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PHD - Trailing on the rhs of image

swag72

By swag72
in Imaging - Discussion

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Psychobilly

Psychobilly

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http://stargazerslounge.com/imaging-discussion/120593-phd-drift-polar-alignment.html

This thread http://stargazerslounge.com/beginners-help-advice/126991-polar-alignment-question.html in particular this post http://stargazerslounge.com/beginners-help-advice/126991-polar-alignment-question.html#post1662032

I'll add more links as I find them....

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themos

themos

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If you know which camera you use you can find out how close the pixels are (p, in microns). If you know which telescope you use you can find out your focal length (f, in mm). Then you can find out how many arc-seconds each pixel sees ( (180/π)*3600*p/f*1000 = (648/π)*(p/f) . Example: 5micron pixels and 700mm focal length yields 1.5 arc-seconds per pixel.

So now you can convert pixels to arc-seconds. If the declination drift due to polar misalignment is 10 pixels in 10 minutes, that makes 15 arc-seconds, in our example.

Simples!

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SteveP

SteveP

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Sarah

I'm guessing that you don't have a permanentb set-up in which case you will need to develop a technique for polar alignment every time you do your imaging.

I'd suggest some form of camera based drift alignment since you're imaging rather than using an illuminated reticule. It avoids havinyg tbo change the EP for your camera every time. Have a look in the help of whatever software you're using for mount control aynd see what it recommends.

Steve

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themos

themos

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Drift alignment is great for measuring your polar error but pretty bad about fixing it. If only PHD could output the polar error in the two directions and you could feed it to EQMOD and ask it to put the mount in the RA/DEC position so that a bright star should be centred if polar alignment was dead on. Then we could just move the mount axis to centre the star. I think that's what PolarAlignMax does but it costs money.

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martin_h

martin_h

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You can check for rotation in DSS when you stack your images. After you have done the stacking etc and it has auto saved, save your image as normal, then re-click the "stack checked pictures" tab, hit cancel when the stacking screen comes up, now look at the data at the bottom, it should show the offsets and angles. The first one will probably be 0.00 and the rest(if you have rotation) will incriment a bit.

If they do show this then you need to refine your polar alignment, by whatever method you choose.

post-16950-1338775233_thumb.jpg

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swag72

swag72

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I spent some time last night between clouds using PHD to do a drift alignment, and by George it works a treat.

Method

- set scope to near meridian and celestrial equator start PHD and do a calibration, start guiding with the DEC turned off, turn on the Graph and change the RA/DEC button to dx,dy.

If you see a steady drift in the dy you move the mount to the East or West until the dy graph remains flat, apart from the seeing causing slight random movement.

Repeat the procedure with the scope to the East or West on the Celestrial Equator, calibrate PHD and guide in only RA, watch the dy trace but this time alter the mount up or down until the dy trace flatlines.

Set scope to near meridian and celestial equator :) Oh dear - Don't know what that means - I'm all for a challenge, but not when I don't know my front from my rear - This was the whole reason the scope stayed in the box for months, I thought I wouldn't understand it, looks like it's proving to be the case right now. Is there one of these written for total numpties?

I do appreciate you all taking the time with me guys - I am losing the will to live.

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jgs001

jgs001

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Very interesting reading on the cause and the drift alignment. I'll be having a play with the PHD alignment next time. I've not had an oppo to watch your video yet Themos, but reading the text and a quick scroll through it makes perfect sense, the reason why that is... now to make sure I can actually do it.

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swag72

swag72

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It is a constant challenge, but that's how you learn!!! I'd like to say worth it in the end, but I don't think there is one...

I'm happy for a challenge, personally I don't learn well, I need real dummy stuff to get it to sink in. Perhaps I should just go back to everyday photography!

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jgs001

jgs001

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I'm happy for a challenge, personally I don't learn well, I need real dummy stuff to get it to sink in. Perhaps I should just go back to everyday photography!

Not given what you've managed to achieve so far in the whole month since you unboxed it all... you can't give up...

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swag72

swag72

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All I do is set the scope up facing north, set the time and date on the polar scope and then get polaris in the right place in the reticule. Get polaris in the scope sight, home the mount then 1 star align.

There are other ways to do it?

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