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Would f4.5 be desireable vs f5 for 12.5"

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#1 stargazer193857

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Posted 22 August 2019 - 03:16 PM

Assuming two mirrors are made to the same quality for the same price, which one would you prefer to use for your scope?

The f4.5 lets you sit less high at zenith and have a wider low power view. The f5 has a smaller obstruction and less coma.

I'm thinking the f5 is better for planets and the f4.5 for zenith DSO. Which would you prefer?


Also, f5 is not enough to hit 20% CO. Obsession gets to 20% with a low profile focuser. That is good for a smaller airy disk, but a long focuser and larger CO are good for fighting stray light for galaxies.

Edited by stargazer193857, 22 August 2019 - 03:19 PM.


#2 ShaulaB

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Posted 22 August 2019 - 03:18 PM

When I look at vendor sites, an f4.5 usually costs more than an f5 because it is harder to grind a good quality faster mirror.


Whichever one you choose, collination will be essential. An f4.5 scope can provide planet views just as good as a (somewhat) longer focal ratio reflector if you have good collimation, the right eyepieces, and a coma corrector.

Edited by ShaulaB, 22 August 2019 - 03:22 PM.

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#3 Achernar

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Posted 22 August 2019 - 03:27 PM

The difference in eyepiece height would be six inches between the two in a telescope of the same design, and if you are a fairly tall man like I am the F/5 mirror would be fine. If you have two mirrors with an F/5 and a F/4.5 focal ratio of the same quality, and you wish to observe while seated on a chair, the F/4.5 focal ratio would be a better choice. You will find coma noticeably worse through the faster mirror, and if you use a Tele Vue Paracorr as I do to get rid of it, you'll have a focal ratio of F/5.2 while using it. If the mirrors are good and well collimated,you will be more than happy with the views of the moon and planets.  With an F/5, you may find there is no need for a coma corrector. My Dobs are both F/4.5's because I wanted to observe from my chair or while standing without need for a ladder. If portability is important, it's better to go with a F/4.5 primary mirror than a F/5, especially for apertures over 12-inches. My 10-inch has a central obstruction of 25%, the 15-inch has a central obstruction of 20%. Both do very well on the planets when the seeing is good, and both are not built especially for planetary observing, they are used more on deep sky objects than anything else.

 

Taras


Edited by Achernar, 22 August 2019 - 03:33 PM.

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#4 WilburTWildcat

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Posted 22 August 2019 - 03:32 PM

Achernar makes a good point about sitting v.s. standing. Sitting also provides a much more stable base which means that details are easier to eek-out compared to the slight shake of standing or a ladder.

 

That said, I'd probably go with the F/5. smile.gif . Better all-around for planets/lunar/DSO. Get a really tall chair. lol.gif



#5 wrvond

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Posted 22 August 2019 - 03:32 PM

The difference in tube length is not enough for me to worry about and all my oculars are TeleVue. Coma is not really an issue either, since I have a paracorr. The difference in obstruction is, to my mind, negligible in use. Stray light is also a non-issue for me since I am in pretty dark skies.

I'd have to go with the slightly wider field of view as the deciding factor.



#6 Garyth64

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Posted 22 August 2019 - 03:36 PM

Again, not much of a difference to even think about.  I mean c'mon, they're most likely going to give the same views for planets and DSOs, and you won't be able to tell the difference unless you were told.


Edited by Garyth64, 22 August 2019 - 03:41 PM.

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#7 DocGP

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Posted 22 August 2019 - 04:01 PM

Again, not much of a difference to even think about.  I mean c'mon, they're most likely going to give the same views for planets and DSOs, and you won't be able to tell the difference unless you were told.

Yeah,  jmho, I would go with what kept my fat butt in the seat the most!! lol.gif lol.gif lol.gif

 

But that's just me.

Doc



#8 vtornado

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Posted 22 August 2019 - 04:29 PM

Hi,

 

I don't have a f4.5 scope, and I don't know how bad coma is, but in dob land at high power,

you generally let objects drift across your field of view, which means coma will affect the views at the

edges of the field.  I don't know how much more f 4.5 to f5, and I don't know if you want to

get a coma corrector.  I thought about getting a corrector, and it is not as simple as just plunking the device in.

It seems the corrector may need to be adjusted for each eyepiece if they are not para-focal???

I could be wrong, but I have enough doubts that I have not gone down that path yet.

 

I also have budget eyepieces and they are already "unhappy" at f5, I don't want to kill them off.



#9 scopewizard

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Posted 22 August 2019 - 05:03 PM

I would go with the 5. My 16" is f5 with a 2.77 secondary. General rule from a master optician is if secondary surface to primary is less than 3%, the contrast lost is almost nil. The lost of contrast is what kills details. Scope like mine is easy to collimate and I have never used a Parracor. My friend has a12" f6. Planet details are slightly better than mine but not by much.

Visual observing is aperture and contrast while imaging is focal ratio. You need to strike a balance if using the same scope. I always recommend one to observe and one for imaging. See my equipment below.



#10 Pinbout

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Posted 22 August 2019 - 05:03 PM

Assuming two mirrors are made to the same quality for the same price, which one would you prefer to use for your scope?

The f4.5 lets you sit less high at zenith and have a wider low power view. The f5 has a smaller obstruction and less coma.

I'm thinking the f5 is better for planets and the f4.5 for zenith DSO. Which would you prefer?


Also, f5 is not enough to hit 20% CO. Obsession gets to 20% with a low profile focuser. That is good for a smaller airy disk, but a long focuser and larger CO are good for fighting stray light for galaxies.

I use to like long fl, but I have since converted and would rather have a 12”f3

 

if it’s well figured it will be every just as special on planets grin.gif



#11 astrophile

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Posted 22 August 2019 - 05:51 PM

No conversion here, yet. If ever. At least not in my ever-increasing LP semirural yard. 12.5 f/6 for me, thank you!
(And an 8” f/7 on the way, for lift-n-go)

Edited by astrophile, 22 August 2019 - 05:56 PM.


#12 Jon Isaacs

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Posted 22 August 2019 - 07:49 PM

My thinking:

 

- A 12 inch F/5 has a 60 inch focal length should be an easy sit down scope. My 16 inch F/4.4 which has a 70.9 inch focal length is a sit down scope with a catsperch Pro chair.

 

- Coma is real.  At F/5 the diffraction limited circle is 2.75 mm. In a 12 inch, thats 0.1 degrees. If you're letting objects drift across the field, at 200x, 21 degrees AFoV is diffraction limited.  Your perfect mirror has a Strehl of 0.8 or more inside that circle.

 

I use a Paracorr 2 with my 13.1 F/5.5. with type 6 Naglers, the planets are clean and sharp across the entire field, without correcting the coma, the sweet spot is small..

 

My 12.5 inch is F/4.06, it's the reason I have a Paracorr. I've had it for nearly 20 years. We've been friends a long time.. it has its advantages and disadvantages but it I were starting from scratch, I'd go F/5. I'd still be using a Paracorr...

 

Regarding Vtornado's questions about coma correctors and spacing: The Paracorr 2 has this nailed. The Tuneable Top is like a very fast smooth, very tight  helical focuser. You can focus with the Tuneable Top, fine focus with the Focuser.

 

The 12.5 inch F/4.06 alongside the 13.1 inch F/5.5 (72 inch FL) person is 5'10".

6362240-Rollie and the Dobs CN.jpg
 
Jon


#13 stargazer193857

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Posted 22 August 2019 - 11:52 PM

I think the eyepiece of a 12.5" f5 would be 58", at least 60" if a low profile focuser is used.

That means a 29-31" chair for me. Many people here could shave 5" off the chair, but their legs are shorter than mine. Interesting though that f5 is preferred. If someone buys a coma corrector, they can go bigger and faster. And if they don't yet have one, f5 may be best.


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