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Help needed to figure out band shift for fast telephoto lens.

NV observing
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#1 slavicek

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Posted 11 October 2019 - 04:03 PM

I am thinking about getting 200mm f/1.8 telephoto lens for wide field observing and for NV intensifier to be used with 5, 7 & 12 nm Ha filter. Filters would be placed between the telephoto lens and the NVD. Is there anyone out there who can help me calculate the bandwidth shift for those filters at this configuration? I do not know much about optics but it seems to me that when the filter is placed into the cone of light created by fast telephoto lens, the light will be hitting the filter at an angle, increasing the light travel path thru the filter, hence changing the filter wavelength? And how to calculate or what would be new center of the filter wavelength now?
Any help will be appreciated.



#2 jdbastro

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Posted 11 October 2019 - 04:44 PM

If you are using a full-frame (35mm format) telephoto, then the band issue is moot.  The image cast by the lens onto the 18mm dia intensifier has mostly near perpendicular incident light rays within the 18mm portion of the image circle.

 

Go get your lens (provided it is full-frame) and have fun.   ;)


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#3 Jeff Morgan

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Posted 11 October 2019 - 07:34 PM

If you are using a full-frame (35mm format) telephoto, then the band issue is moot.  The image cast by the lens onto the 18mm dia intensifier has mostly near perpendicular incident light rays within the 18mm portion of the image circle.

 

Go get your lens (provided it is full-frame) and have fun.   wink.gif

 

My fastest lens is a f/1.4 Canon 50mm.

 

Great results with the two h-alphas I own, 12nm and 7nm.


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#4 OleCuss

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Posted 11 October 2019 - 07:53 PM

The optical design of a good camera lens is different than that of a telescopic objective (often better).  The camera lens will include a straightener so that the photons are not coming in at an angle which would make your filter choice critical.


Edited by OleCuss, 12 October 2019 - 06:43 AM.


#5 GeezerGazer

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Posted 12 October 2019 - 02:30 AM

I agree with all the above.  I use a Nikon 300mm f:2.8, 105mm f:1.8, and a 50mm f:1.4 with no problem using 12 and 7nm H-a filters.  Of the 3 lenses, my favorite is the 300.  I have often thought a 200mm would be perfect for many, extended H-a objects.  Yes, go for it!  


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#6 slavicek

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Posted 12 October 2019 - 11:31 AM

Thank you all for your replies. I am considering Canon 200mm f/2 (new), Canon 200mm F/1.8 (only used available) and Nikon 200mm f/2 (new). I believe they are all "full frame lens". I will start new topic about why this lens soon. bow.gif



#7 GeezerGazer

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Posted 12 October 2019 - 12:12 PM

Slav, a consideration... the faster the lens, the more difficult it is to get to perfect focus... particularly important if imaging.  I find the f:1.4, 50mm lens the most difficult for imaging, and this smaller lens does not have a focus lock which means that a slight touch can move the focus ring.  It performs fine for visual use, but focusing it for imaging is quite the challenge, exacerbated by the phone I use which does not allow "zooming in" to check star focus.  In contrast, the 300mm Nikon has a moving detent that does not lock the focus, but makes regaining perfect focus  very fast, moving the focus ring until it slides into the detent that can be set at any position within the focus range.  I find it very handy and use it all the time. If you can get that feature on your lens choice, I'd highly recommend it, even if you don't plan to image. 



#8 Jeff Morgan

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Posted 12 October 2019 - 05:25 PM

Are you talking about achieving focus, or having if drift under gravity?




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