Star Adventurer - polar alignment tips please?!?

[JamesF]

Just now, Mr niall said:

That is interesting - I’ve just copied what PS align is displaying, perhaps I should be inverting it??

I don't honestly know.  It may be that PS Align already does the image inversion for you.

James

[Davey-T]

Don't know where the app puts it but in the polar scope it's diametrically opposite to the position in Stellarium so you can compare them.

Dave

[JamesF]

I need to check that 1st November is the correct date, too.  I read that it was quite some years back, but I don't know if it's still true.  A quick check on Stellarium (since Dave suggested it makes it look as though 8th November is a more likely possibility, but I'm not sticking my neck out just yet

James

[Scooot]

25 minutes ago, Mr niall said:

That is interesting - I’ve just copied what PS align is displaying, perhaps I should be inverting it??

I think you just copy PS Align. If you look at where NCP is in relation to Polaris on Sky Safari you’ll see it’s opposite to PS Align so the app is already allowing for the inversion. I think  

[Davey-T]

I can fire my iOptron up and that gives the position in the solar scope in the handset.

Dave

[Davey-T]

iOptron says 1.23 so opposite to Stellarium which is all I usually do for the Star Adventurer, look at Stellarium and put it opposite, this is usually good enough, then I tweak it while imaging to keep Polaris tracking round the circle.

Dave

[Buzzard75]

I use PolarFinder. It's not free, but it has reticle options for iOptron, Astor-physics, Losmandy, Skywatcher, Takahashi, Vixen, AstroTrac, and StarAdventurer. It also has options to display as if you were looking through a telescope, naked eye or a diagonal. In the case of my iOptron, I have to select the telescope option to get the correct mirror image I see through the polar scope. It's available on Android.

Best I've gotten so far is 120s subs. Whenever I tighten everything down and then attach my camera, there's inherently some movement, which of course throws everything off. So close enough is all the better I'm going to get.

[JamesF]

Here's the result of my own accuracy check:

[JamesF]

I'm measuring that as about 2mm off the original mark.  The radius of the circle described by the end of the counterweight shaft is about 310mm, giving a circumference of 1948mm, so it's about 0.1% out.  Although that doesn't account for the rotation of the Earth about the Sun.  I'll have to think about that one

James

[Davey-T]

And account for sidereal time being different to real time

I did mine on the dining room table in the middle of the room with the laser aimed at the wall about 8 foot away so it magnified my error to a couple of centimetres.

Dave

[JamesF]

I'm just trying to measure the radius a bit more accurately now

James

[Davey-T]

8 minutes ago, JamesF said:

I'm just trying to measure the radius a bit more accurately now

James

The farther away you aim the bigger the circle and more accurate the calculation, you must have a laser level James with all that construction you do.

Dave

[JamesF]

Oddly enough, I don't   Not that it isn't tempting to get one having seen the builder using one when he did the work in our cellar recently though.

James

[Davey-T]

Mine on test.

Dave

 

[JamesF]

Ok, my more accurate measurement is 305mm, so in a solar day the end of the shaft has travelled 1916mm for the full circle, plus 2mm over.  A sidereal day being 3'56" shorter should therefore mean the end of the shaft travels 5.2mm less, so 3.2mm short of a full circle.  Which would still only be 0.17% out, or about 2187 arcseconds too slow over the course of the sidereal day.  I think that's about one arcsecond error every forty seconds.

At 200mm focal length image scale is 1031 arcseconds per mm and if your camera has, say, 4um pixels that's 4 arcseconds per pixel, so the image should shift one pixel every 160 seconds.  I'm not convinced that would be noticeable and for my 450D it's probably more like one pixel every 210 seconds.  I bet alignment is more likely to be an issue before that's a problem.

James

[JamesF]

I'm tempted to do a more accurate trial now

I have a laser pointer.  I could clamp that up in the mount, press the button to light the laser and mark the starting point, then let it run for a sidereal day, stop it and mark the position again.

Though it might all be overkill given that it looks adequately accurate to start with.  Something for when I'm really bored, perhaps

James

[Davey-T]

I've done 15 minutes using 24mm on mine so the drive is pretty good.

Dave

[JamesF]

Yeah, at 24mm with the 450D based on my calculations you'd expect to reduce the error by a factor of just over eight compared with 200mm, so 15 minutes should be no problem.

James

[Davey-T]

The only limitation at these short focal lengths is light pollution 

I can't do 15 minutes from home.

Dave

[JamesF]

I thought I'd check that the polar scope is aligned with the RA axis this lunchtime, during which process I discovered that the clamp on top of the EQ adapter is loose and can move from side to side.  This bit, if that isn't clear:

So I need to tighten those countersunk screws up first.  I'll check the panhead ones at the bottom at the same time.

I tidied up all my astro Allen keys and put them somewhere safe the other day as part of moving stuff back into the cellar now the work in there is mostly complete.  I don't need to tell you the next bit, do I?

James

[Davey-T]

Mine was the same, I don't tighten those bottom ones too much then you can adjust the PA without touching them, same for the dec it moves enough without loosening the locking lever.

Dave

[JamesF]

I also noticed for the first time that the hour ring only runs from 1700 to 0700.  Must have saved a fortune in white paint, that

James

[Davey-T]

5 minutes ago, JamesF said:

I also noticed for the first time that the hour ring only runs from 1700 to 0700.  Must have saved a fortune in white paint, that

James

I think it's only for decoration, I doubt they expect anyone to actually use it

Dave

[JamesF]

On to step two, having tightened up the screws holding on the clamp (I investigated the others, but couldn't find any significant play so I left them as is).  I also found that tightening up the alt clamp moves the mount body very slightly away from the clamp side, so that's something to think about given that it needs to be loose for polar alignment.  I wonder if a shim might take the play out.

It's been a horribly murky and rainy day so it was a bit tricky to find anything far enough away for the polar scope to get to focus.  This will have to do for the time being, two photos with the RA axis turned 180 degrees between the two:

Obviously there's a bit of shift there.  At the moment I'm trying to decide if there's enough to make it worth trying to improve upon.  I suspect so, but I think I'd like a bit clearer view before I start.

[Davey-T]

21 minutes ago, JamesF said:

 I also found that tightening up the alt clamp moves the mount body very slightly away from the clamp side, so that's something to think about given that it needs to be loose for polar alignment.

It doesn't need to be loose as I mentioned, the alt adjuster still moves easily with it tightened.

I fitted an O ring behind the reticle to keep it held forward otherwise it can fall back when aimed at the sky, it's quite fiddly adjusting it by small amounts.

Dave