In degree or arc minutes - how accurate does a polar alignment need to be?

[Gem (Grant)]

Hi all

Any techies out there want to work this one out (or guess!):
Assuming an 80mm f/5 refractor is used with a DSLR. If you want to take, say 5 minute exposures - how accurate does the polar alignment need to be? How many arc minutes do you need to be within polar alignment?

Curious to know...

[A23649 (Nathan)]

not sure? maybe perfect if not guiding and taking long subs. best bet would be to get as close as you can.

[Gem (Grant)]

I realise the closer the better, but I was wondering if there was a figure. If I can work out how accurate the alignment is initially - then I could know how long to expose for.

[deanm (Dean)]

Try a series of exposures of increasing duration and see where you start to get unacceptable star trails.

Start at 30 seconds and use (as a suggested starting point) 20 second increments.

As Nathan says, best to get alignment as good as possible to start with.

Dean

[[1ponders] (Paul)]

Unfortunately it doesn't quite work that way Grant. It would depend on your altitude and azimuth you want to image and the amount and direction your are out of polar alignment.

But if you want to take 5 minute shots with a DSLR through and ED80 then you will have to be damn close.

What are you using to get polar aligned? Illuminated Reticle or webcam and software? FYI using software polar alignment I have gone more than 10 min unguided in DEC. As for RA it depends on how big your periodic error is and the time for the worm to complete a full cycle. eg losmandy G11 is 8 min but I think the G8 is only 4 min.

[Gem (Grant)]

Ahh... alt and az... didn't think of that.
It was meant to be a theoretical question rather than a practical question.
Having a CGEM mount there is a polar alignment mode where you can align off any star - so I was wondering how close the initial alignment would need to be for a set exposure length. For example, if the initial align was say wtihin 5 arc minutes from a polar alignment then you would be able to expose for X minutes with Y focal length.
Thinking it through now it would depend on sky position, as you say, since the target would affect any trailing.
Oh well - it was a theoretical question, so back to theories!

[[1ponders] (Paul)]

Maybe someone here with more mathematical nouse than I have can answer but I can tell you for a non tracking mount. The exposure time to achieve non-trailed stars using a normal tripod is T=700/fl(mm) for near the celestial equator (ie a 50mm lens will give you 15 sec) and T=1000/fl near the celestial pole.

[Daveskywill (David)]

Hello:

You are right, that the Declination of the object you are trying to image through the equatorial mount determines the about of drift to the north or south in your scope as you are tracking it.

It is hard for me to estimate what this would look like visually in my mind. But I suppose the star would look like an arc eventually and probably with a more gradual one than if you weren't tracking at all.

And if your mount is pointed to the east of celestial pole, the star will track to the north (if you have un-inverted opitcs and you live in the Northern Hemisphere it will appear However, I forgot there for awhile you're in the South. But actually after I thought of it, it's the same there too.

And if you're mount is pointed to the west, the star will track to the south in un-inverted optics it will appear to go that direction too.

PS: I'm not real sure about those drift directions. But I do realize that it would drift one way for to the west or east of pole and is the opposite for the other.

In fact that's one way to align the equatorial mount is through drift.

Yeah someone should be able to figure it out mathematically. Like how much the drift is in mm is in an equation in a book called: The FX Exposure determination Method.

David

[swannies1983 (Dan)]

This is interesting. I finally had a chance to go out and try out drift alignment on my EQ6 + Astromeccanica motors + 8" Newtonian. After many frustrating moments, I think I finally achieved an ok result. I was able to achieve no DEC drift for about 8mins using my 12.5mm reticle. There was still drift in RA but I reduced this by adjusting the sidereal rate. Nevertheless, some RA drift remains but I don't think I can eliminate it.

Overall, I was able to keep a star in the box of my recticle for at least 5mins. Is this acceptable? When I finally start prime focus imaging, I won't have the ability to guide. I assume the aim will be able to get subs in the order of a few minutes. I was interested to know what sort tolerance I would have with this polar alignment.