Telescope & observatory
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In August 2009 I 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 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
Subsequently I installed an observatory dome at my rural observing site. I first had to get a very solid concrete base made, with armoured cable coming up through it to bring mains power from the house 40m away.
Gary and Dave from Pulsar Observatories delivering...
I then had to wait at least a day for the sealant in the joints to harden. After that I installed the rubberised floor and telescope:
The floor tiles were pre-cut but a couple of them needed a little trimming to fit. I also had to cut slots in 4 of the tiles to fit around the joint flanges in the walls. It was really quite easy to do. The whole floor took less than half an hour to lay.
Here are some views taken with a 15mm fish-eye lens:
You can see the power supply for the tripod but you cannot see the mains cable to that because it lies in a channel under the floor, as planned.
Technical advantages of having the observatory are:
- I can get the tripod very accurately aligned with the Earth's axis, once and for all. I have made locating sockets in the floor so the tripod cannot move.
- The scope is sheltered from gusts of wind. And also from a street lamp that has recently been replaced by a much brighter, broader spectrum one much to my annoyance. The new lamp really is not suitable for this rural location.
Both of those factors should make for better photographs because star images will move around less during each exposure.
Practical advantages possibly exceed those:
- I do not have to lug the equipment out, whatever the temperature, and set it up and try to align it every time I want to observe.
- I can take advantage of gaps in the clouds rather than having to wait for a completely clear night.
Disadvantages are apparent too, because the observatory wall limits how low towards the horizon I can point the telescope. I have already raised the tripod legs higher than I originally intended. The trade off is that I have to use steps more to get up to the eyepiece and camera.
I intend to keep the tripod rather than getting a pillar, even though I had the centre of the concrete base made stronger for a possible pillar. The tripod is very rigid and I can conveniently keep boxes of eyepieces and filters, my flat box, and other equipment under it.
Canon EOS 5D MkII ISO1600 233x32s
Sigma 17-35mm lens @ 35mm f/5.6
2011 Nov 6 19:09-21:44 UT
From Rookhope 54.8
Processing this, to combine 233 half-minute images, provided an opportunity to add another option to GRIP. The camera was stationary but simply averaging the images would cause the stars to fade away to nothing against the bright moonlit sky. It was necessary to detect the stars in each frame and boost their pixel brightnesses by 233 (the number of frames) before adding each exposure into the accumulator. Once I had decided to do that, it was easy enough using my existing Java library for GRIP. A new version of GRIP including this star-trail capability will be available for download soon.
It would have been possible to create the trails using ISO 100 (4 stops down in sensitivity) and 2 minute exposures (4 stops up in exposure time) but I wanted to use the frames to make a video too, so I wanted to keep trails short in each frame. That was successful and I might show the video here at some stage.
I did a little light painting with a red torch during this single exposure, to show the telescope in the observatory.
Canon EOS 5D MkII ISO1600 32s
Sigma 17-35mm lens @ 35mm f/5.6
2011 Nov 6 21:55 UT
From Rookhope 54.8