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Re: [ATM] Using Barlow to reduce Coma

Hi,

I suppose that when I have to refocus to have nice pinpoints at the border of the field, I am dealing with field curvature.
I am using Radian eyepieces.
Coma is a arrow figure, however most light is at the tip of the figure. I suspect that this is the reason one has to go far from the optical axis to see the arrow.

Regards,
Adrie

  ----- Original Message ----- 
  From: Tony Gondola 
  To: A. Suijkerbuijk 
  Sent: Thursday, January 26, 2006 4:24 PM
  Subject: Re: [ATM] Using Barlow to reduce Coma


  In a straight newtonian, even one this fast, coma is the main abberation. It's far larger then the effects of field curvature.

  Tony


  A. Suijkerbuijk wrote:

Hi,

I am using a Paracorr on a f/3.2 mirror, but for large magnifications it is 
better not to use it. There is a lot of light around stars, spherical 
aberration perhaps.
It is nice for low magnifications. I think coma is not the main problem, 
field curvature is the real problem.
I think a corrector for high magnifications using such a mirror has to be 
calculated.

Regards,
Adrie Suijkerbuijk
----- Original Message ----- 
From: "Mark" <marco.miglionico@btinternet.com>
To: "Mel Bartels" <mbartels@bbastrodesigns.com>; "'AMT LIST'" 
<atm@atmlist.net>
Sent: Wednesday, January 25, 2006 9:13 PM
Subject: Re: [ATM] Using Barlow to reduce Coma


  O.K., so if I were to pop in a 40mm plossl, then coma would look
as bad with and without the Barlow?

My common sense tells me that as I am only looking at half the field with
the barlow in, so
it would be like looking at the centre area without the barlow -  which 
has
less coma!

By that rationale, what I want to know is comarping the smaller field with
the barlow in the F3.3 scope,
to say a scope with an F6.6 mirror --   what will the difference be?
(Bearing in mind I know
that the Barlowed F3.3 is looking at the central part of the field which 
has
inherently less coma.)

Help.

----- Original Message ----- 
From: "Mel Bartels" <mbartels@bbastrodesigns.com>
To: "'AMT LIST'" <atm@atmlist.net>
Sent: Wednesday, January 25, 2006 4:32 AM
Subject: Re: [ATM] Using Barlow to reduce Coma


    What my question really is: will my F3.3 mirror have the same coma as
an F6.6 newt if I use a x2 Barlow?
        This isn't as easy to answer as one might think since the definition of
'same coma' is fuzzy.  But strictly speaking, no, a Barlow will not
decrease
or increase coma.

Mel Bartels



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