Special Challenges of Short f/ratio Truss Newtonian Telescopes

Short focal length telescopes present several challenges, most are related to the need to achieve greater relative accuracy of collimation, or optical alignment, and holding it.  The entire support structure (telescope) must be more rigid to maintain this more demanding collimation as well as properly support the larger, thicker and heavier secondary mirrors necessary with a short f/ratio telescope.  This requires new tools and techniques for collimating, optical support components that give easy, accurate alignment and hold it, and a support structure utilizing optimized materials and engineering to meet these demands.

Optical Support

Secondary Holder - Our super duty secondary holders are engineered to maintain greater collimating accuracy.  They incorporate a thicker, stiffer housing (bezel), a thicker, stiffer adjustment plate and a larger mount stud.  Also supplied with heavier secondary mirrors (5"+) is a rigid foam insert to position  the secondary mirror securely and accurately inside the secondary holder.  The four screw adjustment also insures the fine adjustments needed for short f/ratio telescopes can be made easily.  Two nuts with nylon washers position the holder laterally and also allow easy adjustment of the rotation.  Use the link below for dimensions, exact weight and sizing.

Spider - Our Super Duty Spiders are engineered to accurately hold the larger , heavier secondary mirrors needed with shorter f/ratio telescopes.  The width at the center hub is 2.5" up to 26" spider size and 3" from 26" - 41.5".  They have 35% thicker vanes than out Heavy Duty Spiders and two mounting nuts on the end of each vane, giving a total of eight mounting screws.  The 26" - 41.5" Super Duty Spiders also have a 1/2" mount hole to accommodate the larger Super Duty Secondary Holders with a 1/2" mount shaft.

Primary Mirror Cell - Our mirror cell incorporates 35% finer collimation adjustment and a zero stretch sling to properly edge support the primary and insure it remains accurately centered.  

Mechanical Support

All the telescopes mechanical support components act a a single unit, so the weakest is the limiting factor in achieving and maintaining accurate optical alignment.

 Cage - The cage must be stiff enough to support the spider and holder.  To hold accurate collimation the spider vanes need to have 70-100 pounds of tension across each pair.  Conventional cage construction is rarely up to this. We have evaluated several materials used as cage ring supports for deflection under load using a 8.5 pound weight placed at the center of a 12" section of material.  

Telescope Components

Focuser - An accurate focuser with no backlash and fine travel is needed for the exacting focus of short f/ratio telescopes.  A low profile (1.5" - 2.1") is preferred to minimize the size of the secondary mirror, already relatively large to obtain  a reasonable illuminated image in a fast f/ratio telescope.

Truss Connectors - Simple clamping systems are no longer adequate to hold accurate collimation.  Indexing the trusses so no variation in length due to thermal variations is best to hold collimation.

Collimation

Collimation tolerance becomes much more critical as the f/ratio decreases.  Under ideal conditions, the maximum distance from focus where coma barely affects the diffraction image from a star is shown in the first two columns in the table below.  The distance where coma introduces a wavefront error of ¼ wave in addition to any other error present is shown in the last two columns.  For high resolution imaging the tolerance can be set somewhere between these values.  The tolerances may be relaxed for wide-field photography, low-power viewing and for instruments of very large aperture where seeing will limit the possible resolution.  Surprisingly, the size of the telescope mirror doesn't come into the calculations, a larger telescope doesn't have a larger "sweet spot" than a smaller. You will notice that short focus telescopes (low f/ numbers) are very much more critical to collimate - that is one price you must pay for the convenience of a short, wide field telescope.

5" holder showing the rigid foam insert for heavier mirrors	5" holder with 1/2" mount bolt, 3/8" adjustment plate and built in offset

Super Duty Secondary Holders

AstroSystems is the largest supplier of secondary holders and spiders to the astronomical community.   There are no tools needed to adjust holders of 2.5" or larger in minor axis size; they come standard with thumbscrews.  All sizes feature our innovative four screw/two-axis adjustment that greatly simplifies collimation.   Two nuts with nylon washers position the holder laterally and also allow easy adjustment of the rotation.  The secondary mirror is held securely yet safely against the front lip of the aluminum bezel with polyester batting on sizes up to 4.5" and a rigid foam insert on sizes over 5".   Super Duty holders are available from 2.5" - 7.0".  See our heavy duty holder page for other sizes.  Use the link below for dimensions, exact weight and sizing.

2.5" to 3.5"

The Super Duty holders have a proportionately thicker bezel for proper support of  thicker, heavier secondaries and a 3/8" mounting stud.  They utilize 3.5" long thumbscrews for easy, no tools collimation. 

4.0" to 4.5"

The Super Duty holders have a proportionately thicker bezel for proper support of  thicker, heavier secondaries and a 3/8" mounting stud.  They utilize 3.5" long thumbscrews for easy, no tools collimation. 

5.0" to 6.5"

To properly support large, heavy secondary mirrors several design changes have been incorporated.  The larger holders have a proportionately thicker shell and a 1/2" mounting stud.  The adjustment plate has been increased to 3/8" thick and a rigid foam insert is used to position the mirror within the holder accurately and maintain this position in any orientation.  They utilize 3.5" long thumbscrews for easy, no tools collimation.

Please state actual dimensions of minor axis, major axis and thickness as well as amount of offset when ordering Super Duty Secondary Holders.

Dimensions and sizing of Super Duty Secondary Holders

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To Order:
We'll need your tube ID to size the spider and tube thickness to supply proper length stainless steel mounting screws.