Jump to content


Photo

Fast vs Slow F/ratio - Why go Slow?

  • Please log in to reply
14 replies to this topic

#1 MDavid

MDavid

    Mariner 2

  • -----
  • Posts: 207
  • Joined: 15 Nov 2011
  • Loc: 29°N (Texas)

Posted 18 November 2011 - 03:36 PM

Okay, I'm wet behind the ears when it comes to Astronomy, so I was hoping maybe someone could explain the advantages of a "slower" F8-F14 scope?
:confused:
It's my understanding the lower the f-ratio the larger and brighter the image. So, I'm scratching my head when I see all the popular cassegrains with such high F-stops... Here is why I ask...I have a Newt with a F/6 and a short tube refractor. Both have relatively "fast" (wider & brighter) images, but I wanted to also get a catadioptric but I can't get past the image I have in my head from a recent star party.

This guy had what looked like a 10” or 12” Schmidt-Cass (big blue monster on a fork drive - possibly a Meade?) pointed at the Herculean cluster...The cluster took up the entire FOV and was very dim (yes my eyes were fully adjusted)… I walked back to my dob thinking WOW, but NOT the good kind of wow...I don't know what EP he used or if the diagonal was a prism or anything else, but what I do know is that I don't want to spend ump teen thousands for his style set up only to see objects so dimly. BTW - seeing was excellent that evening with the Milky Way bright from north to south horizons (sorry, I don't have enough experience to give a more quantitative opinion of the sky).
So, back to my question, why are the “popular” cassegrains so slow? And Why can’t I use an “Astrograph” which all seem to have faster F-ratios for Visual observing? What am I missing?

#2 David Pavlich

David Pavlich

    Transmographied

  • *****
  • Administrators
  • Posts: 27312
  • Joined: 18 May 2005
  • Loc: Mandeville, LA USA

Posted 18 November 2011 - 03:46 PM

In general, a slower scope has a long focal length. The pluses of this is image scale. Small objects like the Ring Nebula or the Dumbell Nebula will be larger using the same eyepiece as you might use in a fast scope. This becomes very evident when imaging is involved.

The negative is that the field of view is relatively narrow. For instance, I have a Meade 10" LX200 SC. It has a 2500mm focal length....I think...and is f10. I can't fit M45 in the FOV where as something like my Tak TOA130F which is around 1000mm focal length and f7.7, the Pleiades look terrific in the scope.

Everything is a compromise. Wide FOV, you want a faster, shorter focal length. For me, about 97% of what I view will fit in the FOV of the Meade that I have. Others prefer that really wide, panoramic view of an f4.5 Newt. My advice....look through a few different styles of scopes to help you with your decision,

David

#3 maakhand

maakhand

    Viking 1

  • *****
  • Posts: 679
  • Joined: 15 Sep 2009
  • Loc: florida, usa

Posted 18 November 2011 - 04:07 PM

welcome to CN.
brightness of an image depends on the exit pupil you get from a given eyepiece and scope combo. a 25mm eyepiece will give you 4.17mm exit pupil but in a f/10 cassegrain its only 2.5mm. so you will have a brighter smaller image compared to dimmer larger in a cass using the same eyepiece

#4 MDavid

MDavid

    Mariner 2

  • -----
  • Posts: 207
  • Joined: 15 Nov 2011
  • Loc: 29°N (Texas)

Posted 18 November 2011 - 04:12 PM

Ahha..Image scale.., hmmm...that's interesting...so let me see if understand...with the longer focal length you can "fit" smaller objects at higher magnification into the FOV? Visually, does this mean you can get "closer" to smaller targets?
Not yet having been lured to the time mastication known as astrophotography...how does using an "astrograph" for visual observation fit in this picture? :grin:

#5 MDavid

MDavid

    Mariner 2

  • -----
  • Posts: 207
  • Joined: 15 Nov 2011
  • Loc: 29°N (Texas)

Posted 18 November 2011 - 04:23 PM

so you will have a brighter smaller image compared to dimmer larger in a cass using the same eyepiece


Thanks for the welcome maakhand.
Okay, so by dimmer larger in a cass you mean the object will appear to fill the FOV because the TFOV will be smaller relative to the typically longer focal length of a cass?

#6 johnnyha

johnnyha

    Fly Me to the Moon

  • *****
  • Posts: 6500
  • Joined: 12 Nov 2006
  • Loc: Sherman Oaks, CA

Posted 18 November 2011 - 04:47 PM

Bingo, this is why a lot of people with SCTs use a dedicated reducer/flattener, for instance the Celestron 6.3 model will reduce the focal length to f6.3.

Its easier and generally cheaper to make a slower scope, once you start getting faster you start needing to correct for edge aberrations like coma, and also for chromatic aberration (CA or "color") in refractors.

#7 maakhand

maakhand

    Viking 1

  • *****
  • Posts: 679
  • Joined: 15 Sep 2009
  • Loc: florida, usa

Posted 18 November 2011 - 04:48 PM

yes. its easier to get higher magnification in a cass for its longer focal length. depending on the mag image scale will be bigger, will look larger in FOV. downside is smaller exit pupil (dimmer view), smaller Tfov for that eyepiece.

#8 craytab

craytab

    Surveyor 1

  • *****
  • Posts: 1500
  • Joined: 10 Jun 2011
  • Loc: Bethlehem, PA

Posted 18 November 2011 - 04:58 PM

Also, in slower scopes you can use less well corrected and cheaper EPs. I don't exactly know why as I am still a bit of a noob, I am sure someone can explain that.

#9 jgraham

jgraham

    Voyager 1

  • *****
  • Posts: 13692
  • Joined: 02 Dec 2004
  • Loc: Miami Valley Astronomical Society

Posted 18 November 2011 - 05:06 PM

Long Newtonians have nice flat fields, reduced coma, and a smaller secondary. Long focal length mirrors also have a much wider tollerance; it's much easier to make an outstanding local focal length mirror and an okay short focal length mirror.

#10 GeneT

GeneT

    Ely Kid

  • *****
  • Posts: 12650
  • Joined: 07 Nov 2008
  • Loc: South Texas

Posted 18 November 2011 - 06:34 PM

At the eyepiece, I don't know you will notice much difference in brightness when viewing in the range between F5 and F10 telescopes. You will notice differences in magnification and field of view. I viewed for about 10 years with an F10 C8, then switched to F5 Dobs. If you do side by side comparisons, you will see the the differences. Imaging is another matter. Faster F ratios = less exposure times.

#11 MDavid

MDavid

    Mariner 2

  • -----
  • Posts: 207
  • Joined: 15 Nov 2011
  • Loc: 29°N (Texas)

Posted 18 November 2011 - 07:01 PM

This is great stuff and I really appreciate everybody's feedback...
I understand that Coma can be a problem for Newts and CA for refractors both of which appear to be dealt with when using more expensive and/or additional equipment (e.g., ED glass, paracorr, premium EPs, etc...) But what I'm still a little fuzzy on is the disadvantages of an "astrograph" as a visual instrument.
To try to exaggerate the point what would be the disadvantages of purchasing a Tak EPSILON-180ED F/2.8 to use only for visual observing (other than the hit to my bank account and possibly my marriage ;-).

#12 panhard

panhard

    It's All Good

  • *****
  • Moderators
  • Posts: 13641
  • Joined: 20 Jan 2008
  • Loc: Markham Ontario Canada

Posted 18 November 2011 - 11:01 PM

F5 and below require better quality eyepieces to get the best views. In slower scope eyepiece quality is not as critical. I have seen the difference in my f4.7 dob.

#13 johnnyha

johnnyha

    Fly Me to the Moon

  • *****
  • Posts: 6500
  • Joined: 12 Nov 2006
  • Loc: Sherman Oaks, CA

Posted 18 November 2011 - 11:23 PM

To try to exaggerate the point what would be the disadvantages of purchasing a Tak EPSILON-180ED F/2.8 to use only for visual observing (other than the hit to my bank account and possibly my marriage ;-).

At f2.8 a Paracorr 2 is NOT an option, and in a 7" Epsilon this is a significant weight to hang off the side. Assuming the goal is widest field possible, you add a 31T5 Nagler and now your eyepiece train weighs quite a bit and sticks WAYY out of the focuser. Easily manageable in a large dob, quite a handful in a 7" GEM newt. Also at f2.8 in order to view at any sort of planetary magnification you will probably have to add a barlow, which combined with a Paracorr is a bit of a nightmare scenario IMHO. That's ONE reason not to get an Epsilon for visual use only. :p

#14 pogobbler

pogobbler

    Viking 1

  • -----
  • Posts: 522
  • Joined: 30 Sep 2008
  • Loc: Central Indiana, USA

Posted 19 November 2011 - 01:03 AM

If we're talking about visual observing, the images in two scopes of the same aperture at the same magnification will be identical in brightness (not doing any nitpicking about their relative obstructions, coatings, etc.) regardless of their focal ratio, but they will need eyepieces of different focal lengths to achieve that same magnification. Of course, with eyepieces of identical focal length, the faster scope will have a lower magnification, thus a smaller, brighter image of an extended object than the slower scope would have. This is akin to the difference photographically, where you're not using an eyepiece, so the faster scope will give a smaller, brighter image and, thus, not need as long an exposure as the slower scope will.

In the medium range of magnification, there's no particular advantage to have a fast or slow scope. A fast scope will likely be able to achieve a lower magnification, letting you see more sky at once... but if you want high power, such as for planetary viewing, you might have to go with really short focal length eyepiece or use Barlow lenses or a combination of the two. While there are a number of objects that you can only get a full appreciation of with a wide field of view, there are quite a few objects, both solar system and deep sky, that are fairly small and can show best with higher powers.

I, personally, like having a wide field telescope-- mine is an ST 120 refractor that I can grab about 4 degrees of sky in-- and a scope more suited to higher powers, of which I have a few of various apertures, with f ratios from about 10 to 15. Each has it's place.

#15 JLovell

JLovell

    Viking 1

  • -----
  • Posts: 850
  • Joined: 12 Jan 2010
  • Loc: Georgia

Posted 19 November 2011 - 08:47 AM

Usually, you can use an astrograph visually. The problem is that they are optimized to produce a fairly wide (the size of a 35mm film frame) image at the focal plane, and at a fair distance outside the OTA. They bring the image to focus at the film or imager plane in an SLR camera. In a visual scope, the focal plane is right at the first lens of the eyepiece, the one closest to the scope. To use an astrograph visually, you usually have to get an extension tube to hold the eyepiece further out from the focuser. Also, depending on the design, some other compromises have been made. The various types of reflectors, for example, tend to have much larger secondary mirrors. This helps the scope evenly illuminate the entire imaging area in the camera evenly. You don't need that in a visual scope. The larger secondary cuts down on the brightness and contrast. The camera can compensate for that by the exposure settings, and enhancement software, but your eye cannot. The images you would see in an astrograph are usually dimmer and less detailed than a similar scope made for visual use. Hope this explains some things for you.






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics