# Thread: focal lengths and Barlows

1. ## focal lengths and Barlows

Hi all,

I am currently using a 10" reflector and a 3 x Barlow to photograph Saturn. I have a 5x Barlow, but when I attempt to use it, I can only bring the planet into defocus, even when using an extender. I don't pretend to understand the math involved in figuring focal lengths, but is there a way I can bring the planet into focus using the 5x Barlow?

2. ## Re: focal lengths and Barlows

What is the F number of the 10" mirror and what eyepiece are you using? Your magnification may be too high.

I am not a barlow expert so there may be something with the 5x I am unaware of.

3. ## Re: focal lengths and Barlows

For visual, a 10" (254mm) aperture will give you a maximum magnification will be 254x. Depending on what eyepiece you are using, a 3x barlow is probably too much. A 5x almost certainly is. The consequence of too much magnification is an image that will not focus.

If you are doing AP, the same math applies, except it is harder to work with magnification numbers. However, the 5x barlow is going to give you a fuzzy image. All is not lost, however. Shoot a video (10,000 frames or so) of the fuzzy image, and software will be able to pull a reasonably clear image from it.

Here's a single frame (the best of 10,000) and a stacked image from the same video, to show you what processing will do:
Best frame of video.jpg S3-postalignment-p1.jpg

4. ## Re: focal lengths and Barlows

The scope is an f/ 3.9 and I am using a ASI224 cam for capture.

5. ## Re: focal lengths and Barlows

Thanks for the repsonses! My problem on using the 5x is focus. I can only bring the planet into defocus, and no further. The 5x has a rather long extender already, and I am not sure if I should attempt to lengthen the extension or if that only adds to the problem.

Thanks again!

6. ## Re: focal lengths and Barlows

Which direction gets the camera closest to focus: pulling it out or pushing it in? That will tell you if you need more extension or less.

7. ## Re: focal lengths and Barlows

Originally Posted by billgray
The scope is an f/ 3.9 and I am using a ASI224 cam for capture.
I believe a F/3.9 scope is best used (and meant to be used) for Deep Sky. Not planetary imaging or observing.

It would be interesting to give Keith's suggestion a try. That is a great stacked image of Jupiter.

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