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Which t-ring adaptor for Sony a6400 and 8" Dob?

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#1 DakotaHoosier

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Posted 21 September 2020 - 10:18 PM

I'm enjoying visual imaging but as a photographer I'm dying to get some pics, simply to help others see what I was seeing. I bought a T-Ring adapter for my Sony a6400 to attach to 8" Dobsonian. Everything fits/connects, but I don't know enough to put the camera sensor at the focus point. If I add a 2x Barlow, I'm getting closer, but I'd like to image without the Barlow.
I either need MORE or LESS distance. Do I need the T-Ring without the extension (I think it is there to account for the difference in set-back between a DSLR and mirrorless camera)? Do I need to add an extension tube or something? I don't mind spending a little money on experimenting but I'd rather head in the right direction if anyone can guide me...
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#2 TelescopeGreg

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Posted 21 September 2020 - 10:35 PM

Generally, a Newtonian telescope that's configured for visual work will have a problem getting a DSLR to prime focus, due to not enough inward travel of the focuser.  Adding a Barlow is a bad fix, as it makes it several times harder to do deep sky imaging.  If you have an extension, by all means try removing it.  That might do it.  You can check things during the day with the proverbial "distant object".  Nighttime focus will be slightly inward from that.

 

All this said, a Dobsonian mount, especially one that's not motorized, will be a tough platform for imaging.  You generally will need exposures measuring 10s of seconds to several minutes in order to grab an image of most deep sky objects.  That cannot work without a sky tracking mount.  Planetary imaging might be possible if the scope is big and fast enough, as the technique is to take a movie and process it for "lucky imaging".  For Planetary imaging, you'll possibly want to put that Barlow back in, depending on your focal length.


Edited by TelescopeGreg, 21 September 2020 - 10:37 PM.


#3 spereira

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Posted 22 September 2020 - 07:12 AM

Moving to DSLR ...

 

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#4 DakotaHoosier

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Posted 22 September 2020 - 07:48 PM

Generally, a Newtonian telescope that's configured for visual work will have a problem getting a DSLR to prime focus, due to not enough inward travel of the focuser.  Adding a Barlow is a bad fix, as it makes it several times harder to do deep sky imaging.  If you have an extension, by all means try removing it.  That might do it.  You can check things during the day with the proverbial "distant object".  Nighttime focus will be slightly inward from that.

 

All this said, a Dobsonian mount, especially one that's not motorized, will be a tough platform for imaging.  You generally will need exposures measuring 10s of seconds to several minutes in order to grab an image of most deep sky objects.  That cannot work without a sky tracking mount.  Planetary imaging might be possible if the scope is big and fast enough, as the technique is to take a movie and process it for "lucky imaging".  For Planetary imaging, you'll possibly want to put that Barlow back in, depending on your focal length.

Thanks, TelescopeGreg. I had the same feeling. I don't have any extensions I can remove and I've ordered a T-Ring adapter that is missing the barrel distance as seen above. That might be needed in a refractor or some other setup (I don't know). Like everything, it will be delayed.

 

I'm quite a aware of the limits on the Dob with a DSLR/Mirrorless, but I'd like to capture some bright objects to catch what I'm seeing visually. Luna (photos) and Saturn/Jupiter video for lucky imaging. The Dob is all I have right now (borrowed from the Indiana Astronomical Society). I have an 8" Meade SCT unmounted that isn't much help (yet) which will be better for digging into those DSOs!

 

[Thanks for moving this to the right place, Soyuz...this felt more like a beginner / setup question so thanks for getting the thread in the right forum.] 



#5 kathyastro

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Posted 22 September 2020 - 08:00 PM

You need to get the camera as close as you can to the OTA.  That means eliminating any extensions, using the shortest adapters you can find, etc.

 

A standard T-ring for a mirrorless camera effectively increases the camera's body depth to 55mm.  That is not what you want.  Instead of a standard T-ring, get a "T-minus" ring for it.  That has the same attachments - Sony bayonet mount at one end and T-thread at the other end - but has minimum length.




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