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Focal Reducer with Fast Newtonian?

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#1 Shadowalker

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Posted 04 August 2005 - 09:50 AM

Hello Everyone.

Is it feasible to use a focal reducer with an F5 Newtonian. I have one I could borrow, so I might just try it, but at 0.5, it would give me F2.5 - seems like there's bound to be problems with that. I'm wondering what they might be.

Sure would be nice to get a wider field when imaging M31 without buying a little APO...

CS,

Tom

#2 markf

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Posted 04 August 2005 - 09:59 AM

Hmm...would a focal reducer make coma more appaerent? Would the Baader MPCC keep up?

Let me know, when you test, how it goes. Be good info to store in the ol' brain. (although I may be running out of space storing all the other knowledge I've been absorbing in the DSLR forum)

Mark

#3 Shadowalker

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Posted 04 August 2005 - 10:18 AM

I've thought of two things that would limit it - coma and vignetting. The MPCC is supposed to correct from F4 on up. If it didn't work at F2.5, I could use my 1.5X barlow. With the 0.5 and a 1.5 barlow, I'd get F4. Which is an improvement.

Vignetting. Bound to be more, but I'd need to run the Newt program with my scope dimensions to get a feel for how much.

#4 Jarad

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Posted 04 August 2005 - 10:33 AM

Coma will be unchanged - the focal reducer doesn't add coma, it's a property of the primary mirror, which will still be f5.

But vignetting will be a big problem - I doubt you will actually see that much more field, unless you have a really large secondary. Most newts are designed to have about 70% illumination at the edge of a 2" eyepiece, with the focal reducer that same illumination will be halfway to the fieldstop. It will get very faint past that pretty quickly (the light dropoff is sort of a bell-curve shape, so you will be hitting the steep part of the curve with the focal reducer....).

Jarad

#5 Jon Isaacs

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Posted 04 August 2005 - 01:14 PM

Coma will be unchanged - the focal reducer doesn't add coma, it's a property of the primary mirror, which will still be f5.
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On the other hand, you will be looking at a field of view that is twice as wide so the coma will be increased from that point of view.

A couple of added thoughts:

1. In general focal reducers are designed to mate to a given optical system, ie refractor or SCT over a small range of focal ratios. I believe this is because they need to correct of inharent aberrations like field flatness because as you go wider, those sorts of aberrations become more critical.

2. Besides the Vignetting, the aberrations at the edge are going to be a serious problem. Widefield is only worth doing if the stars are sharp across the FOV.

IMHO, if one wants a widefield of view with a Newtonian, I think the way to go is a smaller scope.

jon

#6 dgs©

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Posted 04 August 2005 - 01:22 PM

I'm treading in unfamiliar territory here, but would you really be getting a wider field of view? Would the wider apparent FOV of the eyepiece/reducer train be larger than the secondary mirror, so that part of that super wide FOV would be the far side of the OTA? Or is that the Vignetting you are referring to?

#7 BluewaterObserva

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Posted 04 August 2005 - 02:44 PM

This is a case where oversized secondary mirrors make some sense....

What are the full specs of the newt here???

People do reduce newts, but I am not real familure with it myself. I read about mostly at professional observatories have always and still use newtonians.

#8 Shadowalker

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Posted 04 August 2005 - 02:48 PM

It's a Konus 200mm F5. I've taken the dimensions, but I don't have the numbers with me now. I do know that the secondary is a little over 2 inches, which is large for an 8 inch newt.

DGS, yes, that was the vignetting I was refering to. The wider field might just turn out to be the inside of my scope tube. But my hunch is that if I were do 1.5 barlow it first, then reduce it .5, I think I might just get an effective F4 out of it. Just a hunch. Need to work the numbers.

#9 Jarad

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Posted 04 August 2005 - 03:12 PM

On the other hand, you will be looking at a field of view that is twice as wide so the coma will be increased from that point of view.



Yes, twice the field, and half the mag. Since coma incerases linearly with field, it will appear the same as at the higher mag/smaller field. It's kind of funny, but for a given f-ratio, coma only depends on apparent FOV, everything else cancels out.

IMHO, if one wants a widefield of view with a Newtonian, I think the way to go is a smaller scope.



I agree.

You could try the 0.5x reducer with the 1.5x barlow, but I think that you are asking for even more abberations by stacking that many elements together. You'd probably be better of using something like the TV 0.8x reducer rather than stack a 0.5x and 1.5x.

Jarad

#10 Shadowalker

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Posted 04 August 2005 - 04:08 PM

Thanks for all the comments. I seriously coubt F2.5 is practical in my scope. F4 might be. But I suspect the real answer is a 400 or 500mm FL scope.

Still, I might just give it a try. Nothing to lose except precious non-cloudy nights!

#11 Lego Knight

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Posted 05 August 2005 - 11:17 AM

the True FOV is based on two numbers.
focal length of the scope and the field stop in the eyepiece.

(field stop/focal length)* 59 = TFOV in degrees

thus the focal reducer will not let you see more.
Apparent FOV is a different story.

#12 Jon Isaacs

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Posted 05 August 2005 - 12:35 PM

the True FOV is based on two numbers.
focal length of the scope and the field stop in the eyepiece.

(field stop/focal length)* 59 = TFOV in degrees

thus the focal reducer will not let you see more.
Apparent FOV is a different story.
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The focal reducer actually reduces the effective focal length of the scope so that the true FOV would be

57.3 (deg/rad) x Field stop (mm)/(Focal length scope (mm) x Focal reducer ratio)

If vignetting does not occur and the secondary is large enough, then a 0.5 focal reducer will double the field of view.

jon


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