Quote:
Originally Posted by kens
Theres a huge difference between taking shots with a 90mm f/2.8 and a 600mm at f/6.3.
The A58 has pixels 4.3um across. With a 90mm lens that gives a pixel scale of around 10 arc seconds per pixel. At 600m the pixel scale becomes around 1.5 arc seconds per pixel.
The motion of the stars due to earths rotation is 15 arc seconds per second. So at 90mm you get a star trail of 1.5 pixels with a 1 second expsoure. At 600mm that becomes a 10 pixel star trail. But with extended objects the f/ ratio difference means (simplisticaly) you need nearly 5 times the exposure. So now your trails on the 600mm are 50 pixels long.
So you obviously need a tracking mount.
The next issue you face is periodic error - variations in the tracking rate due to mechanical limitations in the mount. A mount such as the HEQ5 (which David quite rightly pointed out is regarded as a bare minimum for good astrophotography) has a periodic error in the order of 30 arcseconds over a 10 minute period. So every 5 minutes it moves 30 arc seconds one way then in the next 5 minutes it moves 30 arc seconds the next way. That's 20 pixels at 600mm FL over a 5 minute exposure. That's the order of exposure time you would expect for faint objects like galaxies and nebulae.
The mount you linked to would typically have considerably more periodic error than the HEQ5 as would the star adventurer.
The way to counteract periodic error is autoguiding. For that you use a second scope and camera pointed at a guide star that takes exposures every few seconds. Guiding software identifies small movements of the star and sends corrective movements to a suitably functional mount. These movements are typically fractions of an arc second.
So now you need a mount that can take not only the imaging scope/lens and camera but also a guide scope and camera.
To get an idea of how much an arc second is when considering vibration, flexure etc. A movement of around 5 microns at 1 metre distance is about 1 arc second.
So now lets consider the Dob. It cannot track the stars accurately because it does not rotate about the polar axis. Even if you had the motors and electronic smarts to track the stars you would get field rotation. That means star trails that form arcs around the central target because, relative to the stars, the camera is rotating as the scope tracks across the sky. An equatorial mount does not have this problem (simplistically - theres then the issue of Polar Alignment error).
In astrophotography the mount is the most important piece of equipment.
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What you say is undoubtedly true. And as I can't afford even the bare minimum I offer my apologies for wasting your time.
Brian