But usually, it's much easier to use a lens at a slower f/ratio than it it was desingned for, than to use it at a faster f/ratio.
For the adaptation to SC-threads or 2"-focusers, please take a look into the manual which you can find in the download section of the website.
In that that case is there any adaptator to be used ?
What if a crayford is used ?
But we are working on a version of the Alan Gee for 2" eyepiece holders like our 2" Clicklock or a regular focuser. We think that it will be available in no lather than early spring (just follow our Newsletter at https://www.baader-planetarium.com/de/newsletter/), it will be more intuitive to use than the regular Alan Gee. This should be the better solution for you as you can easily use it with your 2" focuser.
I religiously followed the instructions from the manual, page 12: I bought the #2454410 centering ring, the short BTA #2408160, kept the two supplied 19mm spacers with the Telecompressor, and used 45.5mm T2 spacers to achieve a 121mm working distance. Surprisingly, that yielded an effective focal ratio of f/D = 5.2, which was way too low (stars in the corner were distorted). I then removed the 45.5mm T2 spacers completely, and got around f/5.6 (still a bit of star distortion but quite less so).
Do you see why the Telecompressor yields so much unexpected reduction? Is the 121mm distance really computed from the Sharpley lens that is on the camera-side, or the one farther in the baffle of the telescope?
With modern CMOS-cameras we see the end of this product coming - since the original SC-optical design does not support airy disc sizes smaller than ~ 8 micron. So no SC-compressoer has a chance to live up to the tiny pixel sizes.
For this reason Celestron had redesigned the SC to incorporate a matched corrector, situated within the optical train at a position we cannot attain with the Alan Gee. So whatever distance you choose - modern imaging devices will mercilessly exhibit the limitations of the system as a whole.
You could e.g. use Baader T Adapter (BTA) for SC and MAK Telescopes (T-2 part #21) # 2408160 and Baader Expansion Ring 2"a/T-2i with 1mm optical path length (T-2 part #28) # 2958242.
But please remember two things: The compressing factor of the Alan Gee depends on the distance to camera or eyepiece, and it is designed to be used inside of the baffle of a Schmidt-Cassegrain, only then it will deliver the best results (which is mainly important for photography). If you want to use it with a 2" star diagonal, we recommnd that you use it as described in the manual between the nosepiece and the body of the star diagonal. If you want to proceed with T-2, we recommend the Universal Alan Gee II - Telecompressor (UAG II) # 2454405 which combines the Alan Gee optics with a 2" nose piece and a T-thread.
Ron DiIulio, Director
University of North Texas, USA
The compressing factor depends on the distance to your camera/spetrograph, more information on calculating this factor can be found in the manual in the download section.
In addition to the Alan Gee & the provided 19mm spacer tubes, I would need:
Baader Varilock 29 (at 20mm position)
Baader Centering ring
Is that correct?
- Centering ring for Alan Gee II Telecompressor # 2454410 to use the Alan Gee at the C925
- Baader T Adapter (BTA) for SC and MAK Telescopes # 2408160
- Baader VariLock 29, lockable T-2 ExtensionTube 20-29mm with spanner tool # 2956929 at ca. 20mm
You can play around a liitle bit with the length of the VariLock to vary the magnification factor, if you want to.
We do not sell these spacer rings alone, but maybe TS has them or even includes them with their Special Edition.
If you have got one of our 2" star diagonals, you can attach it as described on page 7 of the manual, with
Baader Expansion Ring 2"a/T-2i with 1mm optical path length (T-2 part #28) # 2958242
Baader 2" / T-2 Nose Piece and Camera adapter (T-2 part #16) # 2408150
It should also be possible to remove he Clicklock, clamp the UAG in the telescope with Baader Reducing-Ring 2"i / T-2a, with 1.5mm optical length
# 2958244 or Baader T Adapter (BTA) for SC and MAK Telescopes (T-2 part #21) # 2408160 and expand the thread again to SC for the Clicklock with 2"a / T-2i Zero-length reducing piece (zero optical length) # 2454833 - but please always remember that the reducing factor depends on the distance between the Alan Gee and the eyepiece.
Celestron C11 (no edge)
2956233 Baader 2" ClickLock SCL (3.3")
2956100 Baader 2" ClickLock Diagonal Mirror
2956214 Baader 2" to 1¼" ClickLock Reducer (T-2 part #15B) with 31.5 Eyepieces
In the manual it seem need :
2408150 Baader 2" nosepiece to T-2 / M48 (T-2 part #16)
2958242 Baader Expansion Ring 2"a/T-2i with 1mm optical path length (T-2 part #28)
But it's not clear. Can you explain ?
Remove the 2" nosepiece from the star diagonal, then you have access to an SC-thread
Insert T-2 Part #: 28 Baader Expansion Ring 2"a/T-2i with 1mm optical path length (T-2 part #28) # 2958242
Place the collar aof the Alan Gee on the T-2-thread
Secure it with 2408150 Baader 2" nosepiece to T-2 / M48 (T-2 part #16)
Then you can insert the star diagonal as usual into the eyepiece holder of the telescope.
You can change the distance to the eyepiece and by this the reducing factor by adding/removing the included spacer tubes or additional T-2 extensions.
Thanks a lot
Wouldn’t the original Alan gee be much better fit for such configuration? Considering it sits inside the baffle and the stock visual back is used as the locking screw of it.
Also, using your 31/36 aspheric, wouldn’t using the original Alan gee allows to screw the eyepiece to the sct thread with the Alan gee In it’s place? And In general , which is more suitable for C8 with/without focal reducer, the 31 or the 36?
Thanks in advance
A telecompressor can't really give you a wider field of view, it only gives you a wider field of view with the same camera sensor. It is usually better to use an eyepiece with a longer focal length and a larger field stop for visual use than to use a telecompressor.
With a Schmidt-Cassegrain, the inner baffle limits the usable field of view, as it has got a diameter of 37mm. Using a telecompressor behind the baffle (at the back of the telescope, which is the only place where you can put it), it can give you less magnification and a brighter image, but *not* a larger vield of view, as this is limited by the baffle. If you use a telecompressor or an eyepiece with a very large field of view and focal length, there will be vignetting at the edge.
So, you best option would be a 2" star diagonal combined with a long-focal eyepiece like the Hyperion 36; a telecompressoris only useful if you have a smaller camera sensor (like APS-C or smaller) or use 1.25" eyepieces (like in a binoviewer).
Please also take a look at this info from Celestron.com:
I want to use 2 in eyepieces with my SCT and/or Mak. Will there be vignetting from the OTA itself?
If you screw it to a diagonal mirror, will it cover the entire APS-C matrix?
I have a Meade 8 "lx90 telescope. Thank you.
Specifically, I have an AG I want to use again after a pause of a few years. It is currently in a 2" push-fitting without spacers which attaches the telescope to a Starlight AO unit, OAG, filterwheel and 694 CCD. The total from the back glass of the AG to the chip is about 129mm with the most compact connections I've found, or 8mm more than ideal.
I think for this reason, someone once told me to place the AG behind the AO and in front of the OAG/Filterwheel/CCD. However, this seems to me to be less than idea too. At the time, I ended up not using this set-up much in favour of the Hyperstar because the OTA was on a Fork-Wedge combination and quite wobbly. I have now deforked the OTA and put it on a decent EQ mount - so want to try this set-up again.
So my questions are: what is the best configuration? In the back of the optical tube inside the push-fitting with the 129mm to the chip or placed behind the AO and then at the "ideal" 121mm from that point? And what is the relation between the 146mm and 121mm?
The back-focus of an SC of about 146mm is the distance from the telescope's rear cell to the focal plane if there are no other lenses. If you use the Alan Gee, you place it in the desired distance from your camera (to achieve the desired compression ratio) and then focus with the telescope. Moving the main mirror changes the focal ratio of the SC, the data given in the manual refere to the resulting focal lengths when the Alan Gee is mounted as described in the manual.
But please note that the Alan Gee was designed years ago for Schmidt Cassegrains and not for the already flat field of an EdgeHD. For photographic use with an EdgeHD, we recommend the original Celestron reducers which were designed especially for the EdgeHD-optics.
The Alan Gee works fine with classic SCs, where it flattens the field and suppresses coma over a field of ca. 18mm - but with larger chips and smaller pixel sizes, this system is pushed to its limits. It is not recommended for pixel sizes smaller than 6-7my.
But for both EdgeHD and Schmidt-Cassegrains, the Alan Gee is a very interesting product for use with binocular viewers like the MaxBright II, because its working distance matches the back-focus of a binoviewer very good. Furthermore, the optical system of both Schmidt-Cassegrains and EdgeHD provides such a large back-focus, that the Alan Gee can almost double the field of view on most Celestron Schmidt-Cassegrains without visible vignetting. In the course of the development of the MaxBright II binoviewer, we have also developed a new T-2-cell for the Alan Gee. This new product - the AG Universal II - is designed for visual use with binoviewers and works even a little bit better with EdgeHDs, as they have a longer back-focus.
The AG II Universal as special reducer for binoviewing (but not for photography) will come to the market some months after the MaxBright II. If you are interested in binoviewing, we'd recommend not to buy the standard Alan Gee, because it deosn't offer the same connection options as the Alan Gee Universal. The Alan Gee Universal will be mounted directly between the binoviewer and the T-2-star-diagonal. We are looking forward to present this new product in 2020.
The UAG-II can be used with a binoviewer at an EdgeHD, because it still works fine with the relatively small image circle of a binoviewer (1.25") even on an EdgeHD. But we do not recommend to use it so far away from its optimum position with a camera, where you will notice any aberrations further away from the center of the field of view when you look at the image. Even with a binoviewer, you shouldn't add the additional light path of an Off-Axis-Guider.
For photography with an EdgeHD, you should use the original Celestron reducers specifically designed for the EdgeHD - then you have a system which is optimised to work.
Also, If I want to use the Alan Gee telecompressor for Prime photography using Barlows / Eyepiece Projection, do I need to get additional adapters?
We have described them in the manual at https://www.baader-planetarium.com/en/downloads/dl/file/id/213/product/1197/baader_alan_gee_ii_telecompresor_shapley_lens_f_5_9_manual_and_applications.pdf
To use it with any of our 2" star diagonals, you need (as shown in Fig. 3 on page 2 of the manual) to remove the 2"-nosepiece from the mirror and replace it with
1) Baader Expansion Ring 2"a/T-2i with 1mm optical path length (T-2 part #28) # 2958242
2) the Alan Gee (you may have to remove the two extension tubes to achieve the best distance. The 2" Clicklock mirros has got an optical length of 112mm, so you probably have to remove both)
3a) Baader 2" / T-2 Nose Piece and Camera adapter (same as used by SBIG) (T-2 part #16) #2408150 if you want to insert the star diagonal into a 2" eyepiece holder or
3b) Baader T Adapter (BTA) for SC and MAK Telescopes (T-2 part #21) # 2408160 if you want to screw it directly onto your C9 1/4.
If you want to insert the Alan Gee into the telescope itself, you need the Centering ring for Alan Gee II Telecompressor # 2454410 (for C9 1/4), then you can lock it in its position with the standard visual back of your telescope.
I would not recommend combining the Alan Gee with a Barlow lens - you would use one optical element to shorten the focal length and then another one to extend it again. You'd be better of if you use instead one Barlow and adapt its magnification. The Q-Barlow 2.25x / 1.3x # 2956185 e.g. is designed for 1.3x or 2.25x, depending on where it is mounted, and the VIP 2x modular barlow lens, visual and photographic # 2406101 is designed for magnifications of 2x and more, depending on the distance. For calculating the different Magnifications with the VIP-Barlow, please check this PDF: https://www.baader-planetarium.com/en/downloads/dl/file/id/240/product/1062/calculating_different_magnifications_with_the_vip_barlow.pdf
If you are interested in visual use, we'd recommend instead the Universal Alan Gee II - Telecompressor (UAG II) # 2454405. This contains the same optics as the Alan Gee, but it is preconfigured for easy use with a T-2-prism and a binoviewer or an T-2 eyepiece clamp as shown in the manual at https://www.baader-planetarium.com/en/universal-alan-gee-ii-telecompressor-uag-ii.html You just need to screw it to the back of your telescope, or insert it into a 2" eyepieceholder, if available.
For purchase in/delivery to the United States, please contact our distributor www.alpineastro.com.
EAN Code 4047825011572 Manufacturer Baader Planetarium SKU (#) 2454400 Net weight (kg) 0.09 Optical length (mm) < 50 AR-Coating Multi-Coated (MC) Usage 8" Schmidt-Cassegrains Optical Design Field Flattener, Telecompressor compatible telescopes Schmidt-Cassegrain
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