polar alignment vs. guide scope & software

bartman (Bart) · #1 02-12-2009, 03:03 PM

Hi peoples,
Q: Is a precise polar alignment needed for a guide scope?
Say for example: 10" newt, neq6, 80mm edpro connected to gstar ex connected to laptop with software connected to neq6.....
In other words... does the guiding camera with software etc compensate for a non precise polar alignment?...or any mount inaccuracies ie worm gear?
And on top of that, does the guide scope need to be aligned with the main scope just like the finder scope? precisely?

Cheers
Bartman

scarper (Mart) · #2 02-12-2009, 03:21 PM

It will compensate a little for misalignment........but you must be pretty spot on for it to correct it efficiently.
As long as the guide scope and telescope are pointing at the same thing (centred) then the guiding software should run a few tests before Guiding to make sure all is talking and aligned together.

sheeny (Al) · #3 02-12-2009, 03:24 PM

G'Day Bart,

Generally the better your polar alignment, the longer the exposures you'll be able to take without field rotation. If you are happy to take only short exposures at short focal length, polar alignment doesn't need to be super accurate, however, if you want to take long exposure shots, your polar alignment accuracy will limit the length of time you can expose for before the stars noticably rotate around your guide star.

However, since you specifically mentioned the GStar ex... I believe they are capable of image integration with drizzle (

correct me if I'm wrong). So in theory, as long as there is no noticable trailing in the individual exposures the GStar should be able to assemble the equivalent of a longer exposure without trailing and field rotation I believe.

Your guide scope does not have to be perfectly aligned to your main scope, as the finder scope is. Guidescopes are often mounted in adjustable rings to allow them to be adjusted to find guide stars, or you can use an X-Y guidestar finder which changes the optical axis of the guidescope.

Generally, tracking should cope with PE variations, but PEC helps reduce the tracking adjustments.

Al.

multiweb (Marc) · #4 02-12-2009, 03:40 PM

Quote:

Originally Posted by bartman

Hi peoples,
Q: Is a precise polar alignment needed for a guide scope?
Say for example: 10" newt, neq6, 80mm edpro connected to gstar ex connected to laptop with software connected to neq6.....

Depends on the focal length you're going to image at and your exposures time. Up to 1000m FL and 3-4min exposures you don't have to be well aligned. You can get away with a lot.

Quote:

Originally Posted by bartman

In other words... does the guiding camera with software etc compensate for a non precise polar alignment?...or any mount inaccuracies ie worm gear?

It will to some extent guide out drift errors but as Al mentioned you'll still have field rotation on long exposures. If your mount is very good, the guider will send pulses to the mount which mechanically will move where it's meant to go. If your mount is average sometime it will move correctly some other times it'll end up somewhere else. So as a rule if you nail your polar alignment the guider won't have to correct the mount position too often and that's much better.

Quote:

Originally Posted by bartman

And on top of that, does the guide scope need to be aligned with the main scope just like the finder scope? precisely?

Cheers
Bartman

No it doesn't have to.

bartman (Bart) · #5 02-12-2009, 04:47 PM

Thanks Marc, Al and Mart.
You all have answered my questions ...but now have some questions from your replies....

Marc: you mention " If your mount is very good, the guider will send pulses to the mount which mechanically will move where it's meant to go. If your mount is average sometime it will move correctly some other times it'll end up somewhere else "
Is the NEQ6 very good or average?

Al: I've looked at the PEC in the manual and seems like the thing to do...on top of all the other alignments....is this a common adjustment?

and
Mart: " It will compensate a little for misalignment........but you must be pretty spot on for it to correct it efficiently. "

Took me a little while to comprehend this statement...but now understand!!!!

guess I just have to wait for the bits to arrive and the practice!!!!

cause it always makes perfect ....doesn't it?

Bartman

multiweb (Marc) · #6 02-12-2009, 04:52 PM

Quote:

Originally Posted by bartman

Is the NEQ6 very good or average?

I don't know the NEQ6. What you can do is polar align the mount really good, make sure your balance is also good then select a star in the guider , disable the guide output in PHD and check the graph. It'll give you a good indication of how the mount tracks. Then switch the guiding back (guide output on) on and compare the graphs.

bartman (Bart) · #7 02-12-2009, 05:05 PM

unfortunately I cant see the asterism in Octans that is in the polar scope from my back yard (ummmm courtyard 3.5x 4 meters and south facing wall is about 3 m high. Scope is balanced & level and I have yet to try any software guiding the scope.
I certainly will try what you said once I get the gstar.....

If I may make a bold statement here.....
Could somebody do a generic "how to set up a basic guide scope guide" in the ' projects and articles ' section?
or did i miss it?

Thanks Marc
bartman

multiweb (Marc) · #8 02-12-2009, 05:10 PM

Quote:

Originally Posted by bartman

unfortunately I cant see the asterism in Octans that is in the polar scope from my back yard (

Then drift aligning will get you what you want. There are a couple of tutorials online to achieve pretty good polar alignment.

sheeny (Al) · #9 02-12-2009, 05:11 PM

Quote:

Originally Posted by bartman

Al: I've looked at the PEC in the manual and seems like the thing to do...on top of all the other alignments....is this a common adjustment?

I don't think it would be worthwhile on a mount that you setup each night and strip down again. But I can see the benefit in a permanently aligned mount in an observatory.

Al.

bartman (Bart) · #10 02-12-2009, 05:16 PM

Thanks Al ...true point.
Thanks Marc....I had/ have considered that and have / had a look at those tutes....
of to bed now
doing night shift.....
bartmna

[1ponders] (Paul) · #11 02-12-2009, 05:23 PM

Bart a couple of things to remember.
1. If you are not accurately polar aligned (and I mean significantly >1 deg out in Alt or Dec say) then your autoguider may not be able to keep up with the drift. I have seen this a number of times where the autoguider calibrates successfully but during the imaging run doesn't have a hope of keeping up. The trick is to get polar alignment as accurate as you can. ie learn to drift align.

2. The further out you are then the greater the amount of image that will show rotation (assuming you can autoguide). The trick is to get polar alignment as accurate as you can. ie learn to drift align.

3. The further away from the imaging area you are guiding from, even if you are well polar aligned the more rotation you will observe. Generally you won't see it in the image but when you stack them you will notice that each image may be slightly rotated compared to the previous one. It makes if very challenging to carry out multiple night imaging runs and then try to combine all the runs into one image. If its possible try to autoguide with stars that are close to the center of your image if you can.

In your intro you didn't say if you were imaging with the G star or wanting to autoguide with it.

jjjnettie (Jeanette) · #12 02-12-2009, 06:02 PM

If you're imaging with the Gstar, it's very forgiving. You can get good images out of it using just an alt azi mount.
The software even allows you to pause the imaging run so you can recentre the object if necessary.