This note is about mounting lenses on an OTA. In this case all lenses are mounted on a 10" LX200 using Losmandy rails and accessories. The basic ideas are conveyed in the photographs and comments. There are of course many ways to get the rigidity needed and one does not have to use the Losmandy system. I find it so versatile however that I strongly recommend the Losmandy system. The real issues are to take care that the mounting is rigid, free of not only flexure but of vibration as well. In some cases the 10" would in my opinion be overloaded with the lenses shown. But, I have modified the 10" dec bearings so they operate smoothly even with rather heavy loads.
The photographs also show the telescope mounted only in the Alt/Azm mode. (This is only for convenience in photographing the equipment in my basement laboratory.) This is not a big issue for the photographs, but the mounting of large lenses in the Polar mode requires careful balancing of the entire structure. This is sometimes not easy with what are rather large and asymmetrical loads but it can be done. For long exposure piggy back photography of course one needs to use the Polar mounting mode.
I might add, that I have been generally satisfied with the mounting methods for all of the lenses except for the 400 mm f2.8. This lens weighs 11 pounds and really overloads both the 10" and 12" LX mounts. General dissatisfaction with mounting heavy lenses and cameras on the OTA is one factor that has lead me to design a new mount to be used entirely for photography and imaging.
Below left is shown a 200 mm f 2.8 lens with the camera body mounted directly on the Losmandy camera adapter plate DCM. This is a satisfactory mounting provided that the camera body is strong and has a very flat bottom so that it mounts without rocking motion on the plate. Some cameras, like the Canon shown are very good in this respect while others are not. I would not recommend mounting a lens longer that a 200 mm in this way.
On the right is shown a Canon 200 mm f 4.0 lens with the lens mounted on the
adapter plate and the body mounted on the lens. Note that is this case,
the lens itself has a very fine rotating mount so that the camera can be held
in vertical or horizontal positions. This way of mounting a good quality
200 mm lens in fine for piggy back photography. Again the mounting requires
a mounting ring with a generously sized base plate and one that is flat so it
does not rock. The Canon lens is very well designed and built in that
respect. Interestingly, the 200 mm f 2.8 lens which
is shorter and heavier does not have the integral mounting ring. It was
designed as a high speed hand held lens while the f 4.0 lens is an older and
possibly more conservative design. Two reasons for the integral mounting
ring are to balance the lens/camera better and to reduce stress on the camera
body mount for long heavy lenses.
It should also be noted that Losmandy makes a second camera mounting adapter called the DCM1. This plate holds the camera higher off the rail and it has an adjustable pivot to allow the camera to be adjusted vertically. This bracket is useful if one wants to mount much shorter focal length lenses or even wide angle lenses. Such lenses are of such light weight that they are easily held by the camera body and taller Losmandy plate.
Below is shown a 300 mm f 4.0 lens mounted again on its own integral mounting
ring on the DCM mounting plate. The camera is shown in two positions.
One of the nice features of the integral mounting ring is the rotational adjustment
provided. This mounting is again quite good, but getting to the point
that consideration might be given to an additional mounting ring to strengthen
the flexural integrity of the mounting system. The lens and camera are
together very well balanced with this combination.
As one moves to longer, larger and heavier telephoto lenses, it becomes more
and more important to pay attention to flexure of the lens with respect to the
optical tube which it is assumes is being used to do the guiding. The
400 mm f 4.5 lens shown directly below is a fine example of optical design and
has its own integral mounting ring. Notice that the ring is forward enough
to perfectly balance the lens/camera combination on the mount. While excellent
for normal photography, this is a lens that really should have added to it a
bit more stability.
Shown immediately below is just such a simple means to stabilize the long heavy
400 mm lens. One can now see the advantages of the Losmandy rail system.
In an instant, one can add a medium sized mounting ring at the front of the
lens because the rail extends the full length of the OTA. This assures
that the lens will not flex during long exposures. I have easily taken
one hour exposures with this setup that show no differential flexure between
the lens and the OTA.
Then there is the monster, and a monster it is. This is the 400 mm f
2.8 Canon telephoto. All 12 pounds of it. Frankly, this lens is
almost too large and heavy to mount on an LX OTA. It can be done,
as shown below by placing a nice little wedge under the front cell of the lens
for stability. Again the Losmandy rail system shows its mettle by making
placement of the lens mount and the wedge easy and versatile. I have used
this lens on both a 10" and 12" LX200 but not without problems. Even though
the declination bearings on both telescopes have had the Hart transplant, these
mounts are basically not strong enough to carry this lens well.
Incredible stress is placed on the RA bearings in the Polar mount mode.
Remember, that the lens has to be balanced with almost as much weight as it
carries. It is to use such large lenses that I am now designing and building
a mount that will take 150 plus pound loads with ease. By the time
two imaging systems and a guider are mounted on the drive, the weight easily
gets to the 50 pound range and then is doubled for the balancing weights.
In polar mode that takes a compact (short), strong mount. A discussion of mount
design and building is elsewhere on this site.
Another interesting lens is the 500 mm f 5.0 lens that was made many years ago by Nikon. The lens is shown below mounted on its integral foot. This mounting is not satisfactory. Additional braces, wedges or rings would have to be used. Since I do not use this lens for astrophotography, I do not show a suitable mounting method. This is a small mirror lens designed mainly for sports photography. It can be had held. I show it just for the sake of curiosity.
Many smaller mirror lenses are available. Usually 500 or 600 mm f 8.
These I have felt are not suitable for astrophotography. They are designed
mainly for sports photography. They are very light and can be hand held.
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