Polar Alignment of Barn Door Tracker

[aod]

I've recently built myself a motorised barn door tracker - i'm very happy with the construction and calibration (I used Gary Seronik's excellent guide, modified for UK/Metric supplies which should minimise tangent error). I may purchase an astrotrac/polarie in future if I get into this more as a hobby. My interest is wide field and deep sky photography to the limits of DSLR, i'm not really fussed about telescopes.

As far as I can tell beyond the construction there are two things that will give me good pictures - dark skies and good polar alignment. Most barn door tracking builds seem to gloss over the fine details of the equipment of polar alignment on a home made barn door tracker.

As far as I can tell, I could start with a drinking straw accurately aligned on the hinges but i'd rather something a bit more fancy/accurate - i'm aiming to go up to about 300mm focal length if possible. what have other people used or adapted before? I was looking at something like a red dot finder (any good suggestions? - I have seen a well reviewed Telrad one at a reasonable price) - is this suitable?

How do I attach something like that to a piece of wood?

What other options are there for something cheap and simple to guide polar alignment? 

Cheers

[Viewmaster]

I use a laser mounted into a tube withadjustment screws.   The laser is angled at 45 minutes of arc from the tube's axis.

The tube is then mounted parallel with the barn door trackers RA axis.  and rotated to the correcr RA of Polaris, so when laser is on the star the tracker's axis is pointing to the Earth's Celestial axis.

[iamjulian]

You are right, a straw is a poor way to go if you have made all the other parts as accurate as possible.  I mounted a spare finder scope on mine.  You then just need to calibrate it by swinging the top part of your tracker through 180 ish degrees while looking through the finder.  If it is aligned with the hinge, your target (distant TV aerial or something) should rotate, but stay at the centre of your crosshair or red dot, or whatever you are using.

I wrote a bit about it here:

https://plus.google.com/114512556959374515427/posts/hEiGUfJn8Ar

[leeseafish]

have a look here for ideas on finderscope mount for a barn door tracker - http://www.bencooper.karoo.net/AstronomyMountsBarn.html

[Ryan P]

On 9 July 2014 at 17:20, iamjulian said:

You are right, a straw is a poor way to go if you have made all the other parts as accurate as possible.  I mounted a spare finder scope on mine.  You then just need to calibrate it by swinging the top part of your tracker through 180 ish degrees while looking through the finder.  If it is aligned with the hinge, your target (distant TV aerial or something) should rotate, but stay at the centre of your crosshair or red dot, or whatever you are using.

I wrote a bit about it here:

https://plus.google.com/114512556959374515427/posts/hEiGUfJn8Ar

Apologies for dragging up an old thread but your link you posted is dead and I would really like to hear a bit more as I also am struggling to polar align my bdt.

Do you happen to have a new link for your findings? Thanks in advance.

[happy-kat]

I bought a red dot finder (rdf) for my barn door which sits on a home made base cut to the right altitude angle for my location. So I need only align the rdf to the mount hinge and then point the rdf at Polaris. The home made base has leveling feet to ensure it is level and I sit it on a chair or table it is very steady.

You know when the finder or rdf is aligned to the barn door hinge because as you open the two boards the distant object stays in the finder or rdf.

To use, In the northern hemisphere you want the hinge on the left (west) and the opening of the two boards on the right (east) so the finder or rdf points towards Polaris. I didn't own a sturdy enough tripod to mount it on and set the altitude right hence I built a table top wedge base.