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Walking on the Moon

PHD2 Polar Alignment - How good does it need to be?


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Hi

Just trying to refine my polar alignment using the drift alignment tool in PHD2.

While doing the AZ adjustment I've got it down to 1.65' of error, but refining it further is really difficult as the purple adjustment circle is so small.

Is there a zoom option? I can't find it if there is.

Is this good enough and if not then is it a case of guessing with regards to the amount of adjustment?

Ideally 0' of error is the target but realistically what should I be aiming for?

Cheers

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8 hours ago, ollypenrice said:

Under a minute of arc is surely good enough for almost all purposes. It should allow for any sub length you're likely to use. How long exposures do you normally use?

Olly

Thanks Olly, I was after an expert's opinion and you'll do nicely. If you say that's good enough, that's good enough for me.:icon_biggrin:

I had no idea what was necessary to aim for...However I do see posts with claims of subs that are 15, 20 minute or more UNGUIDED let alone guided. Is this really possible? I tried a quick 120s sub and the exposure was too long(2 secs) in PHD and was getting slight trailing, lowering the exposure time(1 sec) gave tight stars. This would indicate, at least to me, that guiding is most definitely required even for a 2 minute sub with said accuracy of PA.

That said, I didn't do the ALT drift alignment, thought I read somewhere this wasn't as important. Might try that tomorrow morning.

Just trying to get everything tuned so PHD has as little to do as possible and to try and eliminate the field rotation I was getting (turns out hot pixels have a use after all:smiley:)

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59 minutes ago, richyrich_one said:

Thanks Olly, I was after an expert's opinion and you'll do nicely. If you say that's good enough, that's good enough for me.:icon_biggrin:

I had no idea what was necessary to aim for...However I do see posts with claims of subs that are 15, 20 minute or more UNGUIDED let alone guided. Is this really possible? I tried a quick 120s sub and the exposure was too long(2 secs) in PHD and was getting slight trailing, lowering the exposure time(1 sec) gave tight stars. This would indicate, at least to me, that guiding is most definitely required even for a 2 minute sub with said accuracy of PA.

That said, I didn't do the ALT drift alignment, thought I read somewhere this wasn't as important. Might try that tomorrow morning.

Just trying to get everything tuned so PHD has as little to do as possible and to try and eliminate the field rotation I was getting (turns out hot pixels have a use after all:smiley:)

Be aware that the term 'unguided' has two entirely different meanings. People with 10 Micron and ASA mounts with absolute, or effectively absolute, encoders use it to mean that they don't guide on a star but on the mount's own encoders along with an astro-cartographic model of the sky which corrects for atmospheric refraction at low elevations. They need an accurate PA but not all that accurate. I've helped our robotic owners reach about half an arcminute on their rigs and this is enough for their version of 'unguided.' Indeed that's probably better than is really needed, depending on pixel scale, FOV, sub length etc. The other version of 'unguided' means letting a bit of electric and mechanical clockwork do its thing and, in this case, PA is important but if you're dealing with a budget mount then, over the eight-ish minutes of a worm's rotation, you might expect a periodic error of 30 arcsecnds. If you have a premium 'clockwork' mount that might be 3 arcseconds 'unguided' in the true sense of the term. (That is, no feedback mechanisms anywhere in the train.)

A little polar misalignment need not be regarded as a bad thing. On worm and wheel mounts it can allow you to correct in Dec in just one direction and so avoid driving the mount into oscillating across the backlash. It also proveides a whiff of natural dither if you haven't built that into your guiding - but you should.

Olly

 

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