A Polar alignment difference in approach

[pmrid (Peter)]

Until I caught the ASiair ZWO bug, I had been using SharpCap for PA and found it pretty darned good. Using a QHY PoleMaster CCD made it particularly easy.

The Polemaster is mounted on the RA axis of the mount - a HEQ5 or an EQ6. So it is aligning that axis. The underlying assumption is that your telescope is pointing to the same spot (cone error excepted).

Then, along came ASiair with it’s own PA routine. It uses your primary CCD and your imaging scope. If anything it is even easier than the SharpCap method but the same underlying assumption remains - is your scope correctly aligned to your mount’s RA axis.

When I first tried it last night, it told me that I was appreciably below altitude - to the extent of about 10 minutes. And also about 6 minutes off in azimuth. I had done a PoleMaster align a few days before and had not moved the mount in between.

Has anyone else had an opportunity to quantify the differences between the two approaches? When I get another clear night I plan to try to do so.

[rustigsmed (Russell)]

hi peter,

i've only used sharpcap recently which i found to be pretty good (coming from synscan hand controller). I've just had my ASIair arrive so will be looking forward to testing it out.

Did the ASIair result in good pointing?

Cheers

Russ

[gregmc (Greg)]

As a check for your altitude adjustment, use an inclinometer (mobile app or some other way to check accurate angles to horizontal).

When at the home position with the OTA pointing at the Pole, check to see if the angle is exactly the same as the latitude for that location (mobile's GPS reading)

[pmrid (Peter)]

Originally Posted by rustigsmed

hi peter,

i've only used sharpcap recently which i found to be pretty good (coming from synscan hand controller). I've just had my ASIair arrive so will be looking forward to testing it out.

Did the ASIair result in good pointing?

Cheers

Russ

I don’t really know yet. The reason is that I seem to have some coma and also some tilt to resolve. I’m using an 8inch newt with an MPCC III. But am still messing about with backfocus spacing. So until I resolve that, I can’t really compare.

I will note that my guiding error seemed to reduce by about 2/3. So I’m pretty positive.

[pmrid (Peter)]

Originally Posted by gregmc

As a check for your altitude adjustment, use an inclinometer (mobile app or some other way to check accurate angles to horizontal).

When at the home position with the OTA pointing at the Pole, check to see if the angle is exactly the same as the latitude for that location (mobile's GPS reading)

The trouble with digital inclinometers is that they need to be accurately zero’d and even then, most report accuracy in the +- 0.1 to 0.5 degrees range. Not all that encouraging if you want to use it for PA levels of accuracy.

The other problem is that we have no visual guide for dead-centering the pole and we also have to make some allowance for atmospheric refraction around the pole so that inclinometers are at best an approximation. The degree of refraction is a function of height and several other things but, for example, at 10 degrees, it can be as high as 5-6 minutes of arc. At say 27 degrees it would be in the vicinity of 3 minutes.

[Zuts]

I think the proof is in the pudding, if you are getting nice tight round stars then it's safe to say that PA and guiding are working well.

I am very happy with my ASIAir plus, the point being that it works for me and does what I want.

I 100% agree with you that it would be nice to compare the methods, but as an intellectual exercise only, as if one method works well then take that dust cap off and start collecting photons

[Startrek (Martin)]

I’ve used the Synscan polar alignment routine since day one ( 5 years ) on various mounts and scopes ( mainly newts ) and it’s worked well as you don’t need a view of the SCP. I just use my old DSLR in the newt connected to BYEOS ( alignment tool ) and de focus the alignment Star into a donut for accurate centering. The live view on my DSLR is much better than my 2600MC Cmos camera , there’s no lag or buffering, the Star centering is instantaneous.
I usually require 2 or 3 iterations to get below 1 arc minute which is satisfactory for PHD2
Back to your original question, something interesting I found in regard to PA , if I redo my polar alignment say a day or two later (out of curiosity ) the error is slightly different to the original Polar alignment error
Because I’m using the main OTA for PA , variances in atmospheric conditions and optical train will affect your PA error to some degree
I try not to get get too wound up about PA as PHD2 will guide satisfactorily with a PA error of up to 5 or 6 arc minutes. If fact the PHD2 developer Bruce Waddington mentioned not to be obsessive over PA error , it won’t make you guide any better

Cheers
Martin

[gregmc (Greg)]

Originally Posted by pmrid

The trouble with digital inclinometers is that they need to be accurately zero’d and even then, most report accuracy in the +- 0.1 to 0.5 degrees range. Not all that encouraging if you want to use it for PA levels of accuracy.

The other problem is that we have no visual guide for dead-centering the pole and we also have to make some allowance for atmospheric refraction around the pole so that inclinometers are at best an approximation. The degree of refraction is a function of height and several other things but, for example, at 10 degrees, it can be as high as 5-6 minutes of arc. At say 27 degrees it would be in the vicinity of 3 minutes.

I did say as a check, use the inclinometer.
I also use a compass stick to get the true north close (and in the middle of the az adjustment range for later polar alignment)
-- a compass still as I like real magnetic compasses and I adjust for the magnetic declination with the stick keeping the compass away from any influence of the tripod

Good info putting numbers re relating inclinometer accuracy to the sky. I must dig into that. I'd also be interested in the impact on compass accuracy and true north. An advantage of living south (in Sydney) where even less atmosphere to look through.

I use SharpCap to drive a polemaster camera to do polar alignment with the atmospheric compensation turned on so hopefully less impact. I intend to try the NINA polar alignment if we ever can get clear skies again.

It's more out of curiosity as I do a small model the sky anyway with my Astro-Physics Mach2, turn tracking on and can image without guiding if I want but like the graphs from PHD2