Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Confused on f/number vs scope diameter

imaging optics
  • Please log in to reply
48 replies to this topic

#1 dalbert02

dalbert02

    Lift Off

  • -----
  • topic starter
  • Posts: 9
  • Joined: 30 May 2020
  • Loc: Miami, FL

Posted 02 July 2020 - 01:30 PM

As I research telescopes for astrophotography, I see it often recommended to use an Apochromatic telescope instead of an SCT because (among a few reasons) APOs are 'faster', which I understand to mean they let more light in.  For example, I read a 4" f/4 APO is better for deep sky imaging than a 10" f/10 SCT.  From a purely photography perspective, I agree an expensive f/1.2 50mm lense would allow me to use a faster shutter speed than an f/5 50mm lense as the f/1.2 lets more light on the film/sensor.  However, with telescopes, wouldn't a 10" SCT at f/10 ultimately let more light hit the film/sensor than a 4" f/4 APO?  I would think diameter would be more important than f/number.  May you please explain what I am missing?


Edited by dalbert02, 02 July 2020 - 04:29 PM.


#2 gezak22

gezak22

    Gemini

  • *****
  • Posts: 3,274
  • Joined: 15 Aug 2004
  • Loc: On far side of moon. Send help.

Posted 02 July 2020 - 01:37 PM

The 10" will collect more light, but because of the longer focal length (f/10) it will also image a smaller part of the sky. And so the photons per pixel will be less than the 4" f/4 apo.

 

As you search for a scope, I recommend you first agree on a FOV and an image scale. That will determine what focal length, pixel size, and sensor size you should use. Once you have those number fixed, get the scope with the largest aperture that your mount/wallet can afford without venturing into f/2 territory where collimation becomes very critical. Personally, a 4" apo is a great scope to star with.


  • drd715, OldManSky and dalbert02 like this

#3 JamesMStephens

JamesMStephens

    Viking 1

  • -----
  • Posts: 872
  • Joined: 25 Jan 2015
  • Loc: Hattiesburg, Mississippi

Posted 02 July 2020 - 01:40 PM

The brightness of an extended object depends on f-number.  A 2" f/4 and a 10" f/4 will show the same object as equally bright, but the image produced by 10" will be five times as large as that produced by the 2".  Image scale is determined by focal length, brighness by f/ratio.  The image produced by the 10" f/10 will have about fify times the image scale of the 50-mm f/5 lens, but the exposure time will be longer.


  • markb and dalbert02 like this

#4 cst4

cst4

    Messenger

  • *****
  • Posts: 415
  • Joined: 06 Jun 2018

Posted 02 July 2020 - 01:41 PM

Ten inches of aperture will let in a lot of light.  A big issue with that size of a scope though is trying to mount it so that it can track something efficiently.  A fast wide field refractor is a much better place to start as it is much easier to mount and track and will fit more targets in the view.  If you were imaging some tiny little planetary nebula than the 10" might be what you want but for many objects in the sky you don't need or want that long focal length.  It is easier to do long exposures with a shorter focal length scope and plus something at F/6 will require much less exposure time than an F/10 SCT... so all in all much easier.


Edited by cst4, 02 July 2020 - 01:46 PM.

  • dalbert02 likes this

#5 SilverLitz

SilverLitz

    Viking 1

  • -----
  • Posts: 833
  • Joined: 17 Feb 2018
  • Loc: Louisville, KY

Posted 02 July 2020 - 01:41 PM

I feel f/ is more important, as it will determine the light per pixel, and is what is important regarding how much integration time is necessary for a good exposure.  Large aperture SCT's have a large aperture, but a high f/, requiring much longer exposures and their FoV may be very small (depending on the sensor size).

 

The difference between AP and typical photography, is that in AP you do not necessarily have scopes that will adequately cover the camera sensor or that the entire sensor may be a very small percentage of the scope's image circle, and you cannot "zoom with your feet" to properly frame the target.

 

In AP, you must factor in both the scope's FL and the sensor's size to estimate the FoV, and factor in both the scope's FL and the sensor's pixel size to estimate the image scale or resolution.


  • dalbert02 likes this

#6 barbarosa

barbarosa

    Mercury-Atlas

  • *****
  • Posts: 2,543
  • Joined: 11 Apr 2010
  • Loc: 139 miles W of the Awahnee Hotel

Posted 02 July 2020 - 01:54 PM

Aperture rules or so they say. But the focal ratio can't be ignored  

 

8" SCT @ f/10 narrow field of view requires longer exposures.  M27 appears large

8" RASA @ f/2 wide field of view, shorter exposures. M27 appears small. (Shown below as custom scope).

 

85mm refractor @ f/5.9 wide field of view, longer exposures than the RASA, shorter than the SCT

 

 

astronomy_tools_fov (4).png


Edited by barbarosa, 02 July 2020 - 01:55 PM.

  • dalbert02 likes this

#7 OldManSky

OldManSky

    Skylab

  • *****
  • Posts: 4,307
  • Joined: 03 Jan 2019
  • Loc: Valley Center, CA USA

Posted 02 July 2020 - 01:54 PM

The beautiful thing about how a focal ratio is calculated is that the same f/number results in the same "light per unit area" on the sensor/film no matter what the diameter of the objective is.  So a 300mm (FL) f/4 camera lens delivers the exact same amount of light per unit area (pixel) as a 50mm (FL) f/4 does, even though the 300mm likely has a diameter many times larger than the 50mm does.  This is why with photographic lenses, you can meter the light at 1/500th at f/4 (for a given ISO value), and use ANY lens at f/4 and get the correct exposure. 

 

The converse applies as well -- no matter how bit the diameter of the lens/telescope, a "slower" (higher number) f-ratio delivers less light per unit area than a "faster" (lower number) f-ratio does.  So the 80mm (D) refractor at f/5 delivers a LOT more light per unit area (pixel) than the 200mm (D) f/10 scope does, even though the 200mm gathers more light overall.


  • markb, JamesMStephens, kathyastro and 1 other like this

#8 Michael Covington

Michael Covington

    Author

  • *****
  • Vendors
  • Posts: 6,253
  • Joined: 13 May 2014
  • Loc: Athens, Georgia, USA

Posted 02 July 2020 - 01:56 PM

Consider a 10-inch f/10 telescope and a 4-inch f/10 telescope.

 

The 10-inch collects more light.  It also forms a larger image.  The light is spread out.

 

Both are f/10, so the images are the same brightness.  The one with more light also spreads it out over a bigger image.

 

A 4-inch f/5 telescope would produce an image half the size and thus four times as bright as a 4-inch f/10.

 

Just like 50-mm f/4 and 200-mm f/4 camera lenses.  Same brightness, different magnification.


  • OldManSky and dalbert02 like this

#9 robbieg147

robbieg147

    Mariner 2

  • -----
  • Posts: 293
  • Joined: 23 Mar 2020
  • Loc: Kent, England

Posted 02 July 2020 - 01:58 PM

It really comes down to what is it you want to image or view? A 4" F4 apo will have a focal length of approx. 400mm, a 10" SCT at F10 would have a focal length around 2.5m

 

Once you know the focal length you need then get the biggest aperture you can afford.

 

Comparing camera lenses to telescopes is a bit dodgy really, if I buy a 50mm F1.2 I am not interested in corner sharpness as I am going to use it to throw the background out of focus on objects at close range. So using such a lens for AP was not what it was designed for. If you buy a lens for AP don't assume fast is necessarily the best.


  • dalbert02 likes this

#10 Midnight Dan

Midnight Dan

    Voyager 1

  • *****
  • Posts: 13,517
  • Joined: 23 Jan 2008
  • Loc: Hilton, NY, Yellow Zone (Bortle 4.5)

Posted 02 July 2020 - 02:02 PM

 I agree an expensive f/1.2 50mm lense would allow me to use a faster shutter speed than an f/5 50mm

 

This is a common point of confusion and the reason is because of exactly what you state in your comment I quoted above.  Your assumption is that you're comparing lenses of the same focal length.  An f/1.2 lens will be much faster than an f/5 lens - *IF* they are both the same focal length and you expect to see the same size image as a result.

 

Let me give you a different example.  What's faster, an f/8 500mm lens, or an f/5 250mm lens.  Well, technically the f/5 is faster.  But it's not a good comparison because the two will produce completely different images.  If you need the magnification of the 500mm lens to get your desired image, then it would take a 2x tele-extender with the 250mm lens to get the same image.  And when you add that in, your f/5-250mm lens becomes an f/10 500mm lens.  So the point is, if you're looking for the particular image scale produced by the 500mm lens, then the f/8-500mm is faster.

 

The same occurs with the two scope you mention.  The f/4 APO you mention is about a 400mm focal length, while the f/10 SCT is 2000mm.  If you were to add a 5x barlow to the 4" APO, you would now have the same focal length as the SCT, but you would be at f/20.  So if you need the magnification provided by the SCT, then the APO would end up slower.  On the other hand, if you're content to image larger targets and stay at the native focal length of the APO, you would be able to image in half the time compared to the SCT.  

 

-Dan


  • StarryHill and dalbert02 like this

#11 Stelios

Stelios

    Cosmos

  • *****
  • Moderators
  • Posts: 9,538
  • Joined: 04 Oct 2003
  • Loc: West Hills, CA

Posted 02 July 2020 - 02:04 PM

As I research telescopes for astrophotography, I see it often recommended to use an Apochromatic telescopes instead of an SCT because (among a few reasons) APOs are 'faster', which I understand to mean they let more light in.  For example, I read a 4" f/4 APO is better for deep sky imaging than a 10" f/10 SCT.  From a purely photography perspective, I agree an expensive f/1.2 50mm lense would allow me to use a faster shutter speed than an f/5 50mm lense as the f/1.2 lets more light on the film/sensor.  However, with telescopes, wouldn't a 10" SCT at f/10 ultimately let more light hit the film/sensor than a 4" f/4 APO?  I would think diameter would be more important than f/number.  May you please explain what I am missing?

Assuming other things are equal, the ratio of exposure times required to get equivalent signal goes by the square of the F/ratio. So an F/10 scope needs 100*100/4*4 = 6.25 times longer exposure to match the signal of an F/4 scope. So faster scopes have a significant advantage.

 

Given the same F/ratio, the larger aperture scope will produce more detail--up to a point, limited by seeing. The best way to evaluate this is image scale (or pixel scale), which also depends on the camera's pixel size (the formula is 206.3*pixel_size_in_microns/focal_length_of_scope_in_mm). For a typical 4mm pixel size and a 10" F/10 scope this becomes 0.33"/px. It takes exceptional seeing to take advantage of such a scale, which means that quite a bit of the extra detail is wasted. Usually ideal pixel scales are around 1"/px for the mainland US, perhaps a little less. 

 

So why not get a 10" F/4 scope and have the best of both worlds? The practical answer is cost--not so much of the scope itself (although a quality F/4 newtonian isn't cheap--you need good focuser and rigid frame) but of the mount needed to carry the scope for astrophotography. The mount requirements increase by scope weight (to which you need to add camera imaging train and guider weight) and also by focal length. Mounts that can carry 10" scopes are prohibitively expensive for most who want to try out the hobby, and can be hard to transport to a desirable observing site.


  • StarryHill, OldManSky, bridgman and 1 other like this

#12 bridgman

bridgman

    Apollo

  • -----
  • Posts: 1,239
  • Joined: 29 Jan 2019
  • Loc: Mosport, ON, Canada

Posted 02 July 2020 - 02:06 PM

The brightness of an extended object depends on f-number.

Added bold for emphasis. There is arguably a difference in result depending on whether you are talking about an extended object (eg nebula, planet etc..) or a point source (eg star).

 

For an extended object the outcome is just like a camera lens - bigger opening gathers more light, but longer focal length spreads that light across a larger area on the film/sensor, so f/ratio is what affects exposure while for the same f/ratio a larger aperture (with longer focal length) affects image scale rather than exposure time.

 

For a point source where in theory all the light ends up on the same pixel even if focal length changes a bit the outcome should be different, with aperture affecting exposure time rather than f/ratio. I said "in theory" and "a bit" because in practice things like diffraction, collimation and seeing result in even point sources being spread across multiple pixels (with longer FL covering more pixels) so it's not as simple as "f/ratio doesn't matter for stars".

 

I suppose one consequence of this is that the relative brightness of nebulae and stars should differ depending on f/ratio, but I haven't advanced enough to think about stuff like that yet.

 

EDIT - as Stelios pointed out, the reasons for recommending an apo are not just f/ratio - the recommendation is usually for a relatively small apo which makes mounting and tracking easier and more likely to be successful. The other advantage of an apo (relative to an SCT) is mechanical - you don't have to deal with things like mirror flop.

 

Relative to an equally fast Newtonian the benefits are a bit less obvious, although in general size/weight becomes a factor since imaging Newt's tend to be quite a bit larger (say 8" to start). AFAIK the reason you tend not to see smaller imaging Newts is because the central obstruction has to be fairly large relative to the primary, which in turn starts to affect efficiency and contrast.


Edited by bridgman, 02 July 2020 - 02:17 PM.

  • dalbert02 likes this

#13 Woody218

Woody218

    Explorer 1

  • -----
  • Posts: 65
  • Joined: 01 Jul 2020
  • Loc: Bismarck, ND

Posted 02 July 2020 - 02:09 PM

You can also add a reducer/corrector to the f/10 SCT scope and bring it down to f/6.3 in the case of the Meade or Celestron reducer.



#14 bobzeq25

bobzeq25

    ISS

  • *****
  • Posts: 20,443
  • Joined: 27 Oct 2014

Posted 02 July 2020 - 02:41 PM

Complicated topic, about which there is much misinformation.

 

Short answer.  Both aperture and f-ratio matter.  How much of each depends on circumstances.  Like much in astrophotography it's complicated.  And definitely not determinative when it comes to selecting a scope to start with.

 

The one thing that's certain is that, pretty much independent of aperture, higher F numbers require longer subexposures.  Add that to the fact that the longer focal lengths found generally (generally, it's not a theoretical requirement) magnify tracking errors, and you start to really need an excellent mount ($$$) to make one work.  And a lot of experience.  It's easy to think that this is all about equipment, an it's not.  Knowledge is crucial.

 

So here's what you're missing, and it's _big_.    Not something I thought up myself.  The fairly uniform recommendation of both beginners and experts.  The best tool for _learning_ DSO imaging for a beginner is a small refractor on a good mount.  A long, heavy, slow scope is quite difficult to learn on.  Even on an expensive mount, much moreso on a typical beginner mount. More than you would think intuitively.  Tracking is a major reason, but the big scope makes _everything_ harder, right down to locating your target.

 

The two big beginner mistakes, seen over and over again here, are an inadequate mount and too big a scope.  Often combined.  Not because they are dumb, but because this is _so_ unintuitive.

 

As I say, not something I thought up.  Here's one sample (and they are representative) of an expert and a beginner. 

 

Scroll down the webpage of this recommended book, to the picture of the author.  That's a $1200 Sirius (aka HEQ5) mount with a $500 70mm refractor.  The expert did not choose those because he had them lying around.  <smile>  That's the best way to spend $1700 on a starter setup.  Yes, you pay more for the mount.  It's more important.  Yes, you can make fine images with a 70mm.  A sample of one of mine below.

 

https://www.astropix...bgda/index.html

 

Here's a very talented beginner looking back on his first year.  He came here looking for equipment advice.

 

"First and foremost is listen to the folks who have been there. The philosophy of 80MM APO and good $1500-2000 mount is great advice for beginners. Sure you can possibly image as a beginner with something that is larger or that you may have but holy cow its hard enough with something small."

 

Holy cow, it is.  <smile>  No one quits imaging because it wasn't challenging enough, especially to learn.  You want a setup that gets out of your way, and lets you get on with the job.  I have many more of those references.

 

Good mount, small refractor.  The right tool for the job.  Your intuition will lead you astray on this one, _particularly_ if you have a background in visual, something altogether different.

 

You seem to have a background in photography.  Here's someone who also had that.

 

"A very short time ago I bought the 8" SCT thinking that heck, I've been into photography for 20 years so how hard can AP be?  Good Lord, I wish I would have found this thread <recommending the good mount, small refractor approach> before I went the route I did.  The SCT isn't too bad but not for starting AP <of DSOs, planets are quite different>."

 

Note that the posts above are essentially about imaging, not about learning imaging.  That they (appropriately) didn't even mention the mount.  The best equipment for an experienced imager is quite often not the best to learn on.  A good analogy would be trying to learn how to drive with a Formula One car.

 

This is why you see people recommend APOs over SCTs.  The subject is usually starting out.  An SCT is not a bad scope for AP, but it's a tool for an experienced imager.

 

M31 SV70T smaller.jpg


Edited by bobzeq25, 02 July 2020 - 03:30 PM.

  • dalbert02 likes this

#15 kathyastro

kathyastro

    Surveyor 1

  • *****
  • Posts: 1,979
  • Joined: 23 Dec 2016
  • Loc: Nova Scotia

Posted 02 July 2020 - 02:49 PM

As I research telescopes for astrophotography, I see it often recommended to use an Apochromatic telescopes instead of an SCT because (among a few reasons) APOs are 'faster', which I understand to mean they let more light in.  For example, I read a 4" f/4 APO is better for deep sky imaging than a 10" f/10 SCT.  From a purely photography perspective, I agree an expensive f/1.2 50mm lense would allow me to use a faster shutter speed than an f/5 50mm lense as the f/1.2 lets more light on the film/sensor.  However, with telescopes, wouldn't a 10" SCT at f/10 ultimately let more light hit the film/sensor than a 4" f/4 APO?  I would think diameter would be more important than f/number.  May you please explain what I am missing?

The comparison to terrestrial photography is valid.  An f/4 100mm camera lens will have a front element that is 25mm in diameter.  An f/4 500mm lens will have a front element that is 125mm in diameter.  Which lens gives you the faster exposure time?  As any terrestrial photographer knows, they will both be the same, since they are both f/4.  The actual aperture measurement in millimetres makes no difference to the exposure time.

 

The same is true for astrophotography, since the rules of optics do not change just because the Sun went down.

 

Aperture is relevant to astrophotography, but not for exposure times.  It is relevant because it determines the maximum resolution you can obtain.


  • Peter in Reno and dalbert02 like this

#16 dalbert02

dalbert02

    Lift Off

  • -----
  • topic starter
  • Posts: 9
  • Joined: 30 May 2020
  • Loc: Miami, FL

Posted 02 July 2020 - 03:45 PM

I want to sincerely say 'thank you' to everyone who posted here.  The very quick outpouring of help, especially to a complete newbie, is not something experienced very often these days. I greatly appreciate the solid advice and the examples used to help me understand.  Thank you!  I've got alot to learn but hopefully I may be able to avoid some of the mistakes others have made.  My wallet also appreciates that!    


  • bobzeq25 and JamesMStephens like this

#17 WadeH237

WadeH237

    Fly Me to the Moon

  • *****
  • Posts: 5,882
  • Joined: 24 Feb 2007
  • Loc: Snohomish, WA

Posted 02 July 2020 - 03:55 PM

Aperture is relevant to astrophotography, but not for exposure times.  It is relevant because it determines the maximum resolution you can obtain.

Really?

 

So assume I have two telescopes that both have 400mm of focal length, but one is 4" aperture and one is 10" aperture.  Let's also say that I have one camera that I swap between the scopes.  At this point, the physical measurements are identical between the two setups except for the aperture.  So if what you say above is true, then a 60 second exposure on each telescope will produce two identical images.  Right?

 

The problem with this topic, whenever it comes up, is that the focal length and the aperture both matter. Focal ratio is a convenient calculation that tells you how much illumination that you can expect over a given area at the focal plane.  In this respect, it's a good way to determine an appropriate exposure time.

 

But it would be incorrect to say that aperture doesn't matter, and it would also be incorrect to say that focal length doesn't matter.

 

At the risk of further complicating things, I am going to make the following statement about the comparison between a 4", F/4 scope and a 10", F/10 scope:

 

If you are imaging a specific object, say a galaxy, the 10" scope will certainly collect more photons from that object, but it will produce a large image at the focal plane, based on its focal length.  Even though the 4" scope collects less photons, the same galaxy produces a much smaller image at the focal plane, again due to its focal length.  So even though you have less photons, they are concentrated into a smaller area, and thus you get a brighter (but smaller) image.


  • markb likes this

#18 nitegeezer

nitegeezer

    Galactic Ghost

  • *****
  • Posts: 7,067
  • Joined: 27 Nov 2007

Posted 02 July 2020 - 03:55 PM

The Meade .33x reducer is hard to find, not made anymore. I have an 8" SCT and can use it at F/10 or with reducers at either F/6.3 or F/3.3. I like having one scope that will give me those options.

#19 markb

markb

    Viking 1

  • *****
  • Posts: 933
  • Joined: 16 Jun 2006
  • Loc: Long Island; in transition to Arizona

Posted 02 July 2020 - 04:21 PM

I have to add my thank you to that of the OP.

 

Despite long experience in regular photography and ownership of a wide range of apertures and f ratios, I had pretty much the same question, which I had trouble answering.

 

The participants in the thread gave some of the best, and most cogent, answers I've seen in a long time on this non-intuitive topic.


  • OldManSky likes this

#20 imtl

imtl

    Viking 1

  • *****
  • Posts: 679
  • Joined: 07 Jun 2016

Posted 02 July 2020 - 04:57 PM

Really?

 

So assume I have two telescopes that both have 400mm of focal length, but one is 4" aperture and one is 10" aperture.  Let's also say that I have one camera that I swap between the scopes.  At this point, the physical measurements are identical between the two setups except for the aperture.  So if what you say above is true, then a 60 second exposure on each telescope will produce two identical images.  Right?

 

The problem with this topic, whenever it comes up, is that the focal length and the aperture both matter. Focal ratio is a convenient calculation that tells you how much illumination that you can expect over a given area at the focal plane.  In this respect, it's a good way to determine an appropriate exposure time.

 

But it would be incorrect to say that aperture doesn't matter, and it would also be incorrect to say that focal length doesn't matter.

 

At the risk of further complicating things, I am going to make the following statement about the comparison between a 4", F/4 scope and a 10", F/10 scope:

 

If you are imaging a specific object, say a galaxy, the 10" scope will certainly collect more photons from that object, but it will produce a large image at the focal plane, based on its focal length.  Even though the 4" scope collects less photons, the same galaxy produces a much smaller image at the focal plane, again due to its focal length.  So even though you have less photons, they are concentrated into a smaller area, and thus you get a brighter (but smaller) image.

Without going into the usual heated debate of A vs. F. You and Kathy are actually saying the same thing. In the example you gave you changed the F ratio when you went to the 10". If you keep the F ratio the same then the exposure time will be the same. Image scale will be different. You both wrote the same thing only in different phrasing.

Eyal


  • Peter in Reno likes this

#21 JamesMStephens

JamesMStephens

    Viking 1

  • -----
  • Posts: 872
  • Joined: 25 Jan 2015
  • Loc: Hattiesburg, Mississippi

Posted 02 July 2020 - 05:06 PM

I want to sincerely say 'thank you' to everyone who posted here.  The very quick outpouring of help, especially to a complete newbie, is not something experienced very often these days. I greatly appreciate the solid advice and the examples used to help me understand.  Thank you!  I've got alot to learn but hopefully I may be able to avoid some of the mistakes others have made.  My wallet also appreciates that!    

Look at Jerry Oltion's article in the December 2017 Sky & Telescope: https://tinyurl.com/y93ql73r



#22 dalbert02

dalbert02

    Lift Off

  • -----
  • topic starter
  • Posts: 9
  • Joined: 30 May 2020
  • Loc: Miami, FL

Posted 02 July 2020 - 05:35 PM

So after a bit further reading, what is better as far as bang for the buck?  A f/5.4 4" APO such as Televue 101 at $4k or a Celestron at f/2 8" RASA at $3.7k?  Assuming same mount for each as they are 10lb vs 17lb ?  I would think the RASA but it would be kinda nice to actually see things with my own eye which steers me to the APO.


Edited by dalbert02, 02 July 2020 - 05:48 PM.


#23 WadeH237

WadeH237

    Fly Me to the Moon

  • *****
  • Posts: 5,882
  • Joined: 24 Feb 2007
  • Loc: Snohomish, WA

Posted 02 July 2020 - 05:46 PM

Without going into the usual heated debate of A vs. F. You and Kathy are actually saying the same thing. In the example you gave you changed the F ratio when you went to the 10". If you keep the F ratio the same then the exposure time will be the same. Image scale will be different. You both wrote the same thing only in different phrasing.

Eyal

We did not say that same thing.

 

I did not make the claim that "aperture is relevant to astrophotography, but not for exposure times".  That is patently false.  Of course it matters for exposure times, but it's not the only thing that matters, and I gave an example that should be quite clear.

 

And yes, I know that the focal ratio changed in my example.  My whole point is that focal ratio is a mathematical way to describe the relationship between aperture and focal length.  It's a useful exercise because it predicts useful exposure times.  But the aperture and the focal length are the physical characteristics that matter.  You can't ignore either of them.

 

Here's another question to ponder:  Why do research institutions pay millions of dollars for large instruments?  If aperture only matters for resolution, why wouldn't they just buy a 14" RASA and call it a day?  Heck, even my little EdgeHD 8 is seeing limited most of the time.


  • sdbodin likes this

#24 Peter in Reno

Peter in Reno

    Voyager 1

  • *****
  • Posts: 10,948
  • Joined: 15 Jul 2008
  • Loc: Reno, NV

Posted 02 July 2020 - 05:47 PM

So after a bit further reading, what is better as far as bang for the buck?  A f/5.4 4" APO such as Televue 101 at $4k or a Celestron at f/2 8" RASA at $3.7k?  Assuming same mount for each as they are 10lb vs 17lb ?  I would think the RASA but it would be kinda nice to actually see things with my own eye which steers me to the APO.

It may be more difficult to image with RASA than 4" APO @ f/5.4. It's always more difficult to get the collimation spot on with compound telescopes (multiple mirrors) than refractors. Refractors generally don't require collimation unless you drop the scope on a hard ground like concrete.

 

But to answer your question, RASA will definitely be faster (shorter exposure times) than a refractor you listed (not counting obstruction size from imaging camera). Two reasons. First, RASA focal ratio is much shorter by a factor of 7x (5.4 ^2 / 2^2 or 29 / 4). Second, the focal length of RASA is shorter meaning the image scale is smaller (larger arcsec/pixel) assuming same camera is used.

 

Two important factors for determining exposure time for a given scope and camera are focal ratio and pixel size (and image scale).

 

Peter


Edited by Peter in Reno, 02 July 2020 - 05:57 PM.


#25 gezak22

gezak22

    Gemini

  • *****
  • Posts: 3,274
  • Joined: 15 Aug 2004
  • Loc: On far side of moon. Send help.

Posted 02 July 2020 - 05:49 PM

I've been in these debates way too many times, and my observation is that people will often compare f/4 vs. f/10 while "keeping everything else the same" ... which of course is non-sense, because when f ratio changes, either focal length or aperture has to change. If people compared f/4 vs. f/10 while also stating how that is accomplished (changing aperture or focal length), there would be far fewer misunderstandings and fewer "myths".

 

 

So after a bit further reading, what is better as far as bang for the buck?  A f/5.4 4" APO such as Televue 101 at $4k or a Celestron a f/2 8" RASA at $3.7k?  Assuming same mount for each as they are 10lb vs 17lb ?

In my opinion, anything faster than f/4 is tricky, even for seasoned photographers. At f/2, your collimation must be perfect. There must be zero tilt if you want pinpoint stars across the sensor. Temperature effects must be taken into account as that changes focus. For me, the sweet spot was always between f/4 and f/5.

 

Edit: I also believe in buying from "my side of the pond", so I would get the Televue. With a reducer, you can image at f/4.5 and get nice images across an APS-C sensor.


Edited by gezak22, 02 July 2020 - 05:51 PM.

  • bridgman likes this


CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics





Also tagged with one or more of these keywords: imaging, optics



Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics