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Improving PA and GOTO - the easy way?


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The version in the handset uses the data it works out for Mel and Maz, and then it slews the scope by that amount from a star and asks the user to djust alt or az (whichever it's attempting to refine) to bring the star back into the FOV. I think that is significantly different from this other method, as in the latter method, one is assuming the only reason the alignment star is not in the centre of the FOV is due to poor PA, but I'm still worried that ANY cone error would upset things. I'm still going to give it a go. It would just be good to have be able to do this in the day time.

James

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OK, you set up every night and need to get decent polar alignment. Again, start by defining what is acceptable. The answer lies in your field rotation acceptance, nothing else. You may be surprised to

James, The method uses the inherent abilities of the GOTO function....doesn't rely on any special accuracy other than the built in stepper pulse rate. When you GOTO the second star, the mount is inter

A guess would be that you should get it to 10 arc-minutes or so. I ran my remote obs on just under a degree of PA error for a few mounts due to misaligned holes in the pier... Round stars but serious

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The version in the handset uses the data it works out for Mel and Maz, and then it slews the scope by that amount from a star and asks the user to djust alt or az (whichever it's attempting to refine) to bring the star back into the FOV. I think that is significantly different from this other method, as in the latter method, one is assuming the only reason the alignment star is not in the centre of the FOV is due to poor PA, but I'm still worried that ANY cone error would upset things. I'm still going to give it a go. It would just be good to have be able to do this in the day time.

James

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I tried the handset version twice, but i just couldn't get it to work. It made everything worse.

But then i'm not convinced the mel and maz data is correct, and again is dependant on several variables which are not resolved by adjusting the az and alt knobs (cone error, periodic error, for me mirror flop, and i'm sure other things). But in my attempts i hadn't fully levelled the mount and i found adjusting the alt and az knobs didn't especially bring the star in question closer to the centre of the fov, just moved it around the centre, and also i found any minor adjustment made the star to jump erratically as the weight of the scope and two counter weights caused significant friction of the mount head on the tripod (i'm still looking for furniture polish to lubricate those surfaces), also i think my initial polar alignment wasn't great.

But then i read more about drift alignment, and i felt that didn't sound so complicated after all.

So still work in progress for me. And still open to suggestions and i may have a go tonight if i can see any stars :)

James

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I haven't seen that instruction....must be with Synscan V3.35. I'm still using V3.27.

Similar, but I use the planetarium program for the first sync on any star and then target any star as the second. Then use that to go straight into Alt/Az adjustment.

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This method does not work for my set up.

I told the handset I wanted to do a three star alignment.

I started with the scope in roughly the home position. The first star was in Aquarius, and was pretty close in a 30mm EP; I centred in that EP then changed to a 12mm reticule and centred that. The second star was in Cassiopeia, again very close in the 30mm EP, then centred in the 12mm EP. Then I selected Vega, again not bad (it was getting better each time). I only used the elevation knob and the azimuth knobs but it was taking massive rotations of these knobs to get the star to move anywhere near the centre of the FoV in the 30mm EP. I gave up as I needing to turn the elevation knob numerous times, and also making massive adjustments in azimuth, clearly far too much than my polar alignment was out.

I suspect I've got cone error, or soemthing else was wrong to start with.

I was annoyed so I've packed up.

Shower and bed.

I won't be trying this technique again! Lol.

James

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Hmmm

James,

reading the above, the difference, to what I was doing, is that I didn't use the Synscan controller to do ANY 1-2-3 star alignment.

I went straight to the planetarium program and synced the first star then GOTO to target.

Having said that.....

I also found the adjustments of the Alt/Az a PITA!!

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It wasn't so much that the adjustments alwete difficult, it's just that it would have taken about 20 turns to get vega in the centre, and i know how much even a quarter of a turn moves polaris in the polar scope so there was clearly something very wrong.

I was fuming as my back was hurting and it had clearly totally giggered up my existing reasonable polar alignment; hence why i packed up.

I will wait to hear other peoples experience and trouble shooting tips before i try again.

I have to say, the fact i've not heard or read about this method before (which does sound very simple if it works) suggests it might not work for everyone reliably.

James

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  • 1 year later...
  • 2 weeks later...

I actually got round to trying this method last night, and I got into a terrible mess (nothing new for me there, then!)

The only slight difference I applied was for c/d by getting the star in CdC first, in order to minimise the amount of movement I needed to apply:
c. Set the scope up to get a known bright star in the middle of the FOV (you can start by using the finder cross-wires).
d. Find the same star on CdC and "sync" to this star.

Precisely what I did was:
(1) start with mount in "home" position, linked to CdC
(2) used CdC to slew to Polaris; it was just on the edge of the 1/2o telrad circle
(3) centred it in telrad & reticle e/p, using alt-az bolts
(4) pressed sync on Cdc & got confirmation

I then slewed to Altair as my second star (nice and bright and over 80o away). It was sitting between the 2o and 4o circles. Once centred (using alt-az bolts) & sync'd, I went back to Polaris, which was now outside the 4o circle, and so the evening continued, with no slew in the next hour or so getting me within the 4o circle again.
 

I guess the best thing I can say about the evening is that the mount did not have "accurate" PA before I started, so at least I didn't make things any worse than they were before.

Knowing me, I probably did something incredibly stupid & obvious, most likely in my "slight difference", but I can't see what it is.

Thanks for you help.

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I think the key to this is to choose two or three stars widely separated and non higher than %60deg declination since the trig gets sensitive to small errors.

Using Polaris as one of the stars is never going to work.

Use the summer triangle for example.

Mike

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James, DP and all other members,

This process does not work!

It is flawed, and comes from my limited early trials.

I've recently relocated my observatory and instead of using the usual drift method to set-up the mount I went back to this process.

I couldn't get it to work for me.

In hindsight it relied on a "rotation" about the first synced point which doesn't happen in practise.

My apologies to the many members who may have spent frustrating time trying to make it work.

Sorry for the confusion.

I've asked the Mods to close and remove this thread to prevent future frustration......

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I use Drift to set PA, but have read other iterative methods.

Skimming through the above I get the impression you are centering on Polaris AND the other star using the alt/az bolts, when you should centre Polaris with the bolts, and centre the star with the handset or planetarium then SYNC.

At the moment you are adding and removing the same correction in an endless loop........

Again I would say I Drift, somaybe  the method you are trying does work for your mount, but sounds wrong to me.#

Michael

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As an aside, what are you guys using to get an accurate time as even a 30second error in the handset will put the goto out by an eighth of a degree.

Cheers

If you have an internet connection then install an NTP time server daemon like Meinberg. I have mine set to 10sec correction interval and it keeps it within 2-3millisec. If no internet then use a GPS dongle.

ChrisH

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I use a USB dongle.

I have decided I cannot be bothered to mess about with Astrotortilla for my PA. The last time I tried it, I ended the session running it 3 times back-to-back with no adjustment in between. Logically, it should therefore have produced identical results. Actually the three results were (1) 12.19' out, (2) 53.29' out & (3) 20.21o out. By anyone's standards that is pretty screwy. No one, either on here or on the AT forum was able to explain those results, which resulted in me putting the mount back in its box for the last two years. [in fairness, I should say that others have found the PA part of AT to work well.]

Much as I hate paying for software, yesterday I splashed out the princely sum of £10.43 (€14) for a copy of alignmaster [there is a 30-day free version, but I got that back in 2013 and with it came free 30 nights of cloud!]. I've read glowing reviews, and the process seems to be so simple that even I won't be able to mess it up ... But maybe I should be careful when making wild predictions like that ...

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Maybe this is simply gas on the fire, but I don't understand the polar alignment frenzy at all. As long as the field rotation in one sub, and the field rotation between first and last in a session, is acceptable, why bother nailing the PA? This, of course, assumes that you will guide or use a mount with modeling.

The guiding, or the modeling in the case of, for instance, my mounts, will track perfectly well as both axes are involved in he constant adjustment process.

So, find your max acceptable field rotation, adjust PA to that accuracy and the spend the time imaging ;)

/per

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If you have a permanent setup, that may be true. But if you are plonking down your tripod every time, you need some sort of means of getting it "acceptable".

I know the line about always putting your tripod feet in the same place, (sometimes easier said than done), and I am working towards that. But (I am not sure how much this would affect it, so correct me if I am wrong), if I put a tripod foot 1mm away from its "correct" position, isn't this going to introduce a large enough error, such that, even if my field rotation was acceptable last time, it won't be this time?

And if I am going to have to get it to an acceptable level, and if alignmaster is really as quick and effective as the reports would have me believe, why not get it completely right.

My 2p ...

Edited by Demonperformer
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Assuming this is the procedure you follow: https://sites.google.com/site/openphdguiding/phd2-drift-alignment, seems a bit more complicated than just moving one star back into the centre of the reticle after the computer has decided how far it needs to  move?

Hi

If you scroll down that page you'll see it's a visual tool and relies on the guide camera image. You just have to click on 'Drift' to get the initial DEC trendline (red), then on 'Adjust' to adjust the bolts to get the DEC trendline horizontal for each of azimuth and altitude (and repeat). You ignore the RA trendline (blue). In fact, if you like, you can simply stop when you feel the PA error is reasonable for your setup. PHD2 does most of the thinking for you. It even chooses the star it uses :).

Louise

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I did scroll down and read the article. I guess it all depends on what suits the individual. If one finds getting trendlines horizontal easier than centring stars in a reticle then I agree, use that.

Well it does tell you what your PA error is, so makes it easy to decide when to stop - or not! The circle and the trendline are helpful in giving you feedback about how much you need to adjust and, with the line, in what direction to turn. By averaging out the seeing fluctuations to make the trendline it's probably more accurate compared to a reticle.  I find it easy to use and reliable :).

Louise

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