Steve's 'BarnDoor' - NCP alignment

If you know all about alignment to NCP, just skip to 'Alignment in practice' below - otherwise continue here (which assumes only minimal celestial knowledge (= like me :-) )

Assuming you have bent the screw bolt and measured the hinge/bolt distance correctly, and the Base is exactly level (and the correct Latitude angle has been built in), then accuracy of the system depends largely on achieving the correct alignment with the North Celestial Pole [NCP].

The first step is to rough align with Polaris. This should be dead easy (if you have built in the correct Latitude angle) since all you need to do is rotate the base until Polaris comes into view.

If we simply align to Polaris, the error will be approx 3/4 degree (45 arc minutes, 2700 arc seconds).

As a 'rule of thumb', for every 10 arc minutes of NCP polar alignment error, you can expect the tracking to be off by 2.5 arc seconds per minute of exposure.

If you align with Polaris, 45 arc minutes off NCP, you can expect to achieve about 10 mins exposure with a 50mm lens (and about 4 mins with a 135mm lens) before any significant star 'trailing' is seen = with most Digital Cameras, this should be more than enough time.

With practice it is possible to achieve better NCP alignment, however you will be lucky to do much better than 15mins of arc (which gives you half an hour exposures at 50mm, 12 mins with a 135mm telephoto or 6 mins with 300 mm).

Tracking errors due to bolt bend errors.

Over half an hour, a correctly driven but rather poorly 'bent' screw rod might cause a tracking alignment drift (due to the bolt being 'out of arc' error) of about 20 arc seconds, whereas a typical 15min NCP polar alignment error will have caused some 100+ arc second drift.

Thus, for all practical usage, tracking accuracy (and hence exposure time) will depend almost totally on NCP alignment. For this reason I decided not worry too much about how well (i.e. how badly) the screw rod had been bent :-)

Even when the 'seeing' is perfect (no atmospheric distortion), as the Earth rotates the 'thickness' of the atmosphere you are looking through changes (as the stars move closer to, or further from, the horizon) which means that the changing refraction will shift the stars at least 15 arc seconds per hour.

First find the 'Great Bear' or 'Big Dipper' - this should be easy enough even in the rather light-polluted sky's of UK ..
	Now follow the 2 stars at the end of the 'bucket' to find the pole star (Polaris) at the end of the 'little dipper' arm (not so easy) ..

Most likely the Little Dipper will look a bit like the image below (left), however with a bit of practice you should have little trouble locating Polaris. It's the bright start at the end of the little dipper 'arm' (in fact, it's usually easier to use Polaris to find the 'little dipper', rather than the other way around) - the second image (right) has been enhanced a bit and the 'little dipper' drawn in :-

Basically we need to align the hinge with the North Celestial Pole (NCP). A low power finderscope should be attached as close as possible to the hinge. Note that attaching the 'scope to the moving arm (instead of the fixed arm) will allow alignment using the drift method (see later).

If the base plate angle is correct (& the base is flat to the ground, for which a spirit level is needed), you should only need to rotate the base plate a few degrees to turn the hinge towards the pole. As I discovered later, a compass really helps assist initial line-up, before getting down on hands and knees to look along the hinge and use the finderscope.

If the base angle is off (or the base is not flat), then you will need to adjust the base angle as well.

	Polaris is about 3/4 degree (45 mins = 2700 seconds) off NCP (shown 'x', left). To find NCP, first track your eye 'back' down the 'little dipper arm'. About half way towards the towards the 'next star' making up the arm, you will find a dim star 'just off the arm' - this is Lambda Ursa Minor. It is quite dim (+6.4 magnitude) and lies about a degree 'down the arm' (or 'up the arm', in the reversed view of a typical finder scope :- ) ) from Polaris. The midpoint of a line drawn between Polaris and Lambda is only 20 arc minutes from the NCP. See in more detail below.
Step sideways for about 1/4 of the distance between them (in the direction of the concave side of the Little Dipper 'arm'). This will put you 'spot on' to NCP. As long as you have cross-hairs on your finder, you should be able to get it centred within 10 arc minutes of the NCP (plus the error in the alignment of the finder with respect to the hinge). NB. Of course you need to be able to see Lambda in the first place. In clear sky it is a naked-eye object, so you should have no trouble with the typical 8 x 30mm finderscope.

The 'drift alignment' method (can be used to 'fine tune' a basic 'aim at Polaris' alignment)

Basically, the idea is to mount a small telescope (or camera with decent power zoom-lens) on the movable arm, choose any star and 'start up the drive'.

Assuming the Drive is perfect, then, if the BarnDoor is aligned correctly, whatever stars you can see through the finder scope will "stand still" !

However if the stars 'drift' in position, then either your Polar Alignment is not 'spot on' - or your drive is imperfect. .. 

In UK, most of the 'up & down' (North / South) drift is due to Polar Alignment error (and thus most 'side to side' drift (East / West) is drive error).

So you just keep adjusting the alignment until (all) the stars stop drifting "up or down" ...

Of course you have to detect any drift by eye - so you will need to use something that is rather more powerful than the original 'spotter scope'.

If your Hinge 'spotter' scope has exchangeable (or zoom) eyepiece (and can be 'swung around' on its mounting), then, after a first quick check on NCP alignment (at low mag), the Hinge spotter scope can be put to dual use (at high mag).

Of more concern is that any errors in Drive speed will cause the stars to drift East/West - and there is no way to 'align out' such errors. I thus spent a lot of time perfecting the Drive system (see later).

 NB. plainly this is not so easy (looking through the scope, moving the barn door and keeping time) when the wheel is manually driven

Setting up in practice

Start by aiming the BarnDoor (hinge) as close as possible towards North (a compass helps, although if you have a 'SatNav' that will operate from internal batteries, you can do even better :-) ). Next, level the base to achieve the correct base (latitude) angle (using the spirit level). If you look along the edge of Hinge (visually or at low mag.) you should see Polaris - so get as close as possible to NCP with the cross hairs in the finder.

Now select a bright star near the horizon and engage the Drive (= start turning the nut :-) ). If the star 'drifts' out of view, then, assuming your latitude angle is 'spot on', the base just needs to be 'rotated' a (tiny) bit. The further the star is from NCP the faster it will drift (if the alignment is off), hence the choice of a target star.

At this stage you will discover how difficult it is to 'rotate' a heavy wooden unit sitting on an un-even surface balanced on 3 screw legs without upsetting the 'level' :-) 

It I ever build another one, I will build a separate base and fit the BarnDoor movable arms as a 'unit' to this with a single pivot bolt. Having first (compass) North aligned and levelled the base, the top part of the BarnDoor unit can then be slightly rotated (to align with NCP) without having to move the Base :-)

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