Some thoughts about spiders in Newtonian telescopes

By Nils Olof Carlin, modified from Olle Eriksson's home page. 3D graphics by Olle

The "spider" is the support that holds the diagonal (also known as the secondary) mirror in its proper place within the tube. The vanes run from the tube walls to the diagonal holder, and the number of vanes can be four (I can think of no reason for more) down to one. There are many ways to make a spider, here are some requirements to keep in mind:

  • It should support the diagonal mirror firmly and without vibrations.
  • It should allow the mirror to be tilted and positioned precisely in 3 dimensions, to allow exact collimation of the optical path (the mirror may also be rotated along the tube´s optical axis, but this is not required).
  • It should be designed not to give worse diffraction effects than necessary.

Some possible arrangements for spider vanes. 1 and 2 are common. 3 and 4 are useful with sled-type focusers where the diagonal mirror moves with the focuser. 5 shows a curved spider. 8 shows a mirror supported by an optical window. If you make a corrector window for a Schmidt-Newtonian, it is natural to use it for support - but do NOT make an optical window just to get rid of the spider diffraction effects!

7 is my own favorite, described below.

Since the spider vanes are in the optical path, they will give some diffraction effects that can be visible and possibly degrade the image contrast, just like the secondary mirror obstruction does.

Each straight piece of vane will give "spikes", seen radiating from a bright star, in the perpendicular direction to the vane. In 1 and 7 there will be 4 spikes like a cross, in 2 there will be six.

If you make the vanes curved, each "spike" will be spread out like a fan over the angle of that circle segment, and be much less conspicuous, but the curved vanes will be longer and thicker than straight vanes for the same support, and the total amount of light spread out is increased. The stars won´t look like "stars" on astrophoto, but for visual use, it is a matter of taste. If you want the widest spread, the curved vanes laid together should make half a circle, that is 180 degrees. Use four vanes, each a 45 degrees sector, or three of 60 degrees. A more gentle curve may get a good result a bit easier.

The thicker the spider vanes are, as seen along the optical path, the more light is diffracted, and the more concentrated it is near the star. This means that if the vanes are thick enough (and many), enough light can be spread out to affect contrast on planetary surfaces, or obscure faint stars close to a bright one.

There is no natural minimum thickness of a vane (unlike the diameter of the secondary mirror), so you ought to chose the thinnest material that will support the mirror well. For instance, if the width of the vanes of a 4-vane spider is 3 percent of the mirror diameter, then about 4 percent of the mirror area is obstructed, and something like 4 percent of the light will be diffracted from its proper place in the image and land where it shouldn't. Also, the thinner the vane is, the wider is the diffraction and the more of the diffracted light will fall outside the planet's surface where it won't affect contrast.

Usually the mirror support is carried by a bolt through the center of the spider. To allow collimation, it is tilted with the help of 3 push-pull screws. The spider vanes are fastened with screws through the tube wall or to support pillars in diagonal cages. Note that the optical center of the mirror is not at the geometrical center of the elliptic surface, so the mirror must be "offset" to be properly centered. You can calculate offset=(width of secondary mirror)/(4* the f/ratio of the main mirror). If you use a 50 mm secondary for a 200 mm f/6 main mirror, the offset is about 2 mm (if the spider design doesn't let you offset the mirror, you can leave it centered with no serious problem).

My own favorite spider, the "hacksaw spider" is adjusted not in the center but at the outer ends. These are clamped to supporting blocks of wood between the rings of my diagonal cage - with a solid tube as shown in the graphic images, the blocks can be put outside (easier to adjust) or inside of the tube wall. The vane ends can be moved along the tube away from or towards the main mirror, and also sideways (since the spider vanes are flexible), as well as nearer or farther from the focuser. This allows full adjustment of the angulation and position (in 3 dimensions) of the mirror. The clamp can be a wooden block or a L profile of aluminium, with another piece tightened on the vane with two screws. I like to use spring washers on the screws, as I can loosen them a little and adjust the vanes by hand, and when this is done I can just tighten the screws to lock the position. On my smallest telescope, I have a piece of rubber to hold the vanes firmly enough for transport, but I can still adjust them by hand.

Once you have done this adjustment, you can leave the screws tightened and do the daily collimation fine-tuning on the primary mirror only. Here is my FAQ about collimating a Newtonian telescope

For spider vanes for a 6" and a 10"I have used cheap hacksaw blades with teeth sanded away, sprayed flat black. The mirror support is a wooden block cemented to the vanes (or screwed - the blades are too hard to drill but I have let the screw heads grip the edges of the vanes). The block has one 45 degree face, and the mirror is cemented to it with silicone RTV, the cement used for aquariums. WARNING do not cement tightly, as changes of temperature or humidity will change the dimensions of the wood, bend the mirror, and give astigmatism. Use 3 or 4 gobs of RTV, and 1-2 mm thick spacers while it sets, to allow some motion (I found out this the hard way!). John Dobson suggests pieces of leather between the mirror and block for this reason.

Here is my 13.1" with a 63 mm diagonal. A hacksaw blade is too short and flimsy for so large a spider, so I have used 2mm x 30 mm aluminium profile. The "block" is also made from aluminium "T" profile.

One advantage with this design is that it is very "low profile" since the vanes are not (much) farther out of the tube than the diagonal mirror is. If you want the most portable assembled tube, with a separate extension for light protection, this helps.

(In use, I wrap a piece of camping mattress around the whole diagonal cage)

Nils Olof Carlin,