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Coma in Dobson + focal multiplier

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#1 AlbertoJ

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Posted 19 September 2020 - 07:54 PM

Hello, has the same coma a f/4 dobson + 1.25x (barlow, OCS, glasspath) as a f/5 dobson?



#2 Keith Rivich

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Posted 19 September 2020 - 09:43 PM

Are you asking: Does an f/4 reflector with a 1.25x barlow have the same coma free field as an f/5 reflector?



#3 Jon Isaacs

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Posted 20 September 2020 - 03:56 AM

Hello, has the same coma a f/4 dobson + 1.25x (barlow, OCS, glasspath) as a f/5 dobson?

 

No.  The barlow, OCS does not correct the coma.  A barlow only magnifies what there, if coma is there, then it's magnified.

 

Coma is corrected by adding reverse coma.  

 

Jon



#4 AlbertoJ

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Posted 20 September 2020 - 04:41 AM

Are you asking: Does an f/4 reflector with a 1.25x barlow have the same coma free field as an f/5 reflector?

Yes.



#5 AlbertoJ

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Posted 20 September 2020 - 04:46 AM

No.  The barlow, OCS does not correct the coma.  A barlow only magnifies what there, if coma is there, then it's magnified.

 

Coma is corrected by adding reverse coma.  

 

Jon

But coma free field depends on magnification (telescope focal lenght and eyepieces focal lenght) as well as eyepiece apparent field of view. Isn't it?



#6 Jon Isaacs

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Posted 20 September 2020 - 04:56 AM

But coma free field depends on magnification (telescope focal length and eyepieces focal length) as well as eyepiece apparent field of view. Isn't it?

 

In an F/4 Newtonian, the coma free field is 1.4mm.  That does not depend on telescope focal length, eyepiece focal length, magnification, apparent field of view.  

 

If you want to calculate the coma free apparent field of view, then it does depend on magnification and the telescope focal length.  

 

For example, a 400mm F/4 Newtonian has a coma free field of   57.3 deg/rad x 1.4mm /(400mm x 4) = 0.0501 degrees.

 

If you magnify the image 200 times, then the coma free field will be 200 x 0.0501 degrees = 10.0 degrees.

 

With a 1600mm focal length eyepiece, 200x requires an 8mm eyepiece.

 

If you use a 1.25 x Barlow, you will now be using a 10mm eyepiece to get the 200x and the coma free field is still 10.0 degrees.

 

I hope this makes sense.  

 

Jon


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#7 AlbertoJ

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Posted 21 September 2020 - 07:41 PM

Do you mean only 10° of apparent field of view can be seen without coma?

Then If you use a Nagler at 200x, 10° useful and the other 72° with bad quality?

What's exactly the coma free field formule or ecuation?




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