Why is my polar alignment so random?

[ryandsimmons]

My normal procedure for setting up my scope for imaging is:

Set up scope, balance etc.  Get a rough polar alignment (in my garden I use similar places every night, but as I am on grass it is not exact.)

I have a Celestron C8 on a CG5 mount.

I do a 2 star polar alignment, with 4 calib stars.  I then do a scope polar alignment using the built in function.

I sometimes do this a second time.

Then I reset the scope again and do a 2+4 alignment and start trying to image.

Now sometimes I get nights when I can do 120 second images, and stars are barely oval (okay, by sometimes I mean it happened once.) but some nights I get 15 seconds max.

I just cannot figure out why some night the polar alignment just doesn't work.  Last night I was aiming for 30 second subs and couldn't even manage that, despite polar aligning twice with the software.

Is there something that I am missing that can cause this?  The only thing I can think of is the scope balance (as in mount balance against the bubble guide, not the weight balance.)  Given the mount only has a small bubble guide I may be slightly off there, would that cause this?

[ryandsimmons]

I wish I could edit:

I don't have a polarscope.  I just do my initial alignment by putting the telescope in the same place, and point it at the same chimney, which gets me reasonably close.

I just spent money on dew control, and I have no budget for a polarscope for at least 3 months...

[ronin]

It is the mount that needs to be accurately polar aligned not the scope.

Since you say you are doing rough polar alignment for the mount I expect that this is the source of the problem.

When you align the scope the softeware is determining the corrections that need to be applied to the scope for the errors in the mount setup and alignment. You really want no errors in the mount alignmnet.

In simple terms it is the mount that need to be rotating accurately around the earths axis.

I can track a target vis a scope with an Alt./Az mount, but they are no good for AP.

Forget the scope, align the mount.

[Leveye]

Your answering your own questions. Your going to need to be much much more accurate in setting up your rig to achieve the longest unguided imaging times. You can't really polar align well enough without a polar scope.When you finally get one your going to want to make sure that it's collimated properly to the mount for the most accuracy. Also align your mount your true north/south position and not magnetic. That can put you off by over 15 degrees.

[ryandsimmons]

It is the mount that needs to be accurately polar aligned not the scope.

Since you say you are doing rough polar alignment for the mount I expect that this is the source of the problem.

When you align the scope the softeware is determining the corrections that need to be applied to the scope for the errors in the mount setup and alignment. You really want no errors in the mount alignmnet.

In simple terms it is the mount that need to be rotating accurately around the earths axis.

I can track a target vis a scope with an Alt./Az mount, but they are no good for AP.

Forget the scope, align the mount.

Hi,

the CG5 mount has a procedure that once you have aligned, it can then show you where true polar alignment is.  So you manually adjust the mount to compensate.

Where I said scope above just substitute for "mount" sorry.

[D4N]

You say you're on grass, do you put anything under the tripod legs to stop them sinking in when the mount is moving? It doesn't take much movement so throw the whole thing out.

Also how accurate are your goto's? Poor goto accuracy can be a sign that something is wrong.

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[ryandsimmons]

Well last night the GOTO was great enough that it was slewing to the accuracy that the target was in my 7mm EP with a barlow.  I do find that the polar alignment and GOTO often don't synch though.  I can have amazing GOTO but rubbish star trailing, whereas other nights my GOTO seems fairly poor, but zero star trails.

The three things that I can narrow it down to at a guess are:

1) movement on the grass as you suggest.

2) The mount is not balanced.  I only use the built in bubble level.  I have no idea if this would matter much though?  And how would you use a proper spirit level?  Would you take the mount head off and balance the tripod first?

3) Software margin of error means that the CG5 mount polar align is a bit dodgy.

[Scott]

I don't know the mount, but my guessing is that if you want good polar alignment (and you do) without a polar scope then drift alignment is probably your best bet. 

 Polar Drift Alignment.pdf

[Stu]

Levelling the mount is not really that important vs actually polar aligning, but in your case it will get you much closer to start with. I level my tripod with a spirit level before putting the head on.

As Scott says, drift aligning is probably your best bet for getting an accurate setup.

The other thing which may be worth checking is your power supply. Do you use batteries, a leisure battery or mains adaptor? Variable or low voltage into the mount can cause them to do random things. I have had situations where Goto accuracy and consistency was dramatically improved by switching to a mains supplied '12v' rather than a battery.

Stu

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[Gazabone]

I find I get easier and better levelling using the bubble level app on my phone rather than the real thing. It flashes red when levelling is way off turning to Amber when I get closer and flashes green when very close. The flashing is mush more rapid the closer I get and I get a constant green light when it's absolutely spot on.

Having said all that, as others in the thread have said, the levelling shouldn't be hyper-critical.

Good luck though.

[ryandsimmons]

Cheers guys.  I guess most of it is down to practice.  I never try drift aligning as I just don't get the time most nights am lazy.  ;-)

I should try it though.  With most things I find taking more time at the start saves time later.  And I think I really need to build a proper area in my garden for the scope, or at least mark out areas on the grass to make set up more accurate.

[ryandsimmons]

While I am asking questions...

Are any of these viable targets with my setup?

C8 scope at F10 with unmodded DSLR.  So probably 30 second subs.

tomorrow looks like the first night in months I can have a proper all night attempt.  These are what will be above 50 degrees for at least 2 hours that I can see.  Dumbell is the only one I have tried before and was a great success.

Heart Nebula

Soul Nebula

Elephant Trunk   Dumbbell Nebula Pacman Nebula Pelican Nebula

[Scott]

This website will tell you how big/little any given target will look on your chip. F10 is quite slow for dso imaging but thats no reason not to try. A 2000mm focal lenght is also going to make tracking very tricky.

http://www.12dstring.me.uk/fovcalc.php

Good luck though

[ryandsimmons]

This website will tell you how big/little any given target will look on your chip. F10 is quite slow for dso imaging but thats no reason not to try. A 2000mm focal lenght is also going to make tracking very tricky.

http://www.12dstring.me.uk/fovcalc.php

Good luck though

:-)

That combined with http://dso-browser.com/ is how I do all my planning.  The problem is sometimes the real world just doesn't conform when I actually try,  for instance the Iris nebula according to the numbers should be an easy target (mag 7, 5') but was impossible, whereas the harder on paper Dumbell (Mag 7.3, 8') was incredibly easy.  So I thought I would ask if anyone has experience.  I am not proficient enough to evaluate targets properly yet.

Ideally I would like a crack at the whirlpool, unfortuantey I just don't have the angle for it.  I am tempted mainly by M33 assuming my neighbours on that side don't have their lights on all night again.

[ryandsimmons]

Aaarg, I hate not having edit yet.  M33 I think is too big though.

[themos]

If you are too much in a hurry for drift alignment but can spend about 2 minutes with a 50mm-200mm lens, you can measure your polar alignment error precisely. You need to take two images of the Polaris region with such a lens, one "portrait" and one "landscape", with your camera piggy-backed on the scope and not touched between the two frames except to rotate in Right Ascension. The "landscape" could be the one with the scope in the "parked position" (and the counterweight bar hanging down) and the "portrait" with the counterweight bar horizontal. Then send me the images, I will plate-solve them, find the common point (where your mount is actually pointing) and then annotate them with the position of where you're pointed at and where you should be pointing.  Like so:

[D4N]

While I am asking questions...

Are any of these viable targets with my setup?

C8 scope at F10 with unmodded DSLR. So probably 30 second subs.

tomorrow looks like the first night in months I can have a proper all night attempt. These are what will be above 50 degrees for at least 2 hours that I can see. Dumbell is the only one I have tried before and was a great success.

Heart Nebula

Soul Nebula Elephant Trunk Dumbbell Nebula Pacman Nebula Pelican Nebula

I think dumbbell is the only one worth trying with an unmodded DSLR, the emission nebula will just be too faint, to give you an idea I was shooting soul last night and it was barely visible in a 5 min sub with a modded DSLR in good seeing. You could expect to take ten times as long as that to get to the barely visible stage with an unmodded DSLR.

You might be better hunting galaxies as they emit light over the whole spectrum.

Try M33 and M101.

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[ryandsimmons]

Cheers chaps.

Looks like Dumbell it is.  The galaxies set over other houses to early.  Although the dumbell does go over Gatwick Airport later which means I fight light pollution...

Actually, that is a good incentive to try to find a decent dark sky around me and stop being so lazy and sitting in my garden!

[Scott]

Where ever you go and whatever you image, you're going to need perfect balance, perfect pa and no wind etc. being to lazy just don't cut it in A.P. unfortunately  

[dph1nm]

Why do you think the problem is polar alignment? The C8 has a huge focal length. With the 1000D you are talking under 0.5" pixels. Periodic error in the mount is likely to get you long, long before polar alignment does.

NigelM

[uhb1966]

The longer your focal length, the more any errors are magnified....

[lensman57]

Hi,

the CG5 mount has a procedure that once you have aligned, it can then show you where true polar alignment is.  So you manually adjust the mount to compensate.

Where I said scope above just substitute for "mount" sorry.

You are not alone, I have been imaging for about 18 months now and my set up is mobile as is yours and I have to set up on grass everytime. The best thing to do is make sure that your heavy set up is on something solid. Paving slabs might be a solution if you don't mind the grass getting damaged. With a mobile set up and the polar align routines performed via the hand set the alignment is not backlash aware so even if the mount is positioned accurately as the night before the PA will be different for a new session due to the mount's backlash. The best thing to do is to make sure that the mount is on solid ground and that the PA routine is done atleast 3 or 4 times each time as it is an Iterrative process. Good luck.

A.G