How good is polar alignment with a polar scope for imaging in Southern Hemisphere?

[HenryNZ]

Hi first post here. I have been visiting this forum for a while but have been mostly posting on cloudynights. However, I would like to get some more local comments on this particular topic, as I found the Americans have a hard time understanding the nature of the problem...

When I bought my mount (ioptron ZEQ25) the dealer says it does not come with a polar scope because there is no point having one in Southern hemisphere. I have since been using alignmaster and PHD drift alignment to get polar alignment for imaging. While alignmaster is quick and dirty, I find that it is limited by the goto accuracy and the backlash of the mount (poor in the case of ZEQ25), and never get perfect alignment with it for this reason. PHD drift alignment is a pain and slow, but with better and usable accuracy. However even with multiple iterations (taking up to 45-60 minutes), I can get at best within 5 arc min. Typically PHD2 will report sub arcmin alignment on the Az and Alt axis individually but when actually imaging there is still drift. I suspect it may be cone error or perhaps the mount is not level / squared or something - I am not sure why.

My question now is, is there any point actually spending money on a polar scope? I have never been able to see sigma octantis with my naked eye but can you see it thru a polar scope? The North Americans seem to be able to get workable polar alignment within couple of arc min using polaris (i.e. better than what I can get with PHD2 drift in 45-60 minutes) - can the same be achieved using a polar scope with sig oct in Southern hemisphere? If so, I may as well use a polar scope and save myself 45-60 min of precious imaging time.

Or is there a better way to get better alignment for imaging (with a portable setup).

Many thanks

Henry

[Amaranthus (Barry)]

If you can get your mount in the same place each time you set up, then I find that drift alignment with PHD2 will get me within 1' after about 10 min. Consistently. A little drift is also okay, if you are guiding.

I've never even bothered to look through the polar scope on my AZ-EQ6! (even though I can see down to 6.7 visually in my skies).

[HenryNZ]

Unfortunately I do not have a site where I can permanently mark the spot (i am imaging on a piece of turf which also gets a lot of foot traffic). I can see how that would be very helpful. Every time I place it on the site even with a compass etc I always start off with at least 5-6 degree of error and converge to alignment over 5-6 iterations.

[SamD (Sam)]

With some practice I now find using the polar scope on my NEQ6 quicker than using PhD drift align, it only take a few minutes. It's accurate enough alignment for guiding. After dark adpating, I can just about see 2-3 of the stars of the asterism in the polar scope. I have quite a lot of light pollution - can only see washed out Milky Way near overhead, and can't see any asterism stars with naked eye.

It's very helpful though to be able to get the mount close to the same position each night. I have three small holes in the lawn. When I drop the mount into these holes I can see the asterism stars in the polar scope straight away. If you are 5 degrees away to start with, star hopping in Octans with the polar scope to the asterism would probably be too much effort.

[PeterEde (Peter)]

I can see 4 stars of the asterism (on a good night), the fourth being very faint. But three is enough. which I can see easily from my Adelaide beach side suburb.
With polarscope I can get 2+ minutes tracking on my EQ3

[HenryNZ]

Thanks. It's good to know people use polar scope for imaging. I typically only have 3-4 hours max for imaging as I need to work early mornings and cannot always stay up till late. A 45 min alignment time is too long. I don't do super long focal length so it's encouraging that good results can be obtained with polar scope alone in a few minutes. I also think that polar scope if calibrated properly may mitigate problem with cone error (I suspect drift alignment is never perfect despite my efforts because my scope axis is not perfectly aligned or orthogonal to the polar axis. So what might be good alignment in one position is not good for another direction).

I have been hesitating to spend money on a polar scope but I think I may give it a try now that I hear what you have to say.

Do you have a figure of your alignment error with polar scope as reported by PHD or PHD Lab?

[SamD (Sam)]

Originally Posted by HenryNZ

Do you have a figure of your alignment error with polar scope as reported by PHD or PHD Lab?

About 2-4' error, which translates to about 70-140 ms of dec guiding per minute (at 0.5x auto guide speed). I prefer to have a bit of error in polar alignment. I find it easier to always guide declination in the same direction in PhD, and avoid oscillating N/S dec guiding.

As a one-time job, I found it quite fiddly to align the reticle inside the polarscope (with the star patterns on it) with the true pole (tiny allen bolts). I think mine is slightly misaligned, but luckily it seems to give me the amount of error (2-4') I need for reliable dec guiding in PhD !

If an error of say up to 5' is going to give you problems with field rotation on longer exposures, then a polarscope alone may not be the best way to go - you would still need to do drift alignment.

[HenryNZ]

I would be quite happy to get 2-4 arc min of error. I get 5 arc min of error after drifting for 45 min! It does seem like having some kind of marker at the site to mark the position is important, as trying to get the scp into the fov of the polar scope sounds difficult if not impossible, is that right?

[SamD (Sam)]

Originally Posted by HenryNZ

as trying to get the scp into the fov of the polar scope sounds difficult if not impossible, is that right?

My NEQ6 polarscope has about 5 degress FOV, so you can afford to be only a couple of degrees out in positioning the mount to get the scp in view. My lawn holes kind of self-drill with about 30-40kg of mount, scope and counterweights pushing down !

[multiweb (Marc)]

It's the quickest way to polar align on the field, by far, if you can see the scp.

[5ash (Philip)]

When I set up my NEQ6 on a permanent pier , I used a G star to drift align. Much easier than looking through the eyepiece. You could use a sensitive webcam or planetary video imager instead of the G star. Another option is to place the camera at the polar scope and observe the asterism and reticule more easily , myastroshop used to sell an adapter for the G star to make this easier. Alas I have sold mine and the G star. Will have to use my guide camera next time!
Philip

[ZeroID (Brent)]

Originally Posted by HenryNZ

Unfortunately I do not have a site where I can permanently mark the spot (i am imaging on a piece of turf which also gets a lot of foot traffic). I can see how that would be very helpful. Every time I place it on the site even with a compass etc I always start off with at least 5-6 degree of error and converge to alignment over 5-6 iterations.

Are you not able to knock in three wooden pegs to just below surface level once you get a 'best' position. Makes it a heck of a lot easier if you can get a repeatable sertup position and getting it level is a big help.

I made a wooden 'T' frame that compensated for the slight slope on my earlier driveway site. The mount stayed same leg length so I just dropped the T frame down on marked spots and then placed the tripod on that. I could be aligned in 10 minutes or less. My marker spots were yellow spray paint blobs. Sunken pegs or similar could gain you consistency in placement.

No handy concrete paths ?

[gbeal]

My roll out pier has three screw down bolts that locate on three washers glued to the asphalt where I use it. Same each time, makes life simple.
Like Brent suggested, three pegs, or possibly three small pavers sunk into the turf so you can mow or walk without smacking them.
Gary

[VPAstro (Andrew and Cam)]

I am north of Melbourne, and the SCP sits right over melbourne, so the light polution makes it very difficult to see octans. On a clear moonless night, I am able to see it though. I use to always use my polar scope, and have placed marks on the ground for the 3 legs of my tripod, which really made it easy to align. When I upgraded to version 3.35 and now 3.36 of my synscan, I just use its polar alignment function. It works really well, and takes me about 5 - 10 minutes to get something I am happy with. I have not looked through my polars scope since. I did try alignmaster a few times, but found that the had controller acheived better results.
Once I have done the polar alignment, I place the scope in home, and it is over to EQMOD, stellariam and PHD.
This works well for our HEQ5

[speach (Simon)]

I've neq6 pro with a polar scope, tired using it but finding Sigma Octans is a pain, never found it. My advice is go with Drift.