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T2 Spacing with various FF/FR

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#1 dwyer2bp

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Posted 10 October 2019 - 12:26 AM

Hi all,

 

I have been doing DSLR imaging so far with a few of my scopes: C8, SW ProED80 w/ FF/FR, 6" newt with coma corrector. With these FF/FR/CCs they all seem to be designed to work directly with a DSLR with a T-ring, so I have not had to do any thought about proper distance to the image plane etc.

 

Now I picked up a QHY163C which has a 17.5mm back focus (distance between image sensor and the front of t-2 (M42) connection. I want some advice on making sure I duplicate the correct back focus from the FF/FR/CCs and I wanted to know "How precise must this really be"?

 

The assumption:

DSLR has a back focus of: 44mm

A typical T-ring adds: 10mm (my guess with ruler)

 

So we want to replicate 54mm distance right?

 

Let's use the Pro ED80 FF/FR as a first example.

  • First I'll subtract the QHY back focus from the target of 54mm (giving me a target of 36.5mm with extensions).
  • To connect to this I need a M48 to M42 adapter (and the one I have now adds ~6mm)
  • I can get a kit on Amazon and combine a 20mm + 10mm
  • 20 + 10 + 6 = 36mm. (not quite 36.5)

 

How precise must one try to be?

 

Also, another question: By using a train of M42 size tubes, should I be concerned with vignetting on a 4/3" sensor diagonal?

 

Thanks! 



#2 Stelios

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Posted 10 October 2019 - 01:40 AM

It's actually 55mm for DSLR + T-ring. But not all FF's and FR's that *claim* 55mm actually deliver. Deviations of 1-2mm are common, and more are not unheard-of. 

 

The best thing to do is to get a variable spacer, such as the 20-29mm Baader. Then you can play with it till it's as good as you can get it. Remember: If corner stars move *away* from the center, then you need to *add* space. If they appear to be circling the center, then you need to reduce spacer. 

 

In addition to the 20-29mm Baader, there's also a 29-46mm and 15-20mm from Baader, and Stellarvue offers an 8-12mm. 


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#3 GregsCNAccount

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Posted 10 October 2019 - 02:20 AM

Cosmos,

 

Is your statement "Remember: If corner stars move *away* from the center, then you need to *add* space. If they appear to be circling the center, then you need to reduce spacer" universally true for all coma correctors, focal reducers and field flatteners?  If so, that makes tuning the spacing much easier since the spacing change direction is easy to figure out.

 

Greg



#4 dwyer2bp

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Posted 10 October 2019 - 10:14 AM

It's actually 55mm for DSLR + T-ring. But not all FF's and FR's that *claim* 55mm actually deliver. Deviations of 1-2mm are common, and more are not unheard-of. 

 

The best thing to do is to get a variable spacer, such as the 20-29mm Baader. Then you can play with it till it's as good as you can get it. Remember: If corner stars move *away* from the center, then you need to *add* space. If they appear to be circling the center, then you need to reduce spacer. 

 

In addition to the 20-29mm Baader, there's also a 29-46mm and 15-20mm from Baader, and Stellarvue offers an 8-12mm. 

This seems like a good solution.

 

When you say "move *away*" are you referring to moving in and out of focus or adjusting the variable extension?

 

Also, any idea if vignetting might become a concern?

 

I received a message from HighPoint. I mentioned the need to use a 2" filter in the setup, and they suggested something like the following:

https://www.highpoin...er-slider-ufc-2

(there are 3-4 other components they suggested I would also need to get to make this work)

 

For the Pro ED80, I have a 2" nosepiece that connects to the FF/FR that has 2" filter threads. Same for the coma-corrector (built in 2" filter threads).

 

However, I am not sure how I could use a 2" filter with the SCT. My thought was getting:

  • 2" nose-piece to T2 adapter, threaded for 2" filter
  • SCT thread to 2" adapter (has a brass ring to hold 2" pieces)

Are there better solutions?

 

Thanks!



#5 Stelios

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Posted 10 October 2019 - 10:57 AM

When I say "move away" I mean this in a focused image (and yes, this applies to everything). Edit: It certainly applies to reducers and flatteners, not sure about comma correctors as I've never used one.

 

And my name is not "Cosmos", that's the 'category' CN puts us in smile.gif  My name is on the header of the post in bold underline (Stelios)


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