How well am I Polar aligned?

[Bizibilder]

I've just spent a couple of hours setting up my new  EQ6     and trying to get the polar alignment spot on.  The mount is EQMOD controlled and I used this for basic PA.  Then on to Alignmaster and, after several iterations I got this:

Several more iterations later and I couldn't improve on the error readings.  I can't find any information as to whether this is "good" or not - does anyone have any idea of the sort of figures i should be able to get?  I have not tried drift aligning yet and realise that that is the "best" method - I'm just interested to see if anyone has any further information on the degree of error that you should be able to get by these methods.

[jambouk]

I've read people suggesting under 5 arcminutes is good; under 1 very good. Mine are often around 10arcminutes out.

I only use the handset, and i'm not convinced the algorithms are that correct as it generates all sorts of weird and wonderful numbers for Mel and Mal and they don't always relate to how quickly something drifts out of the FoV when i'm tracking.... But i only do visual and mess around with planetary imaging so it's not so crucial for me yet.

Good luck with it all.

James

[jambouk]

There are a few old threads; search for "mel maz" on the search box.

James

[jambouk]

My random variation of errors!

James

[Gina]

I use AstroTortilla for PA.  I got the error in both Az and Alt to less than 1 arc min as I recall.  I was hoping to get it better but the clouds came in and I haven't had a chance since.  That was using the Esprit 80ED on the EQ8.  Next clear(ish) night there was cloud low down so instead of more PA adjustment, I did some unguided test subs of the Heart Nebula and managed around 10m without star trailing.  If I can get the PA errors down to a few arc secs I reckon I shall be able to do a lot better.  Oh for some clear night skies

[Gina]

I've just spent a couple of hours setting up my new  EQ6     and trying to get the polar alignment spot on.  The mount is EQMOD controlled and I used this for basic PA.  Then on to Alignmaster and, after several iterations I got his:

alignmaster 2.PNG

Several more iterations later and I couldn't improve on the error readings.  I can't find any information as to whether this is "good" or not - does anyone have any idea of the sort of figures i should be able to get?  I have not tried drift aligning yet and realise that that is the "best" method - I'm just interested to see if anyone has any further information on the degree of error that you should be able to get by these methods.

This looks pretty good to me   Best I got on my NEQ6 was a couple of arc min.  The EQ8 is definitely easier to adjust

[wxsatuser]

Try this page Roger

http://www.starrynights.us/Articles/Polar_Align.htm

[Steve Ward]

Sub arc-minute is very good Roger.

I would say that a drift-alignment session would be a good test of the figures , the DARV method with a camera not a reticle EP I find much easier ( http://www.cloudynights.com/item.php?item_id=2838 ) ... 

I suspect that as you'll be guiding though this will be an unnecessary step , given that read-out.

[Demonperformer]

Try this page Roger

http://www.starrynights.us/Articles/Polar_Align.htm

Fascinating article.

But my idiot-question of the morning is 'what is an STV?'  I have read the article a couple of times and cannot see the answer.  Tried googling it and although I found several pages that referred to it for polar alignment, I have been unable to find out what it actually is.

Thanks.

[wxsatuser]

No idea as well.

I was concentrating on the first bit.

[Gina]

No idea either   But I guess it's not Scottish TeleVision

[Steve Ward]

StarTrailViewer in CCDware ... ?

[Gina]

Thank you for the link to a very interesting article   Not just interesting - very helpful   Seems that with around 1 arc min error I should be able to use 30m unguided exposures with my 1000mm MN190 - that would be great   I'm pretty sure I'm not doing that well as around 15m was about the best I could do with the Esprit 80ED at 400mm FL.  That would correspond to around 5 arc min error.

[Oddsocks]

STV was a video camera and guider package from SBIG, I remember the adverts for it from around five years ago at least.

They are no longer listed on the SBIG web site and only appear under the archive user manuals section so I guess they have long since been superseded by other methods of guiding.

Here is a link to an old web page showing what it looked like...

http://www.company7.com/sbig/products/stv.html

[libraryman]

I used PHD2 and the PA graph, i got down to sub arc min PA within 30min's on my EQ8, however seeing and other atmospheric issues can mean it is not always easy to sustain a good reading

Ray 

[cfpendock]

For me this is an extremely interesting thread.  Thanks to wxsatuser above for the link - this is very interesting.  Usually I can get to within 20" and find this works fine whatever I am imaging.  But I am always guiding for anything over about 5 minutes - perhaps unnecessarily.  But it is so easy.... I don't know how far out of PA I could be and use auto guiding to correct the error - I suppose it would depend on the settings used for guiding.

Chris

[RikM]

It is good to get the PA as close as possible even when guiding so as to avoid rotation error creeping to very long subs.

[jambouk]

RikM, can you explain why rotation is a problem when guiding, as I thought guiding would make very small adjustments in RA and Dec to make sure the object stayed in the same position? Is it the fact that the object drifts in the first place which gives a small amount of rotation, but there would appear to be as much risk of 'trailing' as 'rotation' but from reading your post you are making it sound like rotation rather than trailing. Sorry, I'm not an imager or a guider so have no real understanding of this, but would like to know.

James

[RikM]

I'm afraid I don't know how to describe this effect clearly but I'll give it a (ham fisted) go.

All the guiding software does is keep the guide star in the same place on the chip. If you are accurately polar aligned then as the RA axis tracks round, all the stars will be on the same arc across the field and should star as pinpoints in your image. If your PA is off a bit then the RA axis won't be rotating about the same point in the sky as the other stars in your image. Your guide star will stay in the same place on the chip but the arc the other stars in the field follow won't be the same arc as the RA axis describes and so will start to trail/rotate around the guide star.

If your PA is off the guiding will, in effect, give you a steady up/down/left/right movements similar to the movement of an altaz.

Here's how Jerry Lodriguss describes it: http://www.astropix.com/HTML/I_ASTROP/TRACKED/POLAR.HTM

[jambouk]

Ah, i think i see.

So would in theory guiding be better if it could latch onto four stars say, one in each corner of the chip?

I'll put that article on my pile to read!!!!

Thanks and it was a very non ham-fisted explanation.

James

[RikM]

So would in theory guiding be better if it could latch onto four stars say, one in each corner of the chip?

No, you only need one guide star, the rest is purely mechanical, not correctable unless you fit a field de-rotator in the imaging train. It isn't really a big problem unless you have a huge chip, huge field and long subs. Or if your PA is well out and your guide star is nowhere near the middle of your actual image.