Old 23-09-2012, 08:30 PM
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darrellx (Darrell)
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Help to understand Optics!

Hi All
I am having a bit of trouble understanding what is happening with one of my scopes, and I am hoping someone might be able to help me.

I have a Megrez90 and the issue is the focus when using cameras. When I use my ATIK 314L+ I need a small extension tube and the focuser racked right out (to 3inches) to get focus. All works well.

When I insert the filter wheel into the mix, the focuser needs to be moved in to 1.5inches and I can focus. All works well.

If I understand what has happened to this point, the focus point is a certain distance from the last lense and the CCD in the camera meets that focus point when the focuser is 3inches out (with the extension tube). By inserting the filter wheel, I am effectively moving the CCD further out or away from that point of focus so the focuser needs to move in so the CCD meets that focus point.

In general terms, I think I have that.

Now, when I add either a Barlow or a Powermate, here is where I get confused. I know there is a fundamental difference between the two, but that adds to my confusion.

The Barlow being a "diverging lens" moves the focal point (exit pupil on some web pages) further out. (Diverging means spreading out)
I would have thought that since I was adding a length of something into the light path, I would have had to move the focuser further in. But not so.

So..... take the filter wheel out, and I have the extension tube and the ATIK - focus is at 3inches. By adding a Powermate, I expected the focus point to be pretty much the same, or maybe have to move the focuser inwards to compensate for the length of the Powermate. However, I have to extend the focuser out as far as possible and use a 2inch extension tube and I am only just beginning to see some sort of hugh blobs of stars, but still nowhere near focus. Same with a Barlow. I tried this again last night on the moon.

All the web pages I have read describe the Powermate as "a positive field lens to redirect field rays - result is an exit pupil that stays where the eye piece designer intended".

So I am confused why I would need to move the Powermate at all!
I am also confused why I would need to be moving the barlow out not in!

Then I read "Powermate.... essentially parfocal and with nearly constant magnification regardless of image distance behind top surface except 5x model which increases 1x for every 35mm of image distance increase." I don't really understand this, but my interpretation is that 5x Powermates are somehow different from the 2x or 2.5x or the 4x. It just so happens I experimented using a friends 5x (before I committed to a purchase of my own).

Does this suggest that while I couldn't reach focus with a 5x I might with something like a 2.5x?

Now, the last point. I have my camera attached to an extension tube directly to the focuser. I have read somewhere that I should still use a diagonal. This seems odd to me, since it would just introduce more class and more bends into the light path. I have tried this - the camera into my diagonal and it comes to focus with very little adjustment needed.

Why does the camera (with the Powermate) come to focus in the diagonal, but not when there is no diagonal? Is the diagonal acting like a much longer extension tube?

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Old 28-09-2012, 07:53 AM
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gasman1 (Gary A Sitton)
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Barlows move the focus point back (to a varying degree depending on the configuration) and thus give more apparent back focus. The diagonal is like a bent extender tube which doesn't change the focus distance but does force the focuser in closer compared to no diagonal.
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