IMPORTANT! Long time exposures without purpe corner!

Started Oct 19, 2005 | Discussions thread
OP Nordstjernen Veteran Member • Posts: 6,876
Re: How long is the longest shot you actually take?

No questions are stupid -- some answers might be

The longest exposure with in-camera noise reduction is 30 sec. When we use longer exposures the purple corner will still be an issue. This is one reason for using a separate dark frame. Another reason is that each dark frame (noise reduction) blocks the camera for as long as the light (main) exposure. To gain more time each evening under the stars, it is better to make just a few manual dark frames during the session. For this reason I also standarize all the exposures for the same session, i.e. to 5 minutes.

With your Meade ETX telescope/prime focus photography you should use a guide scope -- a small telescope mounted on your main telescope, with a crosshair eyepiece, to watch the telescope's tracking. You have to adjust for periodic errors -- move the telescope a little now and then with the hand control. The guide scope will point at a star, and you have to ensure that this star is percectly sentered in the eyepiece all the time -- for hours in theory. With DSLR's up to maximum 5--10 minutes. You can also get autoguiding devices (I do not use one). Remember that even a 300-400 mm lens needs very accurate tracking to get perfect star points.

Even better, you could mount the camera on top of the telescope with a telephoto lens, and use the telescope as a guide. When you are more experienced you could do prime focus photography. Anyway, do not give up! It is amazing what training can do!

AS should always be switched off when the camera is mounted on a tripod -- and coupled to a telescope. Why? Because AS can make unwanted shifts when it is turned on. When it is off, you ensure that the sensor is locked in position all the time.

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