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:
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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:
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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