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Zenithstar61 + Flat61A

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#1 OldSchoolAstro

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Posted 25 September 2020 - 08:17 AM

I apologize if this is a really silly question but just want to confirm a few things. I was told that for my Nikon DSLR I needed to "unscrew and lengthen" the Flat61A by 12.9mm to position the sensor exactly to hit the image plane. Then to get stars sharp on my camera I had to focus out the R&P focuser to around a value of 31 on its markings. 

1) If this doublet APO scope is just two glass elements at the front converging light to a specific spot  based on its focal length, why do I need an exact value of 12.9mm on the Flat61A if I still need to just adjust the DLSR sensor's position using the R&P focuser to move everything? Is this to allow focusing at the closest possible distance? So then my 12.9mm along with moving the focuser all the way out to 31 is for "infinity focus" only?

 

2) Could I add something that is 17mm wide in my optical train (filter drawer system) even though its more than 12.9mm extension of my Flat61a? Could I add it in, take out the 12.9mm and put it back to 0, and then make up the remaining 4-5mm from my focuser movement and maybe only come out to roughly 26-27 instead? 


Edited by OldSchoolAstro, 25 September 2020 - 08:18 AM.


#2 drew2578

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Posted 25 September 2020 - 08:44 AM

You are talking about two different types of backfocus.  The first is to focus the Flattener with the Camera sensor and the second is to focus the Primary elements with the Flattener.  You unscrew the flattener to get the required 55mm of backfocus of the flattener.  That is a great feature as not all Nikon mounts are the same.

 

You can add anything you want in front of the flattener  as long as it is below ~31mm it sounds like.


Edited by drew2578, 25 September 2020 - 08:46 AM.


#3 OldSchoolAstro

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Posted 25 September 2020 - 09:05 AM

You are talking about two different types of backfocus.  The first is to focus the Flattener with the Camera sensor and the second is to focus the Primary elements with the Flattener.  You unscrew the flattener to get the required 55mm of backfocus of the flattener.  That is a great feature as not all Nikon mounts are the same.

 

You can add anything you want in front of the flattener  as long as it is below ~31mm it sounds like.

 

Are there optics inside the flat61A? Is there actual glass in there? Obviously would need to have some to flatten the field but just wondering how many elements, and where inside they are positioned?



#4 drew2578

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Posted 25 September 2020 - 09:08 AM

Yes optics are there.  As long as you get the 55mm behind it down I don't see why where they are located in the flattener is important.  

 

Just worry about the T-Mount section in the attached drawing.

Attached Thumbnails

  • gallery_269597_11113_48707.jpg

Edited by drew2578, 25 September 2020 - 09:10 AM.


#5 OldSchoolAstro

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Posted 25 September 2020 - 09:11 AM

Ok thanks, I was looking at this same diagram and I guess I can only to insert the filter drawer between the scope and the Flat61A and take away 17mm from focusing out with the focuser then. 



#6 sg6

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Posted 25 September 2020 - 04:06 PM

These things work a bit by "implication"

The falttener optics/lens has to be at the correct distance from the front objective - hence you attach to the focuser and wind it in and out. But you do noit know what that is and directly neither can it easily be measured.

 

When the flattener is at the correct distance from the objective it will form an in focus and flat field image at a position of 55mm from a datum face on the flattener.

 

So you fix by fair means and foul your camera sensor at that distance - referred to as the back focus.

 

Then you adjust the focuser to achieve a nice sharp and hopefully flat image on the camera sensor.

 

Which Nikon?

The F mount versions have a farly long flange to sensor distance - 46.5mm.

Means all you have to play with is 8.5mm, and a DSLR t-ring is often 8mm.

 

If you have a 12.9mm extension requirement then the DSLR flange distance is 42.1mm and there are no Nikons in that area.

If you already have an 8mm T-ring that would mean a DSLR flange of 34mm which would imply the Nikon S mount of one of their mirrorless cameras. Full Frame item ??

 

Could you confirm the DSLR you are using, and if possible the t-ring, mainly thickness it adds.

 

If it is an S mount and the sort of standard 8mm t-ring they take up 43mm, so you only have 12mm to play with. Putting in more then 12mm means incorrect position of DSLR sensor for a flat field.


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