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Is a Maksutov always considered worse for viewing faint DSO over a Newtonian or Refractor, even at the same aperture? Or it's just exit pupil related?

Visual Maksutov
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#76 Hesiod

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Posted 09 May 2024 - 10:50 AM

Since stardiagonals have a fixed optical path (ca 120-150mm for 2" and ca 70-80mm for 1.25" are common values; every thing else equal, SCT-variants are a tad shorter; prisms tend to be a bit shorter than mirrors) the effective impact is % greater the smaller the telescope is.

In fact, the focal length increase is a function of the displacement and should be the same for every telescope with the same optical "receipt": in my case, with both telescopes (C8 and IM500) having f/2 main and f/5 secondary mirror I estimated an increase between 1:2 and 1:3 (for each millimeter of displacement the focal increases by more than 2mm but a bit less than 3mm).

The 2" SCT stardiagonal I have used in my example has an optical path of ca 120mm, meaning it will increase the focal of the IM500 or C8 by a value of k*(110-A) where k is between 2 and 3, while A is the optical path required to have the nominal focal length.

 

If we assume the same value for A in both the C8 and the IM500, e.g. 70mm, and the same value for k (e.g. 2.5), would obtain a 100mm increase for both, which translates into a 8% increase for the IM500 and 5% increase for the C8.

 

However I observed a different behaviour because in the IM500 the value for A is smaller: its nominal focal length is attained at a closer distance from the physical end than in the C8 (I can not tell the exact reason but have observed that some features do not scale at the same ratio of optics and end being comparatively larger).

 

The case of the C9.25 posted by David Knisely is especially interesting to my musings because this model has a different optical design and a f/2.5 main. Since it seems to hit the nominal f.l. with a refractor-type stardiagonal, which has the longest optical path, I'd expect it to be quite a bit shorter at prime focus, or even with a 1.25" prism


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#77 KBHornblower

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Posted 09 May 2024 - 12:48 PM

To me this is much ado about trifles.  I am not fussy about the exact focal length or ratio in routine visual observing.  I am interested in how well the scope shows faint nebulae or galaxies.  There is not a sharp focal ratio limit beyond which the scope suddenly becomes unsatisfactory.


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#78 Starman1

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Posted 09 May 2024 - 03:16 PM

Thank you David,

 

I have been wondering about this phenomenon and didn't want to start a topic full of conjecture and opinions.

Your effort and work producing the data is much appreciated. waytogo.gif

 

Best regards

Please note that because the magnification of the secondary on the 9.25" SCT is only 4.2x, versus 5x on the other sizes, the focal length changes less per extra mm of back focus than the average SCT.

On the f/10 models with f/2 primary and 5X secondary, adding a mm to the back focus adds 3.1mm to the focal length.


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#79 Hesiod

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Posted 10 May 2024 - 05:31 AM

Well, my ramblings were just a reminder to be sure to know the real focal a MCT is working at before ruling it "dim".

As a side note, the placement of main mirror affects also the amount of SA
for the system.
As an example, I use a GPC with my C8 because have found the views a bit
more mushy if do not use it.
The overall difference is not huge, but can be seen nonetheless and I believe that acknowledging its existence is important when drawing comparisons.

Theoretically it could works the other way depending on the original correction of the system

#80 Starman1

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Posted 10 May 2024 - 10:05 AM

Well, my ramblings were just a reminder to be sure to know the real focal a MCT is working at before ruling it "dim".

As a side note, the placement of main mirror affects also the amount of SA
for the system.
As an example, I use a GPC with my C8 because have found the views a bit
more mushy if do not use it.
The overall difference is not huge, but can be seen nonetheless and I believe that acknowledging its existence is important when drawing comparisons.

Theoretically it could works the other way depending on the original correction of the system

One paper I read on the design of the original SCT was that SA would be as large as 1/4 wave with a displacement of the primary mirror by as little as 1mm.

The paper recommended fixing the primary-secondary mirror spacing and focusing externally.

I haven't done the math, so I don't know whether that figure is true or not, but certainly fixing the mirror also fixes the focal length and f/ratio.

A moving mirror system was selected to fix the position of the focal plane so an external focuser need not be used.

And, of course, that resulted in a whole host of other problems.


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#81 Jehujones

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Posted 11 May 2024 - 12:36 PM

To me this is much ado about trifles.  I am not fussy about the exact focal length or ratio in routine visual observing.  I am interested in how well the scope shows faint nebulae or galaxies.  There is not a sharp focal ratio limit beyond which the scope suddenly becomes unsatisfactory.

I realize that we are not talking about significant values, it was just a curiosity and nothing more for me.



#82 Jehujones

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Posted 11 May 2024 - 12:37 PM

One paper I read on the design of the original SCT was that SA would be as large as 1/4 wave with a displacement of the primary mirror by as little as 1mm.

The paper recommended fixing the primary-secondary mirror spacing and focusing externally.

I haven't done the math, so I don't know whether that figure is true or not, but certainly fixing the mirror also fixes the focal length and f/ratio.

A moving mirror system was selected to fix the position of the focal plane so an external focuser need not be used.

And, of course, that resulted in a whole host of other problems.

...And, of course, that resulted in a whole host of other problems waytogo.gif




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