48 replies to this topic

[nyx]

...and a fast scope for astrophotography, what would be the ideal focal ratio for DSO (visual)?

 

Playing around with Stellarium, I can see that the Andromeda galaxy (one of the largest celestial objects, if I am not mistaken) requires a 2.1° FoV in order to fit in the eyepiece. I can achieve that using 1) a 150/750 (f/5 - fast) scope with a 23mm 70° eyepiece or 2) a 150/1200 (f/8 - slower)  scope with a 36mm 70° eyepiece.

 

So, which of the two scopes is suitable for DSOs (visual only) and why?

 

Clear skies!

[petmic]

IMHO for visual it is all about aperture and not focal ratio. Get the largest aperture you can afford and forget the f number. You are right about Andromeda galaxy but that is an extremely large DSO. The others are smaller and longer focal length is not an disadvantage.

[Araguaia]

Depends on the size of the DSO and of the details within the DSO that you want to see.  The faster scope will give a larger FOV but less magnification.

 

For example, a wide field will show the whole Orion Nebula.  Greater magnification will show more stars in the Trapezium and Bok globules in the surrounding region.

[nyx]

I understand the importance of aperture when it comes to DSOs. I used 150mm as an example, due to the fact that this is the aperture my scope has.

 

My question originates from the way most (if not all) telescopes are marketed. You find the same pattern everywhere:

 

long focal length = planet telescope

short focal length = astrophotography

 

The only exception is dobson telescopes. There you read in the product description that they are excellent for DSOs. And that is exactly what I do not understand. The same tube is marketed differently depending on the mount:

 

alt-az/EQ = planets

dobson = DSOs

 

Assuming the same aperture and different focal lengths, the maximum achievable magnification should be the same. If one can get the same FoV by playing around with eyepieces, why is one marketed as a planet scope and the other for astrophotography? What would be a DSO scope then?

 

I forgot to mention, 2" focuser/eyepieces only.

[MikeTahtib]

DSOs tend to be very dim.  Image brightness correlates to exit pupil.  The bigger the exit pupil, the brighter the image, up to the point where the exit pupil meets the size of the night vision adapted pupil of your eye, at which point, the extra light from a larger exit pupil will not enter your eye, so not produce a brighter image.  Exit pupil is calculated by dividing the focal length of your eyepiece by the focal ratio of your telescope.  A combination of f ratio of f5 with a 30mm eyepiece will give an exit pupil of 6mm, which is a good size for most people, maybe smaller than ideal for young people, maybe bigger than ideal for older people.  30mm is good because you can get wide-field eyepieces (82 degree) at this focal length to maximize your ability to see large objects.  

[nyx]

DSOs tend to be very dim.  Image brightness correlates to exit pupil.  The bigger the exit pupil, the brighter the image, up to the point where the exit pupil meets the size of the night vision adapted pupil of your eye, at which point, the extra light from a larger exit pupil will not enter your eye, so not produce a brighter image.  Exit pupil is calculated by dividing the focal length of your eyepiece by the focal ratio of your telescope.  A combination of f ratio of f5 with a 30mm eyepiece will give an exit pupil of 6mm, which is a good size for most people, maybe smaller than ideal for young people, maybe bigger than ideal for older people.  30mm is good because you can get wide-field eyepieces (82 degree) at this focal length to maximize your ability to see large objects.  

OK, that is a good point! So, faster telescopes (smaller focal ratio) deliver brighter/bigger(?) images to the eye compared to slow ones. And, if I understand it correctly, the same applies for higher magnifications, right?

 

So, at the end of the day, most scopes marketed as astrophotography telescopes are essentially DSO scopes, assuming the aperture is large enough for visually observing DSOs. Right?

[Knasal]

This paragraph from Orion telescopes sheds light on part of your question  

 

One factor that is left out to this point in the thread is “central obstruction.” Their paragraph addresses this: 

 

From Orion:

 

”Most telescopes of any aperture will give pleasing views of the planets, at least of Saturn and Jupiter, because they're so big, bright, and distinctive. The telescope must have clean, high-quality optics, a sturdy mount, and good eyepieces. High-quality refractors have the edge for studying the planets because of their "clear apertures." Any time something blocks a portion of a telescope's aperture (such as a secondary mirror), some image sharpness and contrast are lost. While reflector and catadioptric telescopes need small, secondary mirrors to redirect light to their eyepieces, refractors have a clear light path straight to their eyepieces, keeping contrast at its greatest. Reflectors and catadioptric telescopes work very well on the planets, it's just that a larger aperture is needed to give the same view.”

 

-Kevin

Edited by Knasal, 25 March 2019 - 07:49 AM.

[Tony Flanders]

My question originates from the way most (if not all) telescopes are marketed. You find the same pattern everywhere:
 
long focal length = planet telescope
short focal length = astrophotography

That is an oversimplification at best, and outright wrong at worst.

Short focal ratios are indeed useful for deep-sky photography, because they reduce exposure times. That is, in fact, precisely why optics with short focal ratios are called "fast."

For planetary photography, long focal ratios tend to be more useful. But this all depends on the details of your camera, etc. Moreover, there are many ways to increase the effective focal ratio of a telescope with add-ons such as Barlows lenses.

For visual observing, focal ratio isn't really all that important. A high-quality 300-mm f/4 Newtonian will deliver dazzling planetary views -- far better than a 90-mm f/13.3 Mak-Cas, which has both slightly longer focal length and much longer focal ratio.

 

It is true, however, that telescopes with very long focal lengths cannot deliver wide true fields of view appropriate for observing huge objects such as the Pleiades -- nor huge fields of multiple objects like Markarian's Chain in the Virgo Galaxy Cluster. For that reason, most deep-sky observers prefer to keep their telescope's focal ratios down to f/8 or faster.

[TOMDEY]

Theoretically, F# doesn't matter... only aperture and (low, wide-field) magnification do. But, that leads to relatively fast F#, because slower systems would require gargantuan eyepieces and focusers, to attain the same low powers and wide fields. That is, the ~fast DS Telescope~ is driven by available eyepieces and focusers (two-inch).

[Wouter1981]

I like think about that a little bit different. Planets are small and bright so you need high magnifications and those are easier to achieve with a slow telescope than with a fast telescope. A 150mm f10 with a 10mm eyepiece will perform better than a equally priced 150mm f5 with a 5mm eyepiece. A quality 150mm f10 mirror is easier the produce than 150mm f5 mirror and and therefore cheaper. A 10mm eyepiece will have (often) more eyerelief then a 5mm eyepiece of the same design.

DSO's are dim and sometimes big, so you need as much aperture as you can handle and a fast telescope allows for big light grasp, while being short, portable and able to give wide views.

Why are Dobsons so great for DSO? Because they are the cheapest way to get a big aperture, but a dobson is not a special kind of telescope, it's only a reflector on a simple and sturdy alt-az mount.

 

As you can see, I have several times related to money. Because it's more complicated than that of course. A fast 100mm f5 top quality apochromat  with premium eyepieces will outperform a decent 100mm f10 achromat without a problem for high power planetary. The same with reflectors. A good quality 150mm f2.8 can outperform a decent f8 for planetary views. But those high quality telescopes can cost 10x the money a decent telescope cost. Then I haven't started about coma correctors, filters,... but those ar all expensive.
Another thing that is important is the price of wide field eyepieces. You really can buy a new telescope with the price difference between a 30mm plossl and some 30mm 100° eyepiece. It can be cheaper to buy a fast dobson for wide views than buying a widefield eyepiece to increase the field of view in your slow telescope.

I won't go much into astrophotography, because I barely have any experience there, but be aware with what they market as astrophotography telescopes. I have seen 60mm achromats on a alt-az mount labeled as "suitable for astrophotography".

Edited by Wouter1981, 25 March 2019 - 07:53 AM.

[Tony Flanders]

So, faster telescopes (smaller focal ratio) deliver brighter/bigger(?) images to the eye compared to slow ones.

Not a bit of it! The brightness of the image on your retina depends entirely on the telescope's aperture and magnification. How you achieve that magnification is irrelevant.
 
So ignoring internal losses, a 200-mm f/10 SCT using a 10-mm eyepiece to achieve 200X delivers exactly the same image as a 200-mm f/5 Newtonian using a 5-mm eyepiece to achieve the same 200X.

 

It is true, however, that 1.25-inch eyepieces simply aren't available in focal lengths longer than 40 mm -- the reason being that such eyepieces would have very narrow apparent fields of view. So there is a limit to how bright you can get the image on your retina to be in an f/10 scope with a 1.25-inch focuser.

 

The equivalent to a 200-mm f/5 Newt with a 25-mm eyepiece to achieve 40X would be a 200-mm f/10 SCT with a 50-mm eyepiece. But eyepieces with focal lengths that long are rare in 2-inch barrels, and almost unheard-of in 1.25-inch barrels.

 

If you want to achieve something close to the theoretically lowest useful magnification for your telescope, you need a focal ratio of about f/8 or faster in scopes with a 2-inch focuser, or f/5 or faster in scopes with a 1.25-inch focuser.

Edited by Tony Flanders, 25 March 2019 - 08:01 AM.

[Hesiod]

In most cases "deep sky"="fast telescope", essentially because slow telescopes (f/15 or more) have apertures ranging from small to very small.
Indeed, the mass of commercially abailable telescopes are fast or very fast; the slower ones are for the most the f/10 SCTs, which could be brought to latge pupils with rather "standard" eyepieces.
Most of the DSOs are actually either small or very small, so the ability to frame huge fovs is not strictly mandatory, unless have very specific aims.
IME a 1200 to 1500mm f.l. is a sort of sweet spot between fov, magnification and more practical concerns (e.g tube size in Newtonian reflectors)

[nyx]

Not a bit of it! The brightness of the image on your retina depends entirely on the telescope's aperture and magnification. How you achieve that magnification is irrelevant.
 
So ignoring internal losses, a 200-mm f/10 SCT using a 10-mm eyepiece to achieve 200X delivers exactly the same image as a 200-mm f/5 Newtonian using a 5-mm eyepiece to achieve the same 200X.

 

It is true, however, that 1.25-inch eyepieces simply aren't available in focal lengths longer than 40 mm -- the reason being that such eyepieces would have very narrow apparent fields of view. So there is a limit to how bright you can get the image on your retina to be in an f/10 scope with a 1.25-inch focuser.

 

The equivalent to a 200-mm f/5 Newt with a 25-mm eyepiece to achieve 40X would be a 200-mm f/10 SCT with a 50-mm eyepiece. But eyepieces with focal lengths that long are rare in 2-inch barrels, and almost unheard-of in 1.25-inch barrels.

 

If you want to achieve something close to the theoretically lowest useful magnification for your telescope, you need a focal ratio of about f/8 or faster in scopes with a 2-inch focuser, or f/5 or faster in scopes with a 1.25-inch focuser.

The bigger was referring to the larger exit pupil. The way I see it, eyepieces and mirror quality aside, a faster scope will always deliver a bigger image diameter (exit pupil) than a slower one. But please, do correct me if I am wrong!

[gnowellsct]

If you work the numbers backwards, 0.5 mm is a practical exit pupil to call the magnification ceiling of any scope. And based on my personal tastes 3.5 mm is the shortest fl eyepiece you want to use.

These two numbers alone give you f/7 as a ratio. The magnification with a 3.5 mm ocular will be 2x the aperture of the scope in mm. A 7 mm will give you the scopes aperture as magnification.

So the ceiling of each instrument is 2x the aperture. Go beyond this and you will see stuff but views will dim.

So an 80mm peaks at 160x, a 250 mm at 500x etc.

Now consider your sky conditions. If 300x is the usual ceiling a 250 mm aperture can afford to "spend down focal ratio" (relative to f/7) and still reach 300x. A 250mm f/5 will hit 250x with a 5mm ocular and 300x with 4mm. You are in the reasonable viewing zone. You pay for it at the other end, where large exit pupils mean you cannot physically use the aperture you have paid for. Other issues arise including coma etc.

Now let's say you want a 300x view in a 100 mm telescope. That's 3x per mm of aperture. An f/5 100 mm would require a 1.3 mm eyepiece. (5mm=100x, 300x means 5/3=1.3. Constraining the aperture forces the focal ratio out, but you will never ever get to 300x and stay above a 0.5 mm exit pupil in a 100 mm scope, because the 0.5 exit pupil is an ocular one half the focal ratio, always. Your long focal ratio scopes are thus designed to exceed the organic limits of the eye with easy to use oculars, an f/15 100 mm scope will reach 300x with a 5 mm eyepiece.

The field of view is strictly a function of magnification. At 100x all scopes have the same field of view.

So we conclude your question is wrong. Field of view is determined by magnification. Magnification is determined as a ratio to aperture. If the question is what will give you 2.1 degrees and 300x at 0.5 exit pupil, you fiddle with aperture till you get where you want. A 100 mm can't do 300x at 0.5 mm exit pupil. For that you need 150mm or larger aperture. And as it happens the increased aperture is optimal for DSOs.

To get portability and wider fields reduced apertures and shorter focal ratios do the trick. But it's absurd to call a four inch f/15 instrument optimized for planets. It is optimized to offer dim views at ultra restricted exit pupils.

The notion of long focal ratios as optimized for planets is rooted in the need to control CA at high power and the difficulty in using tiny glass oculars. It has some, but less critical, basis in small Newtonian design under 150 mm where a long focal ratio can have a small secondary and more accurate mirror than say a small f/4.5. GN

Edited by gnowellsct, 25 March 2019 - 09:16 AM.

[Jon Isaacs]

The bigger was referring to the larger exit pupil. The way I see it, eyepieces and mirror quality aside, a faster scope will always deliver a bigger image diameter (exit pupil) than a slower one. But please, do correct me if I am wrong!

 

That's true if you use the same eyepiece in both scopes. Otherwise, it depends on the eyepiece one chooses.

 

Exit Pupil = Aperture / Magnification

 

If the apertures are the same and the Magnifications the same, then the exit Pupil will be the same. Otherwise, it depends on the eyepiece one chooses. An F/10 scope with a 40mm eyepiece will be brighter than an F/5 scope with a 10 mm eyepiece.

 

As far as planetary viewing, it's generally limited by the seeing.  Under a stable atmosphere, it's limited by the aperture and the optical quality. When the seeing is stable, larger reflectors provide the best planetary views, refractors are just not big enough.

 

Jon

[Peregrinatum]

90% of the Messier List are less than 0.25 degree FOV

[nyx]

90% of the Messier List are less than 0.25 degree FOV

That is a good point :/

[dakinemaui]

OK, that is a good point! So, faster telescopes (smaller focal ratio) deliver brighter/bigger(?) images to the eye compared to slow ones. And, if I understand it correctly, the same applies for higher magnifications, right?

Assuming the same aperture [Edit: and eyepiece], faster telescopes are brighter because they squash the image into a smaller area. More photons hitting a given photoreceptor in your eye == brighter. Lower magnifications do exactly the same thing (squash the image into a smaller area) and therefore appear brighter. The telescope focal length appears in both the focal ratio and magnification, which is why they act similarly. Ultimately, focal length is the "independent" variable.

 

A higher magnification yields a dimmer image, which explains why you can't use super high magnifications on a small aperture: there simply aren't enough photons to trigger a response in a given photoreceptor, because a) the small aperture didn't collect that many in the first place, and b) it spread out the ones it did collect across multiple photoreceptors.

Edited by dakinemaui, 25 March 2019 - 10:35 AM.

[Jon Isaacs]

Assuming the same aperture, faster telescopes are brighter because they squash the image into a smaller area. More photons hitting a given photoreceptor in your eye == brighter. Lower magnifications do exactly the same thing (squash the image into a smaller area) and therefore appear brighter.

 

A higher magnification yields a dimmer image, which explains why you can't use super high magnifications on a small aperture: there simply aren't enough photons to trigger a response in a given photoreceptor, because a) the small aperture didn't collect that many in the first place, and b) it spread out the ones it did collect across multiple photoreceptors.

 

Again, it all depends on the Magnification and the aperture.  A 10 inch F/5 at 50x is the same brightness as a 10 inch F10 is at 50x. Both produce a 5 mm exit pupil..

 

There are limits placed on very slow scopes because the necessary eyepieces are relatively rare but there's nothing inherent that limits the brightness of a slow scope.

 

Jon

[nyx]

I want to thank all of you for your input.

 

I guess I am trying to prove that my very first (and only, for now) telescope is not the wrong telescope, as subjective as this may be. I currently own a 2" 150/750 scope that is apparently optimized for astrophotography (secondary mirror is 63mm) and I have three 2" 70° eyepieces, 22mm, 13mm and 5mm. I do not do any astrophotography. Visual and DSOs is all I care about.

 

Living in a class 4/5 Bortle zone and a alt-az mount with a 9kg load capacity, I was wondering if I can somehow upgrade/optimize my rig. The obvious upgrade would be a dobson , though I hate the fact that it needs more storage space than my current reflector. Plus, I am fairly sure it is not as comfortable as my reflector on the mount (I do not use a chair).

 

A 6" reflector is too heavy for my mount and I have no intention switching to an EQ mount. A 4-5" reflector would probably be a downgrade for DSOs and (having tried a Mak in the past) I will probably find looking through it to be a very uncomfortable experience (neck-wise). 

 

Would a 150/1200 reflector be better for DSOs than my current scope or is a dobson the only way to go?

[dakinemaui]

If one can get the same FoV by playing around with eyepieces, why is one marketed as a planet scope and the other for astrophotography?

Because astrophotography typically doesn't play around with eyepieces; instead, they mount a camera directly to the back of the scope. The field of view depends on the telescope focal length and camera array physical size. That said, they can insert focal reducers or focal increasers (a.k.a., barlow lenses), but there is far less flexibility compared to swapping eyepieces.

 

For the record, one can do "afocal imaging", where the light exiting an eyepiece is captured by a camera with its own lens (e.g., a smart phone or webcam). Quality is typically lower than with the other type of imaging.

[dakinemaui]

Again, it all depends on the Magnification and the aperture.  A 10 inch F/5 at 50x is the same brightness as a 10 inch F10 is at 50x. Both produce a 5 mm exit pupil..

Agreed, I meant to say for the same aperture and eyepiece (corrected above), comparing focal ratio is sufficient. In general, comparing the exit pupil covers all the variables.

[havasman]

Mass market scope sellers allow their marketing departments to spew all sorts of misleading and/or false information that can easily be taken as legitimate by less experienced amateurs and throw them off onto less than fruitful paths of investigation.

 

Dobsonian scopes are Newtonian telescopes on innovative alt/az mounts. They allow larger apertures to be efficiently deployed and brought DSO observing to millions of amateurs. Mounting a Newtonian of 10 or 16 inches or even larger on a traditional alt/az or equatorial mount is a more expensive and technologically daunting task than using a Dob mount. Dobs dominate the amateur market for larger aperture scopes, and for very good reasons.

 

The best scope upgrade for a telescope used in a light polluted environment is taking it to a dark site to observe. The effectiveness of this strategy should not be underestimated. I had a 130mm f5 tabletop Dob that was a much more capable instrument from a dark site than my 254mm f4.7 Dob is from inside my very light polluted home location.

 

Aperture is, as stated above, much more important than focal ratio if the goal is increasing the instrument's capacity for DSO observing. Lower focal ratios make the package size of larger aperture Newtonian scopes on Dobsonian mounts (Dobs) more manageable.

 

In brief, if you want to see more DSO's efficiently either take your scope to a dark observing site or increase your aperture and that is most efficiently done via faster focal ratio Dob.

[SeattleScott]

Nyx,

It sounds like you have a 6” F5 reflector with a 2” focuser, on an alt az mount. And you are asking if a 6” F8 will be better for DSO. I would be inclined to say no.

Scott

[nyx]

Nyx,

It sounds like you have a 6” F5 reflector with a 2” focuser, on an alt az mount. And you are asking if a 6” F8 will be better for DSO. I would be inclined to say no.

Scott

In a nutshell, yes, this is what I am asking. Thanks!