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Pros and cons of Fast/Slow scopes

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#1 JamesBaud

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Posted 22 August 2004 - 06:30 AM

I'm trying to understand the pros and cons of "fast" or "slow" telescopes. I seem to hear conflicting information. I understand the benefits of increased aperture, and I understand how the focal length of a scope changes the effective magnification of eyepieces... But I don't understand why one would want a "fast" or "slow" telescope. If I understand correctly, the F numbers indicate the elongation of the cone of light coming into the scope. F/15 would be a very long, elongated cone, whereas F/5 would be short and squat (at least relatively). It seems that, given focal reducers and barlows, you can emulate just about any F number you want anyway.... So whats the big deal?

Here are the pros and cons as I understand them... If you have any comments or I'm wrong, please correct me:

A fast scope can get lower magnification easier, and a slow scope can get higher magnification easier. (easier meaning without barlows and focal reducers, without using 2mm pinhole eyepieces, etc.)

I'm not worried about astrophotography just yet, but I've heard fast scopes are better for it, and will require shorter exposures. That only makes sense to me if a fast scope provides a brighter image, but the brightness of the image should be based purely on aperture and magnification, right? Oi!

I've also heard that extremely fast scopes have trouble with certain eyepieces, but I don't understand entirely why this is.

And I've heard that planetary observing is better with slower scopes, and I've heard people get indignant about that claim too.

So is the speed of a scope really important?

#2 Tim2723

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Posted 22 August 2004 - 07:27 AM

Hi James, and welcome to CN!

Like everything else in the hobby, this subject is argued about.

Fast and slow are terms borrowed from photography to describe the speed at which an image could be captured on a photographic plate, which sort of shows how old the terms are. I'm not an astrophotographer, but I understand that this characteristic of telescopes is still valid today. From a strictly visual standpoint, the speed of the system has no effect on brightness, as the human eye cannot store the light it recieves. For the eye, aperture and magnification are the primary ruling factors. This will be argued, I'm sure, but for my experience, I cannot tell a difference in brightness between my f/5 and f/10 refractors. The field of view is twice as large for any eyepiece in the f/5, and this is a major advantage when viewing extended objects.

Generally speaking, short f ratio scopes are more prone to a variety of aberations, depending on the type of scope. In achormatic refracors, its false color; in reflectors its things like comma. There are exceptions to this as in everything else, for instance, the apochromatic refractor can be free of percievable color yet have an f-ratio that's quite short, compared to it's achromatic counterpart.

Slow scopes can have problems with certain eyepieces, but then again, fast scopes can have problems with other eyepieces. It gets quite confusing, but often has to do with the eyepiece cutting off part of the cone of light, or with it producing an exit pupil that is inapropriate for the application. There is nothing 'inferior' here, just a matter of selecting the right matches.

Planetary observers tend to want the greatest clarity and resolution possible with the least distortion from aberations (but of course, so does everyone else!), and they often use very high magnifications. Long focal length scopes are often the ones that provide this most easily, but there are exceptions, such as the apochromats and large aperture reflectors with superb optics that can be used more critically at shorter focal lengths.

A big advantage of shorter focal lengths is that, at least for refractors and classical reflectors, the scope can be more compact, which makes for easier handling. However, in the case of catadioptrics and certain folded designs, very long focal lengths can be packed into very short tubes.

I know that doesn't firmly answer any of your questions, but that's the point. There are no hard and fast rules here, but a juggling act. We contsantly juggle the attributes of any given system for any given application. This is why so many of us end up with multiple scopes. I like to think of them as tools with different purposes. There's the planetary scope, and the deep space scope, and the widefield scope, and the astrographic scope......

Welcome to the confusion of the astronomy hobby! Half the fun is in hanging around here to debate the answers!

#3 Jon Isaacs

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Posted 22 August 2004 - 10:01 AM

James:

A good question, Tim has given you some good answers. I will add a few thoughts...

1. Long focal length scopes are often restricted as to their maximum field of view and image brightness, this is due to restrictions in the eyepiece formats and also inharent in the telescope design. For example a 125mm F15 Mak that is limited to the 1.25 inch eyepiece format will have a maximum field of view of less than 1 degree a maximum practical exit pupil of not much more than 2mm. A scope like this is not going to knock your socks off scanning the Milky Way no matter how dark the skies are.

2. While barlows are readily available and relatively inexpensive, focal reducers are not generally available, the standard commercial SCTs do have focal/reducer correctors available.

3. It is true that long focal ratio designs often have fewer optical aberrations, this partly because "slow" optics are less demanding to produce.

However it is important to realize that Maks and SCTs are long focal ratio designs that use very fast primary mirrors and then use a convex secondary to provide a longer focal length. Most commercial SCTs actually use a primary that is about F2, spherical to be sure but getting top notch optics is not so easy.

4. Fast scopes are generally more compact that slow scopes. Simple scopes are generally easier to fabricate with premium optics. Most of the reasons people choose different designs has to with the trade off between optics, aperture and portability/transportability.

5. While it is true, fast Newtonians have off axis aberrations, for planetary viewing this is of secondary importance because even at high magnifications, a planet will still be in the diffraction limited field of view. (A Paracorr can be added to correct a much larger field if so desired.)

For these reasons, many planetary observers prefer large fast Newtonians, these days DOB/Driven DOB style. The large aperture provides brightness and resolution that will allow high magnfications when the seeing allows and the simple two element optics can be fabricated to with great precision...

So in general, a scope choice is a compromise. Size, cost, optics, aperture.....

I have a 12.5 inch F4.1 Newtonian, it fits in the back of my Ford Escort sedan. It gives good planetary views. I am sure that an 12.5 inch F8 Newtonian of equal quality would give better views but it would be nearly impossible to use, let alone transport with anything like the same ease.
-------

Bottomline: It is easier to make a fast scope into a slow scope than a slow scope into a fast widefield scope, in many cases it is just not possible.

I did not discuss refractors, that is another whole issue, fast achromatic refractors have significant false color which is not easy to correct. Fast APOchromatic refractors are the diamonds of the telescope world, beautiful to look at/through, but small and very expensive...

Best wishes, clear skies

jon isaacs

#4 BoriSpider

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Posted 22 August 2004 - 12:49 PM

So if I was buying a 8" dob and wanted to view planets
more, then I would buy the f/6 rather than the f/7?
Is that why they come in two f/ratio's?

I was kind of wondering the same Q.

#5 Don W

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Posted 22 August 2004 - 01:10 PM

Some of the others have alluded to this, but let me put it in different terms. With a fast scope, you may run into problems getting enough magnification for lunar/planetary viewing. You end up using eyepieces well under 10mm and eye relief can be uncomfortable. A slow scope can get to the same magnifications with longer fl. eyepieces which afford better eye relief.

On the flip side, with a slow scope it's harder to get the low magnifications for wide field viewing. But let's take a closer look here. Just what huge extended objects are there in the night sky that you need to get that low? Orion nebula is quite extended but doesn't require ultra low power. M31, the Andromeda Galaxy covers over 4 degrees, more than many binoculars can see. The Pleiades can be hard to get in a single view, but not much worse than the Orion nebula. So ask yourself, "Just how much time will I be spending looking at extended objects at very low power?". My answer to that question is "not much".

That would lead me to think that a medium to slow scope would really fill all my needs. So let's look instead at what focal length will give me what I want and pick a scope based on that instead. In my view, something around 1500mm focal length is pretty good. A 10mm eyepiece gives me 150X which is a comfortable magnification for most planetary observing. On a night of good seeing, I can use a 2X barlow to push up to 300X. If I stick with 1.25" eyepieces, I can get a maximum field of view with a Televue 24mm Panoptic for 62X and if I have a 2" focuser I can go with one of several 35mm eyepieces to give me 42X.

To get this sort of set up, I'd need one of the following:

12" f/5
10" f/6
8" f/7.5 (f/8 would be good too)
6" f/10

One of the most popular focal length scopes over the years has been around 1200mm. It's a bit better for extended objects but still not a big problem for higher powers. Two popular configurations in this focal length have been the 6" f/8 and the 8" f/6 newtonians. The mirrors are easy to make and don't require the amount of figuring that faster systems require. Most mirrors in this range are very good optically speaking.

That's my story and I'm sticking to it.

#6 Tim2723

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Posted 22 August 2004 - 03:41 PM

As you can see, you've already received three lengthy answers, each expanding on the other, yet none have been really contradictory yet. That illustrates the complexity of the subject; a complexity that could, and does, fill volumes.

On top of Don's description of the good general purpose telescopes then come the specialist instruments. With enough time in the hobby, many of us choose certain paths that appeal to us. Observers such as myself have spent thousands on equipment, books, and software to study just the moon and planets. Others have spent thousands, even tens of thousands of dollars on equipment that can ONLY study ONE single object, the sun. Take note of Don W's solar telescopes. Very expensive instruments that can only look at one thing, but in ways no other telesopes can. Then there are those who wish to see deeper and deeper into space and challenge themselves to see the farthest and faintest objects. Those observers own gigantic telescopes. Then again are there are those who seek the perfect photograph of distant places. They might own equipment more costly than the national debt of some small countries!

All of this put together is the stuff that we juggle. First as the newcomer who is usually a general observer, and then later as a specialist seeking the optimum performance in very unique applications.

#7 John M

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Posted 22 August 2004 - 03:51 PM

Just a couple of bones to add. Fast mirrors and objectives have more curve then slow optics. It is harder/more expensive to correctly shape more curve then less curve. The retailer here in town suggested that shorter focal length scopes (faster) are, at least partially, a by product of producing a suitable mount for a reasonable price. I met a guy last night with an 8 1/2" f6 Mak Newt. 1300 mm focal length I think. I feel relitivly certain that the mount it was monted on cost more then the used 2000 Dodge Dakota we just bought.

#8 CollinofAlabama

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Posted 22 August 2004 - 06:11 PM

James,

I would have say, yea, to all the above. Generally, for the novice, I would think a short tube refractor is best. You get the best of the refractor world (wide Field of View) without having the collimation nightmares short focal ratio reflectors bring to the table. On the other hand, and to be fair, your planetary observations are gonna be pretty bad. A go-to Maksutov is another good beginner option, but I've always enjoyed fishing the object out of the sky. Maks have such a narrow FOV that many a beginner gets too frustrated with just finding anything to look at without go-to.

To be fair, however, and touch on a point not mentioned yet, is contrast. Refractors are renown for having high constrast. All mirrored telescopes (catadioptics and newtonians), usually have some contrast issues in smaller apertures with shorter F/ratios. Using a longer focal ratio helps to diffuse this issue somewhat, and is one of the reasons Maks, Schmidts and slower newts tend to have better contrast (they also have less collimation issues). Below is a table that shows how small the "sweet spot" gets regarding collimation and focal ratios ...
F/ratio _ sweet spot diameter (mm)

04.5 _ 02.0048
05.9 _ 04.5183
07.0 _ 07.5460
08.0 _ 11.2640
10.0 _ 22.0000
10.1 _ 22.6666
12.1 _ 38.9743
15.0 _ 74.2500

You can see how touchy fast reflectors are. Fast refractors are easier to knock out of alignment for this same reason, but, if reasonably maintained (no Samsonite gorilla monkey business with your ShortTube 80, mind you) there should be no problems.

Hope this sheds some light on the subject. For a beginner, the issue is quite clouded. You've come to the right forum! For now, I'm gonna grab my two short tube refractors and pack 'em into the car, eat dinner with a couple of astronomer buddies, then we're heading out for the 25 minute drive to DARK skies! Yee haw! Our first in MANY DAYS!

Ciao amigos,

CDS

#9 BillFerris

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Posted 22 August 2004 - 06:47 PM

I'm trying to understand the pros and cons of "fast" or "slow" telescopes. I seem to hear conflicting information. I understand the benefits of increased aperture, and I understand how the focal length of a scope changes the effective magnification of eyepieces... But I don't understand why one would want a "fast" or "slow" telescope.


In the simplest terms, "fast" and "slow" relate to exposure times for photography. If you're going to use the scope for photography, then the speed of the optical system is an important consideration as it impacts exposure times and, for a given aperture, image scale.

If you're interested in portability, magnification or field of view, then focal length and aperture are the issues. If you only have room for a 4' optical tube in your car, then a 6", f/8 or an 8", f/6 will equally fit that criteria. Also, an eyepiece will produce the same magnification and true field in both scopes.

Regards,

Bill in Flagstaff

#10 JamesBaud

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Posted 23 August 2004 - 02:45 AM

Thanks for all the help! My telescope is already on the way, a 4.5" newtonian, 1000mm focal length, which I guess would put it at F/8.8 or so... Somewhere in the middle in terms of speed anyway. Unfortunately, my primary concerns were price and GOTO functionality. I'm learning the sky slowly, can pick out 10 or 15 constellations, but I'm impatient!

I can take it on faith that a faster scope requires less time for a picture but doesn't provide a brighter image, but I can't understand it logically.

One post mentioned that slower scopes can have trouble with certain eyepieces... What sort of eyepieces does a slower scope have trouble with? For that matter, what eyepieces does a fast scope have trouble with? Is it certain focal length eyepieces, or is it certain brands or form factors?

My scope will come with 25mm and 10mm eyepieces, and I ordered a 2x barlow and a 40mm that was on closeout for 20 bucks. Probably not the best quality but it'll get me started. Looking at eyepieces, I had no idea they could be so expensive! 600+ dollars for a 31mm nagler? Thats three times as much as my scope! And probably bigger too ;) I may invest more in one higher power eyepiece, because I couldn't handle the tiny eye relief that seems common on higher power cheap EPs.

#11 Jon Isaacs

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Posted 23 August 2004 - 12:43 PM

>>So ask yourself, "Just how much time will I be spending looking at extended objects at very low power?". My answer to that question is "not much".
------------

My answer: Quite a bit...

Some thoughts on widefield viewing:

1. Wide field viewing is about not only extended objects but also about viewing the relationship between objects and just viewing star fields without name. Stepping out of the box, one can infact just roam around and enjoy the view, discovering what may be.... The Milky Way is pretty impressive at 20X with a 3 degree FOV.. Lots of unnamed nooks and crannies, rivers and valleys of nebulosity...

2. The wider the FOV, the easier it is to Starhop.

3. ~1200mm is indeed a nice focal length, especially in an F5 scope. These days mass produced F5 Optics are pretty darn good and a 10 inch F5 scope will fit in just about any car that exists.

jon

#12 Guest_**DONOTDELETE**_*

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Posted 24 August 2004 - 07:15 AM

James, I hope you enjoy the new scope. Welcome to the hobby.

Most of the questions in here have already been answered, but I wanted to add a couple of thoughts. I may be wrong, but it seems that the preference for 1200mm focal lengths is largely based on what will fit across the backseat of a car. A 6" f8, 8" f6, 10" f4.5, and 12" f4 are all about the same length. Even a 4" f10 refractor comes close when the dewcap is considered. A 48" tube is about the max that will fit in the seat of my small car. Larger scopes are often made to dissemble for travel.

As far as eyepieces go, I think that a lot of the older, simpler designs have difficulties in fast scopes. In very slow scopes, even some obsolete types like Huygens are said to work well. I use mostly orthos and a 10 mm Koenig in my slow refractors, and have been very pleased. Some of the newer eyepiece designs are more optimized for fast scopes (and large fields of view.)

Gary

#13 Jon Isaacs

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Posted 24 August 2004 - 07:07 PM

>>Most of the questions in here have already been answered, but I wanted to add a couple of thoughts. I may be wrong, but it seems that the preference for 1200mm focal lengths is largely based on what will fit across the backseat of a car. A 6" f8, 8" f6, 10" f4.5, and 12" f4 are all about the same length.
-------------

A very good point and something to consider when buying a scope. The Orion and GSO 10 inchers, while F5, are wisely designed to be short enough to fit across the back seat of a small car as well.

jon


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