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Is negative backfocus possible?

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#1 SanFranStarGazer

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Posted 20 March 2019 - 11:49 AM

I have an Explore Scientific AR102 refractor. I have successfully taken photos by attaching my Nikon DSLR to the Crayford focuser. But then I can’t look at the rear screen of the camera to check/adjust setting nor to view the previous exposure.

 

So, I thought that I could attach the camera to the diagonal. I did a daytime terrestrial test.  Here’s what I found.

 

With a 20mm EP attached to the diagonal, the scope focuses on an object 1 mile away at Crayford=15mm and 88mm for an object 30 feet away.  When I swap the EP for the camera, the near object comes into focus at Crayford=55mm, but I can’t focus on the distant object.

 

Does this mean I need Crayford = -18mm (e.g. the focuser would need to go past 0 and retreat into the OTA)? Is there some lens that I can put into the optical path to give me -18mm of negative focus?

 

Thanks for any explanation or suggestions.



#2 Jeff Struve

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Posted 20 March 2019 - 12:10 PM

Maybe a better solution would be to forget shooting through the diagonal as that is detrimental to the image quality and invest in BackYard Nikon?


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#3 Alex McConahay

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Posted 20 March 2019 - 12:20 PM

>>>>> But then I can’t look at the rear screen of the camera to check/adjust setting nor to view the previous exposure.

 

Why can't you look at the rear screen?

 

Alex



#4 happylimpet

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Posted 20 March 2019 - 12:35 PM

A very weak barlow does what you're asking. But yeah, stop using the diagonal!!! Gaffer tape a mirror under the viewfinder?



#5 ngc7319_20

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Posted 20 March 2019 - 12:48 PM

You do not want to shoot through the diagonal -- it uses up too much back focus.

 

Maybe you can connect the camera to a laptop, and view the images there in live time?  I've done that with my Canon and it is so much easier than using the tiny display on the camera back.



#6 SanFranStarGazer

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Posted 20 March 2019 - 01:02 PM

Maybe a better solution would be to forget shooting through the diagonal as that is detrimental to the image quality and invest in BackYard Nikon?

I’ve got Backyard Nikon (as you’ll note from my signature line lol.gif ). I wanted to be able to do photos minimalist, without laptops, cables, batteries, etc.



#7 SanFranStarGazer

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Posted 20 March 2019 - 01:06 PM

>>>>> But then I can’t look at the rear screen of the camera to check/adjust setting nor to view the previous exposure.

 

Why can't you look at the rear screen?

 

Alex

Picture a high-declination celestial object. Visualize where the camera is. Now visualize arching your neck beyond it’s rearmost position while sitting on the ground. laugh.gif



#8 OldManSky

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Posted 20 March 2019 - 01:07 PM

I'm guessing your Nikon DSLR doesn't have a flip/rotate LCD screen?  That would solve your problem...without a diagonal :)



#9 fmeschia

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Posted 20 March 2019 - 01:13 PM

A pocket mirror that reflects the live view screen would do the trick.

Francesco 



#10 WadeH237

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Posted 20 March 2019 - 01:26 PM

Pick a star for focus that is not directly overhead, and then point to your object and image.


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#11 ngc7319_20

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Posted 20 March 2019 - 02:23 PM

Haha... put the diagonal on the camera back!  wink.gif


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#12 Alex McConahay

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Posted 20 March 2019 - 04:20 PM

>>>>>> Picture a high-declination celestial object. Visualize where the camera is. Now visualize arching your neck beyond it’s rearmost position while sitting on the ground.

 

Yeah, its possible. It can get awkward, but you only need to do it for the time it takes to get an initial framing.

 

Focusing, etc, can be done someplace else in the sky.

 

Alex


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#13 ngc7319_20

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Posted 20 March 2019 - 04:50 PM

 

 

Yeah, its possible. It can get awkward, but you only need to do it for the time it takes to get an initial framing.

 

Focusing, etc, can be done someplace else in the sky.

 

Alex

What about one of these right-angle eyepiece attachments for framing?  In the old Nikon F days it was called a DR-3 or something.  I see chinese rip-offs selling cheap now.  I use a similar thing on my Canons.

 

https://www.ebay.com...74893685?chn=ps



#14 Alex McConahay

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Posted 20 March 2019 - 06:08 PM

>>>>>> 
What about one of these right-angle eyepiece attachments for framing?

 

The will work, but only for the regular (optical) vierfinder.  And that is often so dim, there is not much to see. 

 

One wants to see the LCD viewfinder, and all that. A simple mirror will work.

 

ALex




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