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Why a refractor over a larger SCT at less cost?

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#201 Jon Isaacs

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Posted 30 August 2013 - 06:16 AM

People are talking about all kinds of things for a question that really doesn't have an answer because it depends on so many individual preferences.

If the OP though wants to know the difference between all of these different telescopes, what they can and can't do, and without all of the subjective fluff, they can learn all about how telecopes work from Telecope optics.



A few thoughts/experiences:

This question does have answers, it has many answers, that is why there are 10 pages of answers. There is no one "correct" answer because indeed it does depend on individual preferences.

The important factors that determine the reasons people actually use different telescopes are not primarily optical, you don't learn them from a Rutten and Venrooij, you learn them in the field. Indeed, it is questions like this one that arise from reading about optics and telescopes without having that experience in the field.

It is the subjective "fluff" that is most important when it comes to choosing a telescope. And that is what members of this forum can provide that a book cannot.

Jon

#202 Mark Costello

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Posted 30 August 2013 - 09:26 AM

I'll just pop in and say that to me SCTs (or any Cassegrain type scope) and refractors (particularly like my achro) make good compliments. And not counting set up and tear down, it seems like it'd be hard to beat them for comfort and ease of use, assuming they're on a good tracking mount and have either a real good finder or a goto mount (which most SCT rigs do have). OTOH, assuming the same quality of build, the optical performance should be better for a Newt.... It's a tough call, good thing my refractor has me hopping.... :lol:

#203 ATM57

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Posted 30 August 2013 - 01:46 PM

Aperture does not always rule. I have never seen an 8" SCT that can come close to the views that my 130 AP delivers. And that includes deep sky objects.


Good aperture does! But it must be good. A "good" C-8 should beat a "perfect" 5" refractor all the time. I don't believe you have looked through a good C-8. The ones you have been comparing to were either of average optics, not cooled or out of collimation. BTW, the collimation must be at the level where you can center the airy disk with the first diffraction ring at high power. I have seen very few C-8's collimated at this level.

I have a C9 that easily spilts 78 Peg, 72Peg, Zeta Herc, etc, etc. The planetary views are stunning.

The pic shows a perfect 130mm on the left and a 1/10th wave C-8 including the 35% obstruction on the right. Under good conditions this should be the result. (I have experienced such things)

BTW, I have owned refractors to 8" (three)

Scopejunkie

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#204 Jon Isaacs

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Posted 30 August 2013 - 07:27 PM

I have to think that a 1/10 wave C-8 is a rare bird. This is the level that Roland Christen strives for when he hand figures optics using an interferometer, this is what you get with the 10 inch A-P Mak or one of Peter Ceravolos legandary Mak-Newts.

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#205 ATM57

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Posted 30 August 2013 - 11:33 PM

I have to think that a 1/10 wave C-8 is a rare bird. This is the level that Roland Christen strives for when he hand figures optics using an interferometer, this is what you get with the 10 inch A-P Mak or one of Peter Ceravolos legandary Mak-Newts.

Jon Isaacs


Thank you for your politically correct way of saying no commercial SCT could ever be this good. I know you have your doubts which means to me that you have never used one this good. I currently have a C9 that I have tested with a 250line glass grating and at two bands they are straight on both sides of focus and of high contrast and easily seen. A grating this fine at F/10 is quite sensitive. Straight lines at two bands is better than 1/10th wave wavefront. The fact that they are so easily seen at two bands is an indication of really good surfaces. The majority are not that good but there are some that are exceptional. I know, I have used some. The star test is excellent as well. Whoever did the secondary correction must have been a good mood that day.

With this instrument I have clearly seen 72 Peg, 78Peg, Zeta Herc, Lambda Cyg. Refractor tight and well seen in average seeing. I will admit that I collimate the scope using the airy disk and the first diffraction ring. Most don't go through this much trouble. I do. SCT's need all the help they can get. When you have one this good you kind of chuckle to your self. It's like you shouldn't be getting away with this level of performance this cheaply. I'm looking forward to Jupiter. Saturn was excellent before it sank into the "soup".

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#206 T1R2

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Posted 31 August 2013 - 01:26 AM

I 2nd that, my C11 offers me almost photographic views, Saturn was awesome this year, all the bands were seeable, plus the ring system was the best I ever seen in a SCT, besides a 14". I can't wait for Jupiter and Mars to rise higher.

#207 Jon Isaacs

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Posted 31 August 2013 - 05:25 AM

A few thoughts...

Double stars are great fun but are not a particularly good measure of optical quality. Rather they are a measure of seeing. I work my 10 inch GSO Dob at the Dawes limit when the seeing permits which is not so uncommon here. Gamma CB. Zeta Bootes come to mind. But don't think the optics are anything special, its just paying attention to collimation and serious attention to thermal issues.

As far as a 1/10 wave SCT's.. I said such a scope would be a rare bird. I think even you would agree.

This thread is about why one would choose a smaller, more expensive refractor over a larger SCT. There are many reasons... Many valid reasons. My NP-101 cost someone about $4000 when all was said and done. That would buy a much larger SCT that would go deeper and under the right circumstances provide more detailed planetary views.

But... It couldn't do those things that an NP-101 does best and I have other choices if I want the good planetary views or want to hunt down the faint fuzzies.

Jon

#208 t.r.

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Posted 31 August 2013 - 06:05 AM

The pic shows a perfect 130mm on the left and a 1/10th wave C-8 including the 35% obstruction on the right. Under good conditions this should be the result. (I have experienced such things)

But in practice, in the field, I find just the opposite occurs, The image on the right is a perfect illustration of the contrast delivered by an apo refractor. The image on the left is more representative of the way an sct displays an image on average. This may be why people would pay the extra money for the refractor.

#209 hfjacinto

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Posted 31 August 2013 - 08:51 AM

Because one pays alot for a telescope it doesn't make it better than a larger scope it just makes it more expensive.

#210 Jon Isaacs

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Posted 31 August 2013 - 09:59 AM

Because one pays alot for a telescope it doesn't make it better than a larger scope it just makes it more expensive.


I think it is a little more complicated than that. There is some correlation between purchase cost and manufacturing cost. One would have some expectation that a 4 inch refractor that cost $3000 would have better optics than a 4 inch refractor that cost $500.

When it comes to comparing scopes of different designs and apertures, then "better" becomes quite difficult to define because it is dependent on the use, the application.

If someone asks, "which is the better scope, a 60 mm spotting scope or a 20 inch Dob? ", the first order of business is to find out the intended use... For the purpose of watching birds, the 60 mm spotter is the clear winner. For the purpose of providing birds a place to build nests, the 20 inch Dob would seem to be the right choice. :)

Jon

#211 hfjacinto

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Posted 31 August 2013 - 11:12 AM

Jon,

You know very well what I mean, a $5000 -130MM AstroPhysics is just that. Its a $5000 telescope. A $1500 11" SCT will show you more DSO's than the $5000 telescope.

#212 RTLR 12

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Posted 31 August 2013 - 11:17 AM

...the Eye of the beholder.

Stan

#213 Dakota1

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Posted 31 August 2013 - 11:56 AM

Who cares how much a scope costs. If the scope you buy makes you happy and does what you want it to so be it. If a person wants to spend $5.00 or $25,000 dollars that's up to that person to get what ever style scope he wants. More power to that person and I see no reason for anyone to worry about what someone spends on a scope or what make brand style he gets that up to him or her what they buy. If you are happy with what you have do not worry what other people have or how much they paid. Its there money to spend as you did for yours.

Thanks Bill

#214 Jon Isaacs

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Posted 31 August 2013 - 12:52 PM

Jon,

You know very well what I mean, a $5000 -130MM AstroPhysics is just that. Its a $5000 telescope. A $1500 11" SCT will show you more DSO's than the $5000 telescope.


I now know what I think you meant but I am not sure you know what I meant. A 12 inch Dob costs $700 and will show more DSOs than an 11 inch SCT, not only because it has a larger aperture but also because it can provide a wider field of view. But does that make it the better scope??

Not necessarily.

There are many ways to judge a scope...

I don't make the claim that my 25 inch Dob is a better scope than my 60 mm spotting scope. They are just different. For some tasks one is a better choice, for others, the other is the better choice.

In my world, in my situation, the NP-101 is a better choice than an 11 inch SCT...

Jon

#215 ATM57

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Posted 31 August 2013 - 12:56 PM

The pic shows a perfect 130mm on the left and a 1/10th wave C-8 including the 35% obstruction on the right. Under good conditions this should be the result. (I have experienced such things)

But in practice, in the field, I find just the opposite occurs, The image on the right is a perfect illustration of the contrast delivered by an apo refractor. The image on the left is more representative of the way an sct displays an image on average. This may be why people would pay the extra money for the refractor.


I agree, but there are reasons why. I stated some. Since this is not the Cats & Casses section. I won't go into these.

The images were generated by software. This would be equal to laboratory conditions. As we all know, the night sky is not so forgiving. Still, resolution is a function of aperture. The pictures show this.

I enjoy the challenge making an SCT perform well. I have had some stunners over the decades.

Scopejunkie

#216 PeterR280

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Posted 31 August 2013 - 01:03 PM

I sold my TV102 and i am using the C102GT as the 4" scope. I am also waiting or my AP130GT that's supposed to come any day now. I hope the 100 times price factor isn't disappointing in the image results as compared to the $59 C102. My C11 isn't getting as much use. There is no question that portability and ease of use is the main factor. As far as a $6,000+ 5 inch refractor, I suppose it's like comparing a Cartier pen with a BIC pen.

#217 Koala117

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Posted 31 August 2013 - 01:30 PM

Weight can also be an issue of why to choose refractor over an SCT. Some have very bad backs, shoulders, elbows, hearts(like myself) and can not go around lifting even a 40 lb. setup.

This is one reason that I like refractors the most! There is a Celestron ST80 on its way to me, right now, and I can't wait. It will compliment the 80mm f/11 that I currently also have very much use for.

Another reason I think refractors are nice is because they need very little cool-down time at 80mm. Granted, I've never heard of an 80mm SCT but I imagine it would have at least a *little* more cool-down time(?) for optimal viewing. This would hinder 'grab n go' for those needing it, I suppose! As one that lives near the Great Lakes, that is also very important since, though it is currently ~85 farhenheit today, in 4 months it will be 20 or less(:().

SCT also needs much more upkeep for collimation than refractors. Heck, dobs frighten me as it is to collimate! While I am certain that SCT *can* provide lovely views, I would be nervous about messing it up and so are others, from what I've read here. =)

SCTs(and newtonians) have the secondary mirror setup which apparently causes bright stars to not look like little pin pricks, if I understand correctly, because of the way secondaries are mounted. <-- If that part is wrong, please do correct me. I prefer having as much proper info as I can. :D

All this is not to say that SCTs are inferior to refractors. Each type of scope, mount, etc. has its purpose and its place, of course! I just wanted to share why *I* would prefer a refractor above all others, if given a choice in similar quality and price.

Clear skies and a great weekend to you all! :D

Mike

edit: phrasing :)
edit2: I just want to add that I may have seemed biased against anything but refractors. I'm not. Telescopes are like kittens, in my opinion. They're all awesome!

#218 ATM57

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Posted 31 August 2013 - 03:23 PM

Nice scope choice. Sometimes my brain thinks 30 years old and my back reminds me that I'm not that young. Using my 10" F/8 reflector, stand up observing hurts after fifteen minutes at the eyepiece. Loading up a C-11 night after night hurt also. That's the real reason I don't have it now. I was able to run 400x consistently with it on Saturn. Last year I could see the Cassini division where it crosses the globe of Saturn. A feat that was very difficult last year no matter what instrument you were using.

My stars are pin points in my C-9 just like they were in my 6" F/8 refractor. The secondary obstruction has no effect on that unless it's warm in which it will produce a spike on bright stars.

Refractors have cool down issues too. One of the reasons they aren't as prominent is because of the location of the lens. My very good 6" F/8 refractor needed 30 minutes to be optimal (star test) but was useable as soon as I set it up.

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#219 loo27

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Posted 10 September 2013 - 01:50 PM

Good post, here's my perspective. I owned a very good refractor and thought it was magic. Then I realized that newtonians and SCTs were just as good, but I had to learn how to use the instrument. After years of practice, I became very good at cooling techniques and collimination. My favorite scope was a C14 in which I observed Jupiter once and it looked just like this picture. The scope had cooled overnight and I was looking at Jupiter right before dawn. I was doing 650x without breaking a sweat.

The lesson for me is most scopes of acceptable quality (1/4 wave or better) and design will show your similar images, given aperture, if you learn to master the use of them. However, there are caveats:

1. Portability matters. It matters because sky conditions dictate results and if you don't use your scope a lot you won't see things at their best. Better to have a smaller scope you use a lot rather than a huge scope you don't have the energy to set up.

2. Understand the scope limitations. The C14 gave me a few of a lifetime, but only once. It's not the C14's fault, but it requires better skies, more cooling and great collimination to max out it's potential. This is generally truer the bigger and faster the scope is.

3. Big scopes show your more, but less frequently. Sky and weather conditions must be better and collimination has to be spot on.

So, rather than just design considerations, consider your local weather conditions, how often you can go to a dark site, how much scope you can physically handle, how frequently you can observe and your skill level. Refractors are nice because you can be less skilled in collimination, you don't have to wait as long for cool down and most are 6" and under and are portable. That is the real value of refractors.

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#220 Spacetravelerx

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Posted 10 September 2013 - 05:40 PM

Good post, here's my perspective. I owned a very good refractor and thought it was magic. Then I realized that newtonians and SCTs were just as good, but I had to learn how to use the instrument. After years of practice, I became very good at cooling techniques and collimination. My favorite scope was a C14 in which I observed Jupiter once and it looked just like this picture. The scope had cooled overnight and I was looking at Jupiter right before dawn. I was doing 650x without breaking a sweat.

The lesson for me is most scopes of acceptable quality (1/4 wave or better) and design will show your similar images, given aperture, if you learn to master the use of them. However, there are caveats:

1. Portability matters. It matters because sky conditions dictate results and if you don't use your scope a lot you won't see things at their best. Better to have a smaller scope you use a lot rather than a huge scope you don't have the energy to set up.

2. Understand the scope limitations. The C14 gave me a few of a lifetime, but only once. It's not the C14's fault, but it requires better skies, more cooling and great collimination to max out it's potential. This is generally truer the bigger and faster the scope is.

3. Big scopes show your more, but less frequently. Sky and weather conditions must be better and collimination has to be spot on.

So, rather than just design considerations, consider your local weather conditions, how often you can go to a dark site, how much scope you can physically handle, how frequently you can observe and your skill level. Refractors are nice because you can be less skilled in collimination, you don't have to wait as long for cool down and most are 6" and under and are portable. That is the real value of refractors.



loo27,

You make a couple of very important points - location and portability.

My 14" LX850 (and heck the 10" LX200) provides fantastic views outside of the monsoon season in New Mexico. Most of the year I have wonderful views, but humidity also runs most of the time below 20% and I am over 5000'. When first moving to the Southwest it was like I got a brand new telescope. In general, I have not had collimation issues. In the fall, winter and through early spring I get many days of spectacular views. I never had this east of the Mississippi - location, location, location. So #2 and #3 really depends on where you live. In New Mexico, at home, the 14" will win almost all the time. I will literally leave it setup for 5-7 day stretches. Once I get the observatory built - 365 days.

However, I will not travel the U.S. with the 14" LX850 (or rarely travel). This is where the refractor works very well for me. So far I have had wonderful views in the 80mm APO, and I am betting the same with my 130mm APO (based on other reports). The 80mm (and I imagine the 130mm) will not be as amazing as the 14" on a clear night in New Mexico, but they will get around the United States and camp sites (car camping). These are simply far more portable. So your point #1 is very true, however in my case I am blessed with a wonderful location but I have the advantage of being able to travel with a refractor.

And of course for the most part you can quickly setup a refractor and have a look-see.

#221 Jon Isaacs

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Posted 10 September 2013 - 07:10 PM

The 80mm (and I imagine the 130mm) will not be as amazing as the 14" on a clear night in New Mexico, but they will get around the United States and camp sites (car camping). These are simply far more portable.



This is one reason truss tube Dobsonians are popular. A 12.5 inch can travel to those dark sites while using up but a small space, mine has a 19"x20" inch foot print and is 29 inches tall, the seat space of one passenger. When all is said and done, it uses up more room than a 80mm refractor but less than a 5 inch refractor in a case.

Jon

#222 BillP

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Posted 10 September 2013 - 07:30 PM

A few thoughts/experiences:

This question does have answers, it has many answers, that is why there are 10 pages of answers. There is no one "correct" answer because indeed it does depend on individual preferences.

The important factors that determine the reasons people actually use different telescopes are not primarily optical, you don't learn them from a Rutten and Venrooij, you learn them in the field. Indeed, it is questions like this one that arise from reading about optics and telescopes without having that experience in the field.

It is the subjective "fluff" that is most important when it comes to choosing a telescope. And that is what members of this forum can provide that a book cannot.

Jon


I think this hits it on the head. It's really all about likes and what dislikes you can tolerate in an instrument and still use it often.

That said, for me my primary preferences are 1) the scope is ready when I am so I hate long acclimation times, 2) small enough that no hassle to take out in one trip with mount, 3) gives me both wide fields and high magnifications without much problem (1 deg FOVs are just too small for me).

So given the above, a 4" or 5" refractor is perfect (I have a 4"). It is a zero problem instrument and like a good dog is eager to go out and have fun.

However, 4" is not enough for me to really enjoy globs...so I have an 8" SCT for that which I mount on a simple Alt-Az so it is light and no hassle. Had it on a driven mount and hated it because just akward. On the alt-az it is a dream to use. But I do get frustrated with the small TFOV since I have no reducer for it and don't like futzing with 2" EPs too much.

Then I have the 10" Dob. Love the pull it has but hate the acclimation time...also not so convenient to take out as an unpleasant carry since not on wheels. But beautiful images when the mirror is freshly cleaned and it is full cooled. Since I got the 8" SCT though I use the Dob even less.

Frankly the 8 SCT and 4" Refractor I find a perfect pair. Oh yes...the 80mm APO is a must for travel, which is usually the only time I use it. But I often wonder if an 8 SCT and TMB 92mm APO might really be the perfect combo :-D

If I had to do it all over having the experience I have now, would probably be a 8" Edge SCT and TMB 92 APO with a Minitower Pro which can drive them both at once if I wanted and a spare alt-az for when I have them separated.

At the moment though, I have the 4" APO and 80mm APO both mounted on the Minitower Pro as it is a joy to have a quality really wide field view and higher magnification view available simultaneously. The the 8" is on an SCT for when I need more light pull. For the past year or so this combo I have been finding quite satisfying and both on their mounts are a quick carry outside...and since the SCT is rarely used for planetary, it is ready quite quickly for satisfying DSO viewing.

#223 Jon Isaacs

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Posted 11 September 2013 - 05:16 AM

I think this hits it on the head. It's really all about likes and what dislikes you can tolerate in an instrument and still use it often.



That's about the long and short of it. One's particular situation is very important.. in that sense, I pretty lucky. I can have a big scope out and ready to go in about 5 minutes.

Like Bill, I like smaller refractors, 80mm-100mm is a good place, very easy to setup, easy to use.

But unlike Bill, I frequently observe with larger scopes. Dark skies are within an hour's drive and the scopes are there waiting, ready to go, they just need to be uncovered and wheeled outside. The seeing is generally not as steady as it is down along the coast, so I don't worry too much about thermal issues, equilibrium...

In terms of the effort to observe that Bill mentions, for me, that teeter-totters between the 16 inch and the 25 inch. The 16 inch is easy, that's because no ladder is involved, the 25 inch takes a lot of energy just dealing with it.. to move around the sky, it's something of an operation.

Jon

#224 Chris Greene

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Posted 19 September 2013 - 11:49 PM

Great thread! I've been a casual visual observer on and off for at least four decades now.

After a period of not owning any telescopes for a few years (I'd sold my Vixen made Celestron 80mm back around 1990) I decided to get a small refractor again. I'd moved to rural Idaho and had really dark skies but only so-so seeing (Magic Valley). I became intrigued with the TeleVue Pronto which was a current model from TV and found a used one with a TelePod on AM for $800-900. Over time, I added some TV and Pentax ep's.

Joining the local astro club, I went to a few local star parties and drooled over the big stuff people had. Eventually, I bought a C8 and was pleased with how well it showed globs but found it a PITA to use (I mounted it on a Vixen GP). Then, I came across a nice 4" f/5 Genesis and bought it too. I decided I preferred the design of TV's mounts more than EQ mounts so picked up a Gibraltar. When a nice NP101 came along, I bought it and sold the Genesis.

I figured I was in telescope heaven - a C8 for the fuzzy stuff, the NP for the stars, and the Pronto for travel. Decided I wanted a Questar, just because and got one of those too.

Then, one day at a star party at Craters of the Moon, I set up next to a guy who had a beautiful 18" DOB (Starmaster?) and I had my little Pronto. We both trained our scopes on M81 and M82. Now, without a doubt, his views were better and significantly brighter. But you know what? I could see them just fine in my 70mm Pronto. We both were pretty amazed how good they were in our side by side comparison. Next time I saw him, he'd bought a TV-85.

Back home, I found myself rarely using the C8 and the NP101. The Questar and I never really bonded even though it was a beautiful piece of equipment. I ended up selling all three of them and have kept only the Pronto. I can take it out of my studio, set it on the adjoining patio and be observing in a couple minutes. As I'm now pushing 63 and have seen all the wonders repeatedly, my mind's eye takes care of the rest when I look at the Messier objects and it's terrific for casual sweeps of the Milky Way.

Sure, the NP101 showed fainter stars; you'd see that with the Double Cluster but it's a much bigger scope and the Pronto was so much quicker to set up on the TelePod.

At this point, I'd be thrilled if I could find a nice used TV-76 or, more likely, the TMB 90 that will be back around by the end of the year.

Point is, for me, while the SCT I owned showed nice resolution in globs, the nuisance factor wasn't worth it. The NP101 was a tougher decision to sell but the scope I used the most over these past 10+ years has been the small refractor for its quick and easy set-up. That's why a small refractor over a larger one or even larger SCT for me.

I'll finish by saying that if I had an observatory where I could have a scope or two permanently mounted, a 14" SCT would likely be my scope of choice along with a 5-6" APO. ;)

#225 Jon Isaacs

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Posted 20 September 2013 - 03:58 AM

Point is, for me, while the SCT I owned showed nice resolution in globs, the nuisance factor wasn't worth it. The NP101 was a tougher decision to sell but the scope I used the most over these past 10+ years has been the small refractor for its quick and easy set-up. That's why a small refractor over a larger one or even larger SCT for me.


It really does depend on how much effort one is willing to put out. Some nights, many nights, a small refractor is plenty good enough for me. But there are many nights when I want the deeper reach, the more detail, the greater resolution of a larger scope.

So many nights, when I have the time and energy, I am willing to make the effort to setup a larger scope.. and a smaller one too... a larger Newtonian/Dob and a smaller, fast refractor, it's a tough combo to beat.

Jon

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