Sizing a Secondary Holder

The secondary mirror size is based on the minor axis, or width.  The secondary holders manufactured by AstroSystems will accommodate a secondary mirror that is within 1% of the nominal holder size.  It is also required that the mirror have an accurate proportion between minor and major axis dimensions.  To check the ratio of the length and width of the secondary mirror, multiply the width by 1.414.  A variation of 20-30 thousandths of an inch (<1/32") is acceptable.  Be sure to measure the outside dimensions of the secondary mirror, not just the polished optical surface.  While it appears that the slit in the side of the shroud would allow it to fit varying mirrors, the forming of a rigid lip limits the adjustability to 1% variance in size.

Minor Axis Major Axis Minor Axis Size Range
1.30 1.84 1.287-1.313
1.36 1.92 1.345-1.375
1.52 2.15 1.505-1.535
1.83 2.59 1.810-1.848
2.00 2.83 1.980-2.020
2.14 3.03 2.118-2.161
2.50 3.53 2.475-2.525
2.60 3.68 2.574-2.626
2.75 3.89 2.690-2.810
3.00 4.24 2.970-3.030
3.10 4.38 3.069-3.131
3.50 4.95 3.465-3.535
4.00 5.65 3.960-4.040
4.25 6.01 4.207-4.292
4.50 6.36 4.455-4.545
5.00 7.07 4.950-5.050
5.50 7.77 5.445-5.555
6.00 8.48 5.940-6.060
6.50 9.19 6.435-6.565
7.00 9.89 6.930-7.070

Sizing a Spider

The spiders manufactured by AstroSystems are made to the size (tip to tip) specified by the customer.  They are welded and not adjustable.  The hexagonal stainless steel vane end is 0.75" long and 0.35" across the tips.  It has a 0.35" deep slit that is welded onto the vane.  This leaves 0.35" of the 10-32 threaded portion available for the mount screw.  Spiders get their rigidity, and hence their ability to hold accurate collimation, by being tensioned.  It is important to size the spider and mount hardware to get the maximum tension possible with the tube materials.  The hardware is provided and is stainless steel, #10-32, with a Phillips drive pan head and #10 washer.

1.  Size the spider with 0.06" or more space between the vane end and tube wall to allow for the fact the tube will deform inwards as the attachment screw is tightened.  Large fender washers or internal reinforcement is required on a cardboard tube.

2.  Have at least 0.75" of space between the edge of the primary mirror and the tube wall, as seen from the front of the telescope.  This keeps the shadow, and added diffraction, of the vane end out of the light path.  This space will also allow warm air to escape out the front with minimal effect on the seeing on the inside of the tube and minimize vignetting (light loss) at the edge of the field.

3.   With truss telescopes, the light baffle material can be drilled for the vane end to protrude.  This gets the vane end close to the ring spacer and out of the light path (see bottom drawing above).

4.   The mount screw should be sized to thread into the vane end at least 5 threads (0.15").  If the mount screw is too long and bottoms or the tube wall deforms to allow the screw to bottom, the spider is not tensioned properly and will not hold proper collimation.

Example 1:  A tube has an inside dimension of 10" and a wall thickness of 1/2".

The spider will be ordered at  9.88" (10" ID less the 0.06" space at each vane tip) with 3/4" hardware.  The mount screw goes through the 0.5" tube wall, spans the 0.06" gap and will thread 0.19" into the vane end (6 threads).

Example 2:  A truss telescope for a 20" mirror has an upper cage inside diameter of 21" with 1.25" spacer tubes 1/4" outside of the light baffle (see bottom drawing above).  The spider will be ordered at 21.38" with 1.5" hardware.  The light baffle is drilled with a 3/8" hole to clear the vane end.  The mount screw goes through the 1.25" spacer, spans the 0.06" gap and threads 0.19" into the vane end (6 threads).

Dimensions for Spider and Secondary Holder

Heavy Duty Holders (1/4" mount shaft)

  A B C D E (2xB) A+B A+B+C Weight
Minor Axis
 in. in. in. in. in. in. in. (LBS.)
1.32 2.7 0.75 1.6 0.75 1.5 3.45 5.05 0.14
1.36 2.75 0.75 1.55 0.75 1.5 3.5 5.05 0.14
1.52 2.7 0.75 2.0 0.75 1.5 3.45 5.45 0.16
1.83 2.9
1.25
2.25
 0.75 2.5
4.15
 6.40 0.19
2.14 3.0 1.25 2.4 0.75 2.5 4.25 6.65 0.24

Heavy Duty Holders (3/8" mount shaft)

  A B C D E (2xB) A+B A+B+C Weight
Minor Axis
 in. in. in. in. in. in. in. (LBS.)
2.50 3.35 1.25 3.00 0.75 2.5 4.60 7.60 0.42
2.60 3.45 1.25 3.00 0.75 2.5 4.70 7.70 0.44
2.76(70mm) 3.50 1.25 3.00 0.75 2.5 4.75 7.75 0.48
3.10 3.85 1.25 3.00 0.75 2.5 5.10 8.10 0.55
3.50 4.05 1.25 3.00 0.75 2.5 5.30 8.30 0.78
4.00 4.8 1.5 2.50 0.75 3.0 6.30 8.80 0.86
4.25 5.25 1.5 2.50 0.75 3.0 6.75 9.25 0.96
4.50 5.40 1.5 2.50 0.75 3.0 6.90 9.40 1.25

 

Super Duty Holders (1/2" mount shaft)

  A C D E  A+E+C Weight
Minor Axis
 in. in. in. in. in. (LBS.)
5.00 5.90 1.2 0.75 3.0 10.10 1.45
5.50 6.05 1.0 0.75 3.0 10.05 1.70
6.00 6.10 1.3 0.75 3.0 10.60 2.10
6.25 7.00 1.3 0.75 3.0 11.50  
6.50 7.50 1.3 0.75 3.0 12.00  
7.00 7.40 1.0 0.75 3.0 11.40 2.55

 

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