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What is the advantage of using 62mm diameter telescopes?

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#1 thornhale

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Posted 11 April 2021 - 09:25 PM

As I am carefully thinking about a possible next investment while trying my best also not to do it right away, I started coming across videos with people using even smaller telescopes, like the 62mm F4.5 Raptor telescope.

 

Please educate me and share with me why buying an even smaller telescope is beneficial? I thought that to some degree aperture is king because in addition to more light being gathered, the ability to resolve two points that are close to each other also increases. Isn't 70mm or 80mm already pretty portable enough? Why would you want to go even smaller?

 

Thank you for answering!


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#2 gwlee

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Posted 11 April 2021 - 09:32 PM

Most smaller aperture scopes offer more portability and a wider FOV. People who want more of both go smaller than70-80mm. 72mm gives me enough of both to satisfy my current wants, but others want the portability and FOV of a 50mm scope.

Of course getting more portability and FOV requires giving up other things, chiefly resolution and light grasp. Expect that many people buying small scopes already have a larger scope that meets their needs for resolution and light grasp.

Edited by gwlee, 11 April 2021 - 09:46 PM.

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#3 thornhale

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Posted 11 April 2021 - 09:35 PM

Most smaller aperture scopes offer more portability and a wider FOV.

Isn't a 80mm F/5 telescope already plenty wide? Even F6...we are looking at 400mm-480mm focal length. With that you can capture all of Andromeda which itself is 3 degrees so very wide. What objects need even wider angles?


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#4 chanrobi

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Posted 11 April 2021 - 09:58 PM

Isn't a 80mm F/5 telescope already plenty wide? Even F6...we are looking at 400mm-480mm focal length. With that you can capture all of Andromeda which itself is 3 degrees so very wide. What objects need even wider angles?

If you're into shooting janky nebulas like heart & soul or rosette or , north american for example

 

But I would just use a lens in that case don't need a "telescope" for that


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#5 gwlee

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Posted 11 April 2021 - 10:04 PM

[quote name="thornhale" post="11026039" timestamp="1618194910"]
Isn't a 80mm F/5 telescope already plenty wide? Even F6...we are looking at 400mm-480mm focal length. With that you can capture all of Andromeda which itself is 3 degrees so very wide. What objects need even wider angles?[/quote

That’s why I said “most”. There will always be exceptions, but 80mm f6 and f7 are probably the most common. 80mm f5 scopes are less popular because although they can provide an even wider field they have other drawbacks.

My 72mm f6 scopes provides me with up to 5* FOV with a 2” 35mm EP, which I enjoy for cruising the milky way. Only experience will enable you to know what you like, which might turn out to be 3*.

Big instruments are for viewing small objects. Small instruments are for viewing big objects. My smallest instrument, a 7x50 binocular is capable of viewing objects that span 7.5*, I use it a lot, but the universe is much larger than that.

Edited by gwlee, 11 April 2021 - 10:22 PM.

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#6 dnrmilspec

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Posted 11 April 2021 - 10:06 PM

According to your sig, you have a 6" Orion.  Is that correct?   And are you interested in these scopes for visual use? 

 

Assuming those two things, I don't think a small refractor is going to please you, and I am a HUGE refractor fan.  Your V6L is a 750mm scope.  This is generously wide.  It is also very much brighter than a 60-80mm refractor.  The resolution will be very much higher.  You will have no false color.  (I don't mind that but you might.)

 

What you may like about the small refractor is the pinpoint stars and maybe better contrast.  But all things considered, unless you go for an apochromat and  use it on planets and the moon, you will be gaining little.  Consider this.  You already have 50mm binoculars.  Look at the brightness through them and you have an approximation of what you would see through a 60mm scope.

 

Out of curiosity, what do you want your new scope to do that your 6" reflector does not?


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#7 T~Stew

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Posted 11 April 2021 - 10:28 PM

Isn't a 80mm F/5 telescope already plenty wide? Even F6...we are looking at 400mm-480mm focal length. With that you can capture all of Andromeda which itself is 3 degrees so very wide. What objects need even wider angles?

Many have imaged with much wider than an 80/5... quite popular is the Samyang 135mm focal length f/2, numerous threads on here about wide field imaging with it, some larger object or combinations (think all of orions belt + horsehead + Orion Nebula), or even whole constellations or large chunk of milky way. But yes, I think for single DSO' 400mm or so covers most. 



#8 bobzeq25

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Posted 11 April 2021 - 10:40 PM

You're getting a lot of answers from visual observers.  Visual is a _completely_ different thing than imaging.  I cannot emphasize that enough.

 

Your eyes are inherently short exposure, the camera can do ones orders of magnitude longer.  That changes _everything_.

 

In imaging smaller telescopes are (sometimes) used for three main reasons.

 

They make learning this complicated art _much_ faster/better/cheaper (in all kinds of ways).  Starting with too big a scope is a _very_ common beginner mistake.  Often made because they were so happy with it for visual, and simply don't grasp just how different imaging is.

 

They provide a wide field for large targets.  You can do M31 with 480mm, but it will require either a large chip camera, or a mosaic.

 

They are both portable in themselves, and enable the use of smaller more portable mounts.

 

Most serious imagers have multiple scopes.  I have refractors from 66-130mm, and an 8 inch RASA, a scope which _cannot_ be used for visual, it's strictly an imaging scope.

 

The Raptor was designed for imaging. 

 

https://radiantelescopes.com/raptor61/

 

It could be used for visual, but that's not what it was designed for, and few visual observers would appreciate the imaging features, and the associated price.

 

Below is an image I did with a 70mm Stellarvue (imaging) scope.  I'm not saying it's great, but imagers can appreciate it, and my larger scopes could not have captured that target. 

 

Better version than the crummy ()required) CN jpg, with acquisition details, here.

 

https://www.astrobin.com/367734/C/

 

Few (no?) visual observers would pay $1000 for that scope.

 

The Radian would be a great scope to learn imaging with.

 

NGC 6992 V5.jpg


Edited by bobzeq25, 11 April 2021 - 10:49 PM.

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#9 gwlee

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Posted 11 April 2021 - 10:45 PM

Many have imaged with much wider than an 80/5... quite popular is the Samyang 135mm focal length f/2, numerous threads on here about wide field imaging with it, some larger object or combinations (think all of orions belt + horsehead + Orion Nebula), or even whole constellations or large chunk of milky way. But yes, I think for single DSO' 400mm or so covers most.

Only you know what you want to look at. Select an instrument with a FOV large enough to frame it, or be willing to look at it a section at a time with a larger instrument.

People who want to frame very large objects generally choose very small instruments, which is their advantage that you asked about.

Edited by gwlee, 11 April 2021 - 10:46 PM.

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#10 MartinPond

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Posted 11 April 2021 - 10:52 PM

You're getting a lot of answers from visual observers.  Visual is a _completely_ different thing than imaging.  I cannot emphasize that enough.

 

Your eyes are inherently short exposure, the camera can do ones orders of magnitude longer.  That changes _everything_.

 

...

....

That is a very spectucular example of scope+tracking+time !

 

Anyone care to guess how much bigger the aperture would need to be to

    see that by eye?   Crickey.


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#11 ravenhawk82

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Posted 11 April 2021 - 11:34 PM

Such telescopes are great for imaging but not the best option for visual use. You can adjust your camera's exposure time so aperture matters much less than f-ratio when it comes to how bright of an image you get at the sensor, and since your magnification is determined by the scope's focal length instead of your eyepiece's, how well it can resolve is usually a moot point. In almost every scenario (except for planetary imaging with its crazy high magnifications) your sensor becomes the limiting factor in resolving something long before your telescope, and seeing sometimes limits you even before that.

For astrophotography, you want a flat field, a fast focal ratio, and a light weight so that you don't have to load your mount down too much. If it's a refractor you also want it well corrected against chromatic aberrations. These things are all much easier and cheaper to accomplish in a smaller telescope. I'd pick something very different for visual use but for wide field astrophotography a small, light, and well corrected scope is a powerful tool. A wide field like this really is worth it too if that's the kind of photos you like to take. The widest scope I have has a 550mm focal length, and at that point I can just barely frame the heart nebula on my 4/3rds sensor camera (as shown in my profile photo). If I wanted to go wider than a focal reducer would allow I'd need to either get a bigger sensor or a wider telescope. A scope like the Radian 61 would be cheaper than an APS or full frame CMOS (something like an AT62ED would be MUCH cheaper) and wouldn't mean buying bigger filters so that'd be the path I'd take.

 

A nice camera lens will fit sometimes fit the bill but terrestrial lenses are built for different purposes and make different compromises... For example, a little field curvature will be almost unnoticeable in a portrait but distracting in an image of a star field. Meanwhile, many of the mechanics that you pay for in a camera lens like automatic focus and f-stop are wasted at night when autofocus doesn't (usually) work and there's no need for the camera to rapidly change aperture.

They're all different tools for different jobs.



#12 barbie

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Posted 11 April 2021 - 11:42 PM

A small aperture lightweight scope tends to get used more than a large and heavy one, especially if you suffer from age related physical disabilities like I do. I use my small scopes often and after having had the "big guns", it's nice to have instruments I can use and use often enough to justify having them in the first place because they are so light and easy to set up.


Edited by barbie, 11 April 2021 - 11:47 PM.

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#13 gwlee

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Posted 11 April 2021 - 11:51 PM

People use different size scopes for some of the same reasons they use different FL eyepieces; it allows them to see things that they could not see with only one eyepiece and one scope. Or, even one scope and many eyepieces. 

 

Four example, with four common eyepieces, my largest scope enables me to see objects in their entirety that span 1.8* to 0.36*. With the same four eyepieces, my smallest scope enables me to see objects in their entirety that span 5.1* to 0.99*. Although there is some overlap in capabilities, the large scope is better suited to viewing small objects, and the small scope is better at suited to viewing large objects, which is its advantage that you asked about. 


Edited by gwlee, 11 April 2021 - 11:59 PM.


#14 ravenhawk82

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Posted 12 April 2021 - 12:15 AM

Anyone care to guess how much bigger the aperture would need to be to

    see that by eye?   Crickey.

Very rough napkin math here, but just for fun:

Bob's photo was taken with 900s narrowband subs at F/6 (ignoring the RGB subs because I'm guessing the narrowband is where most of the detail came from). Our eyes, when night adapted, get down to around F/2, so roughly four stops faster than Bob's scope. This would equate to about 16x less exposure time meaning our eyes would need to expose for about 56 seconds for an equivalent exposure. Now, how fast our eyes can refresh is still uncertain because we process data as a stream of input rather than a steadily refreshing field. This page suggests a lower bounds of about 100ms. It later admits that further studies found people can discern much faster movements, but when our eyes are already straining to process information in the dark I think it's fair to assume that a dark adjusted eye using averted vision has a pretty slow "refresh rate" relative to paying attention to action in good light so I'll stick with 100ms.

This is is 560x less exposure than Bob's equipment (ignoring gain because there's this got out of hand enough already), so our eyes would need a lens that can collect 560x more light. A dark adapted pupil has a diameter of ~8mm max, so an area of ~25mm^2. You'd need an area of 25x560=14000mm^2, or a lens with a diameter of about 4.4 meters to see that level of detail. This actually sounds pretty reasonable, considering the sketches that the author of this article made when looking through the 4.27m Discovery Channel Telescope.

Fair warning though, I'm sleep deprived and not a mathematician so I may have goofed here somewhere ;)
 


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#15 thornhale

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Posted 12 April 2021 - 01:01 AM

You're getting a lot of answers from visual observers.  Visual is a _completely_ different thing than imaging.  I cannot emphasize that enough.

 

Your eyes are inherently short exposure, the camera can do ones orders of magnitude longer.  That changes _everything_.

 

In imaging smaller telescopes are (sometimes) used for three main reasons.

 

They make learning this complicated art _much_ faster/better/cheaper (in all kinds of ways).  Starting with too big a scope is a _very_ common beginner mistake.  Often made because they were so happy with it for visual, and simply don't grasp just how different imaging is.

 

They provide a wide field for large targets.  You can do M31 with 480mm, but it will require either a large chip camera, or a mosaic.

 

They are both portable in themselves, and enable the use of smaller more portable mounts.

 

Most serious imagers have multiple scopes.  I have refractors from 66-130mm, and an 8 inch RASA, a scope which _cannot_ be used for visual, it's strictly an imaging scope.

 

The Raptor was designed for imaging. 

 

https://radiantelescopes.com/raptor61/

 

It could be used for visual, but that's not what it was designed for, and few visual observers would appreciate the imaging features, and the associated price.

 

Below is an image I did with a 70mm Stellarvue (imaging) scope.  I'm not saying it's great, but imagers can appreciate it, and my larger scopes could not have captured that target. 

 

Better version than the crummy ()required) CN jpg, with acquisition details, here.

 

https://www.astrobin.com/367734/C/

 

Few (no?) visual observers would pay $1000 for that scope.

 

The Radian would be a great scope to learn imaging with.

 

attachicon.gifNGC 6992 V5.jpg

I think I understand now. My initial question was: But with smaller aperture your ability to resolve to dots decreases...M3 for instance is said to take at least 200 mm aperture to fully resolve....and here you will probably say that that object needs a different OTA for imaging. Correct? So use the above for big nebular or large galaxies....use other OTAs for things that require resolving power.

 

Flow of conscience: Is this the reason why 80mm F6 telescopes are so popular? They are wide enough to easily image many bigger things. At the same time, they could still reasonably be used for visual observation whereas 70mm and even worse 62 mm telescopes would produce highly unsatisfying views for the visual observer - which the astrophotographer doesn't care for because he/she can compensate for that with longer exposure times).



#16 bobzeq25

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Posted 12 April 2021 - 02:27 AM

I think I understand now. My initial question was: But with smaller aperture your ability to resolve to dots decreases...M3 for instance is said to take at least 200 mm aperture to fully resolve....and here you will probably say that that object needs a different OTA for imaging. Correct? So use the above for big nebular or large galaxies....use other OTAs for things that require resolving power.

 

Flow of conscience: Is this the reason why 80mm F6 telescopes are so popular? They are wide enough to easily image many bigger things. At the same time, they could still reasonably be used for visual observation whereas 70mm and even worse 62 mm telescopes would produce highly unsatisfying views for the visual observer - which the astrophotographer doesn't care for because he/she can compensate for that with longer exposure times).

What you say is correct, _except_ that you're overlooking the important difference between learning imaging, and doing imaging.   That leads to situations like this.  Note the words "learn" and "learning", they're key.

 

"I regret spending the first 6 months trying to learn imaging with an 8" Edge, with that scope it was a losing effort. Fortunately got a nice little refractor, and not only have the quality of my images improved but I'm actually enjoying the process of learning how to do it!"

 

An 80mm F6 is an excellent scope to learn imaging on.  A good one can transition into your big target scope, once you've become an experienced imager.  There is no such thing as a scope that's excellent for both learning imaging and visual.  If you want to do those two things, you really want two scopes.


Edited by bobzeq25, 12 April 2021 - 02:33 AM.

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#17 desertlens

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Posted 12 April 2021 - 02:55 AM

Terms like better and worse are subject to some relativity (and skepticism). Years ago, I decided to be a small aperture visual observer. With realistic expectations, even a 60mm scope can offer some rewarding experiences. You are correct in observing that the 80mm ƒ6 instrument is the obvious choice in this realm. After a 50 year career in photography, I'm over it. The real time experience of visual observing is not to be dismissed, even at small apertures. Here is an old friend:

 

SV80ST0.jpg

Stellarvue 80ST triplet

 


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#18 Ben the Ignorant

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Posted 12 April 2021 - 07:12 AM

why buying an even smaller telescope is beneficial? I thought that to some degree aperture is king because in addition to more light being gathered, the ability to resolve two points that are close to each other also increases.

Sometimes they just want a new toy. waytogo.gif


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#19 BillP

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Posted 12 April 2021 - 08:58 AM

I think I understand now. My initial question was: But with smaller aperture your ability to resolve to dots decreases...M3 for instance is said to take at least 200 mm aperture to fully resolve....and here you will probably say that that object needs a different OTA for imaging.

I would not get caught up in the whole "resolve to the core" nonsense regarding visual on globular clusters (see post #10 by Glenn in the link below).  It is something that is very much a visual community paradigm and not even uniformly defined.  I see stars across the core of "aperture appropriate" globular clusters with most any aperture, even 80mm.  I said "aperture appropriate" because some globs are just too small and too far so very dim and need aperture to see any individual stars.  But bright things like M13 it is a piece of cake to see individual star points across the globular and into the core.  Like anything else in this hobby, answers that only involve equipment more often than not fall short of reality as things like observer skill, experiences, and technique often play the biggest role in outcomes.  IMO I feel most of the resolve to the core silliness is more hubris than anything else.  However, for an older discussion on this topic take a look at the thread linked below, in particular post #4 from Don which is a nice breakdown.  But even in his example of what he considers "fully resolved", we all know that there are more stars in the globular that any aperture you want to pick can resolve, so when you go larger aperture you will always see more stars.  And while one may think they see no "glow" within the globular when they feel it is fully resolved, it is there just appears not to be more likely due to the lack of comparison with a fully dark background sky that might be some distance from the globular itself.

 

https://www.cloudyni...ters/?p=6561610

 

Btw, here is a Hubble pic of the core of M13.  When you look at this, realize that this telescope is still not seeing everything there, and many of those smallest dimmest star points when viewed from earthbound amateur telescopes are seen as single stars when in fact they are groups of several stars due to the limited resolution of both the main objective of the telescope and the seeing.  So upshot is that the cluster is in fact never fully resolved and basically your scope, whatever aperture, under good seeing conditions will fully resolve across the cluster all stars that are appropriate magnitude for your aperture.

 

https://www.nasa.gov...-xlarge_web.jpg


Edited by BillP, 12 April 2021 - 02:02 PM.

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#20 dnrmilspec

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Posted 12 April 2021 - 09:21 AM

Just a side note.

 

One of the reasons that the Starizona Hyperstar for Celestron SCT's is so popular is that it converts (for example)  an 11" F10 into an 540mm F1.9.  So with one scope you have a wonderful visual scope and a great astrophoto scope.  Its complicated but the hits just keep on happening.


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#21 Mitrovarr

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Posted 12 April 2021 - 10:20 AM

Those really fancy sub-80mm telescopes are, for the most part, all astrographs designed for photography. They're intended for high-quality pictures of extended nebulae that are often quite large in the sky, requiring very small focal length (hence, low aperture and very fast focal ratio) and a large well corrected imaging plane. This is accomplished by either being some kind of petzval design or having a reducer/flattener that's often included.

While you could probably have fun using one visually if you already owned it for imaging, it would make absolutely zero sense to buy for that purpose.
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#22 bobzeq25

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Posted 12 April 2021 - 10:42 AM

Very rough napkin math here, but just for fun:

Bob's photo was taken with 900s narrowband subs at F/6 (ignoring the RGB subs because I'm guessing the narrowband is where most of the detail came from). Our eyes, when night adapted, get down to around F/2, so roughly four stops faster than Bob's scope. This would equate to about 16x less exposure time meaning our eyes would need to expose for about 56 seconds for an equivalent exposure. Now, how fast our eyes can refresh is still uncertain because we process data as a stream of input rather than a steadily refreshing field. This page suggests a lower bounds of about 100ms. It later admits that further studies found people can discern much faster movements, but when our eyes are already straining to process information in the dark I think it's fair to assume that a dark adjusted eye using averted vision has a pretty slow "refresh rate" relative to paying attention to action in good light so I'll stick with 100ms.

This is is 560x less exposure than Bob's equipment (ignoring gain because there's this got out of hand enough already), so our eyes would need a lens that can collect 560x more light. A dark adapted pupil has a diameter of ~8mm max, so an area of ~25mm^2. You'd need an area of 25x560=14000mm^2, or a lens with a diameter of about 4.4 meters to see that level of detail. This actually sounds pretty reasonable, considering the sketches that the author of this article made when looking through the 4.27m Discovery Channel Telescope.

Fair warning though, I'm sleep deprived and not a mathematician so I may have goofed here somewhere wink.gif
 

Good stuff, but not even _close_ to the right answer.  Not remotely.  Visual and imaging are just not the same things, at all.  Compare the guy's sketch of M51 to the images here.  _Any_ of the images here, 81 pages of them, and some are pretty bad.

 

https://www.astrobin.com/search/?q=M51

 

One thing you haven't included (far from the only one), that changes the game is processing.

 

You simply can't compare visual and traditional imaging.  Two completely different things.

 

Professional astronomers don't look through telescopes any more.  They also don't make pictures, they gather numeric data.  That's another topic.  <smile>



#23 bobzeq25

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Posted 12 April 2021 - 10:51 AM

Just a side note.

 

One of the reasons that the Starizona Hyperstar for Celestron SCT's is so popular is that it converts (for example)  an 11" F10 into an 540mm F1.9.  So with one scope you have a wonderful visual scope and a great astrophoto scope.  Its complicated but the hits just keep on happening.

Just a side note. 

 

The SCT may be a good imaging scope for small targets when used by an experienced imager.  But it's a horrible choice to try to _learn_ imaging with.  About as bad as it gets.  Anyone considering it needs to watch this Youtube, at least the first five minutes of it.  His is a _common_ experience.  Thinking you'll be an exception is asking for pain.

 

https://www.youtube....h?v=MNQU1hdqz4M

 

Hyperstar makes some of the problems less bad by shortening the focal length dramatically (although it's no longer useful for small targets), but it's _still_ not a good scope to learn with.  Dr. Craig Stark, is a noted astrophotographer, who writes and lectures on the topic.  He wrote the PhD autoguiding program most everyone uses.  Dr. Stark, on the best scope to start with.  It's not just what he says, it's how he says it.  He's dealt with a lot of beginners, understands their misconceptions.

 

"As light as possible.

 

 

 

Seriously.

 

 

 

No, seriously."

 

That's a good place for the Raptor.  If you also want to do visual, and you're on the usual budget, get a 6-8 inch Dob for visual.

 

Two different things.  I cannot emphasize that enough.


Edited by bobzeq25, 12 April 2021 - 11:03 AM.


#24 ravenhawk82

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Posted 12 April 2021 - 11:20 AM

 Compare the guy's sketch of M51 to the images here.  _Any_ of the images here, 81 pages of them, and some are pretty bad.

If you read the article, he's pretty clear that he had limited time for each sketch due to the nature of their visit and that (especially in his M51 and catseye nebula sketches) there was way more detail visible than he was able to put on paper. A 4m+ scope will let you see the same sort of detail but it'll be up to the skilled observer to actually do anything with it ;)



#25 dnrmilspec

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Posted 12 April 2021 - 11:55 AM

The SCT may be a good imaging scope for small targets when used by an experienced imager.  But it's a horrible choice to try to _learn_ imaging with.  About as bad as it gets.  Anyone considering it needs to watch this Youtube, at least the first five minutes of it.  His is a _common_ experience.  Thinking you'll be an exception is asking for pain.

 

I completely disagree with this.  I have seen far to many spectacular first results taken from a city parking lot to agree.  My own experience was that it could not have been easier to learn.  Your mileage may vary.




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