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Old 20-08-2007, 09:45 PM
Dazzled by the Cosmos.

Dennis is offline
Join Date: May 2005
Location: Brisbane
Posts: 11,106
Hi Bostjan

This is scary – you seem to have the same Vixen parts that I have; see my eyepiece projection photo attached!

It is a few years since I last used film, and I understand that a modern ISO400 emulsion is as good as the older ISO100 emulsions that I used to use. So, the extra speed of the ISO400 emulsion should allow for a faster shutter speed and keep camera shake to a minimum.

One old rule was that for a lens of focal length L, use the reciprocal to set the shutter speed. So, for a 200mm lens, use 1/250 sec; for a 500mm lens, use 1/500 sec to minimize camera shake.

Unfortunately, with the long length of a telescope and the camera attached to the focuser, this acts like a long mechanical lever and so even the smallest of vibration becomes magnified and can ruin a photo.

So far, I have only spoken of the Moon which is a bright object allowing a relatively fast shutter speed and the tracking of the mount is not that important.

If you have a motorized equatorial mount, the mount should track at the “Sidereal Rate”, making a complete revolution in 23hours 56mins. With my Vixen GPDX mount and a telephoto lens of up to say, 300mm, I can usually take photos of 10 to 15 minutes duration without any star trailing. This is with the mount accurately polar aligned and the RA tracking motor running at the sidereal rate.

However, if I move up to my Vixen 102mm f9 refractor (918mm focal length), I am usually limited to only 3 to 4 minutes exposures towards the South Celestial Pole (small circles) and 1 to 2 minutes in other regions of the sky (larger circles) before I get star trailing. At focal lengths of over 1500mm I am lucky to get 20 to 30 seconds before stars begin to trail. This is with 35mm film. I have found that tracking with CCD cameras is even more critical and shows up the smallest of tracking errors.

I also have a CCD camera with an internal guide chip. This chip is used for auto-guiding (as opposed to simple tracking) for imaging faint Deep Sky Objects, such as galaxies that require long exposures (30 to 60 minutes). I find a suitable guide star and place it on a reference pixel. The guiding software then makes small calibration movements Up, Down, Left and Right so it calculates how much it must energise the tracking motors to bring the guide star back to the reference pixel if the guide star drifts off due to tracking errors, atmospheric refraction, etc.

Using this auto-guiding technique, you can expose an image at say, 1500mm focal length for 5 or even 10 minutes and still get round stars as the guide chip ensures the guide star remains stationary on the reference pixel.

So, in summary, for the brighter objects such as the Moon and Planets at prime focus, tracking is not an issue as exposures are relatively short. However, when you use eyepiece projection, then the image is highly magnified and become quite dim. To record any detail, you would typically require exposures of say 1/8 to second and these would be a severe test of the rigidity of the mount.

I now use a webcam to take highly magnified images of small areas of the Moon and the Planets, as the webcam can expose the CCD chip at say, 1/30 sec with no mirror shake. Also, the webcam can capture say, 30 frames in 1 second and run for say, 60 seconds so you effectively have taken 30 x 60 photos = 1800 frames.

At least some of those frames should be sharp during the moments of good seeing which may only last for say, 1 to 2 seconds at a time. A program called Registax can then analyse these 1800 frames of the movie file from the webcam and select the sharpest frames before stacking them together and produce a final image that is sharp and detailed.

We’ve covered a lot of ground here, I hope I haven’t confused you.


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