Astro-Physics Telecompressors - 27TVPH & CCDT67
Compression and Coverage
A telecompressor, as its name implies, compresses a given photographic field into a smaller area effectively lowering the focal ratio of
an instrument and giving it a wider field for any given sensor chip or film size while making it faster for photography. In general, theamount of compression varies with the distance from the telecompressor’s optic to the focal plane. To calculate the compression, takethe focal length of the telecompressor; subtract the distance from the center of the telecompressor’s optic to the focal plane; and thendivide the difference by the telecompressor’s focal length. Astro-Physics offers two telecompressors: the 2.7” Photographic-VisualTelecompressor (27TVPH) and the CCD Telecompressor (CCDT67). We actually offer a third set of telecompressors, the 155TCC
and 160TCC, but since they are also correctors (field flatteners), their required distance and therefore their compression ratios are NOT
variable.

The 2.7” Photographic-Visual Telecompressor (27TVPH)
The focal length of the 27TVPH Photo-Visual Telecompressor is 700mm or 27.6". Using the formula above, a distance of 175 mm or6.9" gives the stated 0.75X compression. (700 - 175) / 700 = 0.75 A shorter distance of 140 mm would give a compression factor of0.80X. (700-140) / 700 = 0.80.
For your calculations, the 27TVPH takes up about 57 mm or 2.25" of that distance. The rest is made up of extensions, adapters,
camera backfocus etc. To achieve 0.75X compression then, you would need a total distance of 118 mm (175 - 57) or 4.6” from theback edge of the telecompressor body to the focal plane.
The 27TVPH also has a clear aperture of 56 mm. To get the size of the fully illuminated (non-vignetted) circle, simply multiply the clear
aperture times the compression ratio that you ended up with in the calculation above. 56 mm x 0.75 = 42 mm at 0.75X compression;
56 mm x 0.80 = 45 mm at 0.80X compression. Remember, however, that other parts of your light path like T-rings or filters may restrict
the clear aperture further.
As a further example, let’s assume a digital SLR is being used. The distance from the camera’s bayonet mount to the sensor chip is44 mm. The least amount of compression you can expect would be to use the following: Start with the 27TVPH @ 57 mm; noextensions; the ADA20132 - 2" thin adapter @ 11 mm; the AP16T - 2" / T2 nosepiece @ 3 mm; the appropriate T-ring at 10 mm; andfinally your digital SLR @ 44 mm. 57+11+3+10+44=125 mm. (700-125) / 700 = 0.82X of compression. 56 x 0.82 = 45.9 mm - thefully illuminated circle. Adding extensions or using longer components like the regular 2" adapter (ADA2003) or the PFCT cameraadapter instead of the AP16T will increase the amount of compression and decrease the fully illuminated circle accordingly. (Note, theexample uses the newer ADA20132 with the AP16T. We do not recommend using the older style ADA2013 with the AP16T nosepiece,
but instead suggest the equivalent Baader BP16 nosepiece.)
Please review adapter connection options on the exploded diagram for the 2.7” Photographic-Visual Telecompressor.

The CCD Telecompressor (CCDT67)
The focal length of the CCDT67 CCD Telecompressor is 305 mm or 12.0” Again, using the formula introduced above, a distance of101 mm or 4.0 inches gives the stated 0.67X compression. (305 - 101) / 305 = 0.67 Because the CCDT67’s focal length is much
shorter than the 27TVPH’s, an equal change in distance will have a greater effect on the compression. If we shorten the distance by35 mm as we did above for the 27TVPH, the compression factor becomes 0.78X. (305 - 66) / 305 = 0.78 Changing from 0.67X to0.78X is certainly more drastic than changing from 0.75X to 0.80X as was the case for the 27TVPH. Please note also that theCCDT67’s more aggressive compression makes it less suitable for faster instruments. We strongly recommend using the CCDT67
only on instruments with focal ratios of f/9 and higher.
The CCDT67 takes up 16 mm of the distance in your calculations. The rest of the distance is in the nosepiece, filter wheel, CCD
camera and any extensions you may use. Note also that, unlike the above example with the 27TVPH, the entire nosepiece length is apart of the light path with the CCDT67 since the nosepiece does not get buried inside another adapter relative to the telecompressor.
The CCDT67 has a clear aperture of 44 mm or 1.75”. At 0.67X compression, it will fully illuminate a 29 mm circle - 44 mm x 0.67 = 29mm.
To give a real world example, let’s assume the following: We will set up a system with the CCDT67 @ 16 mm; our AP16T - 2”/T2
nosepiece @ 29 mm; a color filter wheel @ 25 mm; and a CCD camera @ 25 mm. 16+29+25+25 = 95 mm. (305 - 95) / 305 =0.69X
44 mm x 0.69 = 30 mm - the fully illuminated circle.
Please review adapter connection options on the exploded diagram for the CCD Telecompressor.

For All Telecompressors in General
Here are a few more things to keep in mind: The calculation for compression is only valid for systems with fixed focal lengths.
Moving mirror systems like SCT's change the native focal length as they are backfocused resulting in somewhat less compression.
There is no way to calculate the actual compression for an SCT, so the formula only yields an approximation that will indicate morecompresssion than will actually be realized. An accurate compression factor must be measured in practice.
Also, there are limits to how much compression you can achieve with any system: fixed focal length or moving mirror. Compression
increases the backfocus or focuser in-travel requirements of any instrument. Your maximum achievable compression will be
determined by the amount of backfocus your telescope has. You can’t simply add extensions ad infinitum.
Finally, the 27TVPH and CCDT67 do not add any field curvature. In fact, they have a little bit of a flattening effect. However, thetelecompressor takes a larger field and compresses it into a smaller area. If you have a 20 mm square CCD chip, and you arecompressing at 0.75X, you are effectively taking a 27 mm square field ( 20 / 0.75 ) and compressing it onto your 20 mm square chip.
The question to ask yourself is this: Would I show field curvature on a 27 mm square without the telecompressor? If so, then you
may show some on the 20 mm square at 0.75X compression. It won't be caused by the telecompressor, but will simply be yourinstrument's curvature squeezed into a smaller area. To calculate an equivalent uncompressed chip size, take each chip dimension
and divide by the compression factor.
3-12-09

Parts and Backfocus Measurements for the 2.7" Photo-Visual
Telecompressor (27TVPH)
inchesmm

27TVPH

2.7" Photo-Visual Telecompressor (0.75x).

2.250

57.15

ADA20032" Adapter with brass locking ring and 3 large thumbscrews, threads into
drawtube of 2.7" or adapter of 4" focuser.

0.770

19.56

ADA2013

Thin 2" Adapter with brass locking ring and 3 thumbscrews. Not for use
with AP16T. Use when you need 7/16" more in-travel. Threads into
drawtube of 2.7" or adapter of 4" focuser. Note: This information is forcustomers who already have this part. It is no longer available from A-P,
replaced by ADA20132.

0.350

8.89

ADA20132

Thin 2" Adapter with brass locking ring and 3 large thumbscrews. Part isfine for use with AP16T. Use when you need 11/32" more in-travel.
Threads into drawtube of 2.7" or adapter of 4" focuser.

0.430

10.92

ADA2042" Adapter to use SCT accessories and threads into drawtube of2.7" Focuser and Traveler. Has SCT threads on accessory side.
Also threads into adapter of 4" Focuser.

0.300

7.62

ADA2767

6x7 Outer Bayonet for AP 2.7" Thread - attaches 6x7 Pentax,67RT, 67RTEOS, ADA671, ADA672, A1260, A1261, A1262 andcustom adapters to AP 2.7" Thread (including 27TVPH).

0.200

5.08

ADA672

Adapter to attach STL with 5-position Color Filter Wheel to AstroPhysics 2.7" and 4" Field Flatteners and 6x7 Adapters. Includes
four 6-32x1/2 SHCS. Overall length: 1.930"

1.930

49.02

ADA671

Adapter to attach STL with 8-position Color Filter Wheel to AstroPhysics 2.7" and 4" Field Flatteners and 6x7 Adapters. Includes
four 6-32x1/2 SHCS. Overall length: 1.580".

1.580

40.13

A1260

0.5” CCD Adapter for older SBIG ST configurations with gap-mounted
color filter wheel and for use with extensions to create intermediate
spacings. Use with 2.7” or 4” Field Flattener or with 6x7 Outer Bayonet
Adapter.

0.500

12.70

A1261

0.75” CCD Adapter for current SBIG ST series cameras with close
coupled color filter wheel and additional applications where the0.7500.75” length is appropriate. Use with 2.7” or 4” Field Flattener orwith 6x7 Outer Bayonet Adapter.

19.05

A1262

1.75” CCD Adapter for cameras without a color filter wheel. Use
with 2.7” or 4” Field Flattener or with 6x7 Outer Bayonet Adapter.

1.750

44.45

A3503

Thread-on Focuser Extension for 2.7" and 4" Focusers andTraveler. Length is 0.75" to use with Telecompressor (27TVPH)
and ST-8/10 and Finger Lakes Instrumentation CCD cameras(with a CFW).

0.750

19.05

A3504Thread-on Focuser Extension for 2.7" Focuser, 1.25" long.

1.250

31.75

A3502Thread-on Focuser Extension for 2.7" and 4" Focusers andTraveler. Length is 1.75" to use with 2.7" Photo-Visual
Telecompressor and ST-8/10 and Finger Lakes Instrumentation
CCD cameras (without a CFW).

1.750

44.45

A1008-A

Thread-on Focuser Extension for 2.7" Focuser, 2.5" long.

2.500

63.50

AP16T

2"/T-2 Tapered Nosepiece for CCD Cameras, SBIG ST7, Baader
Planetarium Giant Binocular Viewer or any item with T-2 threads.
Fits into 2" openings, 48mm filter threads. I.D. is painted flatblack. Compatible with all compression/locking rings. Only theshoulder contributes to backfocus when used with the 27TVPH.

0.125

3.18

PFCT

Prime Focus Camera Adapter without T-ring.

1.350

34.29

T-RING

T-ring to attach to PFCT or AP16T. Backfocus measurement isapproximate and will vary slightly according to your specific camera's Tring.

0.375

9.53

inchesmm

Parts and Backfocus Measurements for the CCD
Telecompressor (CCDT67)

CCDT67

CCD Telecompressor for SBIG CCD Cameras. Clear aperture is1.75". Compression ratio is 0.67x with SBIG ST7/8/10 + Color
Filter Wheel installed, 0.75x without the Color Filter Wheel.
Suitable for telescopes with focal lengths of f7 to f18 including
Maksutov-Cassegrains, Schmidt-Cassegrains, Ritchey-Cretiens
and Refractors.

0.625

15.88

AP16T2"/T-2 Tapered Nosepiece for CCD Cameras, SBIG ST7, Baader
Planetarium Giant Binocular Viewer or any item with T-2 threads.
Fits into 2" openings, 48mm filter threads. I.D. is painted flat1.150
black. Compatible with all compression/locking rings. Backfocus
measurement includes nosepiece, since the CCDT67 screws ontothe front.

29.21

BP25AT-2 Extension Tube, 15mm long.

0.591

15.00

BP25C

T-2 Extension Tube, 7.5mm long.

0.295

7.50

PFCT

Prime Focus Camera Adapter without T-ring. Note: This part is listed
here for use with CCD cameras without a color filter wheel withapproximate backfocus of 25mm (ie., SBIG ST4000). This combination isNOT recommended for use with a DSLR or 35mm camera.

2.350

59.69