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I have recently (August 09) obtained a Sky Watcher Newtonian reflecting telescope, 254mm (10 inch) aperture, f/4.8 (so the focal length is 254 x 4.8 = 1,219mm).
It mounted on my existing HEQ5 equatorial mount with just one small snag: I needed a longer rod for the counter-balance weights to avoid having to put excessive weights on. The telescope weighs about 15kg and I needed 25kg to balance it with my EOS camera on it; as a temporary measure I visited a sports shop for extra weights. Longer rods are available and I have now received one. It enables me to use only 10kg of balance weights, putting less strain on the mount.
Another slight snag at first was that it was not immediately obvious how to attach my SLR camera (Canon EOS 5D MkII). The focal plane of the telescope, after reflection from the secondary mirror, is quite close to the tube. That is fine for focussing eyepieces but the camera body needs about 50mm more distance. The telescope was advertised as having a direct SLR connection but there was no documentation explaining what was intended. I eventually noticed that the 2x Barlow lens included with the instrument has a screw thread at its exit end. A little experimenting showed that my Canon fitting T-mount, that I already had for my Meade ETX, would screw onto the Barlow lens. So that must be the intended method of camera attachment. The method is called negative projection (see, eg, Covington's book, "Astrophotography for the amateur"). It makes the effective focal length somewhat larger (by about 2 times) and the f-ratio correspondingly smaller. Never mind, the f-ratio is less significant in astro- than in ordinary photography; what matters is the light gathering capability of a decent sized primary mirror. My first images with this set-up showed that I can photograph stars down to magnitude 16.5 even from my light-polluted suburban site.
Here is a picture of the new scope:
No manual of any kind was provided with either my HEQ5 mount or my SkyWatcher telescope. I think that is appalling considering their cost. A Google search found a manual at http://www.skywatchertelescope.net/swtsupport/Instruction_Manuals/HEQ5andEQ6130405V1.pdf but that still lacks a lot of important detail. I have therefore written a page about my own discoveries of how to use the various parts of the telescope, linked here
The effective focal length of the photographic set-up can be measured by using GRIP. Photograph something with a known angular dimension (eg, the separation of a double star) and measure its size in pixels in the image. One of my first photos was of M57, the planetary nebula in Lyra (see next page). Its documented size is 1.8 x 1.4 minutes of arc ('). In GRIP use the straight line option on the measurement menu of the image:
That shows that an angular dimension of 1'.4 ends up as 150px on the image. The image size (in my case) is 5616 x 3744 px and the detector (in my case) is 36mm x 24mm. So 1'.4 becomes 36 x 150 / 5616 = 0.962mm on the detector. The effective focal length, F is then given by the formula
F = w / 2.tan (a/2)
where a = angle subtended (remember to convert to radians: 1 radian = 57.3 degrees) and w = width on detector (in the same units as F). In my example this gives F = 2380mm. This means that the Barlow lens is multiplying my telescope's focal length by 2380 / 1219 = 1.95. I repeated this on the other dimension of the nebula and got the same result, within the number of significant figures quoted.
It might be thought that the best way to measure the field of view would be to stop the motor drive on the telescope mount and time the movement of a bright star from one side of the camera's viewfinder (or live view screen) to the other. Unfortunately neither of those is likely to show the full extent of a photograph. So instead use the calibration facilities of GRIP. On the measurement menu again, select calibrate:
Having done that, measure the whole image (simply select whole image from the measurement menu). The resulting display will show the image size in the calibrated units.
In my case that showed that the field of view in the negative projection set-up described above is 50' x 33'.
Well, it's just a good sized shed that stores all the equipment and has room for me to sit at a table when photographing from my PC. I considered making a sliding roof and mounting the telescope permanently inside but decided that making it properly waterproof and yet light enough for easy sliding, at the size that would be needed for access all around the telescope, was not practical. So instead I have concentrated on making it quick and easy to set up the telescope.
The shed is beside a paved area in the corner of the garden, so it is sheltered by 6ft high fences. Being paved it provides a solid base for the telescope mounting. The mounting is a tripod so I have drilled 3 holes in the paving for the tripod feet to sit in. Smaller holes in the base of those holes go right through the paving so that rainwater drains away. I have painted round the holes so I can find them easily in the dark.
So the tripod is easily erected in exactly the same spot every time. Or at least, exactly enough for the kind of photography I am doing, where GRIP software takes care of small drift. Therefore I have aligned the tripod on the celestial north pole just once and do not need to do that every time I set up.
The plastic water pipes in this photo carry cables from the shed to the telescope. I have used water pipes because the diameter is a bit bigger than normal DIY trunking for electric cables. The two sections of pipe make it very easy to simply fold the assembly up inside the door of the shed when not in use. A couple of hose clips and a strip of red velcro enable one of the pipes to be attached to the top of a tripod leg quite firmly but in a way that is easy to detach again.
There are 4 cables through the pipes:
A fifth cable may be added if I can find a focussing motor that will fit my telescope. I will then be able to focus from sitting at my PC as well as drive the telescope around and take photos.
Everything in the shed is connected more or less permanently but it is all easily unpluggable when I want to transport the telescope elsewhere.
Here is a close-up of the cables at the tripod end. The helical plastic enables enough cabling to hang out of the end of the pipe so that the tripod head can move to all positions without fouling or straining the cables. It is of course important to check that. The two connectors hanging down are USB and power for the camera. The other two cables are plugged into the mount.
This arrangement is not only very quick to set up and take down but it also considerably reduces the scope for cable entanglement (which can be quite a nuisance in the dark).