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Possibly stupid question regarding tweaking polar alignment

Johnny666666

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Johnny666666

Johnny666666

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Hello to all from a newbie!

I have purchased a Skywatcher 150P with EQ3.2 mount, polarscope and RA and Dec motor drives (budget limitations) and I'm reasonably happy with the set up as an introduction. I can just about see Polaris from the garden so setting up the polarscope is not usually an issue although a pain. I can manage between about 30 and 60 seconds (maximum) before star trails become obvious.

I'm aware of the limitations of this mount, being unguided, however I wondered whether tweaking might be possible of the Alt and Dec bolts of the mount by monitoring what happens to long exposures. My thinking is that if star trails are apparent in one particular direction of say a 2 minute exposure, could the Alt and Dec bolts be adjusted fractionally to at least partly counteract this? Or am I talking nonsense and these mounts simply aren't capable of greater than 1 minute exposure time using the 150P?

I'm sure if the RA motor, control and gearing aren't matched up well then this might not be possible, but if it's a case of the polarscope not being aligned well enough (let's face it - rotating it upside-down, fiddling about with the setting dial and its greater than 10-minute markings, and trying to get Polaris dead centre in a tiny circle whilst looking up at 50 degrees from a damp floor might introduce some alignment errors) then I wonder if fine tuning the mount using star trails on photographs might improve exposure times. Has anyone tried this? Current weather conditions are stopping me from having an experiment myself.

Be kind...!

John

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Alien 13

Alien 13

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It sounds as though you are not far off but these are a couple of tips that might help.

1. Get yourself a RA camera viewfinder and just hold it against the polar scope, no more contortions.

2. Make sure that the polar circle is acurately positioned against a chosen datum mark best done in daytime against a plumbline.

3. With a now accurate indicator for the position of the polar circle use stellarium to rotate the Dec axis to the correct hour angle (this can be checked with something like Polar Finder to make sure you are in the approx position).

Alan

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Johnny666666

Johnny666666

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Fantastic - thanks for the info and the link, as a newbie to all this I like to read detailed explanations and descriptions. At least my thinking is on the right lines, I've not seen this technique before so I'm pleased my brain independently came up with something to try! Maybe I subliminally understand more than I think I do :grin:

I have a 'horizon' issue though because I'm basically surrounded by houses. In fact I can just about see Polaris over the roof tops although I can get a lower angle at other compass points. The technique talks about using a star on the horizon - this is completely impractical for me so I assume I can use a star set slightly higher up (but as low as possible) although this might not give as accurate a result?

Many thanks in advance

John

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wxsatuser

wxsatuser

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You do not need a star on the horizon for this method.

Carefully read what Robert has written.

For azimuth ........." Set your telescope to point due south and at 0 degrees DEC."

0 degrees DEC due South is 40degrees above the horizon in the UK.

For altitude......" simply move the telescope to a star along the Eastern or Western horizon and at 0 degrees Dec"

again.....0 degrees Dec along Eastern horizon will be at least 20degrees above the horizon.

There is a bit of leeway in this method, you don't need to be spot on to these points, just get as close as possible.

My setup is in an obsy, so once done I'm good to go anytime, it's a bit more time consuming for portable.

Think once mastered it's a relatively simple way of getting good PA.

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wxsatuser

wxsatuser

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Got me to pretty decent 1-1/2 minutes unguided with the Mak180Pro at 2700mm and 5 minutes with ease with the ED80Pro ... then I moved the tripod legs chasing the Sun one day so need to redo ...  :rolleyes:

Thats very good.

I originally used this method to check Alignmaster, now if I need to check PA will just use this method.

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Russe

Russe

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"Alignmaster" I guess is otherwise the way to go if you don't have an open southern view. My problem...

But nowadays after EQMOD PA, I slew to three stars surrounding my target and sync them with EQMOD. That might already be enough for unguided imaging (have not tried unguided).

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Johnny666666

Johnny666666

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OK - thanks for the clarification, but I'm still a little confused...

So... I set my tripod roughly north facing and align using the polarscope in the way I have been doing until now. Then I rotate the Dec axis (if needed) so that it is at 0 degrees and lock it, then rotate the scope using the RA axis so that it points south in order to align the azimuth. All Ok so far because I think I have a reasonable view south, but how do I align to a star in the south using just the RA axis? If there is none on the arc of the RA axis, and I can't move the Dec axis from 0 degrees, then I'm a little confused as to how I can centre anything in the scope apart from just waiting for something to appear...?

But, assuming I can do this and have aligned the azimuth using the bolts on the mount, I then unlock the RA axis (keeping the Dec axis at 0) and rotate the scope due east or west? If I do this my scope is completely horizontal and all I will be able to see are mine and next door's fence! So again, I'm confused because if I need to keep the Dec axis locked at 0 degrees, then to stay above the fence line and neighbouring houses any star I might capture in the scope may not be anywhere near the horizon and possibly south west or south east at 45 degrees...

Sorry if I sound like a total noobie - it's because I am! Once I get this sorted in my head I'll wonder what all the confusion was, but for now I'm confused! And I only have an EQ3.2 mount with motors but no GOTO, so EQMOD or Alignmaster (as far as I have read) are of no use to me at all...

Confused... :confused:

John

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wxsatuser

wxsatuser

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Zero degrees DEC is the Celestial Equator., don't set your mount to zero.

Setup and polar align best you can.

Then just slew your scope to due South and find a star around the Equator, there will be plenty there.

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Johnny666666

Johnny666666

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Now I'm totally lost...

And the page at the end of the link seems to have disappeared...

I can use Stellarium for this? From my position the current nearest obvious star due South at 0 degrees DEC is Mintaka? And I can just release my Dec and RA locks to locate this in my scope? I still don't understand about looking at anything due east or west at 0 degrees Dec because Stellarium shows this at just about below ground level (I'm about 53 degrees north) if I'm interpreting Stellarium correctly (enabling the equatorial grid).

I'm not getting this at all... :huh:

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Johnny666666

Johnny666666

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That's brilliant, thank you very much for taking the trouble to explain things to me. I hope to have a try at this on the next available cloudless night!

However, the original DA article appears to have disappeared - I just get "The page you requested could not be found" when I follow the link. I've also tried searching for the author and article title but it's gone (for me anyway). Does anyone have a copy of this? Was I right in thinking that there was a PDF version of the article available (if so, that link didn't work either!)

Many thanks - John

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Steve Ward

Steve Ward

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Hi John ,

The DARV article is unavailable for the present unfortunately . 

"Cloudy Nights" , the source of the article is undergoing a revamp and the old articles haven't yet been transferred to the new software , but I'm reliably informed that they will be there soon.

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Johnny666666

Johnny666666

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Hi John ,

The DARV article is unavailable for the present unfortunately . 

"Cloudy Nights" , the source of the article is undergoing a revamp and the old articles haven't yet been transferred to the new software , but I'm reliably informed that they will be there soon.

My apologies for bumping this thread - but I still can't find this article on the revamped Cloudy Nights site. Has anyone seen it recently and can give me a link to it, please? I've tried searching for it and its author but I get nothing. I'd really like to have a go at this tomorrow night as the weather looks 'promising...'

Many thanks in advance - John

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