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Importance of telescope field of view

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#1 VeloBob

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Posted 25 August 2019 - 09:37 PM

As a newbie looking for information about scopes, one of the things I have come across repeatedly when the discussion turns to different types of scopes, is the much wider field of view of a Dobs versus an SCT.  For instance, comparing the Celestron Nexstar 8SE to the Orion SkyQuest XT8G; both are 8" aperture go-to mount scopes, the Celestron is f10, the Orion is f5.9.  I have seen a FOV quoted for the Celestron of 1.2 degrees, for the Orion, one person said 3 degrees (I couldn't find it in the Orion spec lists).  So how important is this, assuming a go-to mount?  I can see where the lager FOV would be a big help if one were manual star-hopping, but it seems to me that it would be much less so in a go-to scope.  Are there many large scale DSO, visible through this size aperture, that would be visible through the Orion and not the Celestron?  Inquiring minds want to know.


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#2 LDW47

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Posted 25 August 2019 - 09:53 PM

As a newbie looking for information about scopes, one of the things I have come across repeatedly when the discussion turns to different types of scopes, is the much wider field of view of a Dobs versus an SCT.  For instance, comparing the Celestron Nexstar 8SE to the Orion SkyQuest XT8G; both are 8" aperture go-to mount scopes, the Celestron is f10, the Orion is f5.9.  I have seen a FOV quoted for the Celestron of 1.2 degrees, for the Orion, one person said 3 degrees (I couldn't find it in the Orion spec lists).  So how important is this, assuming a go-to mount?  I can see where the lager FOV would be a big help if one were manual star-hopping, but it seems to me that it would be much less so in a go-to scope.  Are there many large scale DSO, visible through this size aperture, that would be visible through the Orion and not the Celestron?  Inquiring minds want to know.

I am only interested in the TFOV ( True Field Of View ) not the FOV ( Field Of View ) !



#3 Jon Isaacs

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Posted 25 August 2019 - 10:00 PM

Bob:

 

The maximum possible field of view with an 8 inch F/6 is about 2.2 degrees.

 

There are objects this big, the Pleiades for example and objects even larger, the Andromeda galaxy, the Veil nebula. And there are star fields and fields of nebulosity that are best viewed under dark skies with a large exit pupil (low power and bright) and a wide field of view.

 

This is an aspect of deepsky observing thats missing with an SCT.

 

However, for the most part, most objects are smaller and fit nicely in the field of view of an 8 inch SCT.

 

Jon


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#4 ShaulaB

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Posted 25 August 2019 - 10:42 PM

There is an accessory called a focal reducer that can be added to an SCT. If the focal reducer gives a 0.6 reduction of focal length, then the SCT at f10 should give a similar field of view, with a given eyepiece, as an 6 8 inch f6 Dob.

 

SCT or Dob? Buy both, see which you like best, and sell the one you don't like.



#5 Jon Isaacs

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Posted 25 August 2019 - 11:14 PM

There is an accessory called a focal reducer that can be added to an SCT. If the focal reducer gives a 0.6 reduction of focal length, then the SCT at f10 should give a similar field of view, with a given eyepiece, as an 6 8 inch f6 Dob.

 

SCT or Dob? Buy both, see which you like best, and sell the one you don't like.

 

The focal reducer does change the effective focal ratio but it does not change the telescope itself. The basic limitation of the 8 inch SCT is the 38 mm rear port, the focal reducer does not change this. 

 

One can try to push the field of view beyond about 1.2 degrees but vignetting becomes a problem. The F/6.3 reducer makes that 38 mm rear port look like it's 24 mm in diameter. The focal reducer basically is like an 8 inch F/6.3 with a 1.25 inch Focuser.

 

The Dob will provide a 2.2 degree field of view, the SCT won't come close.

 

Jon


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#6 TOMDEY

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Posted 25 August 2019 - 11:22 PM

This is a topic which generates a lot of confusion among many/most novices and even many (half?) of more experienced avocationals.

 

If we reasonably assume that you will be getting eyepieces that avail the full available field of your telescope, it then it boils down to only two properties of your telescope:

 

>telescope focal length F

>draw-tube diameter d

 

~nothing else~

 

at best, available full-field diameter = 52d/F (deg)

 

This is based on the reasonable assumption that 90% of the diameter of the hole in the drawtube, at the focal plane, will be seen by your widest-field eyepiece, without unreasonable vignetting by anything comprising the telescope itself, including any other accessories like coma corrector, focal reducer, field elements, etc. And that is, indeed, a reasonable, even generous assumption. Note that a focal reducer does not affect the available field!

 

e.g. Your referenced 8-inch F/10 scope has a max available field of 1.3 deg [and a telecompressor does not improve that]

e.g. Your referenced 8-inch F/5.9 scope has a max available field of 2.2 deg

 

That's all there is to it!    Tom


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#7 Sky Muse

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Posted 26 August 2019 - 12:14 AM

It's that short, compact tube of the 8" f/10 Schmidt that's so very attractive, that reels them in; so much so that the relative enormity of the secondary-obstruction is overlooked, swept under the subconscious.

 

A Schmidt is a compact simulation of a long-focus Newtonian, but without the secondary spider-vanes of the latter; a plus.  But another aspect presents itself, and when squashing down the tube of that long-focus Newtonian to fit inside a Schmidt's tube: the secondary-obstruction enlarges, and seemingly exponentially.  For example, this is the secondary-obstruction of a 114mm(4.5") f/8 Newtonian....

 

kit3a.jpg

 

See how tiny the obstruction of the main mirror is?  Here it is again, and highlighted...

 

kit3b.jpg

 

I love that.  The obstruction of a 127mm(5") f/10 Newtonian would be of the same small size, or even smaller quite possibly.  Now, compare that obstruction to that of this 5" f/10 Schmidt...

 

http://scopeviews.co...es/image002.jpg

 

That of an 8" f/10 Schmidt is larger than that of course, as the aperture is larger.  You can't get past the ol' physics...

 

https://y7i4h2m2.sta...T_1-800x533.jpg

 

In all cases, the secondary-obstruction acts as a cataract of the human eye, blurring the image and reducing the contrast; and with Schmidt-Cassegrains possessing the largest "cataract" of all mirrored designs.

 

But aside from that, it's mighty comfy-cozy sitting behind a Schmidt, and just as it would be sitting behind a refractor.  Why, it's like driving an automobile, but to the stars.


Edited by Sky Muse, 26 August 2019 - 12:22 AM.


#8 sg6

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Posted 26 August 2019 - 02:21 AM

Field of view is relevant on goto's also.

They do not go to a target perfectly and so objects may not be in view especially if the final is narrow (= high magnification).

 

Software initially has to assume you have done everything 100% perfect when it slews to the first star. A narrow FoV can easily mean the first is not in view. Even after the scope has done all it's stuff do ot expect everything central.

 

Might be me might be general but a wide field of view also seems more comfortable and/or relaxing to use.



#9 TOMDEY

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Posted 26 August 2019 - 03:25 AM

Alan (Sky Muse) makes good points re' other relative merits of SCTs vs Newts. My 36-inch F/3.75 Newt has a 17% diam (3.0% area) central obstruction. Best I can scale from an 8-inch F/10 SCT image... the central obstruction is 35% diam (12% area). The SCT is elegant, attractive, and convenient. Those are all significant attractors. But the optical and maintenance advantages all go to the Newtonian.

 

If your Newt needs a bath, you wash the two mirrors, put them back in, do a simple alignment and are good to go! If your SCT needs a bath, you either send it to a willing service shop and hope it survives transport... or take it apart at home and then realize you'll never be able to put Humpty back together again.    Tom


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#10 Jon Isaacs

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Posted 26 August 2019 - 04:41 AM

If your Newt needs a bath, you wash the two mirrors, put them back in, do a simple alignment and are good to go! If your SCT needs a bath, you either send it to a willing service shop and hope it survives transport... or take it apart at home and then realize you'll never be able to put Humpty back together again.    Tom

 

 

I am a big fan of Newtonians.

 

But realistically, Newtonians are open telescopes and require frequent cleaning. SCTs are a closed design and can easily go 30 years or more with only the exposed surface of the corrector plate being cleaned.

 

The corrector is much easier to clean than a Newtonian mirror, no disassembly required, no soft coatings to worry about. Cloudy Nights member Marty bought his 8 inch SCT in 1977. I believe the mirrors have never been touched.

 

In my mind, in terms of cleaning, the SCT has a real advantage over a Newtonians. I clean my Newtonians mirrors at least once a year, sometimes more often depending on how much use they get.

 

In all cases, the secondary-obstruction acts as a cataract of the human eye, blurring the image and reducing the contrast; and with Schmidt-Cassegrains possessing the largest "cataract" of all mirrored designs.

 

 

The central obstruction is not a cataract, a cataract is partially transparent and blurs the image as a bad place in a lens does. The CO is simply part of the mirror that is covered. The CO actually slightly improves the resolution.

 

An 8 inch F/6 Dob has a CO of about 25%, an 8 inch SCT of about 35%. There is a difference but it's relative small and primarily affects fine scale planetary contrast.

 

I have done the experiment of adding a 40% CO to a Refractor which has no CO. The effect was visible viewing Saturn but it was surprisingly small. 

 

As I said, my favorite scopes are Newtonians but I would not be worried about the somewhat larger central obsruction of the SCT.

 

Jon


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#11 Mike W.

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Posted 26 August 2019 - 07:18 AM

Tom, yes a newt is much easier to disassemble than an SCT, as noted by Jon, might take upwards of thirty years for an SCT to need any service what so ever,

 

My recent return to the hobby, actually now several years ago,,,, anyway I purchased an SCT, but when I went out the first night I noticed what looked to be a bug splat on a windshield in the fov, I spent a nice Saturday marking and removing just the corrector plate, I could clean the primary without removing it from the ota, I cleaned the corrector and secondary in the sink like any Newt mirror, satisfied with the results, took a nap and waited for the Sun to finish it's job on this side of Earth.

 

Drove to my dark site and again there was the bug, So Sunday I spent again removing the corrector and there on the primary was a spot, it was a spot of grease from the focus system of the SCT, it had dripped down from when I had run the mirror all the way forward Saturday for cleaning, but it was a much different shape than the "Bug Splat" I had seen Friday night so I disassembled my diagonal and cleaned the prism, set everything up and tried to see if I could see the problem during the day, I didn't so it was in the diagonal until the next Friday night.

 

Short story long, turned out the "Bug Splat was actually in my eye, but point of the story, you can take Humpty Dumpty apart to clean and put him back together again.

 

This was all before I had even heard of Cloudy Nights, and yes I've had SCT's stripped to the shell of the ota, mirrors in the sink, now with the recent invent of SCT cooling fans I'm sure many more will come apart.

 

But I'd rather clean a Newt.


Edited by Mike W., 26 August 2019 - 07:21 AM.


#12 whizbang

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Posted 26 August 2019 - 08:50 AM

Sg6 is spot on.  A wider FOV makes it easier to find objects whether you are star hopping or using go-to.

 

The narrow FOV ina SCT means it has more inherent magnification than a DOB.  So, the SCT is easy on eyepieces.

 

SCT's are usually on EQ or Alt-Az mounts that track.  Tracking is really convenient.  Most DOBs need continuous nudging especially at higher powers.

 

Re: the discussion about cleaning:

From what I have seen in older used scopes, closed OTAs protect their optics.  Many 70s orange tubes are good servicable scopes.  Dobs on the other hand are open to the elements.  Some combination or exposure and repetitive cleaning must degrade the mirrors.  I have yet to see a 40 year old DOB that I would spend my hard earned dollars on.


Edited by whizbang, 26 August 2019 - 08:58 AM.


#13 whizbang

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Posted 26 August 2019 - 09:02 AM

Velobob,

 

I would encourage you to consider the XT8i or the XT8g.  The 8i is a simple, reliable design.

 

The 8g is a boat anchor.  Lugging that heavy base is a back breaker.



#14 Jon_Doh

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Posted 26 August 2019 - 10:05 AM

Here's a nice simulator that will let you see what various objects might look like in different telescopes and eyepieces.  It really puts things in perspective.


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#15 aeajr

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Posted 26 August 2019 - 10:12 AM

As a newbie looking for information about scopes, one of the things I have come across repeatedly when the discussion turns to different types of scopes, is the much wider field of view of a Dobs versus an SCT.  For instance, comparing the Celestron Nexstar 8SE to the Orion SkyQuest XT8G; both are 8" aperture go-to mount scopes, the Celestron is f10, the Orion is f5.9.  I have seen a FOV quoted for the Celestron of 1.2 degrees, for the Orion, one person said 3 degrees (I couldn't find it in the Orion spec lists).  So how important is this, assuming a go-to mount?  I can see where the lager FOV would be a big help if one were manual star-hopping, but it seems to me that it would be much less so in a go-to scope.  Are there many large scale DSO, visible through this size aperture, that would be visible through the Orion and not the Celestron?  Inquiring minds want to know.

FOV, field of view, has nothing to do with Dob vs. SCT but with focal length.  Also remember that a Dob is really a Newtonian reflector mounted on a Dobsonian mount. So you are really comparing a Newtonian OTA vs. an SCT OTA.  We can add MCTs and refractors into the discussion as well.  Doesn't matter.

 

If you have a Dob/Newtonian with a 1500 mm FL and an SCT with a 1500 mm FL, when using the same eyepiece they will have the same magnification and the same field of view.

 

FL scope (any kind) / FL eyepiece (any kind) = magnification

 

AFOV eyepiece ( spec provided by the MFG)  / Magnification = approximate the field of view.  Close enough.

 

Doesn't matter what kind of scope it is.  However, it is more common for SCTs to have longer focal lengths for a given aperture.   

  • An 8" SCT will typically have an F10 focal ratio which gives you about 2000 mm FL.   
  • An 8" Newtonian/Dob will typically be about F6 yielding a 1200 mm FL. 
  • Using the same eyepiece in each will give you a lower power/wider view in the Newtonian because it has a shorter focal length.

 

How important is FOV?

 

Some targets are wide, such as the Pleiades (around 2 degrees) , the Andromeda Galaxy ( around 3 degrees) , the North America Nebula (around 4 degrees).  Your field of view defines how much of it you can view at once.

 

The Moon is about 1/2 a degree and planets are much smaller than that.  The vast majority of what we view is 1 degree or less.  But there are some beautiful sites that are over 1 degree.

 

If you have a manual mount then FOV defines your drift time in the FOV before you have to bump the mount or turn the slow motion controls to follow the target.

 

If you are star hopping, a wider FOV makes it easier to find things.   Typical 9X50 magnifying finder scope has a FOV of about 5 degrees. 

 

If you are using a GoTo , finding the target is not much of a concern.  Then the ability to fit and frame the target is the main concern when it comes to FOV.

 

 

You can widen your FOV by using eyepieces that have a wider apparent field of view, AFOV.   

  • A typical Plossl has an AFOV of 50 degrees
  • An AT Paradigm is about 60 degrees
  • Explore Scientific and others have them in 68, 82, 90, 100 and even 120 degrees

 

Finally, the size of your focuser will define the limit of your FOV for your scope.   If you have a 1.25" focuser, the maximum FOV will be attained with a 32-40 mm Plossl.  If you have a 2" focuser, the same scope can have a wider FOV.

 

Hope that helps.  These are related topics.

 

 

Refractor vs. Reflector – Which is better?
https://telescopicwa...tor-telescopes/

 

Different types of Telescopes
https://telescopicwa...-of-telescopes/

 

Understanding Telescope Eyepieces- There are recommendations, based on budget, but the meat of the article is about understanding the issues when selecting eyepieces.
https://telescopicwa...cope-eyepieces/


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#16 aeajr

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Posted 26 August 2019 - 10:23 AM

Velobob,

 

I would encourage you to consider the XT8i or the XT8g.  The 8i is a simple, reliable design.

 

The 8g is a boat anchor.  Lugging that heavy base is a back breaker.

All depends on where you store it and how you move it.  If you can store it at ground level then you can move things on a cart or hand truck and weight becomes much less of an issue.   My 12" Dob is almost 100 pounds yet I move it easily with a hand truck and it pops into the car with very little effort. 

 

I can bring out the 12" dob almost as easily as I bring out my 80 mm refractor. 

 

However if you live in a 3rd floor walk up, then things are different. 



#17 Sky Muse

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Posted 26 August 2019 - 11:39 AM

The central obstruction is not a cataract, a cataract is partially transparent and blurs the image as a bad place in a lens does. The CO is simply part of the mirror that is covered. The CO actually slightly improves the resolution.

How does it do that?  Also, does it improve sharpness and contrast as well?



#18 Jon Isaacs

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Posted 26 August 2019 - 12:01 PM

How does it do that?  Also, does it improve sharpness and contrast as well?

 

Alan:

 

It does not improve contrast and sharpness, most a CO reduces sharpness and fine scale contrast to some relatively slight amount.

 

The reason the resolution is slightly improved is this:

 

As you know, the stars you see in a telescope are not truly points of light, rather they are tiny disks surrounded by a faint series of diffraction rings. The size of this tiny dog determines the resolving power. (The larger the aperture, the smaller dot.)

 

The effect of the central obstruction is to move energy from the central disk out to the rings. This makes the central disk slightly smaller this improving, in theory at least, the resolution. But the added energy to the diffraction rings reduces the fine scale contrast.  In general, the effect of a CO is not positive.

 

My concern here is that this thread stay focused on the important factors. The CO can have an effect on the image but it's relatively small and there other more important factors in choosing between the 8 inch Dob and the 8 inch SCT.

 

Jon



#19 Sky Muse

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Posted 26 August 2019 - 12:20 PM

In general, the effect of a CO is not positive.

Thanks, but it's never positive, in my opinion.  As for importance, such is subjective.



#20 VeloBob

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Posted 26 August 2019 - 12:43 PM

OK, after a little research, the XT8G lists obstruction as 23% and 5% by area, where the Celestron 8SE says 31% and 9.77% by area.  The 8g is 52 pounds, the 8SE is 33 pounds (and the 8 EVO is 43 pounds), with the 8G breaking down into two parts, a relatively light OTA and a heavy base, where the 8SE breaks down into three parts of roughly equal weight.  From what has been said so far, the field of view matters for a few objects, but it sounds like for most of the things I'd be looking at, it isn't a major issue.  The 8SE will be very slightly dimmer, thanks to the greater obstruction, but I can't imagine that the 4.77% difference in area would be noticeable to me.  Thanks for continuing my education



#21 Jon Isaacs

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Posted 26 August 2019 - 12:44 PM

Thanks, but it's never positive, in my opinion.  As for importance, such is subjective.

 

Unless one does an actual comparison that removes all other variables, the actual visual effect of the CO will be difficult to evaluate.

 

If one wants to qualitatively evaluate to effect of the central obstruction, the easiest way is to add a central obstruction to a scope without a CO of add a larger one to a scope with a CO.

 

I have done both. Adding a 40% CO to a 120 mm Apo did somewhat reduce the contrast on Saturn but it was still crisp.

 

In splitting the unequal 1.4" double zeta Herculis, the CO had essentially zero effect.

 

Going from a 25% CO to a 35% CO will have a much smaller effect.

 

Jon



#22 Sky Muse

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Posted 26 August 2019 - 01:08 PM

OK, after a little research, the XT8G lists obstruction as 23% and 5% by area, where the Celestron 8SE says 31% and 9.77% by area.  The 8g is 52 pounds, the 8SE is 33 pounds (and the 8 EVO is 43 pounds), with the 8G breaking down into two parts, a relatively light OTA and a heavy base, where the 8SE breaks down into three parts of roughly equal weight.  From what has been said so far, the field of view matters for a few objects, but it sounds like for most of the things I'd be looking at, it isn't a major issue.  The 8SE will be very slightly dimmer, thanks to the greater obstruction, but I can't imagine that the 4.77% difference in area would be noticeable to me.  Thanks for continuing my education

In the end, given all of the other advantages and variables discussed, go with the Schmidt.  Most likely, I would myself.



#23 Tony Flanders

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Posted 26 August 2019 - 03:31 PM

The 8SE will be very slightly dimmer, thanks to the greater obstruction, but I can't imagine that the 4.77% difference in area would be noticeable to me.  Thanks for continuing my education


That is indisputable. A more significant difference is that the SCT has two extra optical elements: corrector, primary, secondary, star diagonal vs. just primary and secondary for a Newt. Each extra element absorbs, scatters, and reflects a bit of light, and the overall loss is much more than 4.77%. Nonetheless, the difference ends up being anywhere from invisible to small.

 

As for field of view, it is true that 99% of all DSOs fit nicely in the smallish field of view of an 8-inch SCT with a 1.25-inch optical back. It is also true that the remaining 1% include some of the sky's greatest showpieces.

 

But for me, the biggest advantage of a Dob's wide field of view is the ability to view objects in context. Most galaxies are part of galaxy groups, and galaxy groups as a whole are wonderful to view. For instance, an off-the-shelf 8-inch SCT cannot do justice to Markarian's Chain in the center of the Virgo Cluster. Nor to M24, nor to the nebula/cluster field stretching from M8 to M21.


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#24 MalVeauX

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Posted 26 August 2019 - 03:45 PM

Importance of potential FOV is huge. It tells you your widest FOV that you can get. This is very important because some subjects are LARGER than what some long focal lengths can produce a FOV to look at. A really long scope producing barely a 1 or 2 degree FOV cannot fully show you in one look some objects like M42 + running man, or M31, etc. It matters a lot. It's a lot harder to put a 1 degree or less, FOV setup, on a subject even with a tracking mount. And depending on the scope design it matters a lot too as it means a lot in terms of the optics. A F4 mirror versus a F6 mirror is significantly different and it relates to FOV but relates to so much more in terms of eyepieces, performance, etc. And in refractors, lenses, it means a ton too because the really short ones (F4~F6) can provide a really wide FOV but it will cost you something, namely, color correction.

 

Very best,


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#25 TOMDEY

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Posted 26 August 2019 - 04:02 PM

But, but. but... everyone should keep in mind that an SCT is just not as good as a Newtonian. And that's why Newton didn't bother inventing the SCT... he went right for the good stuff. Ditto John Dobson. Everyone remembers Isaac and John... no one remembers Bernhard.

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