What a telescope spider is
In this context, the “spider” is not an animal. It is the support structure in a Newtonian-style reflector that holds the small secondary (diagonal) mirror in the middle of the tube, in the path of the incoming light. It is called a spider because its thin arms, or vanes, radiate from a central hub, somewhat like legs. Searches for “star spider” or spiders in space refer to something else.
What the spider does
The spider and its central holder:
- Hold the secondary mirror in the center of the tube, above the primary mirror.
- Keep it steady so that it does not move when the telescope swings from one target to another.
- Usually allow the secondary to be centered and, in many designs, adjusted so the telescope can be collimated.
Because the spider sits in front of the primary mirror, it blocks a small amount of light and affects the star images, so makers try to keep the vanes thin and rigid.
Vane count and diffraction spikes
The vanes bend, or diffract, a small amount of starlight. On bright stars this produces spikes. A common four-vane spider gives four-pointed spikes, usually seen as a cross, and a three-vane spider generally gives six spikes. Many observers do not mind the spikes, while others find them distracting on planets and double stars, where they can reduce contrast slightly. Astrophotographers sometimes like the look.
Straight vs curved vanes
- Straight vanes are simple, strong and common. They produce the usual crosses.
- Curved vanes spread the diffracted light over a wider arc instead of forming a sharp line. This reduces the obvious spikes, which some observers prefer for planetary or double star viewing. Curved designs can be harder to build and may need more care to keep them tight. Mechanical stability still matters more than shape.
Spider designs and adjustment
Typical designs include thin steel vanes with tensioning or end screws, thicker stamped or cast vanes, and some arrangements that attach to the tube wall with a ring or tabs. Some spiders offer ways to center the secondary and tweak its position; others are fixed. The right design depends on the telescope size and weight of the secondary.
How the spider relates to collimation
Collimation depends on the secondary being stable. If the spider is slack, the secondary holder may shift, and the secondary can fall out of alignment between sessions. Basic steps for centering and tilting the secondary are in our guide on how to collimate a telescope without a laser, and the wider process is in the telescope collimation guide.
If the secondary looks off-center in the focuser, this can be intentional. Some scopes offset the secondary slightly so that it looks centered from the focuser, so check your maker’s instructions before changing it.
Common problems
- Loose vanes: vibration or a drooping secondary.
- Bent vanes: uneven spikes or a tilted secondary.
- Stuck adjusters: corrosion or over-tightening.
- Secondary rotating or shifting: too little friction in the holder.
Be gentle. A secondary mirror is delicate, so support it when loosening anything.
Upgrading or replacing
Aftermarket spiders are available for many sizes, and some DIY builders make curved-vane versions. An upgrade can help if the original is flimsy or hard to adjust, but it is not a cure for poor seeing or misaligned primary mirrors. When ordering, match the tube diameter and the holder’s weight rating, and follow the maker’s instructions.
FAQ
Do all telescopes have a spider?
No. Refractors do not, and many compound telescopes use a different secondary support. It is mainly a Newtonian feature.
Can I remove the spiked look?
Curved vanes or a different design can soften the spikes. You can also use image processing in photos.
Is a spider the same as a secondary mirror holder?
Not exactly. The spider is the arms, and the holder is the central part that carries the mirror. They are often sold together.