NCP alignment reticle
To track the stars accurately as they traverse the sky, the pin of the barn door tracker's strap hinge must be aligned parallel to the Earth's axis of rotation so it points at the North Celestial Pole, a point in the sky very near the star Polaris. The important word here is near. Polaris is actually 40 minutes of arc, equivalent to one-and-a-third full Moon diameters, from the NCP, and that's not near enough to ensure accurate tracking.
I've mounted a 6x30 finderscope on the tracker to assist with polar alignment. The procedure for aligning the finder parallel to the hinge pin is easy enough: Sight a distant object in the finder's crosshairs while opening and closing the hinge; keep adjusting the finder's positioning setscrews until the crosshairs remain fixed on the distant object.
The procedure for aligning with the NCP is more complicated, for when it's sighted in the finder, the finder's crosshairs are aimed at empty space with Polaris nearby. Moreover, because all the stars rotate about the NCP, including Polaris, the precise location of that empty space relative to the nearest thing one can see – Polaris – changes throughout the day. Alignment is made even more complicated by the optics of the finderscope: it has no integrated mirrors or prisms, so it produces upside-down and mirror-reversed images. It takes mental gymnastics to relate the sky seen with the eyes to the view seen through the finder.
There are several methods that can be used to align on the pole, but I don't like any of them, for they are either time consuming or insufficiently accurate. I want something fast and foolproof.
I wondered: what would I see in the finder if its crosshairs were on the NCP? Polaris would be 40' to one side, the star λ Ursae Minoris would be 1° off to the other, and δ Ursae Minoris would be 3.5° further along. Why not replace the finder's crosshairs with a reticle that shows that image? Aligning the tracker would then be easy, for I would only have to adjust the tracker's position until the stars matched the pattern on the reticle.
Here's how I made such a reticle:
I measured the finder's True Field Of View by observing a meter stick mounted at a known distance: 7.4° to the eyepiece field stop.
I used Skytools, a planetarium and observation planning software package, to simulate the finder view of the NCP. Skytools needed to know the finder's True Field Of View and that the finder produces a mirror-reversed, flipped image.
I imported a screen shot of Skytool's simulated view, as shown in Image 1 above, into a drawing package (PowerPoint) so I could draw the reticle shown in Image 2. The Bayer designations for stars are not mirror-reversed and flipped, because the eyepiece only magnifies the reticle; it does not otherwise alter the view.
I exported the reticle into Photoshop to resize it to precisely 18 mm so it would fit inside the field stop of the finder eyepiece (Image 3). I then printed it on a transparency, cut it out, and placed it on the eyepiece crosshairs.
I tested the reticle tonight under light-polluted, moonlit skies. Polaris and δ Ursae Minoris fit into place, but I couldn't see magnitude 6.4 λ Ursae Minoris, which would help most with NCP alignment. While δ was visible, it was too close to the edge of the field of view, where the optical distortion was large, to declare polar alignment. The slightly milky transparency in the eyepiece's light path was obviously evident in the view. Perhaps there's an optically clearer transparency material for use with laser printers?
Just to check, I removed the reticle and sought λ Ursae Minoris again. Transparency or not, λ is not visible under tonight's light polluted skies.
Making a reticle the way I have is easy to do, and the optical quality of the transparency is passable, but it isn't great. I found a site, here, that describes how to engrave an NCP reticle onto Lexan, which would be optically superior, but I doubt I have the manual dexterity to make one.
NCP alignment reticle
To track the stars accurately as they traverse the sky, the pin of the barn door tracker's strap hinge must be aligned parallel to the Earth's axis of rotation so it points at the North Celestial Pole, a point in the sky very near the star Polaris. The important word here is near. Polaris is actually 40 minutes of arc, equivalent to one-and-a-third full Moon diameters, from the NCP, and that's not near enough to ensure accurate tracking.
I've mounted a 6x30 finderscope on the tracker to assist with polar alignment. The procedure for aligning the finder parallel to the hinge pin is easy enough: Sight a distant object in the finder's crosshairs while opening and closing the hinge; keep adjusting the finder's positioning setscrews until the crosshairs remain fixed on the distant object.
The procedure for aligning with the NCP is more complicated, for when it's sighted in the finder, the finder's crosshairs are aimed at empty space with Polaris nearby. Moreover, because all the stars rotate about the NCP, including Polaris, the precise location of that empty space relative to the nearest thing one can see – Polaris – changes throughout the day. Alignment is made even more complicated by the optics of the finderscope: it has no integrated mirrors or prisms, so it produces upside-down and mirror-reversed images. It takes mental gymnastics to relate the sky seen with the eyes to the view seen through the finder.
There are several methods that can be used to align on the pole, but I don't like any of them, for they are either time consuming or insufficiently accurate. I want something fast and foolproof.
I wondered: what would I see in the finder if its crosshairs were on the NCP? Polaris would be 40' to one side, the star λ Ursae Minoris would be 1° off to the other, and δ Ursae Minoris would be 3.5° further along. Why not replace the finder's crosshairs with a reticle that shows that image? Aligning the tracker would then be easy, for I would only have to adjust the tracker's position until the stars matched the pattern on the reticle.
Here's how I made such a reticle:
I measured the finder's True Field Of View by observing a meter stick mounted at a known distance: 7.4° to the eyepiece field stop.
I used Skytools, a planetarium and observation planning software package, to simulate the finder view of the NCP. Skytools needed to know the finder's True Field Of View and that the finder produces a mirror-reversed, flipped image.
I imported a screen shot of Skytool's simulated view, as shown in Image 1 above, into a drawing package (PowerPoint) so I could draw the reticle shown in Image 2. The Bayer designations for stars are not mirror-reversed and flipped, because the eyepiece only magnifies the reticle; it does not otherwise alter the view.
I exported the reticle into Photoshop to resize it to precisely 18 mm so it would fit inside the field stop of the finder eyepiece (Image 3). I then printed it on a transparency, cut it out, and placed it on the eyepiece crosshairs.
I tested the reticle tonight under light-polluted, moonlit skies. Polaris and δ Ursae Minoris fit into place, but I couldn't see magnitude 6.4 λ Ursae Minoris, which would help most with NCP alignment. While δ was visible, it was too close to the edge of the field of view, where the optical distortion was large, to declare polar alignment. The slightly milky transparency in the eyepiece's light path was obviously evident in the view. Perhaps there's an optically clearer transparency material for use with laser printers?
Just to check, I removed the reticle and sought λ Ursae Minoris again. Transparency or not, λ is not visible under tonight's light polluted skies.
Making a reticle the way I have is easy to do, and the optical quality of the transparency is passable, but it isn't great. I found a site, here, that describes how to engrave an NCP reticle onto Lexan, which would be optically superior, but I doubt I have the manual dexterity to make one.