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F/4.5 -F/5:Any significant differences?

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#1 Dragonwatcher

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Posted 26 March 2013 - 02:47 PM

I am toying with many possibilities in a Dob right now from getting new to buying a mirror and having the structure built for me.

One issue is keeping the eyepiece at zenith close to 5' 8" or so. I am thinking ultralight and a mirror of 1" or less (already have two good candidates in the 14" -15" range). Is the difference between a focal ratio of f/4.5 pretty inconsequential versus a 4.8 except maybe for a small difference in eyepiece height? Does it really matter in terms of balancing, collimating, etc. I guess I am leaning towards a 4.8 hoping I might not need a paracorr.

#2 ed_turco

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Posted 26 March 2013 - 02:59 PM

Not much difference in performance between the two so go for the eyepiece height you want.

In any case, you'll need a Paracorr even at f/4.8. You'd have to go to f/6 or a little above to avoid the use of the coma corrector. Then you'd need a stool!

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#3 Lamb0

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Posted 26 March 2013 - 03:11 PM

I doubt you'd see much in the way of an appreciable difference between an f/4.5 or f/5. However, with your Ethoi I definitely recommend a Paracorr. With such a large TFoV the Ethoi will never deliver their best w/o a Paracorr. Even a used Type I should be sufficient, save perhaps for the 21mm Ethos or 31T5 where the Type II Paracorr is recommended for best results. As you're planning on building, (or built for you), I'd plan on the SIPS integrated system for ease of use. Try your Ethoi in scopes with similar f/ratios with and without a Paracorr. IMHO, premium optics deserve a Paracorr! YMWV ;)

#4 Jarad

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Posted 26 March 2013 - 03:21 PM

Does it really matter in terms of balancing, collimating, etc.


In terms of balance for an ultra-light, every bit counts. Since you are using a thin mirror, the mirror box will be light. The longer the FL, the more leverge the UTA has.

You can certainly use either f4.5 or f5, but as the FL gets longer you will need to either add weight to the mirror end, use larger altitude bearings, or use a double truss design with the altitude bearings further up from the mirror. The third option will result in a scope that is less sensitive to balance shifts when changing eyepieces, but it will require a taller base.

I have a very similar scope - 14.7" f4.4, 0.7" thick quartz mirror. PM me if you would like to discuss it.

Jarad

#5 Joad

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Posted 26 March 2013 - 04:39 PM

If you want to keep the eyepiece at 5'8" at zenith with a 14-15" scope, I think you'll have little choice but to go f/4.5, for which you will want a Paracorr. My f/5 is borderline on the Paracorr issue (I don't use one, but I think other viewers who are more concerned with the outer edges than I am would want one), and at 12.5" the eyepiece is certainly within the 5'8" limit at zenith, but I doubt that a 14-15" f/5 would be.

#6 sniperpride

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Posted 26 March 2013 - 11:44 PM

I never use a paracorr in my 4.5, I used it for a while, but it just doesn't bother me that much to use it every time

#7 Mirzam

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Posted 27 March 2013 - 05:50 AM

I have built an ultralight 14" with a thin mirror and agree with the comment that f/4.5 is much easier to balance. When designing the structure, try to recess the mirror as deeply as possible into the mirror box so that the mirror weight serves more efficiently as a counterbalance.

You will want a Paracorr although it does add considerable weight at the UTA.

JimC

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#8 Achernar

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Posted 27 March 2013 - 09:27 AM

The differecnes between an F/5 and an F/4.5 mirror of the same aperture will be more coma through the faster mirror, and a lower eyepiece height. That means the eyepiece will be about 7-inches closer to the ground with an F/4.5 mirror than a F/5 mirror, and the field of view will be wider too with any given eyepiece. Eyepiece height was a major factor in my decision to opt for a 15-inch F/4.5 mirror over the F/5 I originally wanted. Now I can look anywhere in the sky, without a step stool or step ladder. I feel a coma corrector is needed for either F/ratio, so I would base my choice on how high you want the eyepiece to be above the ground.

Taras

#9 Jon Isaacs

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Posted 27 March 2013 - 09:44 AM

I am toying with many possibilities in a Dob right now from getting new to buying a mirror and having the structure built for me.

One issue is keeping the eyepiece at zenith close to 5' 8" or so. I am thinking ultralight and a mirror of 1" or less (already have two good candidates in the 14" -15" range). Is the difference between a focal ratio of f/4.5 pretty inconsequential versus a 4.8 except maybe for a small difference in eyepiece height? Does it really matter in terms of balancing, collimating, etc. I guess I am leaning towards a 4.8 hoping I might not need a paracorr.


My 16 inch F/4.42 has a 70.8 inch focal length, almost the same as a 14 inch F/5. The eyepiece height at the zenith is between 68 and 69 inches. A 14 inch F/5 is doable, especially if you use a reasonable sized secondary that allows reasonable illumination with a standard height focuser. There can be a couple of inches right there.

But I would suggest looking through some faster Dobs with and without a Paracorr. Clean sharp views across the field of view are possible at F/4.5 and faster with a Paracorr. They are not possible at F/5 without a Paracorr/coma corrector though many find them passable.

Jon






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