Alignment vs guiding...any good answer out there?

[emadmoussa]

How much wiggle room do I have for the accuracy of alignment in order for the guiding to work? In other word, can the guide camera compensate for, say, 10% inaccuracy in the polar alignment? Am I making any sense here?

[K3ny0n]

I understand what your saying but unfortunately don't know the answer so I'll stick around .

Rob.

[michael.h.f.wilkinson]

A guider can guide an Alt-Az mount in principle, so you can be very far off and still get the guide star to stay put. When polar alignment is off, you will however get field rotation (everything other than the guide star rotating around the FOV). This will limit the maximum exposure time of the subs you can take. So accurate polar alignment allows long exposures, poor polar alignment not. How bad the problem is depends on many factors. Most important is the focal length of the scope: the longer the focal length, the more the problems get magnified. The only way to find out is to experiment.

[Aenima]

Me too

The guy that wrote 'Metaguide' is incredibly knowledgable on the subject - heres a link to get an idea. http://www.astrogeeks.com/Bliss/MetaGuide/MetaGuide5xx.html#__RefHeading__4_1601665524

Slighly brain boggling for me but a simple answer doesnt seem likely either.

Regards

Aenima

[Aenima]

A guider can guide an Alt-Az mount in principle, so you can be very far off and still get the guide star to stay put. When polar alignment is off, you will however get field rotation (everything other than the guide star rotating around the FOV). This will limit the maximum exposure time of the subs you can take. So accurate polar alignment allows long exposures, poor polar alignment not. How bad the problem is depends on many factors. Most important is the focal length of the scope: the longer the focal length, the more the problems get magnified. The only way to find out is to experiment.

...and theres your simple answer

Aenima

ps feel silly now

[RikM]

I get much better star shapes if I drift align the mount vs. a quick polarscope line up, so it is well worth getting it pretty close.

[michael.h.f.wilkinson]

...and theres your simple answer

Aenima

ps feel silly now

Why should you feel silly?

[emadmoussa]

I usually managed to get the alignment at least 80% correct, and mostly more accurate than that. But it seems since I got the NEQ6, the alignment seems a more difficult task especially with no clear skies out there to experiment on. Yesterday I was using EQMOd to slew the mount to the designated object but it ended up far off the required position. I am not quite certain whether it was a settings problem or a screwed up alignment...Then again that was indoors, and didnt have real targets to hit.

[michael.h.f.wilkinson]

I get much better star shapes if I drift align the mount vs. a quick polarscope line up, so it is well worth getting it pretty close.

Drift aligning is the proof of the pudding. Alignment on a polar scope depends on how precisely the polar scope itself is aligned. My Vixen GP ploar scope seems to be very close to spot on, as the few times I have done longer exposures (donkeys' years back, manually guided), not much guiding was needed to keep the C8 with focal reducer on track (1260mm focal length, F/6.3 using Fujichrome 400 pushed to 1600 ASA) for 15 minutes. I did a 12 min unguided exposure with no star trails and nice round stars (granted at only 85mm focal length, piggyback).

[michael.h.f.wilkinson]

I usually managed to get the alignment at least 80% correct, and mostly more accurate than that. But it seems since I got the NEQ6, the alignment seems a more difficult task especially with no clear skies out there to experiment on. Yesterday I was using EQMOd to slew the mount to the designated object but it ended up far off the required position. I am not quite certain whether it was a settings problem or a screwed up alignment...Then again that was indoors, and didnt have real targets to hit.

I do not really understand what you mean by 80% correct. Given that you can be off by at most 90 degrees (after that you just "guide in reverse" ) being 20% off would mean 18 deg off. I assume that is not what you mean. Furthermore, even with an accurate polar alignment, the scope may get positions on the sky wrong in terms of declination. Such a mismatch is a problem for goto (which needs absolute coordinates), but not for guiding, which only looks at changes in guide star position with respect to the guide camera.

[emadmoussa]

I do not really understand what you mean by 80% correct. Given that you can be off by at most 90 degrees (after that you just "guide in reverse" ) being 20% off would mean 18 deg off. I assume that is not what you mean. Furthermore, even with an accurate polar alignment, the scope may get positions on the sky wrong in terms of declination. Such a mismatch is a problem for goto (which needs absolute coordinates), but not for guiding, which only looks at changes in guide star position with respect to the guide camera.

Sorry, meant to say...80 per cent first time set up without any adjustments. just bang on, then adjust...Not sure I get the bit about declination...

[michael.h.f.wilkinson]

Sorry, meant to say...80 per cent first time set up without any adjustments. just bang on, then adjust...Not sure I get the bit about declination...

Sorry I got it wrong, I meant right ascension, not declination. When you have polar alignment, the declination coordinate is correct: for any object on the celestial equator, the goto system will set the right value. RA is harder, as it depends on time and location, not just on polar alignment. It is a bit like finding the longitude (RA) vs finding the latitude (DEC) of a ship. For the latitude, you just measure the altitude of Polaris, or the altitude of the sun at noon, do some very basic maths (subtraction really) and you are done. For longitude, you need to measure time.

[Aenima]

Why should you feel silly?

I just found it funny that after saying it was unlikely there is a simple answer, one was supplied immediately

I have a knack for these things

Regards

Aenima

[Freddie]

I usually managed to get the alignment at least 80% correct, and mostly more accurate than that. But it seems since I got the NEQ6, the alignment seems a more difficult task especially with no clear skies out there to experiment on. Yesterday I was using EQMOd to slew the mount to the designated object but it ended up far off the required position. I am not quite certain whether it was a settings problem or a screwed up alignment...Then again that was indoors, and didnt have real targets to hit.

Is there a mix up here between polar alignment and GOTO alignment. What you are describing above is GOTO alignment which is not related to polar alignment.

[emadmoussa]

Is there a mix up here between polar alignment and GOTO alignment. What you are describing above is GOTO alignment which is not related to polar alignment.

Mmm...probably mixed up! As far as I understand you need the mount correctly ''polar aligned'' in order for the the GoTo to work....No?

[E621Keith]

Mmm...probably mixed up! As far as I understand you need the mount correctly ''polar aligned'' in order for the the GoTo to work....No?

That depends on your GOTO. Some well designed GOTO system will track even if the polar alignment is way off. However, you may get field rotation as a results.

GOTO determines where your telescope points, polar alignment determines how well it tracks