heads-up".

As I remind enthusiasts, there is no such thing as a "perfect
polar alignment". In other words, there is no Holy Grail
angle by which you can set your mount in elevation in order
to achieve zero field rotation for all zenith distances.

Instead, there is only an optimal polar axis elevation angle
for any one point in the sky.

The problem is due to atmospheric refraction. Particularly for
large zenith distances, it impacts upon both the optimal
point to which one should align the polar axis and also affects the
instantaneous tracking rate.

As a 'whole-sky' compromise, it is usually best to align the polar
axis with the refracted pole.

However, the reality is that for most enthusiasts performing digital
imagery these days, autoguiding has become the norm and
imaging times are usually kept modest so that any residual
field rotation is rarely an issue.

Quote:
Originally Posted by iceman
For long exposures, drift alignment is the only way to ensure you get it as accurate as you can.
Another common myth among enthusiasts is that the drift test
is the Gold Standard with regards polar alignment.

The problem with a drift test is that it does not take into
account refraction nor the geometric, gravitational flexure
and eccentric bearing errors within the mount/OTA.

Therefore best practice is to perform a mount error
analysis from a star pointing test and from that derive the
two polar axis misalignment terms as part of that process
and then finally correct the elevation term for the refracted pole,
which is dependent upon your latitude, temperature and barometric
pressure.

The above prescription would be recommended, say, for anyone
with a mount in a fixed observatory or anyone with a portable mount that
plans on leaving it setup for an extended session.

The techniques I broadly described above above are the same
as employed in most professional observatories and many amateurs
do the same. For example, Monte is a regular contributor on the
forum and commonly uses this approach when imaging.

As many of the fine images that appear on forums and in competitions
are testimony to, if imaging times are kept short, then a drift test alone
can deliver the goods.

Best Regards

Gary Kopff
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Old 23-12-2008, 05:28 PM
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True.. I should have worded it "As perfect as is possible."

Cheers.
Alex.
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  #13  
Old 23-12-2008, 05:39 PM
gary
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Quote:
Originally Posted by AlexN View Post
True.. I should have worded it "As perfect as is possible."

Cheers.
Alex.
Hi Alex,

Merry Christmas!

Perhaps a good description would be "as optimally as possible".

All the best.

Gary
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