Farpoint Primary Mirror Center Marking Kit LARGE — 12" to 26"

Product Overview

The Farpoint Primary Mirror Center Marking Kit LARGE marks the exact centre of a big Newtonian or Dobsonian primary, covering mirrors from 12″ to 26″ in diameter. It is the step that comes before collimation, since accurate collimation depends on knowing where the centre of the primary actually is — without a centre mark, a laser collimator and a Cheshire eyepiece have no reference point to work from.

The kit holds one large precision-cut acetate template for locating that centre and eight triangle centre-mark targets. The triangles are sized so their edges touch the outside of the illuminated field seen through a Cheshire, which turns centring into a direct reading rather than an estimate.

Who It's For

This is a good match if you run a large Dobsonian or a big Newtonian — anything from a 12″ up to a 26″ primary — and the mirror is unmarked, or the existing mark has worn or lifted after a cleaning or a recoat. It also suits a mirror-making or upgrade project where a new primary has gone into an existing tube, and the case where a truss Dobsonian travels often enough that collimation is checked at every setup and the mark has taken the wear.

Key Features & Design

• Large precision-cut acetate template: cut for primary mirrors from 12″ to 26″, locating the exact geometric centre.
• Eight triangle centre-mark targets: enough for several mirrors, or for replacing a mark years later.
• Triangle geometry matched to the Cheshire field: the triangle edges touch the outside of the illuminated field through a Cheshire, so centring is read rather than judged.
• Sized for big apertures: the large template covers the range the standard 12.5″ template stops at.
• Works with every collimation method: laser collimators, Cheshire eyepieces and autocollimators all read the same mark.
• Made in the USA: by Farpoint, alongside their collimation tools.

Optical Design

A centre mark is the datum that turns collimation from an estimate into a measurement. A laser collimator aligns by returning its spot onto that mark; a Cheshire aligns by reading the mark's concentricity within the illuminated field. Where the mark is off-centre, both tools align the telescope faithfully to the wrong point and give no sign that they have. Farpoint notes that small centring errors amplify on larger mirrors, which is what makes the marking step matter more as aperture grows rather than less.

The triangle targets are cut to a specific size for the same reason. Through a Cheshire, the primary presents as an illuminated field with a defined outer boundary, and the triangle's edges are dimensioned to touch that boundary when the mark sits centred. The reading becomes a comparison of two edges instead of a judgement about how near a dot sits to the middle of a bright circle — a far finer thing for the eye to resolve, and finer still on a large mirror where the field is correspondingly large. The acetate template addresses the other half of the problem, locating the true geometric centre of a 12″ to 26″ disc so the target lands where it belongs.

Recommended Uses

• Marking a newly recoated large primary: a mirror back from aluminising returns without a mark.
• Replacing a worn mark on a big Dobsonian: spots lift over years of cleaning and handling.
• Mirror-making and aperture upgrades: a new large primary in an existing tube needs its own reference point.
• Completing separately bought tools: a laser and a Cheshire acquired on their own have nothing to align to until the mirror is marked.
• Truss Dobsonians in regular transport: frequent collimation checks wear a mark, and eight targets are supplied against that.

Compatibility and Accessory Notes

• Mirror size: primary mirrors from 12″ to 26″ in diameter.
• Smaller mirrors: the standard Farpoint centre-marking kit covers primaries up to 12.5″, and we stock it.
• Telescope types: Newtonian and Dobsonian reflectors. Refractors, catadioptric and SCT designs collimate by other means and use no centre mark.
• Tool compatibility: the mark reads the same to a laser collimator, a Cheshire eyepiece and an autocollimator, regardless of brand.
• Farpoint collimation kits: the 1.25″ and 2″ Collimation Kits and the Super Collimation Kit ship with the standard 12.5″ template, so a large mirror is served by this kit alongside them.
• Tell us your scope and we will confirm the template size for your mirror.

Good to Know Before You Order

• This is the LARGE template, for primaries from 12″ to 26″. A mirror of 12.5″ or under is covered by the standard kit, which we also stock.
• The kit marks the mirror; it does not collimate it. A laser collimator, Cheshire eyepiece or autocollimator does the aligning, reading against the mark this kit places.
• Marking is a one-time job on a mirror in hand. The primary comes out of its cell for it, and the mark then lasts as long as the coating does.
• The triangle targets sit at the centre of the primary. That area lies in the shadow of the secondary mirror, so the target intercepts no light that reaches the eyepiece.
• Centring accuracy matters more as aperture grows. Farpoint notes that a small centring error amplifies on a larger mirror, which is why the large template is cut rather than scaled by eye.
• Eight targets are supplied. That covers several mirrors or several re-marks over the years.

Frequently Asked Questions

Is it difficult to install?

No. The primary comes out of its cell, the large acetate template locates the mirror's centre, and a triangle target goes on at that point. It is a one-time job and the mark then lasts the life of the coating.

Does a centre mark affect the image?

No. The target sits at the geometric centre of the primary, inside the shadow the secondary mirror already casts, so it blocks no light that would otherwise reach the eyepiece.

Why does centring matter more on a big mirror?

Farpoint notes that small centring errors amplify significantly on larger mirrors. The collimation tools align the telescope to whatever point the mark defines, so on a 20″ or 24″ primary the consequence of a mark placed a little off-centre is correspondingly greater.

Why a triangle rather than a ring or a dot?

The triangle's edges are cut to touch the outside of the illuminated field seen through a Cheshire. That turns centring into a comparison of two edges, which the eye judges far more finely than the position of a dot within a bright circle.

My mirror is 10″. Is this the right kit?

The LARGE template starts at 12″. A 10″ primary takes the standard Farpoint centre-marking kit, which covers mirrors up to 12.5″.

Can it be used on an SCT or a refractor?

Those designs collimate by other methods and carry no centre mark, so the kit has no role there. Tell us what you have and we will point you at the right approach.

Bottom Line

In short: a large precision-cut acetate template and eight triangle centre-mark targets for Newtonian and Dobsonian primaries from 12″ to 26″, sized so the triangle edges meet the illuminated field in a Cheshire. It sets the reference point that every laser, Cheshire and autocollimator on a big mirror reads against.