Astro-Physics 160 EDF Triplet Refractor

The Astro-Physics 160 EDF triplet air-spaced refractor sitting atop the 1200 GTO mount in the southern Arizona desert. An incredible mechanical and
optical package is perhaps the best way to describe this telescope. In its native format it's an f/7.5 system with a focal length of 1200mm, but can be
reduced to a fast f/5.75 at 920mm focal length with its dedicated flat field telecompressor.
I added a Robo-Focus stepper motor for remote focusing. The Robo-Focus bracket is one offered for sale by Astro-Physics for the AP 160 4" focuser. I've noticed that the massive 4"
focuser is extremely precise and fine focus can be done "on-the-fly" while auto-guiding
without any shift in position of the guide star on dual head SBIG cameras. As advertised
on the Astro-Physics website, this greaseless focuser truly has no image shift at all.

The drawtube end of the focuser is equipped with the standard AP locking compression ring fitting, as is the 4" extension piece. A built-in threaded collar at the upper end of the focuser allows for 360° rotation of the focuser if desired. This is a big, beefy focuser with the
precise movement of a finely crafted Swiss watch. No effort was spared in the design
and construction of this mechanism. It's just what you'd expect from AP.

     
This is a shot looking down the OTA from the focuser end. There are an astonishing number of baffles machined down the length of the
optical tube. At the far end you can see the reflection of the camera flash off the inside of the objective's aluminum dust cap.
 

The 160 f/5.7 Telecompressor Corrector

This 3 element high index glass telecompressor/flattener reduces the native f/7.5
focal ratio of the 160 EDF refractor down to f/5.7 (912 mm focal length) while
providing diffraction-limited, pinpoint stars at the edge of the chip on the large
SBIG STL-11000M CCD camera. The tapered input end on this unit fits
precisely into the back of the telescope's 4" focuser and is securely held
in place by a locking brass compression ring fitting.

  An exploded view of the 160 f/5.7 Telecompressor Corrector

This exploded view shows both the front and rear caps for the telecompressor
corrector. The large front cap fits snugly over the finely machined taper on the
input side of the telecompressor while the smaller, threaded rear cap screws
onto the 3.25" diameter threads on the output side of it. This is one heavy
assembly at 4.5" in diameter and approaching five pounds in weight!
 

The 160 f/7.7 Field Flattener

This dedicated field flattener allows the imager to use the AP 160 EDF refractor with
large format chips such as found in the SBIG STL-11000M CCD camera and obtain
diffraction-limited results to the very edge of the field of view. The tapered input end
and threaded output end are identical to the f/5.7 Telecompressor Corrector.

 

An exploded view of the 160 f/7.7 Field Flattener

From left to right, the components of the 160 f/7.7 Field Flattener are as follows: threaded
end cap (used during storage), 18mm long spacer for the correct metal back distance from
the flattener to the focal plane, the field flattener cell containing the optics, and a machined front cap (used during storage)

The 160 f/5.7 Telecompressor Corrector and f/7.7 Field Flattener

Here's a photo showing the two optics side-by-side, giving the reader an idea of the
relative size of each unit. The f/5.7 has a working length of 1.00" (25mm) and a total
overall length (including end caps) of 2.625" (66mm). The f/7.7 has a working length
of 2.25" (56mm) and a total overall length (including end caps) of 3.75" (95mm).
The front caps of both units are 4.5" (114mm) in diameter and the rear caps are
3.375" (85mm) in diameter.

 

Telecompressor Corrector to STL Adapter

This adapter allows the SBIG STL Large Format cameras to be used with the 160 Telecompressor, providing the correct back distance to the camera's chip. The adapter
bolts directly onto the STL camera mounting block with four hex head cap screws provided
by AP. The telecompressor screws into the threaded input end on the adapter. The extra
large bore of this adapter allows for full illumination of the STL chip.

 

Telecompressor Corrector to ST 7/8/10 Adapter

This adapter allows the SBIG ST 7/8/10 series cameras (with CFW filter wheel attached) to be used with the 160 Telecompressor, providing the correct back distance to the camera's chip. The adapter screws directly onto the telecompressor output threads. A 2" AP16T tapered nosepiece threaded onto to the CFW-8 front cover plate then slips into the 2" locking brass compression ring fitting.

     
View of the AP STL adapter mounted on the SBIG STL 11000M CCD camera
     
 

Here's a solar filter cell I designed and fabricated for the telescope. It's made from 8" Sch. 80 PVC pipe turned on a lathe to exacting dimensions. It consists of a top ring counterbored to accept #10 allen head cap screws that fasten to threaded holes in the lower body, which in turn slips over the dew shield of the telescope. This cell is much more substantial than one made of cardboard, yet is surprisingly light.

   A view of the finished solar filter cell from the back side, showing the thin layer of felt
affixed to the ID of the lower body. The small gap in the felt lining allows for escaping
air when the cell is pushed down over the telescope's dew shield. The Baader Astro
Solar ND5 film is firmly sandwiched between the top ring and lower body, held in
place by eight #10 cap screws.
     
 
 

I machined this front end cell to accomodate an 80 mm Daystar ERF filter and fit over the dewshield of the AP 160. The assembly has a tiny bit of tolerance to allow the ERF to "float"
in its cell. Made out of aluminum, it's still pretty heavy but offers a solid mounting on the
front end of the telescope for H-alpha solar imaging.

  

Here's a diffraction spike generator that can be fitted over the dewshield to create star images with that traditional "reflector-look" . It uses plastic guitar string firmly held in place with set screws threaded into the PVC body. Refractor purists hate these things for imaging and I tend to agree with them. They do however have use in visual observing when trying to split close double stars of greatly unequal magnitude, such as the star Siruis and its companion.