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So, why would you choose a refractor over a SCT?


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A genuine question, because I am obviously missing something. :smiley: ( Visual only! )

A little background: I have three refractors, one a very old 50mm F10; a Skywatcher 102ST and a new Skywatcher 80ED. I like them and had the 80ED out for only the second time last week, which went OK, except for CA! I was not expecting it to be so noticeable. I did consider the scope to be little better than my 102ST, so thought it would be good to see them side by side. I did just that tonight and OK, my memory is obviously faulty, the ED80 is much better than the 102 ST. :rolleyes2:

However, it is just no where near as good as my little C5 SCT, in any way, to my perception. I know the ED80 is not exactly top of the range, but thought it was not all that shabby, by reputation. My intention was to use it for white light solar with my wedge, as well as to get an idea just how good these scope were, as I have been considering the higher rep 120ED, which is just a little bit more expensive.  Now I know it will still be good for the wedge, but I now think I will be avoiding the 120ED, invest the money on improvements for my SCTs, such as a better mount and or focuser. Maybe even consider more investment in the direction of a classic Cassegrain of one sort or another.

The thing is though, that a refractor just looks like a telescope should, if you know what I mean?

I am not even considering how the two sorts compare to reflectors either! ( I do have a 150mm/705 Skywatcher however, which I may give another spin too... )

In a major way, I am not bothered by my mind wandering over which type is best, I am lucky that there are so many options at a reasonable price and it is fun to experiment, to try and find that forever scope. Some people seem to get there, but I know I am far away from that. Maybe I should just cut out all the middle men and try to get a Tak? :grin: ( But should that be a Mewlon? :rolleyes2: )

Edited by Greymouser
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You send me two instruments, an 8 inch TEC apochromat and an exquisitely figured 8 inch Newt, both F8. I test them. I report my findings. At the end of the test I return the Newt. 😁lly

I would love to put my two cents in, the OP's question could not have come at a better time, considering I just got my first refractor. I have had the good fortune of owning many scopes but, they were

What's not to like about refractors? As you can see from my sig, I have two of them. 70mm is ultraportable and can be taken anywhere on a whim, used for white solar and Ha and will travel by airp

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This is a good question and both scopes have their advantages.

I find, all other things being equal, that contrast and sharpness is better in refractors as they have no central obstruction. However the difference in aperture between a C5 and an ED80 is a lot and this will help the C5, but on the other hand a C5 gives you the extra aperture for about  the same cost so in a value sense it is fair to compare them directly. 

I've got onto using a 4" ed refractor recently and the reason I am using it a lot is just that it is a good all round scope that is simple to use and quick to cool down.

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4 minutes ago, Paz said:

I am using it a lot is just that it is a good all round scope that is simple to use and quick to cool down.

Pretty much why I like my C5. :smiley:

It is not just the extra aperture either, or so I feel, the view seems cleaner somehow, if that makes any sense. Though I am realising that my C5 is a very good example of one of its type, as I prefer it to my C9.25 too!  :shocked:

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Another thing to note is that the SCT is a mirror system so doesn’t suffer from chromatic aberration. You probably need a triplet refractor to get minimal CA. Another difference is that SCT can need collimation, whereas refractors rarely need it.

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My Ed80 shows no CA that I can see. What Ep's are you using, is it the ep causing the CA? I brought the Ed120 based on my experience with the 80 and I think it's incredible for the price. I use starguider Ed ep's.

Steve

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It ain't what you see, it's the way that you see it. Assuming more aperture, the SCT will take you deeper. But if it's an exquisite view you're after then you might prefer a classy refractor. To be honest the claims made for refractors are mainly valid for the very good ones. Another member once said that an SCT gives an image with more information in it, yet it's a scruffier image. I reckon that says it well. My concern about small SCTs is that they have limited aperture but a long FL when what I want out of a small scope is a wide FOV.

Olly

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I've owned both a C5 xlt and a number of ED80. The C5 was excellent on it's little goto mount and I was really pleased with the views once I added a 0.63 reducer. All the ED80's I've owned have been superb too, and haven't shown any noticeable amounts of CA to my eyes.

The C5 did surprise me, considering how big the central obstruction is I would have thought contrast would have been less, but the ED refractors have such high contrast and pinpoint stars that the effect is almost 3D! They enable you to see the tiny pin prick stars on the edge of your perception which give the views a real depth! 

If I had to have one it would be the refractor but I wouldn't be sad to own a C5 ot C6 either. Both great scopes :)

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On 27/02/2020 at 22:18, Greymouser said:

A genuine question, because I am obviously missing something. :smiley: ( Visual only! )

A little background: I have three refractors, one a very old 50mm F10; a Skywatcher 102ST and a new Skywatcher 80ED. I like them and had the 80ED out for only the second time last week, which went OK, except for CA! I was not expecting it to be so noticeable. I did consider the scope to be little better than my 102ST, so thought it would be good to see them side by side. I did just that tonight and OK, my memory is obviously faulty, the ED80 is much better than the 102 ST. :rolleyes2:

However, it is just no where near as good as my little C5 SCT, in any way, to my perception. I know the ED80 is not exactly top of the range, but thought it was not all that shabby, by reputation. My intention was to use it for white light solar with my wedge, as well as to get an idea just how good these scope were, as I have been considering the higher rep 120ED, which is just a little bit more expensive.  Now I know it will still be good for the wedge, but I now think I will be avoiding the 120ED, invest the money on improvements for my SCTs, such as a better mount and or focuser. Maybe even consider more investment in the direction of a classic Cassegrain of one sort or another.

The thing is though, that a refractor just looks like a telescope should, if you know what I mean?

I am not even considering how the two sorts compare to reflectors either! ( I do have a 150mm/705 Skywatcher however, which I may give another spin too... )

In a major way, I am not bothered by my mind wandering over which type is best, I am lucky that there are so many options at a reasonable price and it is fun to experiment, to try and find that forever scope. Some people seem to get there, but I know I am far away from that. Maybe I should just cut out all the middle men and try to get a Tak? :grin: ( But should that be a Mewlon? :rolleyes2: )

I think the aperture of your 5" SCT is where its magic lies, and may actually be the most suitable scope for your needs. I'm not surprised you prefer it to your 80ED, but the 80ED will give sharper star images. Moving to a 120ED you'll find a significant jump in performance both compared to the 80ED and your 5" SCT. The ED is a much more capable, all round performer. It's lunar and planetary views will be better defined and its deep sky views will be brighter. It's star images will be sharper and it will give you both low power rich field views and sharp high power views. Even so, your 5" SCT is a highly manageable instrument, and there is a lot of advantage in using a telescope that is easy to use.

The 120ED will give better all round performance, but it is bigger. (Looks great though!)

If you were actually being serious about going down the Takahashi route, I would personally suggest choosing from one of their refractor lines. Their FC100D series are light weight yet extremely capable, and any one of the line would give you better performance than your 5" SCT. If you really needed greater aperture, then the 120TSA is worthy of serious consideration. Larger than that and Tak refractors become heavy and very expensive. They do make some of the world's best telescopes though, so if you're looking for that lifetime keeper, then you've chosen a top tier manufacturer. The Mewlons offer greater aperture at reasonable prices  (Takahashi prices),  but inch for inch a refractor has the edge in sharpness.

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3 hours ago, Lockie said:

If I could only have one scope, period, this would be it :)

Hi Chris,

The 4" ED refractor is a formidable scope to have and very much versatile.

I have often compared my C100ED with others like the 120ED, even though I would love another 120ED I am not sure that I could handle one due to my recent health condition, but saying this just investing in a reducer for the C100ED would give me a very close FOV to the 120ED and cheaper. The 120ED without a reducer sits quite comfortably in between the C100ED at f9 and the 130PDS.

I did look through a XLT127 SCT once and it did blow me away !

I was thinking in investing in a 6" SCT or Mak for lunar or planetary imaging, but I do always seem to refer back to my C100ED, cool time is faster and without optical obstruction and no CA !

Nadeem.

Edited by Skyline
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Thanks for that everyone, lots of food for thought. :smiley:

I use Explore Scientific eyepieces for all my scopes and have never had any CA with my SCT's, ( nor the 6" reflector, ) so I cannot see it being from them causing CA in the ED80. It is only a very small amount on the bright objects, but I expected it to be better corrected, after reading so many glowing reviews. ( Perhaps unrealistic expectations. :sad: ) It is certainly better corrected that the ST102, but that is to be expected, but it seems I will have to spend a lot more to get what I am looking for, if it is even achievable. I suspect that there is a law of diminishing returns with scopes too, as with so many things in life. :smiley:

It also has occurred to me many times, that the different telescopes I have/have tried, all have varying quality, even though otherwise very similar. I suspect this is because none of them are top of the line, in part at least. But it really does seem to me that my C5 gives better views than my C9.25. Not deeper, just flatter and better. There is also the small matter of my own health, a bad back and two frozen shoulders! It has been a while since I have had the C9.25 out, but feel sure it is going to be very manageable, just that the C5 and the AZGTI is just so much easier :grin:.

I did try the 6" Classic Cassegrain, but have found that disappointing too, so am unsure where to go next. The classic does cool very fast, but I just don't know. How much more experimentation can one person do? :undecided:

Added to which I do worry about my deteriorating eyesight, just how much can I justify spending on another maybe scope? Maybe I should follow Gina's excellent example and move towards imaging. Though that opens up a whole new kettle of fish! :grin:

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I think I read somewhere that smaller apertures look through a smaller ‘plug’ of atmosphere, so are less susceptible to seeing effects. This could explain why the C5 looks better than the C9.25 if you have poor to moderate seeing. Conversely, the larger apertures would look better if seeing was excellent.

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5 minutes ago, BlueAstra said:

I think I read somewhere that smaller apertures look through a smaller ‘plug’ of atmosphere, so are less susceptible to seeing effects. This could explain why the C5 looks better than the C9.25 if you have poor to moderate seeing. Conversely, the larger apertures would look better if seeing was excellent.

Yes I often hear this thing about smaller apertures cutting through the seeing but I don't buy it.

If you look through a 4" F8 scope and an 8" F4 scope , you're looking through the same sized patch of atmosphere.

I think the reason the views in the larger scope may look worse is that the larger aperture has the higher resolving power and resolves the bad seeing better, which makes it more noticeable to your eye, if that makes any sense! 

For example, say if you're seeing is at 1 arc second, and you have a small scope with a resolving power of 1.5 arc seconds and a larger scope of resolving power of 0.5 arc seconds. The small scope wont even notice the bad seeing and the image will look perfect, the but the larger scope will and you'll see the wobblyness of the image, but really the amount of detail seen will be higher in the larger scope, it's just the image will look a bit scruffier.

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4 hours ago, CraigT82 said:

Yes I often hear this thing about smaller apertures cutting through the seeing but I don't buy it.

If you look through a 4" F8 scope and an 8" F4 scope , you're looking through the same sized patch of atmosphere.

I think the reason the views in the larger scope may look worse is that the larger aperture has the higher resolving power and resolves the bad seeing better, which makes it more noticeable to your eye, if that makes any sense! 

For example, say if you're seeing is at 1 arc second, and you have a small scope with a resolving power of 1.5 arc seconds and a larger scope of resolving power of 0.5 arc seconds. The small scope wont even notice the bad seeing and the image will look perfect, the but the larger scope will and you'll see the wobblyness of the image, but really the amount of detail seen will be higher in the larger scope, it's just the image will look a bit scruffier.

That sounds like a reasonable hypothesis Craig. But just to throw another spanner in the works based on my own experience with larger telescopes.  At my local astro club there is a very nice 8.5" refractor and many reflectors of various aperture, both large and small. If its due to seeing that a scope is held back, then why does the 8.5" refractor always outperform all the reflectors on the same night, including those of equal aperture,  every time, as regards sharpens, contrast and definition?  Could it be that reflectors need to be figured to a much higher tolerance ( at least 4 times more accurate on the mirror surfaces) than anyone of the four surfaces of the refractor, and thus by nature are more susceptible (sensitive) to bad seeing? 

Edited by mikeDnight
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Yes that's interesting, are you saying that that big frac was not held back by the seeing by the smaller reflectors were?

It is true that a good frac may have a system strehl of 0.98, and in order for a newt to get the same system strehl both the mirrors would need to be figured to about 0.9995 - due to the strehl reducing effect of the central obstruction. Not many mirrors outside of NASA will be figured to that accuracy! 

Perhaps that big frac is actually superbly figured and is just optically better than all the other scopes there, or maybe with the objective at the front it isn't as susceptible to dreaded tube currents and the boundary layer that newts suffer from? And you cant ignore the potential miss-collimation effects with the reflectors either! Also the light has to pass through the tube twice, and so is doubly affected by any currents in there.

Edit:  just thought... If the frac is of a much longer f ratio than the reflectors, the depth of focus will be greater and the wavefront disturbances caused by turbulent seeing would be less visible at the eyepiece?

 

 

Edited by CraigT82
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41 minutes ago, CraigT82 said:

If the frac is of a much longer f ratio than the reflectors, the depth of focus will be greater and the wavefront disturbances caused by turbulent seeing would be less visible at the eyepiece?

 

This is similar to my way of thinking i.e. a large depth of focus gets around fluctuations in seeing conditions, as turbulence effectively changes the focal point slightly.

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4 minutes ago, Lockie said:

This is similar to my way of thinking i.e. a large depth of focus gets around fluctuations in seeing conditions, as turbulence effectively changes the focal point slightly.

Yes... so perhaps when people say "smaller scopes cut through the seeing" they actually mean "longer F ratio scopes cut through the seeing"!

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2 minutes ago, CraigT82 said:

Yes... so perhaps when people say "smaller scopes cut through the seeing" they actually mean "longer F ratio scopes cut through the seeing"!

I think so, but as discussed there is a school of thought that you're collecting photons through a more localised area with a smaller aperture scope, so you're taking a smaller 'cookie cutter' to an area of turbulence with a small scope. (and small scopes cool quicker, so people are less likely to confuse bad seeing with tube currents)

It's probably a bit of all these, but I like to believe the main reason is depth of focus. You should have seen how steady the views were through my old 60mm f16.7!

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The greater depth of focus may also have some truth in it, but here comes another spanner. If depth of focus is the problem, then how can a short focal length apo nearly always deliver an image with better definition? Ive no idea, just thinking out loud. 

Here's another thought. If a primary mirror needs to be at least 4X more accurate on its surface than anyone of the at least four lens elements of a refractor, if a secondary flat isn't also four times better than the combined lens accuracy, has it compromised the system?? :icon_scratch:

It's enough to make my brain hurt!

 

 

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"

The greater depth of focus may also have some truth in it, but here comes another spanner. If depth of focus is the problem, then how can a short focal length apo nearly always deliver an image with better definition? Ive no idea, just thinking out loud. 

Here's another thought. If a primary mirror needs to be at least 4X more accurate on its surface than anyone of the at least four lens elements of a refractor, if a secondary flat isn't also four times better than the combined lens accuracy, has it compromised the system?? :icon_scratch:

It's enough to make my brain hurt!"

 

 

Just when you think you have it figured out :icon_compress:

Edited by Lockie
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1 hour ago, Lockie said:

I think so, but as discussed there is a school of thought that you're collecting photons through a more localised area with a smaller aperture scope, so you're taking a smaller 'cookie cutter' to an area of turbulence with a small scope. (and small scopes cool quicker, so people are less likely to confuse bad seeing with tube currents)

It's probably a bit of all these, but I like to believe the main reason is depth of focus. You should have seen how steady the views were through my old 60mm f16.7!

I've heard this argument using the term 'cells of turbulence.' The fewer turbulent cells your scope looks through, the less it is affected. I have no means of commenting on this since I've no useful experience.

Olly

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