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How much increase in aperture to see a difference?

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#76 noisejammer

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Posted 07 June 2017 - 08:30 AM

Is permanent collimation on a dob possible? Must be a good reason why I haven't seen it.

Of course it is .. but (like everything else in life) it's a trade off.

 

It's quite easy to design mechanical structures that come apart and then re-assemble with tolerances of a few microns. Making them can be quite easy too. The point is that most Dob owners would rather trade larger aperture for no collimation. I'd greatly prefer a compromise - make it possible to tweak the collimation from the eyepiece. This is quite easy if done electrically using simple motors and a toothed belt drive.



#77 Jon Isaacs

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Posted 07 June 2017 - 11:10 AM

 

Is permanent collimation on a dob possible? Must be a good reason why I haven't seen it.

Of course it is .. but (like everything else in life) it's a trade off.

 

It's quite easy to design mechanical structures that come apart and then re-assemble with tolerances of a few microns. Making them can be quite easy too. The point is that most Dob owners would rather trade larger aperture for no collimation. I'd greatly prefer a compromise - make it possible to tweak the collimation from the eyepiece. This is quite easy if done electrically using simple motors and a toothed belt drive.

 

 

In my view, it's a question of simplicity and weight.  A Dob that did not require collimation would both heavier and more complicated.  Given that collimation only requires a moment or two,  personally I prefer the simplicity and lighter scope. 

 

What is often forgotten is that of all the telescope designs,  because it has only one curved surface,  the Newtonian has the least critical alignment/collimation requirements..  This is the reason Newtonians can be user collimated.  

 

A 6 inch F/8 apo triplet needs to have the elements centered within a few microns.  That's a factory job and requires a carefully constructed tube to maintain alignment.. 

 

An 6 inch F/8 Newtonian,  the collimation tolerances are measured in millimeters.. Something like 4 mm for the secondary tilt and 2mm for the primary..  Imagine a refractor with the elements 4 mm off center.. 

 

Jon


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#78 JimP

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Posted 07 June 2017 - 09:14 PM

I'd rather not get into the reflector vs refractor debate but I have owned a number of each and ALL, every one, of my AP, TEC, Takahashi, TMB (LZOS) have been perfectly collimated from the day I got it and none, not one, ever needed collimating. On the other hand ALL, every one, of the Starmasters with Zambuto(18") or Mike Lockwood (20" Quartz)mirrors, or the Obsession (20") Pegasus mirror, required constant collimating and re-collimating until I got sick of it. Now, that may be just me. I have owned telescopes since 1965 when my Mom gave me my first one, over 50 years ago and I am happy to give up portability and aperture to own the refractors and maksutov that I now have.
And I know many others are just as happy with their Dobs and Schmidt-Cassegrains. To each his own and after 50+ years I know what works best for me.


Edited by JimP, 08 June 2017 - 07:32 AM.

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#79 Jon Isaacs

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Posted 08 June 2017 - 12:35 AM

I'd rather not get into the reflector vs refractor debate but I have owned a number of each and ALL, every one, of my AP, TEC, Takahashi, TMB (LZOS) have been perfectly collimated from the day I got it and none, not one, ever needed collimating. One the other hand ALL, every one, of the Starmasters with Zambuto(18") or Mike x (20")mirrors, or the Obsession (20") Pegasus mirror required constant collimating and re-collimating until I got sick of it. Now, that may be just me. I have owned telescopes since 1965 when my Mom gave me my first one, over 50 years ago and I am happy to give up portability and aperture to own the refractors and maksutov that I now have.
And I know many others are just as happy with their Dons and Schmidt-Cassegrains. To each his own and after 50+ years I know what works best for me.

Exactly.. 

 

The refractors had to be built so that they did not shift collimation because if they had,  it would mean a trip back to the factory.. 

 

If you want a portable 20 inch scope,  a scope that one person can setup in 10 minutes,  collimation will be required..  That's reality.  

 

Jon


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#80 infamousnation

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Posted 08 June 2017 - 02:14 AM

The bottom line is the difference in seeing from a good night and a bad night will have a greater effect on planetary detail observed than the difference in aperture will provide, which will be noticeable only if they are set up side by side on a very good night.  You only gain additional planetary detail when seeing is above the threshold for that aperture, which at 5-6 inches, is pretty high in my experience.  Those who live in a place with excellent seeing on a regular basis will get more benefit out of a larger aperture than those who live in places with more average seeing.


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#81 Codbear

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Posted 08 June 2017 - 02:18 AM

I'd rather not get into the reflector vs refractor debate but I have owned a number of each and ALL, every one, of my AP, TEC, Takahashi, TMB (LZOS) have been perfectly collimated from the day I got it and none, not one, ever needed collimating. One the other hand ALL, every one, of the Starmasters with Zambuto(18") or Mike x (20")mirrors, or the Obsession (20") Pegasus mirror required constant collimating and re-collimating until I got sick of it. Now, that may be just me. I have owned telescopes since 1965 when my Mom gave me my first one, over 50 years ago and I am happy to give up portability and aperture to own the refractors and maksutov that I now have.
And I know many others are just as happy with their Dons and Schmidt-Cassegrains. To each his own and after 50+ years I know what works best for me.

JimP you said it perfectly.

 

As far as I'm concerned, no man should ever have to explain or defend his preferences for telescopes, women or wine! Not necessarily in that order by the way tongue2.gif


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#82 Jon Isaacs

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Posted 08 June 2017 - 03:07 AM

The bottom line is the difference in seeing from a good night and a bad night will have a greater effect on planetary detail observed than the difference in aperture will provide, which will be noticeable only if they are set up side by side on a very good night.  You only gain additional planetary detail when seeing is above the threshold for that aperture, which at 5-6 inches, is pretty high in my experience.  Those who live in a place with excellent seeing on a regular basis will get more benefit out of a larger aperture than those who live in places with more average seeing.

 

Living in a place where the seeing is quite good and probably noticeably better on average than it is in Illinois, I find the differences in aperture to be quite significant when it comes to viewing the planets.  And if one wants to see color, a larger aperture provides a brighter image which does allow the eye to see color better.. 

 

And too, somehow, the discussions in the refractor forum, they often focus on the planets and there's a lot more out there to look at and needs to be considered in the equation, doubles, all sorts of DSOs.. 

 

As far as I'm concerned, no man should ever have to explain or defend his preferences for telescopes, women or wine!

 

 

Very true.  No one should have to defend their decisions but at the same time, no one should be prevented from discussing their preferences and experiences, that is the premise of these forums.  

 

Jon


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#83 JimP

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Posted 08 June 2017 - 05:25 AM

Which is exactly what we have both done. So, let's move on.
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#84 JimP

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Posted 08 June 2017 - 07:34 AM

JimP you said it perfectly.

 

 

 

As far as I'm concerned, no man should ever have to explain or defend his preferences for telescopes, women or wine! Not necessarily in that order by the way tongue2.gif

 

 

LOL!! Definitely not in that order!


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#85 Jeff B

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Posted 08 June 2017 - 09:43 AM

"If you want a portable 20 inch scope,  a scope that one person can setup in 10 minutes,  collimation will be required..  That's reality."

 

"Lord, grant me the serenity to accept those things I cannot change".



#86 JimP

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Posted 08 June 2017 - 09:59 AM

"The courage to change the things I can

And the Wisdom to know the difference"


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#87 Scott99

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Posted 08 June 2017 - 12:01 PM

AP's Maksutov is permanently collimated, and precious few other instruments.  I've always wondered why more companies don't go AP's route and offer permanently aligned scopes.  I think it's possible for mirror scopes, but maybe not practical for production scopes.  
 

I remember reading about some custom 16" Newt than APM installed in Europe for someone's "dream" observatory.  It had an optical window so the tube was sealed like a refractor, small obstruction, etc, I think you can build refractor-like, custom machined aluminum reflectors if you want.  It's going to be large and heavy, require a massive mount, and be very expensive.

 

totally agree on collimation, the last thing I want to do with an expensive new telescope is struggle to align it and then hope I've got it right.  Who doesn't like viewing through huge dobs though, I definitely enjoy it.  Just not my choice of scope to own.


Edited by Scott99, 08 June 2017 - 12:02 PM.


#88 Keith Rivich

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Posted 08 June 2017 - 04:25 PM

Nice discussion...I see echo's in the reflector forum.

 

If any of you are in, or will be in, the Houston area, and have a little spare time, the observatory I volunteer at has a 6" and an 11" refractor (both f/15 doublets) mounted to our 36" scope. If the gods (weather, moon, wife) allow we can do a little comparison observing with all three. The skies aren't the greatest but oh, well...it is what it is!

 

 

 

 

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#89 Jon Isaacs

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Posted 08 June 2017 - 04:36 PM

I remember reading about some custom 16" Newt than APM installed in Europe for someone's "dream" observatory.  It had an optical window so the tube was sealed like a refractor, small obstruction, etc, I think you can build refractor-like, custom machined aluminum reflectors if you want.  It's going to be large and heavy, require a massive mount, and be very expensive.

 

 

I suspect you are thinking of this scope:

 

Aries 16" Modified Newtonian

 

That scope was Markus Ludes's own personal scope. 

 

I imagine that if it sat in the back of a pickup truck and rode 30 miles down a washboarded dirt road,  you'd check the collimation..  

 

Newts take a beating.. 

 

Jon


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#90 gnowellsct

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Posted 08 June 2017 - 09:52 PM

I have an AP 130 F/6 which I use for visual observations primarily of double stars although anything is game. I have read so many impressive things about CFF I have thought about getting one. What is your experience in terms of the need for increased aperture moving from a 130 refractor to see a difference? Is a move to 140 enough or would I really need to get a 160 to make a visible difference?

Here's a couple of answers.

 

1.  Refractor to refractor, the jump from 102mm to 130mm, which is just a tad over an inch, is *very noticeable.*  I think refractor owners are very much in agreement on this.

 

2.  The other jump I've made, worth discussing, is from 8" to 9.25", when I sold the c8 for a 9.25.  (Then I sold the 9.25 for a c14, and now I have a c8 again, plus the c14, but not the 9.25).  The differences between a good 8" SCT such as are currently being made and a 9.25 are much more subtle than the differences between a 102mm refractor and a 130mm refractor.   There definitely is a difference, but you have to pay attention.  It doesn't hit you over the head (the jump from 9.25 to c14 hits you over the head).   (I should clarify that I didn't sell the c8 till after I had a 9.25 and I actually side-by-sided them).

 

So, to conclude:  

 

A one inch jump over the smaller 4 inch aperture is very noticeable.  

 

A 1.25 inch (32mm) jump from a c8 to a c9.25 is noticeable but you need to pay attention.  And when you have them side by side you can be struck by the fact that the c8 keeps up comparatively well.  Indeed, I once tested the c8 vs. a c11, this was about fifteen years ago, on a series of NGC galaxies and out of 24 objects or so, the c8 brought in about 21.  I wasn't making a particular effort to rank them by surface brightness or magntiude, just cruising next to a friend with a c11 and looking at what he looked at.  

 

I don't think 10mm is enough over the 130mm to "make a difference."  I think it will be even less noticeable than c8 vs. c9.25.  So to the extent that you want specific targeted advice based on comparing aperture jumps, I think you're going to have to go for the 160mm for reasons stated.  That 30mm is 1.2 inches and is going to deliver, proportionately to the five inch, more than the 1.25 inches does in the 9.25 vs c8,   The 10mm is less than half an inch increment on a 130mm aperture which has already crossed a noticeable threshold from smaller scopes.  

 

That 10mm helps the TEC compete against the TOA 130 and the AP GT and GTX 130.  There's an edge particularly relative to money spent.  But the 160 is going to be the scope that that gives the "oh yeah this is much better" reaction.

 

My 2c.

 

Greg N


Edited by gnowellsct, 08 June 2017 - 09:53 PM.

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#91 Jon Isaacs

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Posted 08 June 2017 - 10:02 PM

I don't think 10mm is enough over the 130mm to "make a difference."  I think it will be even less noticeable than c8 vs. c9.25.  So to the extent that you want specific targeted advice based on comparing aperture jumps, I think you're going to have to go for the 160mm for reasons stated.  That 30mm is 1.2 inches and is going to deliver, proportionately to the five inch, more than the 1.25 inches does in the 9.25 vs c8,   The 10mm is less than half an inch increment on a 130mm aperture which has already crossed a noticeable threshold from smaller scopes

 

 

I think in terms of ratios..  A 102 to a 130 is a 28% increase in resolution, a 62% increase in light gathering.   A 8 inch to a 9.25 is a 16% in resolution, a 34% increase in light gathering, much less. 

 

A 130mm to a 140mm, that's 8% resolution, 16% light gathering.. Inconsequential.  It's the difference between 100x and 108x.  

 

Jon


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#92 gnowellsct

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Posted 08 June 2017 - 10:22 PM

 

I don't think 10mm is enough over the 130mm to "make a difference."  I think it will be even less noticeable than c8 vs. c9.25.  So to the extent that you want specific targeted advice based on comparing aperture jumps, I think you're going to have to go for the 160mm for reasons stated.  That 30mm is 1.2 inches and is going to deliver, proportionately to the five inch, more than the 1.25 inches does in the 9.25 vs c8,   The 10mm is less than half an inch increment on a 130mm aperture which has already crossed a noticeable threshold from smaller scopes

 

 

I think in terms of ratios..  A 102 to a 130 is a 28% increase in resolution, a 62% increase in light gathering.   A 8 inch to a 9.25 is a 16% in resolution, a 34% increase in light gathering, much less. 

 

A 130mm to a 140mm, that's 8% resolution, 16% light gathering.. Inconsequential.  It's the difference between 100x and 108x.  

 

Jon

 

That about sums it up.  

 

But insofar as he wanted "experience," I realized that I had done extensive comparisons of 4 to 5  in refractors and 8 to 9.25, thus bracketing the baseline and two potential jumps that he laid out.    I don't think I'll ever be springing for a 6 or 8 inch apo.  Maybe if I was considerably richer.  I'd keep the 8" apo on a 16" RC and fly to NM every new moon to use them.   But I don't see anything like that happening, and I keep forgetting to buy lottery tickets.   GN



#93 t.r.

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Posted 09 June 2017 - 05:20 AM

Step back a bit and look at this from a different angle...if you didn't already have a 130mm, what aperture would you get? Probably the answer is the most you can comfortably afford, wield and use...kind of makes the 140 or 160 the best choice taking the prevailing seeing conditions into account. Even though Fried puts prevailing best seeing resolution at most locations somewhere between 100-150mm (hence the 130 class niche), the comment in Telescope-Optics.net about the eye not really getting the photons it needs to work best at resolving low contrast planetary detail with less than 150mm aperture should steer most to the 6 inch class and the reason it appears to be the most recommended as a benchmark for planetary observing. I would like to settle in with a 6+ inch apo someday, not because my seeing will support its full resolution often but rather it will support my aging eyes better to see with! But alas, the 5 inch "niche" is comfortable to afford, wield and use!! My penny.gif penny.gif .

 

http://www.telescope...nd_aperture.htm

 

Last paragraph, bottom of the page.


Edited by t.r., 09 June 2017 - 05:47 AM.

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#94 bandazar

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Posted 09 June 2017 - 06:02 AM

I'd like to use a simpler formula of roughly doubling the aperture to make a difference worthwhile to me, imo.  Yes, there are almost always going to be differences in resolution with small jumps in aperture if the design is the same/similar with similar eyepieces.  Whether or not it is worthwhile is something that is subjective and dependent upon user's taste.


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#95 Jon_Doh

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Posted 09 June 2017 - 08:38 AM

 

I have an AP 130 F/6 which I use for visual observations primarily of double stars although anything is game. I have read so many impressive things about CFF I have thought about getting one. What is your experience in terms of the need for increased aperture moving from a 130 refractor to see a difference? Is a move to 140 enough or would I really need to get a 160 to make a visible difference?

Here's a couple of answers.

 

1.  Refractor to refractor, the jump from 102mm to 130mm, which is just a tad over an inch, is *very noticeable.*  I think refractor owners are very much in agreement on this.

 

2.  The other jump I've made, worth discussing, is from 8" to 9.25", when I sold the c8 for a 9.25.  (Then I sold the 9.25 for a c14, and now I have a c8 again, plus the c14, but not the 9.25).  The differences between a good 8" SCT such as are currently being made and a 9.25 are much more subtle than the differences between a 102mm refractor and a 130mm refractor.   There definitely is a difference, but you have to pay attention.  It doesn't hit you over the head (the jump from 9.25 to c14 hits you over the head).   (I should clarify that I didn't sell the c8 till after I had a 9.25 and I actually side-by-sided them).

 

So, to conclude:  

 

A one inch jump over the smaller 4 inch aperture is very noticeable.  

 

A 1.25 inch (32mm) jump from a c8 to a c9.25 is noticeable but you need to pay attention.  And when you have them side by side you can be struck by the fact that the c8 keeps up comparatively well.  Indeed, I once tested the c8 vs. a c11, this was about fifteen years ago, on a series of NGC galaxies and out of 24 objects or so, the c8 brought in about 21.  I wasn't making a particular effort to rank them by surface brightness or magntiude, just cruising next to a friend with a c11 and looking at what he looked at.  

 

I don't think 10mm is enough over the 130mm to "make a difference."  I think it will be even less noticeable than c8 vs. c9.25.  So to the extent that you want specific targeted advice based on comparing aperture jumps, I think you're going to have to go for the 160mm for reasons stated.  That 30mm is 1.2 inches and is going to deliver, proportionately to the five inch, more than the 1.25 inches does in the 9.25 vs c8,   The 10mm is less than half an inch increment on a 130mm aperture which has already crossed a noticeable threshold from smaller scopes.  

 

That 10mm helps the TEC compete against the TOA 130 and the AP GT and GTX 130.  There's an edge particularly relative to money spent.  But the 160 is going to be the scope that that gives the "oh yeah this is much better" reaction.

 

My 2c.

 

Greg N

 

I would say the one inch gain on a refractor and SCT are different because of the SCT's obstruction.  With a refractor you're talking about more clear light reaching the eye with the inch gain.  With SCT's in my experience you have to go up 2 inches to notice a difference.  I noticed a huge difference when I upgraded from a 6 inch SCT to an 8 inch SCT.  However, would the difference be a big had I gone from an 8 inch to 10 inch SCT?  I think the law of diminishing returns comes into play when you begin dealing with larger apertures as you are already gathering a lot more light than when you start with smaller apertures where a two inch increase makes a bigger difference.  So, to me at least, gains in aperture are relative to the type of telescope we're talking about and the size of the aperture you're starting with.



#96 Jon Isaacs

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Posted 09 June 2017 - 08:48 AM

I would say the one inch gain on a refractor and SCT are different because of the SCT's obstruction.  With a refractor you're talking about more clear light reaching the eye with the inch gain.  With SCT's in my experience you have to go up 2 inches to notice a difference.  I noticed a huge difference when I upgraded from a 6 inch SCT to an 8 inch SCT.  However, would the difference be a big had I gone from an 8 inch to 10 inch SCT?  I think the law of diminishing returns comes into play when you begin dealing with larger apertures as you are already gathering a lot more light than when you start with smaller apertures where a two inch increase makes a bigger difference.  So, to me at least, gains in aperture are relative to the type of telescope we're talking about and the size of the aperture you're starting with.

 

This is a scaling problem.  Scaling is about ratios.  

 

If one is scaling up the same design,  then it's pretty much about the aperture only.. 

 

Jon



#97 JimP

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Posted 09 June 2017 - 10:09 AM

Step back a bit and look at this from a different angle...if you didn't already have a 130mm, what aperture would you get? Probably the answer is the most you can comfortably afford, wield and use...kind of makes the 140 or 160 the best choice taking the prevailing seeing conditions into account. Even though Fried puts prevailing best seeing resolution at most locations somewhere between 100-150mm (hence the 130 class niche), the comment in Telescope-Optics.net about the eye not really getting the photons it needs to work best at resolving low contrast planetary detail with less than 150mm aperture should steer most to the 6 inch class and the reason it appears to be the most recommended as a benchmark for planetary observing. I would like to settle in with a 6+ inch apo someday, not because my seeing will support its full resolution often but rather it will support my aging eyes better to see with! But alas, the 5 inch "niche" is comfortable to afford, wield and use!! My penny.gif penny.gif .

 

http://www.telescope...nd_aperture.htm

 

Last paragraph, bottom of the page.

A very interesting article for sure. Thanks very much!!

 

The truth is, and I say this as a lover of refractors on the refractor form, that I understand all the advantages that large aperture Newts offer but I am satisfied after 50 years of observing to give up on the aperture fever chase (which for many years I was an active member) and be happy with what I have been able to acquire with refractors. If you chase aperture where do you stop? Your 16" will not be enough, so you will want the 18", then in order to move up to 20" and maintain some degree of comfort at the eyepiece, you need to move to a shorter focal length, like F/3.0 requiring more time with collimation. Then of course comes the 24", the 30" and, yes, even the 50". Where does it stop? At what level will you be happy? How much aperture is enough? FOR ME, a refractor, an apochromatic refractor, doublet or triplet, provides what I need and anything from about 120mm and up is great for double stars, one of my primary interests and overall viewing pleasure. For the Moon and Planets specifically, I want at least 7" aperture. And that is enough aperture for me.


Edited by JimP, 09 June 2017 - 11:26 AM.

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#98 Rollo

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Posted 09 June 2017 - 10:18 AM

I have an AP 130 F/6 which I use for visual observations primarily of double stars although anything is game. I have read so many impressive things about CFF I have thought about getting one. What is your experience in terms of the need for increased aperture moving from a 130 refractor to see a difference? Is a move to 140 enough or would I really need to get a 160 to make a visible difference?

Definitely get the 160mm.  Forget the 140mm scope if you have a 130mm.   lol.gif 



#99 Jon Isaacs

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Posted 09 June 2017 - 03:59 PM

If you chase aperture where do you stop? Your 16" will not be enough, so you will want the 18", then in order to move up to 20" and maintain some degree of comfort at the eyepiece, you need to move to a shorter focal length, like F/3.0 requiring more time with collimation. Then of course comes the 24", the 30" and, yes, even the 50". Where does it stop?

Life doesn't really work that way.  Does one avoid buying a Nissan Frontier mini truck because the next step is a half ton pickup and then comes the three quarter ton and then comes the two ton and then the Peterbuilt? 

 

I am not one who chases aperture.  I am not one with aperture fever.  I just like the good views and choose equipment to suit my interests and my situation at the time..  I know what I like to do and I know the equipment that is well suited for what I like to do.

 

So there I was, retired, had a dark sky hideaway where the skies were most often clear and dark.. There's a good sized garage and having a large aperture scope ready to go might take 5 minutes, maybe 10..  I had the time the time to spend a week to 10 days a month observing the deep sky..It took me about a year, but it finally dawned on me, "Hey Jon, you could really use a 20-25 inch scope out here?"

 

We all try to choose equipment that suits our needs.  Obviously a large scope didn't work for you.  But no need to be afraid of "chasing aperture."  If one interests are observing the night sky, the reality of the observing experience is one's guide.  There's plenty to be seen in most any scope but some show more than others. 

 

Jon



#100 mogur

mogur

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Posted 09 June 2017 - 06:02 PM

Chasing aperture only goes so far. The bigger you get the heavier and more inconvenient it gets. I once had a 15" dob. Loved it. Nice Galaxy mirror. Never had a scope give better views. Sold it. Got too heavy for me to lug around. Now my biggest is a 10". It seems the minimum I'm willing to go. Resolves most globs nicely. Shows some dim galaxies. Easy to set up and lift. I'm happy with it and will never go bigger.


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