Frequently Asked Questions about the 

SFL TeleKit

(Short Focal Length f/3.3-f/3.9)

What's in a SFL TeleKit or Quantum finished SFL TeleKit?

The TeleKit comes with all the components plus the adhesive to construct a full featured truss telescope.  It does not include the primary mirror, secondary mirror, eyepieces or finder, unless ordered separately.  Many of the TeleKits made are to re-house optics in an existing telescope or for existing optics.  AstroSystems can supply premium optics made by Terry Ostahowski, optician at very competitive prices.  There are also several recommended suppliers listed in our catalog and under products-primary mirrors.  Eyepieces and the finder are not included, customers either already own them or have specific preferences. 

The following is an itemized list of what is included in the SFL TeleKit:

  • Upper Cage rings, spacers, glue dowels and double-sided tape
  • Light Baffle
  • Moonlight focuser or the optional Feathertouch focuser and mount plate
  • Super Duty Spider and mounting hardware
  • Super Duty Secondary Holder
  • Upper Truss Fasteners with cam hardware
  • 4 position filter slide with 5th clearance position, pre-assembled with mounting hardware
  • Primary mirror mount and hardware
  • Primary mirror cooling fan with gel-cell batteries, DC charger and AC float charger
  • Lower Truss Tube Clamps with mounting template
  • Altitude Bearings, PTFE bearings and roller altitude bearings and hardware
  • Lycra light shroud
  • Truss tubes with black polyolefin coating
  • Mirror Box
  • Rocker Box with bottom laminate
  • Pivot bolt, pre-drilled for setting circles
  • Ground board with feet
  • Azimuth roller bearings, azimuth brake and axles
  • Transport handles, wheels and hardware

For assembly:

  • Glue spacer for rocker and mirror box
  • Lower clamp block glue template
  • Epoxy glue kit with stirrers, application brushes, gloves and beakers
  • Scrap Baltic Birch plywood for sanding, routing and finishing practice
  • Scrap plastic sheet for mixing small amounts of glue or as a cutting mat


How does a SFL TeleKit or SFL Quantum finished TeleKit differ from other truss telescopes?

The two primary goals guiding the SFL TeleKit design are performance and aesthetics.  We opted to simplify the overall appearance to give the TeleKit a clean, uncluttered look that allows the furniture grade wood to really show.  A lot of time went into combining these two goals.  For instance, removing the lower truss fasteners from the outside of the mirror box to the inside corners requires that the mirror box be larger so the trusses form a triangle with a wide enough bottom angle for a rigid truss system.  A larger mirror box allows the upper cage to store in the mirror box while being transported.  By eliminating screws, dowels or inside reinforcements of a plain butt joint on the mirror box and rocker box, a finger joint was used.  This is not only stronger but looks great. 

Some highlights of the TeleKit design are:

*The QuickSwitch filter slide.  This is an integral part of the focuser mount and enables you to have four filters at your fingertips.  You may load with bandpass filters such as OIII, UHC, H-beta, Deep Sky or color filters and go back and forth between positions quickly, easily and safely.  The focuser is mounted close to the bottom of the cage giving optimal contrast and dew protection.

*The eyepiece is tilted up from the horizontal 15 degrees on 18" and larger and 22 degrees on 10"-16" TeleKits.  This makes low altitude observing much more convenient, especially with the 10"-16" TeleKits.

*Upper truss clamps are operated with fast and positive cam clamps (no tools).  The truss tubes are held together in pairs which makes transportation and setup a breeze.

*Lower truss clamps are positioned in the mirror box corners for fast and accurate set up, and are secured with four large knobs (no tools).

*The Baltic Birch plywood is a top quality plywood and is both strong and attractive.  It also has a single sheet veneer instead of being spliced, giving the exterior veneer an unblemished continuity.

*The large diameter altitude bearings feature laminate riding on PTFE bearings against textured laminate on 10" - 16" or one PTFE pad and one set of roller bearings per side on 18"-32" TeleKits.  This results in smooth motion and low maintenance.  The altitude bearings are drive and digital setting circle compatible.

*The azimuth bearings feature innovative sealed precision roller bearings mounted on stainless steel axles.  This allows the TeleKit to move with zero backlash with very low maintenance.  Movement at zenith is smooth and effortless and gives the TeleKit the same resistance in both directions, achieving precise movement at any power.

*The TeleKit primary mirror cell features a drop down tailgate for easy mirror access or removal, a transport position so the primary is at rest on a solid cushioned support while being transported.  The primary mirror cell also comes complete with cooling fans, charging system, 12V dc and 110V ac float chargers and gel-cell batteries.  The innovative stainless steel sling eliminates sag and loss of primary mirror collimation, troublesome with most other truss telescopes.

*All the trusses are 6061-T6 aluminum, the finest quality tubing available.  Supplied too, are the Olefin truss covers for professional appearance and protection. 

*The Lycra light shroud is included as a standard part of the TeleKit and offers protection from stray light, thermal currents and dew.  This advanced material doesn't loose its elasticity so it will always maintain it's great conformal look.

*The AstroSystems spider and secondary mirror holder are an unbeatable combination for strength and precise collimation.  The secondary holder utilizes four collimation screws for fast intuitive adjustments and no-tool thumb screws on 2.6" and larger holders.

*Stainless steel hardware is used throughout and offers excellent corrosion protection and always looks good.

*Pneumatic tires and Baltic Birch handles allow you to safely load, unload and move your telescope with ease.

By building your own telescope you have the following advantages:

Flexibility - By building from "the ground up," you have complete control over the appearance of your TeleKit.  Color, type of coatings, number of coats, degree of sanding before finishing are all under the control of the builder.  By putting in the time and effort to achieve a superior finish, a TeleKit can easily surpass the finish and appearance of any commercially available telescope.

Personal satisfaction - Successfully completing a project of this magnitude can only result in the well earned pride and satisfaction deserving of the effort.  Some owners have expressed that they use their telescopes more and get more satisfaction since they feel closer to their "own" telescope.

Confidence - The thorough understanding you gain in completing the TeleKit is an excellent springboard to move on to other projects and telescopes.

Education - Once the TeleKit is completed, the builder understands Newtonian optics, collimation, truss tube telescopes and wood finishing.  This kind of understanding only comes from hands-on construction.

Accessories and modifications - What's to keep you from modifying or adding any accessories to a telescope that you know inside and out? Nothing.  Digital setting circles, drive systems, dew removal systems and finders are easy to install and adjust.

Repair and maintenance - The owner becomes completely familiarized with the TeleKit and knows what components may need periodic maintenance and understands exactly how to do it and what tools are needed, without instructions!  Should any mishaps mar or damage the TeleKit, a thorough understanding of the construction and finish make repairs a snap.

Being an ATM (Amateur Telescope Maker) - You'll be able to discuss topics with other ATM's from a position of experience.  You are now in the company of other ATM's such as: Galileo, Alvan Clark, Isaac Newton, William Hirschel, Russell W.  Porter, Robert E.Cox, Bernhard Schmidt, Earl of Rosse, John Dobson, Clyde Tombaugh and Leon Foucault.

Bragging rights - Let others know that you are proud of what you can accomplish.

What is a Newtonian telescope?

The Newtonian telescope was first conceived by Isaac Newton in 1678.  A Newtonian telescope is characterized by a concave primary mirror and a flat secondary mirror.  The primary reflects and focuses the light toward the front of the tube.  The secondary mirror or diagonal reflects the light being focused by the primary to the side of the telescope to allow an eyepiece to magnify the image.  The preferred curve of the primary is a parabola.  Both mirrors have their reflective material on the front side of the mirror, unlike a common household mirror.

What is a Dobsonian telescope?

The term Dobsonian or "Dob" is often incorrectly applied to any telescope with an Alt/Az mount (moves up and down in altitude and rotationally in azimuth).  John Dobson himself prefers to refer to the telescopes he made popular as a Newtonian.  This telescope is characterized by the use of very inexpensive materials and simple design.  In it's original form it had a Newtonian optical set with the primary made from a thin, light ship porthole and a homemade rectangular secondary made of plate glass.  A cardboard tube holds the optics and the mount is constructed of particleboard.  The movement is smoothed by PTFE pads riding on Formica.  The first Dobsonians used paint stir sticks for the spider and a particleboard disc covered with bubble-pak for the primary mirror holder.  This inexpensive approach was needed since John was building telescopes while living as a monk and had to scrounge the materials, having taken a vow of poverty.  The design was further popularized by the Coulter Optical company offering a manufactured Dobsonian in sizes up to a 29", unheard of aperture in the late 1970's.  What this approach ultimately accomplished was move amateur telescope makers into a new realm of larger apertures, opening a whole new field of objects that could be observed.

What eyepieces should I buy?

Eyepieces provide a focused image for the eye that the brain then interprets.  As such, eyepieces are the most important telescope accessory, providing the critical link between the telescope and the eye.  The choice of an eyepiece is a very personal one, depending on such things as desired field of view, object under observation, the telescope, location, sky conditions and observer experience.  With so many variables it is not possible to choose eyepieces based on the telescope alone, but here are some considerations worth noting. 

Most eyepiece designs will perform satisfactorily within a certain range of f/ratios (the ratio of the focal length to the aperture).  The highly corrected, wide-field type give outstanding performance with f/ratios over 6.  They are preferred for f/ratios between 5 and 6, and are necessary for quality views with f/ratios under 5.  These include such eyepiece designs as the Ethos, Nagler and Panoptic by TeleVue, the SMC by Pentax and similar designs. 

Medium corrected eyepieces such as the Erfle and similar designs give excellent views with f/ratios over 5.  Below f/5 they have noticeable loss of sharpness at the edge.  The least corrected eyepieces such as the Plossl, Kellner, Ramsden and similar designs have narrower fields of view and noticeably less sharpness over a large part of the field when used with telescopes under f/6.  Two inch eyepieces are preferred for wide, low-power fields of view, minimizing vignetting, or edge of field light loss.  Two inch eyepieces usually have a larger eye lens and long eye relief; a great advantage to viewer comfort.


This is found by dividing the telescope's focal length by the eyepiece's focal length (using the same units).  A telescope with a focal length of 1524mm (60 inches), when used with a 26mm eyepiece would yield a magnification of 58.  The highest magnification possible is most often limited by atmospheric turbulence or tube currents from a primary mirror that is still attaining equilibrium with the air temperature.  When experiencing a lack of atmospheric transparency, it is helpful to use higher magnification.  This darkens the sky background at a faster rate than the brighter foreground object dims, resulting in increased contrast.

It is recommended that the eyepiece giving minimum magnification is the first purchased.  This will be your most often used eyepiece because it has the largest field for finding objects and will be the sharpest when sky conditions are marginal.  It is also recommended that the eyepiece magnification proceed in geometric steps.  If the lowest power eyepiece gives say 50X then the next eyepieces would be 100X, 200X and 400X.  This progression is necessary because our brain does not perceive an increase in image size to be obvious when only a 20-30% increase takes place.  Three to four eyepieces and a Barlow is usually sufficient for all the magnification ranges needed under most observing conditions.

Field of View
The field of view, or the portion of the sky that the telescope is actually viewing is determined by the eyepiece design and the focal length of the primary mirror.  Most observers agree that eyepieces with wider fields offer more pleasing views; our eyes and brain being accustomed to wide-field views.

Eye Relief
This is the distance from the eye lens you need to place your eye to see the entire field of view.  Short eye relief will require the observer to place their eye very close to the eyepiece.  This is less comfortable and will contaminate the eye lens with skin oils.  Short eye relief also makes removal of eyeglasses necessary.  There is a general trend of shorter focal length eyepieces having less eye relief.

Exit Pupil

10" 36 39 42

This is the size of the image (of the objective) exiting the eyepiece.  It can be seen by pointing the telescope at the sky or a white wall and moving back a foot or so from the eyepiece.  The image will appear as a small circle of light, sometimes seeming to float just above the eye lens of the eyepiece.  Low-power eyepieces will have a larger exit pupil.  This should fit into the pupil size of one's dark-adapted eye to make the best use of the telescope's light gathering power and sets a practical minimum on magnification.  The table at right gives the minimum magnification for a given size telescope to achieve the exit pupil sizes at top.  Our eyes ability to dilate diminishes with age, so a young observer would choose the magnification that gives a 7mm exit pupil while someone 40-50 years old might choose the 6.5mm column and someone over 60 would choose the 6mm column.  This is only a rough guide since everyone's eyes are different and change at different rates.

Eyepiece Quality
Superior manufacturing techniques such as blackening the edges of the lenses to eliminate "ghosts" (internal reflections) and anti-reflection coatings that minimize light loss and increase contrast are desirable.  Glass that has low absorption and better imaging capabilities will also contribute to a quality eyepiece.

When purchasing an eyepiece, closely inspect for blemishes, scratches or bubbles in the lens surface.  Also, check for dirt, grease or foreign material on the internal lens surfaces.  This is made easier by using a strong light or direct sunlight.  Check the barrel fit in the focuser as well as the condition of the filter threads.  Check for "ghosting" by viewing a bright planet like Jupiter or Venus.  An eyepiece's transmission can be evaluated by viewing a faint galaxy or nebulae, but use a known eyepiece for comparison since this test is greatly influenced by sky conditions.  Last, determine where in the field the image begins to degrade and to what degree.  When these points are deemed acceptable and the eyepiece is comfortable to use and within your budget, buy it.  Good eyepieces are an investment that will greatly enhance your observing pleasure.

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