Drift Alignment Without East/West Horizon

[russellhq]

I was thinking about this the other night as from my site I don't have a view of the East or the West but I'm not sure if it would work or not. If I start my drift alignment facing due south and eliminate all drift from my Azimuth, can I safely assume that when I point as far east (or west) as I can, then any subsequent drift I see will only be down to Altitude misalignment?

Also, what effect does moving away (in DEC) from the equator have on drift alignment? I've read that you should drift as close to the equator as you can but I've never understood why?

[reddoss]

As you have an EQ5 and EQMOD you can use the Alignmaster program and fix on two stars and the move the Alt and Az bolts to the required amount by the program and you have very good polar alignment, as good as drift alignment according to many. You get a month free to try it out after down loading it, and 12 euro's to but it out right. Although this does not answer your question, it may help your not being able to see East/West problem.

http://www.alignmaster.de/

[IanL]

The reason you point due South (for Az) and due East or West (for Alt) is to separate the Alt/Az misalignment components. If you point SE or SW, then any drift may be due to polar misalignment in either Alt, Az or both, if you use software (as above), it can calculate the components for you, but working manually it is not really feasible to do so.

Your idea that if you get Az right first that any remaining component must be due to Alt is logical, but the further you are away from the Eastern or Western horizon, the harder it becomes to detect the drift. (Think about it this way, if you are pointing at the Eastern horizon, then all of the drift is due to Alt misalignment, but when you point South none of the drift is due to Alt, and so drift due to Alt must reduce from maximum at East to nothing at South).

In practice, you can easily track a star near to the equator due south, but you will not be able to see a star on the horizon at due East or West, so everybody has to compromise (usually 20 or 30 degrees above the horizon at least). How well you will do depends on how high you are above the horizon for the E/W part of the process.

You aim at the equator to minimise the amount of time before drift becomes noticeable. The further you get from the celestial equator, the less distance a star moves across the sky in a given amount of time, so the longer you have to wait to see if it is drifting due to misalignment. Again it isn't necessary for the star to be on the equator, just near it.

[russellhq]

Your idea that if you get Az right first that any remaining component must be due to Alt is logical, but the further you are away from the Eastern or Western horizon, the harder it becomes to detect the drift.

If that's the case, then if I can eliminate drift from my FOV across an exposure, then I would assume it shouldn't matter if I can't see East or West as I wouldn't be imaging in the direction anyway? If that makes sense.

[IanL]

If that's the case, then if I can eliminate drift from my FOV across an exposure, then I would assume it shouldn't matter if I can't see East or West as I wouldn't be imaging in the direction anyway? If that makes sense.

Provided the drift is undetectable by the camera over the length of the exposures you are taking, then yes that is correct and you should be good to go. Bear in mind that your camera may pick up drift that you can't detect by eye, or it may not, depending on the pixel scale produced by your camera/focal length vs. the magnification of your eyepiece/focal length. If you are drift aligning using the camera, then hopefully it goes without saying that if the camera can't see drift when aligning, it won't see it when imaging.

The only thing I am unsure about is what happens as you move nearer to the pole vs. where you drift aligned at the equator. Gut says you should be okay as the apparent movement of stars per unit of time reduces nearer to the pole, but I may be wrong there.

Two other things to think about here:

a) If you are imaging and have a functional guider set-up, does polar alignment matter for good guiding? Not so much. In ye olden days when people guided manually it did matter since they had to manually adjust the pointing of the main scope by looking through a guide scope or OAG for long periods of time. Now the computer does it for you, so if your guiding is good who cares whether it is working hard or not? There is an argument that having PA slightly off in Alt (if imaging East or West) or Az (if imaging South) is a good thing. It forces the Dec drive to work constantly in one direction and avoids backlash due to the gears changing direction frequently, which is one of the usual culprits when guiding is poor.

I only polar align my portable setup using the polar scope and I can get 15 minute guided subs no problem (at about 510mm focal length). A longer FL scope would be more demanding, but personally I'd only sweat over a drift alignment if I was making a permanent set-up or had guiding issues due to the mount (backlash or similar), long focal length, etc.

Does polar alignment matter for any other reason? Possibly, depending on exposure length. The main imaging issue with bad PA is field rotation. Let's say you have If you have really poor PA, your guidescope and imaging scope are aligned perfectly to each other, and you guide on a star in the centre of the FOV. That one star will stay nice and circular, but if you image for long enough you will find stars further away from the centre start to image as arcs due to field rotation. (Think about it another way, you can actually do long exposure imaging with an Alt-Az mount if you put a camera rotator on the back of the scope, which rotates the camera over time to counter the field rotation. Functionally an Alt-Az mount moves the scope in the same way as a GEM with the polar axis aligned to the zenith - take a look at the Meade LX80 mount for an example of a mount that can do both.)

So if you can take your desired length of exposures without field rotation causing trailing, then you have good enough PA. Of course over the course of a few hours, the FOV of each sub will rotate compared to previous subs. Most stacking programs can handle that, and actually field rotation in that case is just another form of dithering. Like 'normal' dithering, not all subs will cover the whole frame though, so you will end up having to crop the image to some extent.

[russellhq]

Thanks Ian, that conformed what I've been thinking. If I drift as low as I can and as E or W as I can then any target less E or W and higher up should show even less drift?

I had the scope out last night and it had a slight drift (about 1 arc sec in 5 min IIRC) when pointing south, so when I was west of the meridian I had PHD set to North Only and the guiding seemed to go well. When I slewed to the East and did a flip, I recalibrated PHD and switched to South and everthing seemed to go well again.

With the full moon out, I'm making the most use of the clear skies to try and really understand how my mount behaves and get it working as best I can for when the conditions are good for imaging.

[chrisshillito]

Might I recommend the articles by Frank Barrett here: http://celestialwond...ment/index.html There are four pdfs on that page that cover just about every aspect of polar alignment - its a little mathematical in places but you don't have to follow the maths to appreciate the significance of what he says.

Chris.

[kirkster501]

I use the EQMod polar align tool if imaging and I find it absolutely fantastic and can get 10 min subs spot on. Very simple to use too. 2 mins to polar align even for a numpty like me.....

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