First, we know that a lens in the system will tend to invert the image and any mirror in the system will
result in a mirror-imaged view where left and right are reversed. Now with my refractor telescope setup there is the one lens, one star diagonal, which has a mirror, and then the camera. I sometimes replace the camera with an eyepiece, for instance when the camera is mounted on the telescope in piggy back style. What do all of these components do to the image that shows up on the camera compared to what your own eyeball would show if you were to look at the same object?
Well, we could look at this problem theoretically and examine the optical path and the effects of various components to rotate or reflect the image, and we will do some of that, but mostly let's just compare what we see by looking at real images taken with some of the components missing.
So, for starters, since I am getting lazy in my old age, I didn't set up the scope on its tripod and mount, I just sort of held the scope and balanced it on my lap or on a swivel chair in the observatory office and just sort of pointed it out the window at local landmarks. The first image, made with the camera and normal camera lens attached, shows what the target, in this case a bell shaped fixture on a light standard. This is also what the scene looked like with my very own eyeball just looking out the window.
|Test Image with normal camera and lens setup appears just like the eye sees it|
Note the bell shaped fixture is on the right side of the image and points down toward the ground. Now what happens when the camera is attached directly to the scope without the star diagonal installed? See below.
|Test image with camera installed on telescope at primary focus|
Hey, look at that, in this case the image shows up just as it would if you used your own eyeball. No upside down or mirror-imaged view here. In fact this setup is what the more serious amateurs would do if they were attaching a CCD camera.
The next step in this experimental analysis was to take the camera off and put an eyepiece in place of the camera. Now, with a lot of balance and risk, I suppose, I couldn't leave the scope resting on my knees, but instead choose to rest in the office swivel chair and put myself on my knees so that I could look directly through the eyepiece. Without the star diagonal, you have to be directly behind the scope to look into the eyepiece. Now what did I see? Yes, you guessed it, I saw an inverted image where the whole scene was upside down. It was not mirror-imaged, in the sense that you could in theory just rotate the image around and bring it right side up again and the two views would lay right on top of each other.
Now, let's take a look if we install the star diagonal first and then attach the camera to it. Now the image in this camera is shown below.
|Same test image taken now with star diagonal in place|
This image cannot be rotated around and made to lay right on top of the other images so far. It can't be done because this image is seen through a mirror and left has been turned into right, etc. Now, I tried rotating the star diagonal, much like you would do it you were trying to adjust the viewing angle for a youngster or what not, and got the image shown below.
|Same image, but with star diagonal rotated|
Ok, ok, simmer down. So, what did we learn with all of this? I think we see from this series of views, is that the orientation of the image depends on the telescope imaging setup. So, the next time you are at a star party and roam around looking at what other folks are imaging, just keep that in mind and ask if you are seeing a mirror imaged view of the object or not.
One thing I learned from this little exercise is that since I do most of my telescope viewing with a the camera attached, and very little eyepiece viewing, I can just take the star diagonal accessory off the scope and just attach the camera directly. This will make it easier for me too when I upgrade toa CCD camera or put on my larger spectrometer and won't want the star diagonal there anyway.
Working through this issue and the previously described issues of being able to slew the scope accurately to a location specified only by right ascension and declination to a dim object, which might not be naked eye visible, and being able to track that object accurately for several minutes, are all capabilities needed before the spectrometer can be mounted up with the telescope. The spectrometer weights about 5 pounds and I don't want the star diagonal installed at that time. Developing the skills and capability to find dim objects, goto them and track them has taken a long time to get there. We will see in the next months whether is all comes together and works out. Stay tuned!
Most scopes are designed to accommodate the star diagonal and when it is removed it is usually necessary to add extra length in the optical axis to allow focusing. This operation would make the length of the optical path a little linearly longer, because a focal length extender tube has to be installed to get enough focal length for good focusing, and would not interfere too much with other observers being able to see the image because the camera Liveview LCD screen could be rotated for ease of viewing. Oh, oh, now we have to worry about how that view might be upside down or not. Oh, well, there is always something to worry about on this journey and wandering path of the physicist wannabe through amateur astronomy.
Until next time,
Resident Astronomer George