Telecompressor Alan Gee II (Shapley-lens f/5,9) Especially suited for use with CCD cameras and Baader binoviewers Manual Spacer Ring Flanged Endring Spacer Tube Spacer Tube Lens Cell Fig. 1) The desire for two telescopes in one is old, but due to the laws of optics, all approaches to it must be a compromise. A telecompressor should illuminate the 24x36mm picture format as well as possible (= to minimize vignetting), and it should avoid image distortion especially the inevitable astigmatism. You should also be able to use it with accessories like star diagonals or binoviewers which have a long optical path. The standard compressors of the major manufacturers are designed for a rather short distance to the sensor/film and are not suitable for use with binoviewers. You can reconfigure the Alan Gee II telecompressor to adapt it to different optical accessories. For photographic application, you may have to insert the multi-coated achromatic lenses into the baffle tube of the telescope, or for the visual use with a binoviewer mount the lens carrier together with the lens holder immediately in front of the entrance aperture of a binoviewer. The construction takes two conditions into account: To make maximum use of the baffle s diameter, and to keep the lock secure. Despite the resulting thin housing, the individual tubes are securely connected by fine threads. The optics were designed specifically for Schmidt-Cassegrain telescopes according to the recommendations of Alan Gee and Roland Christen. The two air-spaced lenses have a focal length of 259mm and a flat field. To achieve the best correction, they should be placed according to theory at no more than half of that focal length away from the focal point of the eyepiece or the image plane of the camera. The ideal distance between the camera-side lens and the focal point (field stop / camera sensor etc.) is 121mm, according to Roland Christen. Because of this, there are different combinations of the spacer tubes for each use. Calculation of the compression-factor M according to Michael Covington: Astrophotography for the Amateur. Telecompressor focal length F 2 = 259mm Flange focal distance for DSLR-cameras with T2-adapter (all manufacturers): 55mm Distance between rear lens of the telecompressor and T-ring with T-adapter and all spacer tubes according to Fig.2: 66mm This results in a distance S 2 of 121mm between the rear lens of the compressor and the camera sensor. The (negative) elongation factor M is calculated as follows: M = (S 2 -F 2 ) / F 2 = (121-259) / 259 = 0,533 The minus sign can be ignored for the further calculations. If you focus with the inserted telecompressor, the focus of the telescope must be moved relatively far to the outside, by the distance S 1. The calcalutaion is: S 1 = S 2 / M = 121 / 0.533 = 227mm. This value is added to the focal length of the telescope. A Celestron 8 with f=2030mm for example will have a resulting effective focal length of 2257mm. This would mean an effective f-stop of 2257/203 = 11.1 for the C8 without telecompressor. With the telecompressor, it has got an effective f-stop M eff = 11.1 x 0.533 = 5.9 (= focal ratio). This value agrees well with the values measured directly at the C8. The shortening of the exposure time results from M 2 = 0.35, that is about 1/3.
1. Visual Use with a 1,25" Star Diagonal 1.1 Insert the telecompressor without the second extension tube completely into the C8 baffle tube 1.2 Insert the white plastic spacer ring into the filter notch at the telescope s adapter thread 1.3 Attach the visual back at the telescope, as usual 1.4 Insert the star diagonal into the visual back and secure it with the locking screw 1.5 Now you can use any 1,25" eyepiece, as usual 2. Visual Use with the 2" Maxbright Universal Star Diagonal 2" Eyepiece 1,25" Eyepiece 2" 1,25" reducer Standard-position of field stop 1,25" Eyepiece Standard-position of field stop 2" Eyepiece T-2(a) T-2(i) If you use 1,25" eyepieces, you should remove the spacer tubes to achieve the perfect focus point. 2" Maxbright universal star diagonal Extension ring T-2/2" AG II Short C8 T-adapter for every 2" SC-adapter thread 3,3" nut for C11/C14 (and Meade) If you use 2" eyepieces, you should use both distance tubes to achieve the perfect focus point. 2" / T-2 adapter and 2" filter holder 2" Deluxe eyepiece holder (for all SCs) Fig. 3) In combination with a Baader 2 ClickLock star diagonal, you can mount the Alan Gee II Telecompressor directly in front of the mirror between a T-adapter (#21) and a reducer ring (#28). For mirrors by other manufacturers, you may have to mount the Alan Gee II closer towards the baffle of the telescope, which may reduce the field of view. Do not use eyepieces with a focal length of more than 40mm.
#16 #26 #28 You may have to remove these two spacer tubes to bring the lenses of the Alan Gee to the perfect position. To achieve the best sharpness, the lens facing the camera should be ca. 120mm away from the field stop of the eyepiece or the sensor of the camera. Examples for fields of view: Visual use of the AG with binoviewers Baader Zeiss binoviewer on 8" SC telescope C8, connected with 16mm T-adapter + T-2 prism + 40mm T2 tube. Telecompressor lens with flange mounted directly in front of the Bino (only in this configuration the focus can be reached)! True field measured with the drift method: 2x35mm Baader-Eudiascopic eyepieces: effective field size is 1 12' ( without Alan Gee II telecompressor: the field size only measures 30'). Actual field gain is: 576%! Baader Zeiss binoviewer equipped as above and attached to the Celestron 14: True field measured with the drift method: 2x35mm Eudiascopic eyepieces: 42' ( without Alan Gee II telecompressor: 23'). The effective gain of field amounts to: 333%! For comparizon only: older 60 Baader Bino on 8" SC telescope connected with 16mm T-adapter. Telecompressor lens mounted (with this 16mm T-Adapter) immediately prior to the Bino; only in this configuration, the focus can be achieved! 2x30mm Eudiascopic eyepieces: 1 ( without Alan Gee II telecompressor: 35'). Field gain: 309%! 60 Baader Bino connected to the C14 with 16mm T-adapter (same way as to the C8); only in this configuration, the focus can achieved! 2x30mm Eudiascopic eyepieces: 33 ( without Alan Gee II telecompressor: 18 ). Field gain: 378%! 3. Visual Use with the 60 Baader Binoviewer 3.1 Attach the short T-adapter at the rear side of the telescope. 3.2 Insert the Alan Gee II into the T-adapter until it stops. 3.3 Attach the binoviewer at the T-adapter. The telescope should face a bit downwards when you mount Alan Gee telecompressor and binoviewer this way. T-2/T-2 inverter ring holds the AG II, if the bino is used with the 2" eyepiece holder 60 Binoviewer (with rotatable 2" sleeve) for refractor telescopes Fig. 5) Alan Gee II (may require a shortened tube when used with a refractor) Short SC- T-Adapter (240 8160)
4. Visual Use with the Baader Giant-Binoviewer Remove Glasspath-corrector T-2 Quickchanger Short C8 T-Adapter for every 2" SC-thread Giant binoviewer Fig. 6) AG II without spacer tubes T-2 spacer tube T-2 star diagonal 5. Photographic Use with a (D)SLR (D)SLR-Kamera Alan-Gee II standard configuration T-ring Short C8 T-Adapter Plastic spacer ring Fig. 7) Setup (potographical use): Remove the visual back from the back of the C8/C925. In case of the C11/C14, the reducer from 3" to 2" thread remains at the back of the telescope. 5.1 Insert the Alan Gee II into the baffle of the telescope until it stops. 5.2 Insert the white plastic spacer ring into the filter notch at the telescope s adapter thread. 5.3 Attach the short T-adapter with the 2"-coupling-ring at the telescope. 5.4 Attach your DSLR with a matching T-adapter to the T-thread of the short T-adapter.
6. Photographic Use with a CCD-Camera CCD-Cameras ST-4 and ST-5 with Alan Gee Telecompressor at the C8 Configuration #1: Attach the CCD-camera without the telecompressor directly at the (Celestron) 1,25" visual back. The focus is reached at F eff = 1790mm. This results in (1790/203)= f: 8.8. Configuration #2: The complete Alan Gee II telecompressor (with both extension tubes and the white spacer ring) is inserted into the baffle and held in place by the visual back. The focus is now at F eff = 1210, which delivers a focal ratio of 1210/203 = ca. f: 6.0. Configuration #3: The Alan Gee II telecompressor is equipped with only one spacer tube and mounted in the baffle with the 16mm- T-adapter. Attach the CCD-camera at the T-adapter. We recommend our (#6) T-2 quick changer for this. It makes several aspects of the work with a CCD-camera easier. The focus is now at F eff = 1175, which results in a focal ratio of f: 5.8. CCD-Camera (ST-7/8/9/10) CCD Adaptation Proposal with T-2 Quickchanger T-2 extension tube 15mm Short C8 T-Adapter for every 2" SC-thread T-2 Changer & Change Ring Spacer Ring for holding the AG II in the filter notch in the 2 SC-thread You can easily use another Quickchange Ring combined with a Deluxe Eyepiece Holder to control the field of view! 1,25" Eyepiece T-2 Quickchange- Ring 1,25" Eyepiece holder T-2 Deluxe Fig. 8) Using the AG II at SCs with 2" Eyepiece Holder #26 3,3 nut for C11/C14 and Meade (larger than 12 ) (D)SLR-Kamera T-ring #25C #16 #20 2" Deluxe Eyepiece holder for C8, C925 and Meade Fig. 10)
Important: Focusing a Schmidt-Cassegrain Please note that with all Schmidt-Cassegrains the main mirror has to be focused upwards! This is the only way to make sure that the weight of the mirror is on the upward facing side of the focussing spindle! Otherwise, the mirror will shift downwards (there are no threads without tolerances) while you re taking a picture, and the stars will gradually loose sharpness. If you turn the focuser the right way (counterclockwise), you ll avoid this mirror-shifting. Simply get used to do the last focusing steps always from the correct side, then mirror-shifting will be no problem, and the object will not move on your camera sensor. General information: The field curvature which is typical for all SC-telescopes will be further reduced if you use a telecompressor but it will not disappear completely. You will achieve the best sharpness for visual observation if you use the center of the image for focusing at first. Turn the focuser counterclockwise (See the box about Focusing a Schmidt-Cassegrain above). Once you see a sharp image, turn the focuser for another 1/12th of a turn in the same direction. For photography, you ll achieve the best focus if you rotate the focusing knob counterclockwise until you see a sharp image in the viewfinder or display of the camera. Then move it for another 1/24th of a turn in the same direction. If your are observing at high magnification or taking photos of the planets, please remove the Alan Gee II telecompressor. As long as you don t intend to do planetary work the AG II may remain mounted at the telescope permanently. In this case, it also works as a seal for the tube assembly. The best 1 1/4" eyepiece for rich-field-observation is the 35mm Eudiascopic eyepiece (indentical in construction with the (long gone) 35mm Celestron Ultima eyepiece). It has the largest field stop which can be used in 1,25 eyepieces.
Locking Screw (or Locking Ring) Celestron C8 with Alan Gee Telecompressor II Fig. 11)
5. Fixed mounting in the BTA Adapter The Alan Gee can also be mounted permanently inside of the BTA Adapter (#240 8160). This is necessary e.g. for Schmidt-Cassegrains with a groove, and it makes the handling of the Alan Gee easier: You can simply screw it on your telescope. To do so, you have to disassemble the BTA Adapter. A small screwdriver is helpful. 1.) Remove the inner ring of the BTA Adapter. The easiest way to do so is fixing the inner ring with a screwdriver which you insert into one of the holes, and then rotating the lower ring to unscrew it (left image). Now the adapter is disassembled into three parts: The lower ring with make T2-thread, the coupling ring and the inner ring (image below, from left to right). 2.) Now place the Alan Gee in the lower ring. You do not need the white spacer ring for this. 3.) Put the coupling ring onto the lower ring. At the moment, it is not fixed. 4.) Finally insert the inner ring into the adapter and tighten it. This a little bit tricky because you can t grip it. The best way is to put it into the Ultrashort adapter and hold the lower ring of the Ultrashort adapter in its place. Now rotate the inner ring with the help of a screwdriver slowly and without pressure, until the threads fit. Now you can lock it it works quite fine if you hold the inner ring in place with the screwdriver and rotate the lower ring. From now on, you can easily srew the Alan Gee on your telescope. The Alan Gee is placed ca. 12 mm farther to the outside than if you insert it directly into the telescope.