Interesting facts
The use of binoviewers :
As already mentioned I think that the viewing of the moon and planets is greatly improved when viewing with 2 eyes through a binoviewer, the CA is also reduced.
Experience has shown me that the Denkmeier binoviewer equiped with the Power x switch and the OCS mounted on a Wiliam Optics 2" diagonal mirror reaches focus on all the FrT telescopes in all 3 magnification settings. Unfortunately he seems to give some annoying reflections when using the middle and high magnification settings of the Power x switch (from the information found on different forums this seems to be a general problem with all high F-ratio instruments, so not just on my models).
So the use of the Denkmeier is limited to the lowest magnification setting, you need to put in eyepieces of shorter focal length to raise the magnification.
I have found the Baader Mk V binoviewer to not have that particular problem with certainly the 1.25x and 1.7x glaspath correctors (I haven't tried the 2.6x corrector).
Mounting long focus refractors :
It is undoubtedly true that a long focus refractor demands a bigger mount than short focal length refractors. In fact, it is often said that such a refractor combined with the required mount will cost the same or even more than an short focus apochromatic refractor/mount combo. But, apart from the fact that most people buying a top quality APO refractor rarely put them on a cheap mount, I find that the so-called required size and quality of the mount is frequently overexagerated.
As already mentioned I think that the viewing of the moon and planets is greatly improved when viewing with 2 eyes through a binoviewer, the CA is also reduced.
Experience has shown me that the Denkmeier binoviewer equiped with the Power x switch and the OCS mounted on a Wiliam Optics 2" diagonal mirror reaches focus on all the FrT telescopes in all 3 magnification settings. Unfortunately he seems to give some annoying reflections when using the middle and high magnification settings of the Power x switch (from the information found on different forums this seems to be a general problem with all high F-ratio instruments, so not just on my models).
So the use of the Denkmeier is limited to the lowest magnification setting, you need to put in eyepieces of shorter focal length to raise the magnification.
I have found the Baader Mk V binoviewer to not have that particular problem with certainly the 1.25x and 1.7x glaspath correctors (I haven't tried the 2.6x corrector).
Mounting long focus refractors :
It is undoubtedly true that a long focus refractor demands a bigger mount than short focal length refractors. In fact, it is often said that such a refractor combined with the required mount will cost the same or even more than an short focus apochromatic refractor/mount combo. But, apart from the fact that most people buying a top quality APO refractor rarely put them on a cheap mount, I find that the so-called required size and quality of the mount is frequently overexagerated.
When I read for example that people find a Vixen GP-DX mount way too light for a 100 mm F12 refractor I wonder why they say that. I guess that in general when people talk about a mount they talk about the mount/tripod combination and not about the mount itself. I can indeed understand that the said Vixen mount with the standard tripod is not sufficient for a 100 mm F12, it is also not tall enough. If however you put that same mount on a taller and rigid tripod, or even better on a steel or concrete column it will carry that telescope with ease. Bearing in mind that that Vixen mount can be found on the used market for as little as 300-400 Euro, there is no reason to think that a long focus 100 mm refractor is nearly impossible to mount unless on a hugely expensive mount.
The same goes for 125 mm F12 refractors and to a lesser extent for 150 mm F12 refractors : they can be easily carried by for instance a Losmandy G11 mount which in my opinion is probably the best budget mount/tripod combination on the market for visual use of such refractors. If you put that mount on a sturdy steel or concrete column you will experience little vibrations, unless under windy conditions.
FrT127 on Losmandy G11 mount/tripod with 12" column extension.
F12 or F15 ?
A lot of people seem to wonder about this : is it worth it to go to F15 for the best image? I asked myself that same question for a long time so when an add came up for a used Vixen 100 mm F15 I jumped on it.
First viewing was with the telescope as is with the original Vixen focuser (adapted by a previous owner to accept 2" accessories), on my big observatory mount.
Jupiter was ideally situated in the sky so it was obviously my first target. With a 9 mm Nagler at x167 I was immediately struck by the sharp contrasty view of the cloud bands. There was of course quite some CA visible but, as I know from experience, putting in my binoviewer would reduce that CA to a much lower level. Unfortunately, I could not reach focus because of the 2" adapter mounted (glued?) on the focuser.
Since I had a single speed 2" Moonlite focuser lying around I decided to mount that on the tube but an appropriate flange had to be ordered.
A pitty really because the original focuser was certainly more than adequate for the job (but non-rotating. which was not so practical)
Since I had a single speed 2" Moonlite focuser lying around I decided to mount that on the tube but an appropriate flange had to be ordered.
A pitty really because the original focuser was certainly more than adequate for the job (but non-rotating. which was not so practical)
I used the time to also order some new tube rings and dovetail to complete the 'make-over' of the telescope. You can see the result on the picture on the left with the telescope mounted on my G11. It looks like a new, modern telescope !
Soon after, I was able to observe Jupiter with this setup and my Baader Mark V binoviewer and 11 mm and 15mm Televue eyepieces with which I was now able to reach focus.
Wooow, the view was really wonderful with very little CA still visible, in my opinion certainly not enough to have much influence on the details visible.
Compared to my FrT1OO F12 telescopes there is a little less CA visible and the level of contrast seems to be a small tad higher (that could also be the quality of the lens of course which seems absolutely first class) but the same details are visible in the F12 telescope.
All in all I would say there is very little difference between them : some less CA, somewhat easier to focus... , on the downside : a bit longer and more difficult to mount, more sensitive to wind, a smaller widest field... Personally I like them both and use them both depending on the mood I am in, but I would say that F12 is really the most sensible compromise for a long focus achromat.
But, if you are a long focus refractor nut and have the opportunity to buy one used for a good price, by all means just go ahead. Just be aware of the disadvantages above and probably the presence of a non-rotating original focuser, if you are used to a modern rotating focuser that can be an annoyance.
A new mount for my big refractor
Some time ago I reacted to an add for a used AOK Swiss mount for a very reasonable price (1000 Euro). Included was also a wooden AOK tripod, that showed the necessary tear and wear as the next picture shows.
It was very sturdy and the height can be changed but that takes a lot of time and at the maximum height the stability is not really good enough, maybe because of the wear and deformation of the wood. So I decided to make a tripod with fixed length wooden legs using the hardwood Merbau (which is amongst the most rigid woods you can get) and the hardware components coming from the tripod.
The result is a nearly 1,80 m high extremely sturdy tripod (not really a lightweight though) that took only a few hours to make.
The result with the mount and my 150 mm F12 telescope is shown below. It doesn't really show on the picture but it is really BIG, the fulcrum of the mount must be at about 2.10 m height. There seems to be some play on the drive axis of the mount itself but a little desequilibrium can solve that.
This is a good example of how to mount your big refractor using an older type mount that can be purchased on the used market for a reasonable price. A mount that is no longer desirable for the astronomer that needs an accurate GOTO and very precise gears for critical guided photography but that has all that is necessary for visual observing with a long focus refractor (it also has slip clutches which are really practical for such long tubes).
FrT150 on AOK mount and tripod.
Front view of the FrT150 on AOK mount
