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Frequently Ask Questions (FAQ)
Straight-vane secondary mounts

(last updated July 25, 2010)


1) Should I use a 3-vane or 4-vane spider?

2) My secondary mirror isn't one of your sizes. Can I still use a Protostar holder?

3) Can I swap different sizes of secondary mirrors in the same spider?

4) How thick is the vane material that Protostar uses on the spiders?

5) What does "low total diffraction" really mean?

6) Why are Protostar vanes made of stainless steel (and not aluminum, or carbon fiber)?

7) How many mounting lugs are on each spider vane?

8) Can I order mounts with a special overhang dimension?

9) Why are there lead-times for secondary mountings?

10) Can I order just a spider or a holder by itself?

11) I don't want to use an Allen wrench for collimation. Can I replace the collimation adjusting screws with finger-operated thumbscrews?

Antidew heater questions

12) Should I get the anti-dew heater option?

13) What kind of power supply is required for the anti-dew heater?

14) How much current does the anti-dew heater draw?

15) How do I attach power wires for the antidew heater?

16) Does the anti-dew heater work if I adhesively mount the mirror?

17) Why don't I don't see the LED power indicator light?

Built-in offset questions

18) Should I get the offset built-in to the spider?


1) Should I use a 3-vane or 4-vane spider?

The choice between a 3-vane or 4-vane spider is largely an aesthetic preference, and the consequence of your decision will be the pattern of the diffraction spikes that emanate from bright objects. A 4-vane spider produces four brighter spikes. A 3-vane spider produces six, dimmer, spikes. A 3-vane spider is a good choice for telescope tubes up to about 18 inches in diameter.

3-vane spiders are the best choice for most applications, but they are also a particularly good choice for planetary and lunar observers. The brightness of the spikes is inversely proportional to the viewing magnification. So, at the high powers typical of planetary observation the spikes disappear, and the view is more "refractor like".

4-vane spiders are a good choice for large truss-style telescopes. Typical upper cage designs use four metal strut pipes around the outside of the cage assembly, and these struts are frequently the only place to attach the spider to.

4-vane spiders are also a better choice for photographic telescopes. Having fewer spikes in the photograph is usually preferable to having dimmer ones. An aesthetic preference for four spikes is also a perfectly defendable reason for choosing a 4-vane spider.

A more detailed analysis of diffraction from spider vanes can be found in H. R. Suiter's book Star Testing Astronomical Telescopes (available from Willmann-Bell or Sky Publishing Corporation).

2) My secondary mirror isn't one of your sizes. Can I still use a Protostar holder?

Yes. Our holders permit two different methods of attaching the mirror; a metal retaining shroud, and gluing the mirror to the holder face. If you have a mirror that is a different size than one of our "standard" sizes, choose the next smallest holder, and adhesively mount it. We include spacers and instructions for adhesive mounting with all our secondary mounts.

3) Can I swap different sizes of secondary mirrors in the same spider?

In general, this isn't possible (or at least it is impractical). The central stem on the diagonal holder varies in diameter from 1/4" up to 1/2" depending on the secondary mirror size, so they're not all interchangeable. Also, the overhang dimension changes with each secondary holder size, and there usually isn't sufficient axial travel in the holder adjustment to accommodate two sizes at once without moving the position of the spider.

4) How thick is the vane material that Protostar uses on the spiders?

Most of the spiders use 0.018" (0.46 mm) thick stainless steel, though the large heavy-duty spiders (3.50" and 4.00" mounts) use 0.026" (0.66 mm) thick vane material. These are the thinnest vanes available, which reduces the brightness of diffraction spikes.

5) What does "low total diffraction" really mean?

Total diffraction from the secondary support is a function of the edge-on area of the vanes divided by the area of the mirror. This value is called the obstruction ratio. The appearance and intensity of diffraction artifacts like spikes is a function of the obstruction ratio, and not the thickness of the vanes by itself.

Using geometry, you can compare the total diffraction of different spider designs. For the example to the right, the edge-on area of the vanes is 600 mm², and the area of the primary mirror is 17,688 mm². Thus the obstruction ratio is (600 / 17,688), or 0.034. (When making the calculation, do not include parts of the spider vanes outside of the mirror's diameter, and ignore the size of the secondary itself.)

A different secondary support with 4 mm thick vanes would have an obstruction ratio of 0.068, and thus diffract twice as much light energy.

6) Why are Protostar vanes made of stainless steel (and not aluminum, or carbon fiber)?

Aluminum has a superior strength-to-weight ratio compared to steels, but for a spider a high strength-to-volume ratio is desired. In order to keep the vanes thin, it's necessary to fit as much strength into as small of a volume as possible. A spider made of aluminum would need to be about 2.5 times as thick as a steel vane to have equivalent rigidity. (The modulus of elasticity of steel is three times aluminum's.)

A similar thing is true for carbon fiber materials, but to a lesser degree. The modulus of common carbon fiber materials is about 24-28 Mpsi, whereas steels are around 30 Mpsi. This means the vane would have to be slightly thicker to have the equivalent rigidity of a steel vane. There is also a problem processing carbon composites that are as thin as our vanes (~ 0.5 mm). This reason alone usually forces a carbon composite vane to be 1.0 to 1.5 mm in thickness.

7) How many mounting lugs are on each spider vane?

The number of lugs on each vane has been determined through testing and experience, and depends on the size of the secondary mirror and the tube size.

Mirror holder size

Number of lugs
per spider vane

0.75" to 1.30

One lug for all tube sizes.

1.52" to 2.14"

One lug up to 12.5" tube ID; two lugs for larger tubes.

2.60" to 4.00"

Two lugs for all tube sizes.

8) Can I order mounts with a special overhang dimension?

As a general rule, this isn't possible. The overhang ('A' dimension in the graphic) is predetermined by our tooling. However, you do not have to drill the spider mounting holes in your tube with high precision, as there is about 1/2" of total adjustment travel (i.e., +/- 1/4") to accommodate for measuring tolerances and other variables.

9) Why are there lead-times for secondary mountings?

Due to the almost limitless combination of tube diameters and available options (i.e., offsets, heaters), it is impossible to stock the spiders as on-the-shelf items. We make each one to your specifications, but sometimes the leadtime may grow to several weeks during busy times of the year.

10) Can I order just a spider or a holder by itself?

Yes, we sell spider and mirror holder subassemblies separately, but be aware that these two components work together to allow for collimation. (The collimation screws are in the spider hub, while the clutch disk and semi-flexible shaft are part of the mirror holder.) In other words, one part without the other probably won't work unless you have a custom design in mind. For the same reason, our holders will not work in other manufacturer's spider designs. If you need either a spider or holder to replace a damaged component, please see the Replacement Parts Price List for details.

11) I don't want to use an Allen wrench for collimation. Can I replace the collimation adjusting screws with finger-operated thumbscrews?

Off-the-shelf solutions for thumbscrew adjustment are to be strictly avoided. The problem is that standard bolts of sufficient length do not have threads fully formed along the entire bolt length. If you turn the screws in too much, the non-threaded portion of the bolt shaft will begin to engage the thread inserts in the spider hub. Once this happens, the entire spider is usually ruined, as you can't get the bolt back out without spinning out the thread insert. The insert and the bolt bind together.

The other problem is that most plastic thumbscrew heads have a diameter that's too large, and they will partially protruding into the light path. This is a particular problem for the 1.83" and 2.60" sizes, since the spider hub is nearly the same size as the secondary mirror.

12) Should I get the anti-dew heater option?

The antidew heater is recommended for observers who frequently stay up well past midnight when dew is most likely to form, or live in a region where dew is a more persistent problem. Also, secondary mirrors in open truss-style cages tend to dew up more frequently than closed tube telescopes.

13) What kind of power supply is required for the the anti-dew heater?

Any battery with a voltage of 13 volts or less will power the antidew heater. The heater will work fine at full power (i.e., 13 volts), but only about 1/3 to 1/2 of this voltage is required to keep dew from forming. To lengthen the battery life, we recommend a solid-state controller like the Kendrick Controller to throttle down the power consumption of the antidew heater.

14) How much current does the anti-dew heater draw?

The 1.52", 1.83", and 2.14" holders draw 0.08 amps of current (1.1 Watts of power) when operated at full voltage (~13 volts). The 2.60" and 3.10" holders draw 0.16 amps of current (2.2 Watts), and the 3.50" and 4.00" holders draw 0.24 amps (3.3 Watts). Operating the heater at lower voltages reduces this power consumption.

When used at full power (12 V), the heater will clear dew that has already formed within 10-20 minutes. However, it is best to use the heater in a preventative mode at a reduced voltage. In most cases, even 3 or 4 Volts is sufficient to keep dew from forming.

15) How do I attach power wires for the antidew heater?

There are many clever ways to attach power wires to the two conductive vanes. The two most common methods are shown in the figure to the right. The wires can be hidden from view by attaching them internally to the vanes inside the tube. A #8-32 nut holds each spade lug to the vane tip.

Making an external connection by slipping the spade lug under the spider mounting screw head is even easier, though the wiring will be visible. (Note: Spade lugs are included with secondary mounts sold with the antidew heater option.)

If your telescope tube conducts electricity (e.g., aluminum, steel), you will need to electrically isolate the connections to prevent a short circuit. We supply Nylon bushings in your kit for this purpose.


16) Does the anti-dew heater work if I adhesively mount the mirror?

Yes, the heater will keep dew off regardless of how you mount the mirror (i.e., with adhesive or with the supplied metal shroud).

17) Why don't I don't see the LED power indicator light?

The LED light is embedded in the spider hub at the bottom of a small hole on the hub's face, and it is dim enough that you might miss it in daylight. It is designed not to create unwanted light inside the telescope tube which could hinder astrophotography.

18) Should I get the offset built-in to the spider?

Offsetting the secondary is the practice of shifting the secondary mirror away from the focuser by a small amount so that its geometric center is no longer in the center of the tube. From a purely optical standpoint, offsetting has no effect on optical quality, collimation, or vignetting. Incorporating offset will do no harm, but there is no optical benefit either. (The complete explanation is beyond the scope of this FAQ, but there are Internet resources that explain this in great detail.)

There is one case where offsetting is important (but not for optical reasons). If you are mounting a Newtonian onto a German equatorial mount (GEM) with a computerized GOTO system, you should incorporate offset. Otherwise, there will be a small pointing error. The computer assumes that the optical axis is the same (or at least parallel to) the axis of the telescope tube, and that the right ascension and declination axis are perpendicular to each other. Offsetting the secondary mirror away from the focuser by the proper amount ensures that the primary mirror is pointed down the middle of the tube (i.e., parallel to the tube's axis). If the mirror is pointed slightly away from the tube's axis, there will be a pointing error.

If you decide you need to offset the secondary, small amounts of decentering (less than about 0.1") can be achieved by simply decentering the spider when you install it. (There is about 1/8" (3 mm) of clearance between the tip of the spider and the tube inside wall.) For larger amounts of offset, it is best to have the spider built asymmetrically (i.e., unequal length spider vanes) for a cleaner installation. This can be ordered as an option with Protostar straight-vane mounts.

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