Baader Alan Gee II telecompressor
Posted 28 March 2012 - 04:00 PM
Tonight I had first light on my new (used) Alan Gee II telecompressor from Baader Planetarium, used on C8 and Maxbright bino.
More details tomorrow.
Posted 29 March 2012 - 01:00 AM
Posted 29 March 2012 - 01:07 PM
The AG is part of the Baader T2 system, so it can be configured in myriad ways and be adapted to almost anything, but it is primarily designed to be used with SCTs and Maksutovs. The lens diameter is about 35mm and my first impression was: "There's going to be vignetting - lots of it!" Well, I was wrong!
Since I didn't have the 40mm T2 extension tube the AG must normally be fitted into, when installed in front of a Baader binoviewer, I had to improvise. The T2 system is a bit like playing with LEGO and I was really good at playing with LEGO bricks as a kid. The AG must be installed just in front of the binoviewer, in order to get the working distance right. It should be close to 120mm. I put the AG in a T2/2" nosepiece and screwed it on the Maxbright. Then I put a M48/T2 adapter ring in the bottom of the 2" nosepiece, threaded a male quick-coupler ring into the M48/T2 adapter and inserted the whole shebang into the female quick-coupler on the T2 Baader/Zeiss diagonal, which was attached to the C8 with a Baader T2/SCT adapter. Whew! A Baader T2 tour de force!
I think you can guess what the weather turned to, once I had the system up and running...
But much to my huge surprise, it suddenly cleared a few hours after sunset. Even though it was getting late, I was very eager to test the system, at least just to check the focus and sharpness. The Moon hung high in the west in an already dark sky. I inserted a pair of 20mm 70* AFOV GSO Superview eyepieces and took a first peek. I was delighted to find that the AG came to focus with no issues and I was even more delighted when I began to inspect the image. Not only was it sharp, it was also wonderfully free from ghost images and reflections. The view of the crescent Moon was extremely superb. The field was just under a degree across, but I have yet to determine exactly how wide it is, so that I can calculate the exact magnification. There was not the slightest hint of any vignetting in the 20mm GSOs. None. I also tried 15mm and 10mm GSO eyepieces and they performed splendidly with the AG in front of the Maxbright. In fact, the new focal length, and correspondingly lower magnifications, are much more in agreement with my local seeing than the original, higher magnifications of the uncompressed C8 were. With the AG, I reach approximately 75x, 100x and 150x with the 20mm, 15mm and 10mm GSO Superview pairs, very useful magnifications. With the uncompressed Maxbright, I used to be at approximately 110x, 147x and 220x. That's getting a little too much for lots of deep-sky work and the fields are getting uncomfortably small on many objects. As a result, I turned to my 150mm f/8 refractor for much of the time, because it delivered much wider fields of view, framing the deep-sky objects in a way the C8 couldn't. Until now.
I turned to M35 and immediately fell in love with the new capability of the C8. The field was much, much wider than I was used to and the view exquisite. Not sharp to the edge, but fairly compact stars, and with an overall comfort that was hard not to just love. I don't know exactly what the AG do to the eyepieces, but it does it extremely well. The views with the 20mm GSOs were superb. I looked at many objects, NGC 2903, Leo triplet, M81/82, M51, M3. The views were stunning, despite the light from the Moon. The SQM said 20.35.
All in all, I am extremely happy with it already. I can't wait to check out some deep-sky objects with it under darker skies.
Posted 29 March 2012 - 02:57 PM
I don't have any 2" eyepiece to test this so if you can make such a test with a 30mm 2" wide field eyepiece please let me know the results.
Celestron 0.63 reducer is no match for the versatility of the Alan Gee in my opinion. A C8 + Alan Gee must be very close to a C8 EdgeHD image quality (except coma probably) for visual observing and at a much lower price considering EdgeHD reducer's price too.
Yes, the T2 system is like a Lego but what a useful Lego!
You can also use the Alan Gee + Baader Glass Path Corrector + Maxbright bino. This is an unusual combo I have tested.
With the Alan Gee stars seem rounder and brighter and are more pleasing to view.
I haven't encountered yet any vignetting with the Alan Gee in visual mode or DSLR mode considering the 35mm field stop.
Posted 29 March 2012 - 07:25 PM
I considered the Glee, but my concern is that the back focus required for the telecompressor will be pretty extreme, causing the C8 to perhaps work at only about 150mm of effective aperture.
I am hoping that you have followed that thread and will consider trying the test with the Glee in place.
I have studied the ray traces for the C8 and C14, and I cannot for the life of me see how it would be possible to use the Glee for a binoviwer and not loose considerable effective apetrure.
I think the only time the SCT can use the Glee and not suffer from aperture reduction would be to use it in the baffle tube with a 1.25" diagonal and eyepeice.
Anyway, I am hoping I can get you to try the test to see if there is significent effective aperture reduction.
Posted 30 March 2012 - 04:11 AM
Yes, there's some reduction in effective aperture. Using a green laser pointer and a ruler, I measured the effective aperture with the AG in place to be ~182mm. Removing it, but without refocusing (the scope was focused accurately from last observing session to the exact eyepiece used in the test), I measured the effective aperture to 203mm.
Shouldn't the aperture still be reduced, if I didn't refocus the scope, since the reduction comes from the primary mirror being too close to the secondary? I think I am doing the test correctly. I have used it before. It is a wonderfully simple and accurate test.
EDIT: I tried looking through the bino with no eyepieces in it and the reduction in aperture is directly visible. You have to move your eye around a little to see the whole primary mirror reflection. If I remove the AG, I can see the whole mirror, without having to move my head. Apparently the AG acts as a sort of magnifier, which magnifies the image (it's a positive lens) of the primary mirror enough, that the image won't all fit through the free aperture of the AG. That sounds odd, since we're talking about a focal reducer, but optics are weird like that.
I'll try some more experiments, to see if I can increase effective aperture.
EDIT II: Tried putting the AG in front of the diagonal and the bino in the diagonal. Now effective aperture was down to 165mm! Not the way to go. The reason for the reduced aperture seems to be that the baffle tube is no longer wide enough, since the system is now effectively f/6something instead of f/10. The baffle tube either needs to be wider or shorter, in order to accept the fatter light cone. Perhaps the secondary also needs to be bigger.
Posted 30 March 2012 - 09:05 AM
It is very hard to use a telescompressor and not get effective aperture reduction in an SCT. Even if the aperture of the telescompressor is rather large, if put it too far away from the baffle, or if you put the focal plane to far behind the focal reducer, aperture loss is going to occur.
And of course the central obstruction goes up by percentage.
I estimated that the C14 was working as an 11" scope with the low power arm of the Denkmeier supersystem. Also, I think the edge of field illumination even with low power eyepcies was only about 30%.
My guess is the Glee would buy me a little but I would still be using a 12" scope with poor off axis illumination.
No way out of this box in a standard SCT without given up some aperture, and I am not willing to do that personally.
I love the Maxbrights for planets though.
Thanks for taking the measurement....
Posted 30 March 2012 - 09:24 AM
Tried putting the AG in front of the diagonal and the bino in the diagonal. Now effective aperture was down to 165mm! Not the way to go. The reason for the reduced aperture seems to be that the baffle tube is no longer wide enough, since the system is now effectively f/6something instead of f/10. The baffle tube either needs to be wider or shorter, in order to accept the fatter light cone. Perhaps the secondary also needs to be bigger.
Yes, Baader says that the Glee should only be used in the central baffle for imageing.
I am not sure that having a bigger aperture would make a difference. I think it is just the way the reducer works.
For example, the standard SCT telescompressor mounts directly to the rear cell, and it has 38mm of clear aperture, but you will still see effective aperture loss as soon as you go back past about 110mm of back focus in a C8.
So, making the aperture of the reducer larger would not improve anything.
And of course makeing primary baffle larger would mean that it has to be shorter, which means the secondary baffle would have to be deeper or wider to properly baffle the system.
At f/10, there really is only a very small range of baffle geometries available to still have a "general use" telescope.
This locks us into a pretty hard and fast maximum true field. Lots of people don't believe this, but now that there is an easy test to use, maybe more people will figure this out for themselves.
Did you refocus to infinity when you put the Glee further up? I think had you done this, some of your apetrue would come back...
But not all of it.
Posted 30 March 2012 - 11:12 AM
When doing the 'flashlight' aperture test, to be meaningful the eyepiece must be at the focus for whatever configuration the system is in. You cannot simply relocate the eyepiece while leaving the moveable primary in place; the system needs to be refocused accordingly.
The only reason you're losing aperture is because you are forced to push the focus well back of the nominal position. The inner end of the primary baffle becomes the aperture stop. And this has nothing whatsoever to do with the new, effectively smaller f/ratio; that's only occurring behind the reducer, and as far as the scope's concerned it's still f/10 (actually longer, due to the extended back focus.)
Posted 30 March 2012 - 03:54 PM
Also, I took some more measurements. I found an extension tube roughly the same length as the binoviewer+AG and put it into the diagonal and measured the aperture: 203mm. This was with the system focused for the AG, but not with it in place, so the eyepiece was not in focus. It seems that at the position of the primary used when observing with the AG, the whole aperture should be available, if not the AG was used.