Drift alignment without ASCOM

[Jarrod Smith]

My philosophy on imaging is to keep my equipment as small and simple as possible, so I image with a 7" tablet and use ST4 guiding.  This cuts down on clutter and computer resources.

I was experimenting with drift alignment in PHD2 last night and this is clearly designed for ASCOM guiding.  I did read through the tutorial and read the ASCOM note specifically.  However, I had a lot of problems and was not able to achieve a satisfactory alignment (30+ arcmin error - much worse than what I can achieve with my mount's simple internal routine).  I assume I did something wrong.  Here are the places where I had to deviate from the tutorial:

1. PHD2 was suggesting RA/Dec values (especially for the alt adjustment) that pointed my scope in a direction that the sky was not visible.

2. My hand controller allows me to goto a specific RA/Dec location, but it expects me to enter RA in hours:minutes:seconds, not degrees.  This was confusing to me.  For example, when I switched to adjusting alt, the program suggested -65degrees RA and 0 degrees dec.  How do you convert -65 degrees RA into hours:minutes:second?  I ended up guessing, but again, see (1).

Related to the above: are the celestial coordinates displayed by PHD2 merely suggestions, or must the mount be pointed exactly at that point in the sky?  I was assuming the former (really, had no other option given item #1 above), and so was getting the guide scope pointed to a nearby location, followed by a recalibration in PHD2 (without ASCOM, PHD2 is not able to convert my calibration to a different location in the sky).  If the answer is the latter, can you change the values yourself.  I tried clicking on the up/down arrows next to these values but did not get a response.  This could have been an issue with my touchscreen.

Related feature requests:

My screen is small.  A zoom option would be very useful for the adjustment phase.  It was extremely difficult to see the star while I was making the az/alt adjustments.  Sometimes it seemed like the star remained selected during this process, which helped some.  Sometimes the selection box disappeared.  Not sure what is the correct behavior, or again, if I did something wrong.

Thanks in advance for any advice.

[bw_msgboard]

The Drift Alignment tool is not “clearly designed for ASCOM guiding” – it works fine without ASCOM. It’s just that with an ASCOM connection, the tool is able to make the process quicker and easier. It sounds like you’re confused about what you see in the UI. Have you read the online docs and worked through the tutorial that’s posted on the web site?

 

https://sites.google.com/site/openphdguiding/phd2-drift-alignment

 

Drift alignment concerns itself with two approximate pointing positions: It starts at a point near the celestial equator and the celestial meridian, and then wants to repeat the drift measurement at 25-30 degrees above the east/west horizon, again near the celestial equator. The UI is showing the angular distance from the meridian, not the RA. So the tool was not proposing that you move to -65 in RA (there’s no such thing) – it was asking you to move 65 degrees away from the meridian (25 degrees above the horizon). These slewing positions are very approximate, so you can just slew the scope by hand and get reasonably close. If you can’t work that close to the horizon, that’s ok – but it may cause the process to take longer, with more iterations to close in on the best polar alignment.

 

FWIW, the internal alignment routine in your mount is not adjusting your polar alignment – it is simply building a very crude pointing model to help with go-to’s. Drift alignment for guiding is a mechanical process, requiring you to physically adjust the mount so the RA axis is pointed close to the north celestial pole.

 

Good luck,

Bruce

 

 

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[Jarrod Smith]

On Oct 23, 2015, at 9:59 AM, bw_msgboard <bw_m...@earthlink.net> wrote:

> It sounds like you’re confused about what you see in the UI. Have you read the online docs and worked through the tutorial that’s posted on the web site?

Yes, and yes.

> So the tool was not proposing that you move to -65 in RA (there’s no such thing) – it was asking you to move 65 degrees away from the meridian (25 degrees above the horizon).

I see. I think the "Slew To" prompt is what confused me. I interpreted it as an RA coordinate (the other box is in fact a dec coordinate, right?). I interpreted the column header that says "Meridian Offset" to apply only to the ASCOM readout directly below it (which was not populated in my case so I completely disregarded it). IMO this could be made more intuitive, but I think I understand now. In fact, why not give it in RA, so non-ASCOM users can just plug the coords into our HCs directly? P.H.D. ;-)

> These slewing positions are very approximate, so you can just slew the scope by hand and get reasonably close. If you can’t work that close to the horizon, that’s ok – but it may cause the process to take longer, with more iterations to close in on the best polar alignment.

That's good to know - and is how I interpreted those values. But I do need to recalibrate each time I move the scope (because I am not communicating with ASCOM), correct?

> FWIW, the internal alignment routine in your mount is not adjusting your polar alignment – it is simply building a very crude pointing model to help with go-to’s. Drift alignment for guiding is a mechanical process, requiring you to physically adjust the mount so the RA axis is pointed close to the north celestial pole.

I wasn't talking about the pointing model. I was talking about the ASPA routine.

Thanks for your help!

Jarrod

[Kevin S]

Jarrod,

I doubt everyone knows what ASPA (all star polar alignment) is except of course Celestron Mount users.  Many mounts have no such routine and if they do it's probably not as good as ASPA...hence the desire for other methods.  Not trying to dissuade you from drift alignment but, ASPA done properly should be all you need....

Kevin

[Jarrod Smith]

Kevin, that is an interesting comment.

The reason I decided to try the drift tool is because, after watching the Google Hangout video on 2.5.0, I gave the Guiding Assistant feature a try. It concluded that my polar alignment (which I made with ASPA) wasn't that great at ~25 arcminutes off which would produce some field rotation.

That's what made me decide to give the Drift Alignment tool a shot. Except that I wasn't able to do much better (actually worse) than ASPA. Granted, there was confusion on my part so I should (and will) go try again.

However, the PHD2 drift align tool has made me realize that I probably need to take a great deal more care in selecting which star I sync on when doing an ASPA alignment. I know not to sync on something close to the poles, but have never really thought much about pointing closer to the horizon. Maybe this is the source of my sub-par polar alignment, and not a short coming of ASPA?

Jarrod

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[bw_msgboard]

-----Original Message-----
From: open-phd...@googlegroups.com [mailto:open-phd...@googlegroups.com] On Behalf Of Jarrod Smith

Sent: Friday, October 23, 2015 10:26 AM
To: bw_m...@earthlink.net
Cc: Open PHD Guiding
Subject: Re: Drift alignment without ASCOM

 

 

On Oct 23, 2015, at 9:59 AM, bw_msgboard <bw_m...@earthlink.net> wrote:

 

> It sounds like you’re confused about what you see in the UI. Have you read the online docs and worked through the tutorial that’s posted on the web site?

 

Yes, and yes.

 

> So the tool was not proposing that you move to -65 in RA (there’s no such thing) – it was asking you to move 65 degrees away from the meridian (25 degrees above the horizon).

 

I see.  I think the "Slew To" prompt is what confused me.  I interpreted it as an RA coordinate (the other box is in fact a dec coordinate, right?).  I interpreted the column header that says "Meridian Offset" to apply only to the ASCOM readout directly below it (which was not populated in my case so I completely disregarded it).  IMO this could be made more intuitive, but I think I understand now.  In fact, why not give it in RA, so non-ASCOM users can just plug the coords into our HCs directly? P.H.D. ;-)

 

If we don't have an ASCOM connection, we have no idea where you are.  You could literally be anywhere on Earth.  In that case, we have no idea what your local sidereal time is, so we can't possibly know what RA maps to "15 degrees above the horizon." 

 

> These slewing positions are very approximate, so you can just slew the scope by hand and get reasonably close. If you can’t work that close to the horizon, that’s ok – but it may cause the process to take longer, with more iterations to close in on the best polar alignment.

 

That's good to know - and is how I interpreted those values.  But I do need to recalibrate each time I move the scope (because I am not communicating with ASCOM), correct?

 

You will want to recalibrate if you change the declination by a significant amount.  You won’t need to recalibrate after a meridian flip – you can just use the “flip calibration” function in the Tools menu.  Reducing the need for re-calibrations is just one of many reasons we push people toward using ASCOM.  “Cutting down on clutter and computer resources” is not bad, I guess – but I would opt for making the best use of my imaging time and getting the best possible results. <g>

 

> FWIW, the internal alignment routine in your mount is not adjusting your polar alignment – it is simply building a very crude pointing model to help with go-to’s. Drift alignment for guiding is a mechanical process, requiring you to physically adjust the mount so the RA axis is pointed close to the north celestial pole.

 

I wasn't talking about the pointing model.  I was talking about the ASPA routine.

 

Ok, I don’t know what “ASPA” is  There are many techniques for getting good polar alignment although drift alignment is kind of the gold standard.  As long as the technique involves making mechanical adjustments to the mount alignment and provides some measure of alignment error, it’s probably fine.

 

Bruce

 

Thanks for your help!

 

Jarrod

 

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[Jarrod Smith]

On Oct 23, 2015, at 1:09 PM, bw_msgboard <bw_m...@earthlink.net> wrote:

> If we don't have an ASCOM connection, we have no idea where you are. You could literally be anywhere on Earth. In that case, we have no idea what your local sidereal time is, so we can't possibly know what RA maps to "15 degrees above the horizon."

Oh, oops. Gotcha.

> Ok, I don’t know what “ASPA” is There are many techniques for getting good polar alignment although drift alignment is kind of the gold standard. As long as the technique involves making mechanical adjustments to the mount alignment and provides some measure of alignment error, it’s probably fine.

Yeah, it's just Celestron's way of using the pointing model (which I think is not that crude - 6 stars, in my case using a 10mm reticle EP to calibrate them) to point the scope where a star you've selected *should be* if you had a perfect polar alignment. Then you make mechanical adjustments to get that star in the reticle, thus in theory removing the alignment error.

Thanks again

Jarrod

[bw_msgboard]

Just to tie this off, it sounds like you got into this because you were
getting conflicting reports on your polar alignment error. If you want to
use the Guiding Assistant to measure polar alignment, you really need to run
it for an extended period of time - 60 seconds is probably not nearly long
enough. I would run it for at least the duration of one worm period on your
mount and then would watch to see if the computed error was stabilizing.
Large, uncorrected periodic error can foul up the measurement process, which
is why you want to run for at least one worm period. One advantage to the
Drift Alignment tool is that it continues to guide in RA, thus reducing any
PE impact. There's no rocket science involved in the measurement - you can
use the PHDLogViewer or even Excel to measure the amount of drift you have
in declination over an extended period of time. You basically want to fit a
straight line to the points, since each point has measurement error
associated with it. Once you know the overall drift rate, usually arc-sec
per minute, there's a well-known equation for converting that into polar
alignment error.

Bruce

-----Original Message-----
From: open-phd...@googlegroups.com
[mailto:open-phd...@googlegroups.com] On Behalf Of Jarrod Smith
Sent: Friday, October 23, 2015 11:16 AM
To: bw_m...@earthlink.net
Cc: Open PHD Guiding
Subject: Re: Drift alignment without ASCOM

[Jarrod Smith]

Thanks - I'd though of the PE issue, but admit to only having had patience for 50% of the worm period (5 min). I have in fact calibrated the mount's PEC function. But I never had good luck guiding while PEC was turned on (I may have to start a new thread for that at some point :-)

I'll give this all a new spin with the added knowledge I gained here. Many thanks.

Jarrod