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Your choice of focal ratio?

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#1 Simoes Pedro

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Posted 12 July 2013 - 01:49 PM

Hi all,

if telescope length were not a concern, nor were optics type (achro, Apo), which focal ratio would you pick for visual observation, both some deep sky and planetary.

Too short and eyepieces will suffer, but too long and exit pupil will diminish.

What do you think?

I think I can live in the f8-12 range.

#2 Jon Isaacs

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Posted 12 July 2013 - 02:13 PM

Hi all,

if telescope length were not a concern, nor were optics type (achro, Apo), which focal ratio would you pick for visual observation, both some deep sky and planetary.

Too short and eyepieces will suffer, but too long and exit pupil will diminish.

What do you think?

I think I can live in the f8-12 range.


How it works for me:

- Field of view will suffer with longer focal lengths so shorter focal lengths and faster focal ratios are desirable. There are eyepieces that work well with fast scopes and there are fast scopes that are corrected for field curvature.

- Again, this is how it works for me: Mirrors are the solution to having sufficient aperture for planetary observation...

I think of refractors as smaller scopes and companions to large reflectors. As such, they need to do the things a larger scope is poor at or cannot do. At the top of the list is wide field viewing under dark skies.

Jon

#3 terraclarke

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Posted 12 July 2013 - 02:23 PM

Vixen used F9 to great advantage in their 4 inch Fluorite apo. That was a sweet scope capable of bright images, great contrast, good color correction, and relatively wide fields with a 2 inch diagonal and 2 inch w.f. eyepiece. For an achro, the Stellarvue 80/9D (F9.4) was also a very capable scope. I also loved the Televue 102; that was F8.6. I would choose something around F9 (and 90 to 102mm) if I were to have a single, multi-purpose telescope.

#4 Eddgie

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Posted 12 July 2013 - 04:57 PM

To me, F/8 to f/9 is Ok in a smaller scope. You can get a 2.9 degree field out of a 100mm f/9 telescope. It is rare to need more than this. Very rare. At least for me personally.

I have owned Genesis (102 f/5) and TV 101 (102 f/5.4) and a couple of longer focal length 4" refractors (F/8 and F/9 ED/APOs) and I was just as happy with them as I was with the Telvues for visual.

For imaging, f/5.4 TV 101.

But for visual observing, it has to be very well corrected (not necessarily AOP but pretty close). That is far more important to me than the focal length or focal ratio.

Otherwise, I an not sure why it would matter other than the cost of getting a fast, well corrected scope, which can be very expensive as they get bigger than 100mm.

#5 Simoes Pedro

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Posted 12 July 2013 - 05:55 PM

F8-9 seems to be the sweet spot.

#6 karstenkoch

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Posted 13 July 2013 - 03:03 AM

Two ways of looking at this question:

1) What focal ratio would you choose for a single, multipurpose scope? (either refractor or reflector)
2) What single, common focal ratio would you choose for a range of scopes? (that includes both refractors and reflectors)

I'm not sure this is the ideal answer, but given that my three refractors are f/6, 6.5, and 7, I'm going to be looking for a reflector in that same range (but with larger aperture and focal length) when I decide to make that future purchase.

So, considering the second way of answering the OP's question, what focal ratio would you choose?

#7 Jon Isaacs

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Posted 13 July 2013 - 03:38 AM

So, considering the second way of answering the OP's question, what focal ratio would you choose?



I think it is impossible to divorce focal length and focal ratio from the physical size of the scope. They are just too intimately tied together. The difference between a 10 inch F/5 and a 10 inch F/7 is there to be seen at the eyepiece but it is small compared to the differences each presents in terms of observing ergonomics and transport.

That said, the scopes I use on a regular basis, refractors and reflectors, they are all in the F/4-F/7 range. F/7 is pretty nice for an ED/apo refractor without a built-in field flattener... Newtonians can be faster and faster Newtonians are more practical.

Jon

#8 russell23

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Posted 13 July 2013 - 05:31 AM

I think more in terms of overall focal length for refractors. I really like the 700-1000mm FL range. That range is long enough to get high magnifications easily but small enough to get wide field views. So then the focal ratio is a matter of the scope aperture. My Vixen 140 is an f/5.7 petzval design with 800mm FL which provides a nice flat field that is easier on eyepiece performance than a regular F/5.7 would be.

Let me add that for F/7.5 and slower I'm good with a regular refractor design. Faster than F/7 and I want the Petzval design.

Dave

#9 Illinois

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Posted 13 July 2013 - 06:51 AM

For me is between f8 and 9! My new ES 127mm is 7.5 and I love it!

#10 Scott Beith

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Posted 13 July 2013 - 07:34 AM

My 3" and 5" refractors are f/7 and my 4" refractor is f/7.8.
If I was using achromats as my baseline I would definitely change the focal ratio and my DSO observations would center on globs and planetary nebulae with one exception. I would find another 80mm f/6 Stellarvue Nighthawk for widefield and travel. I had more fun with that scope than any other since I began observing.

I find my current collection good for anything from widefield to high mag planetary.

My personal preference for targets is Solar, Lunar, and Planetary.

#11 vahe

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Posted 13 July 2013 - 08:43 AM

To me the sweet spot for achr is f/15 and for apo is f/9. If you want shorter scopes, no problem, you just have to accept the inevitable tradeoffs.
My 155 f/9 EDT is just right for both wide angle and high power observations, it is 12” longer than the more popular F/7 version but the weight is the same and the same mount will carry them both.

Vahe

#12 Jon Isaacs

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Posted 13 July 2013 - 09:32 AM

To me the sweet spot for achr is f/15 and for apo is f/9. If you want shorter scopes, no problem, you just have to accept the inevitable tradeoffs.
My 155 f/9 EDT is just right for both wide angle and high power observations, it is 12” longer than the more popular F/7 version but the weight is the same and the same mount will carry them both.

Vahe


It's all relative... if a 1.9 degree field of view qualifies as "widefield". I think of 1.9 inches as widefield for a 12 inch scope.

If one thinks in a different paradigm, that is that a fast focal ratio is not about shorter scopes but rather larger aperture scopes of the same focal length, then the losses due to choosing a slower focal ratio are greater. A 6 inch F/9 versus a 9 inch F/6..

Jon

#13 russell23

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Posted 13 July 2013 - 10:11 AM

To me the sweet spot for achr is f/15 and for apo is f/9. If you want shorter scopes, no problem, you just have to accept the inevitable tradeoffs.
My 155 f/9 EDT is just right for both wide angle and high power observations, it is 12” longer than the more popular F/7 version but the weight is the same and the same mount will carry them both.

Vahe


It's all relative... if a 1.9 degree field of view qualifies as "widefield". I think of 1.9 inches as widefield for a 12 inch scope.

If one thinks in a different paradigm, that is that a fast focal ratio is not about shorter scopes but rather larger aperture scopes of the same focal length, then the losses due to choosing a slower focal ratio are greater. A 6 inch F/9 versus a 9 inch F/6..

Jon


That's how I look at it. At faster focal ratio's I really like the Petzval design which I think has 2 advantages. First, the objective will actually be longer FL than the final. I think my vixen 140 has an F/8.4 objective. So the CA is not as bad as if it was a regular F/5.7 achromat.

Second, the Petzval design does provide a nice flat field. At F/8 and longer the Petzval design is not necessary because the field of the refractor is pretty flat.

Dave

#14 Wilsonman

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Posted 13 July 2013 - 11:35 AM

For me, I'd prefer an achro be F/15 and I think an ED refractor is best at F/9. I would not want to go slower mainly because of the lower magnifications the ep's would yield.

#15 jrbarnett

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Posted 13 July 2013 - 01:23 PM

Visual only?

f/8 or f/9. 6" and under in aperture.

You still get "richest field" effect with widefields like the 41mm Panoptic and 31mm Nagler Type 5, but likewise have no trouble logically and systematically ramping up magnification for other targets using well-spaced eyepiece focal lengths an never once needing to resort to a Barlow. You also do not need to be selective about eyepiece design - simple-old, complex-new all work well at f/8 and f/9. At f/7, older eyepiece designs start to fall apart at the edge of the FOV, limiting both versatility of the eyepiece and the telescope.

I'm assuming an ED or fluorite doublet or ED triplet design. Achromats in these do-everything focal ratios are a mess on the CA front which compromises their high magnification suitability IMO.

Regards,

Jim

#16 Astrojensen

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Posted 13 July 2013 - 01:50 PM

The logic in me says that I should say "f/8", but then I come to think of the superb performance I've seen in several very long focal ratio instruments and I want to say "f/20".


Clear skies!
Thomas, Denmark

#17 sg6

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Posted 13 July 2013 - 02:26 PM

I seem to start thinking at about the f/7.5 mark.

#18 dennilfloss

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Posted 13 July 2013 - 03:03 PM

I settled on F/7 for my 'Jack of all trades' ED refractor as portability was a major concern while still getting decent chromatic correction. It allows me to view 3.5 degrees in widefield mode but I can still reach a good amount of magnification without having to put up with an overly long tube.

#19 vahe

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Posted 13 July 2013 - 03:40 PM

The logic in me says that I should say "f/8", but then I come to think of the superb performance I've seen in several very long focal ratio instruments and I want to say "f/20".


I was not going to say it, but now that you mentioned it F/20 is in fact the magic number for dedicated planetary observer, I have two f/20 Maks.

Vahe

#20 Astrojensen

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Posted 13 July 2013 - 05:25 PM

And it's not only magical on the planets, but also on double stars and small deep-sky objects; well basically any object, where you need high magnification. As I seem to have a preference for high magnification, a long focal ratio makes a lot of sense to me.

My old 85mm f/19 Zeiss A doublet apochromat has given me some absolutely stunning, aperture-defying views of a lot of different stuff. I wish someone would make a modern 5" f/20 ED doublet in a lightweight OTA with a 3" focuser with extension tubes and lots of travel, so I could put a focal reducer on it for moderately wide deep-sky, have enough back focus for a binoviewer on a 2" Herschel wedge and reaching focus without barlows.

It would be long, yes, but I firmly believe it would be worth it. It would be a modern version of the 130mm Zeiss A.


Clear skies!
Thomas, Denmark

#21 Simoes Pedro

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Posted 13 July 2013 - 05:50 PM

For such long focal ratios ~f20. Would you consider a split tube design where the two parts would come apart for easy storage and transportation?

#22 BKBrown

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Posted 13 July 2013 - 06:33 PM

I prefer my refractors in the f/7 to f/9 range. I'd love to have a 203mm f/9 Apo! :pulpdnc:

Clear Skies,
Brian

#23 Astrojensen

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Posted 14 July 2013 - 01:53 AM

For such long focal ratios ~f20. Would you consider a split tube design where the two parts would come apart for easy storage and transportation?


Absolutely.


Clear skies!
Thomas, Denmark

#24 Scott99

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Posted 14 July 2013 - 10:42 AM

To me the sweet spot for achr is f/15 and for apo is f/9. If you want shorter scopes, no problem, you just have to accept the inevitable tradeoffs.
My 155 f/9 EDT is just right for both wide angle and high power observations, it is 12” longer than the more popular F/7 version but the weight is the same and the same mount will carry them both.

Vahe


I was on AP's waiting list for this f/9 ED scope for many years - f/9 would be my preference too. This list went inactive and they went with their new 160mm design, so I'm limping along at f/7.5. Managing to live with it OK though!

If money was not a factor and I was choosing a scope for a home observatory installation I'd probably go more along the lines of f/12. APM makes some larger f/12 apo dream scopes. Would love to sit down with some Brandons and Claves with one of those bad boys! Maybe a 12-inch f/12, that sounds about right.


#25 Astrojensen

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Posted 14 July 2013 - 03:00 PM

they went with their new 160mm design, so I'm limping along at f/7.5.


Even if you would have rather liked it to be f/9, I can think of worser fates than having to live with a 160mm f/7.5 Astro-Physics refractor...


Clear skies!
Thomas, Denmark






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