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Barlow and diminished image quality

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#1 eckloss

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Posted 11 July 2018 - 03:31 PM

Will a Barlow lens in any way reduce the amount of light coming to a CMOS imaging camera, and thus affect the quality of the image exposure?

 

ie - I want to image the Whirlpool galaxy tonight. I'd like to get a more naturally zoomed image set if possible.

 

 



#2 xiando

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Posted 11 July 2018 - 03:46 PM

From my experience, although this will likely end up another banal academic argument with the wealthy big shots posting walls of text to the contrary, a barlow will effectively reduce the light. That is, when I used a barlow, the individual subs were far dimmer at the same exposure time than the non-barlowed data.

 

From my experience applying barlows to increase magnification of targets like The Whirlpool Galaxy, M27, M57, etc, If you use a 2x barlow, you'll need to increase your exposure time by at least a factor of 2 to gain the same amount of info as you did when sampling at "N" seconds. IME, simply increasing total integration time (ie doubling the number of subs) doesn't cut it, but then at the time I wasn't using 300s samples either, where maybe just doubling or tripling the number of subs might do the trick...

 

You'll also need to pay more careful attention to guiding, as the main camera's view positional sensitivity will  increase proportionately to the increased magnification brought by applying the Barlow, but I suspect you already know that.

 

...And there's star bloat to deal with as well, which for me meant many subs I had to toss because DSS couldn't figure out what was a star and was was just a blob of light. This was both with an inexpensive 2x Barlow and a relatively expensive corrected 2.7x Barlow. 

 

I'd say give it a try. The whirpool will be up for weeks to come, so if it all goes to hellinahandbasket you can just image it again later via normal means.

 

Admittedly when I was doing so it was unguided and with ~30s subs, so "with a grain of salt" and all that...


Edited by xiando, 11 July 2018 - 03:48 PM.

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#3 photoracer18

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Posted 11 July 2018 - 04:24 PM

Yes. What a Barlow does is double the size of the image then cut out the center part and pass that along. So technically the image is dimmer because the same amount of light has been spread out over a larger area then cropped.
You can also look at it as doubling the focal length of the scope. So if the scope was F5 now its effectively F10, which is "slower" photographically and requires longer exposures to get the same amount of light into the camera.

Edited by photoracer18, 11 July 2018 - 04:27 PM.

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#4 Jerry Lodriguss

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Posted 11 July 2018 - 04:25 PM

Will a Barlow lens in any way reduce the amount of light coming to a CMOS imaging camera, and thus affect the quality of the image exposure?

 

ie - I want to image the Whirlpool galaxy tonight. I'd like to get a more naturally zoomed image set if possible.

 

It reduces the light trivially due to loss in the glass and air-to-glass surfaces.

 

But a Barlow does spread the light out on the sensor. Therefore each individual pixel gets less light.

 

If you use a 2x barlow, you focal length is doubled, so it's 2 extra f/stops light loss,  and you need 4x more exposure to compensate.

 

You need to look carefully at your sampling.  If you are optimally sampled, you don't need any more magnification, it's not really going to do you much good. 

 

If you are undersampled, then increasing the focal length might help.  But that also depends on whether the seeing will support it.

 

Jerry


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#5 bobzeq25

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Posted 11 July 2018 - 04:48 PM

Jerry covered it well.  Note the exposure 4X thing.  That applies both to subexposures and total imaging time.  So, to get the same signal to noise ratio as 1 hour, you'd need 4.

 

That's a major reason people don't generally use Barlows for DSOs (there are exceptions), while the opposite, reducers, are extremely popular.  You're thinking about (possibly) improving resolution, but SNR is crucial to good images.


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