IMPORTANT! Long time exposures without purpe corner!

Started Oct 19, 2005 | Discussions
MatsM New Member • Posts: 4
Re: IMPORTANT! Long time exposures without purpe corner!

Nordstjernen.

I've read your articles about about astrophoto with digital DSLR. Since I'm very interested in astrophoto they inspired me so much that I bought the the KM5D. There aren't much information about astrophoto with KM DSLR out on the internet. So thank you very much for the articles! Very good work! And very nice astrophotos!

Mats

Nordstjernen
OP Nordstjernen Veteran Member • Posts: 6,876
Cool

... and I think that you soon will be able to surpass my results. There is still a huge potential for bettering the picture quality!

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TPeter Forum Member • Posts: 62
Re: How long is the longest shot you actually take?

some (maybe stupid) questions as I am new to Astrophotography as well as to the KM 5d:

  • I realize that using the Noise Reduction function of the 5D the purple corner is gone. Is your only reason for not using this feature the extended time or are there oither reasons (quality?)?

  • I got a Meade ETX telescope and am using the the 5D as Prime Focus. Unluckily neither in Alt/Az nor in Polar mode (even after motor calibration and drive training) can I get exposures longer than about 15 sec without shift in the stars (the equatorial mount is stable, no vibrations, am using remote shutter, without camera perfect tracking for hours...). Any recommendations???

  • Read somewhere in this forum that AS should be switched off when camera mounted on tripod. Why? Does this also apply when attaching the 5d to a telescope? (I tried both but can't see a difference)

would really appreciate some help / tips !
Thanks

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Nordstjernen
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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TPeter Forum Member • Posts: 62
Re: How long is the longest shot you actually take?

thanks a lot Nordstjernen for the fast and very explanatory reply !! Am waiting for the next clear night to practice ..

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DigitalWright New Member • Posts: 8
Re: IMPORTANT! Long time exposures without purpe corner!

For anyone with access to Matlab, the following code will remove noise on a jpeg:
----------------------------------------------------

original = imread('original.JPG', 'JPG');
noise = imread('noise.JPG', 'JPG');
imwrite(original-noise, 'edited.JPG', 'jpeg', 'Quality', 100);

-- hide signature --

where:
original.JPG is the file name of the original (noisy) file.

noise.JPG is the filename of the noisy 'dark frame' of the same time as the original.
edited.JPG is the filename of the output file.

Sorry this doesn't work on RAW files, but it is possible to convert RAW images to TIFF and replace JPG with TIFF in the code.

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