Astro-Physics AP27TVPH Focal Reducer Testing on Stellarvue  SV130EDT Triplet Apo Refractor

Wade Van Arsdale
Little Rock, AR., USA
January 11th, 2011


Stellarvue SV130EDT triplet apo refractor on AP-1200GTO mount

Visual back, showing Baader binoviewer and Starlight Instruments FT3035 focuser

 

I.  Test Images:

Test #1:

Test Date:  December 18th, 2010
Skies:  Clear
Seeing:  7 out of 10, with some interference in atmospheric steadiness from jet-stream
Transparency:  6 out of 10 with some light scatter caused by high, thin clouds
Temperature:  32 degrees F
Dewpoint:  26 degrees F
Wind:  N, 5.10 mph
Test camera:  SBIG ST-2000XM/CFW8 filter wheel, Custom Scientific LRGB filters

Focal Reducer Used:  AP 27TVPH
Native focal length without reducer:  900mm
Focal length with reducer in use:  702mm
Focal length reduction factor:  0.78x

Deep Sky Object M52:


Test #2:

Test Dates:  December 26-27, 2010 and January 2-3, 2011
Skies:  Clear
Seeing:  7 out of 10, with some interference in atmospheric steadiness from jet-stream
Transparency:  9 out of 10
Temperature Range:  32 degrees F down to a minimum of 16 degrees F (Note: no signs of optical pinching occurred at 16 degrees F).
Dewpoint:  15-26 degrees F
Wind:  N, 5-10 mph to calm
Test camera:  SBIG ST-2000XM/CFW8 filter wheel, Custom Scientific LRGB filters

Focal Reducer Used:  AP 27TVPH
Native focal length without reducer:  900mm
Focal length with reducer in use:  702mm
Focal length reduction factor:  0.78x

Deep Sky Object M33:

II.  Parts Matrix:

Parts matrix and backfocus use, stacked in order from focuser and drawtube to camera:

Starting Backfocus:  171.00mm (from end of FT3035 drawtube)
Parts Matrix, and backfocus used: --------------
1)  Starlight Instruments FT3035 focuser --------------
2)  Optional "AP Endcap" from Starlight Instruments --------------
3)  Astro-Physics AP27TVPH focal reducer collar:    19.00mm
4)  1 x 1.0 inch Optec AP spacer:    25.40mm
5)  Astro Physics PN ADA204 SCT adapter:      7.62mm
6)  Baader T2-21 adapter for manual camera rotation:    23.00mm
7)  SBIG ST-2000XM camera and CFW8 filter wheel    44.25mm
8)  Optical Backfocus removed by focal reducer:    46.00mm
9)  Final Focuser drawtube position @ focus:  ~  6.00mm
Native focal length without reducer:       900mm
Focal length with above parts matrix and reducer mounted:       702mm 
Optimal metalback chip-to-focal reducer collar spacing:       118mm  (gives 0.75x focal reduction)
Actual metalback spacing in above configuration:       100mm
Focal reduction factor in above configuration:        0.78x

III.  Summary and Conclusions:

1)  The Astro-Physics AP27THPV focal reducer did not cause any coma or otherwise degrade the image in the 15mm image cone diameter of the ST-2000XM camera.
2)  The AP27TVPH provides a viable way to achieve approximately 0.78x focal length reduction for better target framing on some extended deep-sky objects.
3)  The focal reducer attaches by rigid threaded screw-down which provides for a way to have rigid, sag-free camera attachment all the way through the camera payload from focuser to camera.  This is an advantage over most clamping types of attachments, especially with a heavy camera.
4)  The focal reducer will work and provide good results with all the SBIG "ST"-series cameras.  No conclusions should be drawn however about performance with other brands or models of cameras, or the reducer's performance with this telescope when used with other APS-C sized or larger CCD chips.  I have not tested larger-chip cameras with this configuration on this specific telescope model.

Wade Van Arsdale
January 11th, 2011
Little Rock, AR., USA