Omegon Main Mirror 304 mm f/5 Mirror Set

Omegon Primary Mirror 304 mm f/5 Mirror Set: optics for a self-built 12-inch Newtonian

Anyone who builds a large Dobson or Newtonian telescope themselves starts with the mirrors. The rest of the telescope, the tube or truss, the rocker box, and the focuser, is woodwork and mechanics that you can make yourself. The optics are not: they determine what the instrument will ultimately show.

This set supplies both mirrors you need. The 304 mm parabolic primary mirror and the elliptical secondary mirror are matched to each other, so you don't have to calculate the size of the secondary mirror yourself.

Parabolic, not spherical

At f/5 a parabolic mirror is necessary. A spherical mirror at this ratio would produce visible spherical aberration, preventing stars from ever becoming truly sharp.

The primary mirror is made of borosilicate glass. It expands less than regular window glass, so the mirror retains its shape after cooling.

Secondary mirror with small 70 mm minor axis

The elliptical secondary mirror has a minor axis of 70 mm. That size was chosen as a balance between two things: large enough to fully illuminate the field of view, small enough to limit the central obstruction.

A smaller secondary mirror yields higher contrast on planets, while a larger one illuminates a larger photographic field. This size suits the versatile use for which an f/5 Newtonian is intended.

What f/5 means in practice

With a 304 mm aperture at f/5, you get a focal length of 1500 mm. That is short for this aperture, which keeps the tube manageable and yields a wide field of view.

The flip side is that a short f-ratio shows coma at the edge of the image. For visual work that is not too bad, for photography a coma corrector is recommended.

Included

• 304 mm parabolic primary mirror at f/5
• Elliptical secondary mirror with a 70 mm minor axis
• Both with aluminum and silicon dioxide coating

Featured accessories

A laser collimator is indispensable for a self-built Newtonian: you will need to align the optics regularly during construction and afterwards. Starting from this aperture, a moon filter is a pleasant addition for observing the full moon.

For whom yes and for whom no

• ✓ DIY builders of a large Newtonian or Dobson
• ✓ Anyone wishing to equip an existing tube with new optics
• ✓ Observers looking for a wide field of view with high light output
• ✗ Beginners: this is separate optics, not a complete telescope
• ✗ Anyone with no experience in collimation and mirror cells
• ✗ Photographers without coma corrector: at f/5 one is needed

Specifications

You can find more telescope parts for this merk on the Omegon merk Page.