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Using CLS filter on my 5D MkIII soon



Hi folks ...

I recently purchased a clip-in CLS filter to fit inside my 5D MkIII.

I have never used one of these filters before and I have a question to ask ... maybe somebody can help? I am still learning how to use BYE.

Do I have to increase the exposure time significantly when taking an image when I have a CLS filter fitted?

What is a typical "Light Frame" exposure time & ISO setting for a deep sky object (eg: a galaxy) with ... and .... without a CLS filter???




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The CLS will usually require about 15-20% longer Exposure Time.

The general Exposure Time requirements will vary depending on your Scope.  SCTs, RCs, and "slower" Refractors will require the Longest Exposure time; while "faster" Refractors, Newtonians, and Astrographs will require Shorter Exposures; and HyperStars and Camera Lenses will require the Least Exposure.

How long is that Exposure??  Long enough to separate the Hump of a Histogram from the Left (Black) Edge.  What is the Limit on Length of Exposure??  Guiding and Tracking Accuracy and Skyglow - something where the Larger and Most Expensive Mounts have a significant advantage.  Once your Stars start to become Elongated, or the Background becomes a steady Orange Glow, it's time to Stop your Exposure Duration (and start to increase the Number of Images).

All of these Parameters are different for different Rigs and Skill Levels and Environments...

But, start by shooting for 2-3 minutes of Exposure, then Increase until you start to get consistently BAD Images.

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Since the filter (any filter) blocks some of the light that reaches the sensor you may need to increase the exposure. You should look at the histogram to help you decide on the best exposure. How much you might need to increase the exposure depends the actual wavelengths that are blocked compared to the predominant wavelengths from the target object.

There is really no "typical exposure time" for deep sky objects. Galaxies can have a wide range of brightnesses. It depends on too many factors that vary from setup to setup. I could suggest an exposure of 10 minutes, but if you are imaging from a light polluted area with a fast scope or a low end mount, 10 minutes could be too long.

You need to arrive at the optimal exposure by examining the histograms from a sequence of images taken at a variety of exposures of your images. For example, shoot 6, 8, and 10 minute exposures. With those exposure lengths, you will have to autoguide. Look at the histogram of each image, the amount of noise in the image, and the roundness of the stars when deciding your optimal exposure.

ISO is just a gain/brightness control. Typically, you will see more noise in images with higher ISO's. This is caused by faint background noise being amplified. I would suggest starting with an ISO of 800.


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