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How much increase in aperture to see a difference?

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#26 NHRob

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Posted 05 June 2017 - 07:19 AM

I had a TEC140 once, and compared it side-by-side with a TMB130 with LZOS optics ... on Saturn and a few open clusters.

 

I found the differences were detectable but, subtle. Probably, what you had for breakfast that day made as much of a difference in your view.

 

If I had a 130 I would not go for a 140.  Go bigger if you want a better view.


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#27 daveCollins

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Posted 05 June 2017 - 07:48 AM

Jim,

 

you already have large scopes and so you can see details when desired. Based on this fact, I'd say there is no difference between a 130 and a 160 in the following sense.

 

  • Put together a target list which pushes the capabilities of a 130. If you like the scope and connect with it, then the fun is in seeing what it can do. How close can you get to the limits of the scope.
  • On Friday I used my 130 GTX. Seeing was great and this "little scope" amazed me. I split binaries down to 1.1 arcseconds with many between 1.4 and 1.6 range.
  • On Saturday, going after the same targets as Friday, I used my LZOS 175.
    • Seeing was marginal
    • The little GTX simply crushed the LZOS 175 in what I could see.
      • So in this case the GTX was a joy to use and it made my weekend. The poor LZOS couldn't overcome the disadvantage of seeing.
      • Caveat
        • Sorry GTX, but on the open cluster M 11, the LZOS dominated ... no competition. Of course open clusters aren't really affected much by seeing.

Edit: When one proves a theorem in math, it is bad form to omit stating the final conclusion, so with that in mind:

  • Here is a case of obvious aperture difference, but a case were the experience at the eyepiece was superior to that of a larger scope. 
  • Regardless of seeing conditions, it is the expectation which dominates a session for me. By adjusting the expectation, just as much fun is possible in smaller scopes.
    • If one wants to see the refractor image of challenging objects, and seeing and price aren't limitations, then obviously the larger the aperture, the more difficult the target that can be seen. Yes, I am stating the obvious. But this isn't the situation that we (virtually everyone) finds ourselves in. So in the end, one can have just as much fun with a smaller scope as with a larger one.
    • And as was obvious this weekend, smaller scopes are easier to deal with in most regards.

Edited by daveCollins, 05 June 2017 - 10:48 AM.

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#28 BillP

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Posted 05 June 2017 - 08:27 AM

If I had a 130 I would not go for a 140.  Go bigger if you want a better view.

 

Agree.  In fact, I think the OP should keep the 130 also.  So whatever larger aperture he decides, the 130 should stay because whatever he gets bigger will not be as easy and effortless as the 130.  IMO, nothing smaller than a 180 would be worth it.  Going from a 130 to a 160 is like going from an 80 to a 100...yes a difference but not exactly earth shattering because only seeing a 50% increase in light gathering.


Edited by BillP, 05 June 2017 - 08:28 AM.


#29 daquad

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Posted 05 June 2017 - 08:29 AM

In terms of light grasp, I think one would want to go at least 1 magnitude deeper - a total brightness ratio of 2.512.  Think about the difference in a sky with a naked eye limiting magnitude of 5 compared to one with a limiting magnitude of 6.  Or a limiting magnitude of 6 compared to one with a limit of 7.  To me the difference in seeing the Milky Way, for example, is significant in each case.  

 

So a one magnitude increase requires an aperture increase of SQRT(2.512) = 1.585.  I have a 6" refractor and a 10" reflector.  6X1.585 = 9.5.  So the reflector goes about I magnitude deeper than the refractor.

 

In terms of resolution of fine detail, I think Edggie has said it best in other topics. To see more detail one needs aperture.  A 160 mm scope will be able to show a larger image than a 130 mm with the same brightness by a factor of (160/130)^2 = 1.515.  In other words, you can observe extended objects with the larger scope at the same brightness level, but they will appear 50% larger.  A 140 mm will allow (140/130)^2 = 1.16, i.e., a 16% larger image at the same brightness level.

 

Based on the above, which upgrade will provide the best value, keeping in mind that the upgrade will also require a bigger mount?  My personal formula for deciding on scope size: 1) Determine the largest scope(s) I can comfortably handle.  2) Get the best I can afford or wish to spend, as opposed to THE best.

 

Dom Q.


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#30 BillP

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Posted 05 June 2017 - 08:33 AM

Instead of trying to extrapolate from all the math, here's the differences from a 140 to a 200 at the eyepiece (see sketches):

 

https://www.cloudyni...c-200-apo-r2682


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#31 elwaine

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Posted 05 June 2017 - 09:45 AM

Great article, Bill!

 

On the other hand, some of us stressing the very slight improvement at the eyepiece when going from 130mm to 140mm, or from 130mm to 160mm have written many excellent posts about how to tweak out the very last detail using our small to medium sized refractors; e.g., by using the appropriate eyepiece or diagonal, or training our eyes by sketching, etc.. If every last detail is what we seek, than a 20% or more increase in light gathering and resolving power should make a noticeable difference at the eyepiece... as much as switching from a Nagler ep to a Tom Beck Superplanetary, or from a dielectric diagonal to a Zeiss prism, etc.  So it seems to me that what we are really discussing is, is it worth (spelled $$$) going from 130mm to 140mm or to 160mm. And only the OP can make that determination.

 

Just for reference:

___________________

 

Aperture   Resolving power   Limiting Magnitude   Increase in light gathering from 130mm

 

130mm     0.89 arc"               13.4                          

140mm     0.83 arc"               13.6                           16%

160mm     0.72 arc"               13.9                           51%

 

 

 



#32 daveCollins

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Posted 05 June 2017 - 10:00 AM

Great article, Bill!

 

On the other hand, some of us stressing the very slight improvement at the eyepiece when going from 130mm to 140mm, or from 130mm to 160mm have written many excellent posts about how to tweak out the very last detail using our small to medium sized refractors; e.g., by using the appropriate eyepiece or diagonal, or training our eyes by sketching, etc.. If every last detail is what we seek, than a 20% or more increase in light gathering and resolving power should make a noticeable difference at the eyepiece... as much as switching from a Nagler ep to a Tom Beck Superplanetary, or from a dielectric diagonal to a Zeiss prism, etc.  So it seems to me that what we are really discussing is, is it worth (spelled $$$) going from 130mm to 140mm or to 160mm. And only the OP can make that determination.

 

Just for reference:

___________________

 

Aperture   Resolving power   Limiting Magnitude   Increase in light gathering from 130mm

 

130mm     0.89 arc"               13.4                          

140mm     0.83 arc"               13.6                           16%

160mm     0.72 arc"               13.9                           51%

The eye doesn't respond linearly to light levels, so the eyepiece experience may not relate well to the numbers. A 51% may be noticeable, but may not be particularly significant.



#33 BillP

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Posted 05 June 2017 - 10:11 AM

Great article, Bill!

 

On the other hand, some of us stressing the very slight improvement at the eyepiece when going from 130mm to 140mm, or from 130mm to 160mm have written many excellent posts about how to tweak out the very last detail using our small to medium sized refractors; e.g., by using the appropriate eyepiece or diagonal, or training our eyes by sketching, etc.. If every last detail is what we seek, than a 20% or more increase in light gathering and resolving power should make a noticeable difference at the eyepiece...

 

Oddly, I would not think so.  When one optimizes their eyepieces, they are typically working at refining their optical chain as much as possible.  And when that is done the aperture stays fixed of course and you start working on upgrading other components like the diagonal, baffling, mount.  And in these optimizations, the expectations is small incremental improvements, and those expected improvements are typically more about quality of the view, scatter reduction, achieving a starker background sky, improving perceived visual contrast, improving perceived color saturation/vividness, etc.  So not so much looking for light gathering gains when optimizing eyepieces IMO.

 

But moving aperture classes is a whole other exercise.  With this, the expectation changes and one typically looks for the next major level of performance and wants more major changes than one can get from optimization of an exiting optical chain.

 

FWIW, I've used masks on my scopes to determine how I personally react to different percentage changes in light gathering.  Anything less than a 40% increase I find underwhelming.  Above 40% is where for me the differences "start" to show some.  But that is just the start.  Given that extra costs and ergonomic burdens that comes with larger aperture, for me I just do not consider that the aperture gains outweigh the ergonomic/thermal/cost losses unless that light gathering gain is around 250%, and I feel that at the 400% level is where the "WoWs" start happening.


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#34 Jeff B

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Posted 05 June 2017 - 10:58 AM

FWIW, I've compared my TMB 130SS with my TEC 140ED on several nights.  Both are are excellent samples optically and mechanically.

 

In terms of detail, I can indeed see a bit more via the 140ED if seeing cooperates and I'm specifically looking for it.  But I have a method.

 

I will spend a lot of time with the 130SS, for example, on Jupiter and/or the Moon looking for details that seem at the margins of visibility in the 130SS.  Then, I'll switch views to the 140ED and look for those specific details.  And I can consistently pick out those details a bit more easily in the 140ED ( I've learned that I really need to carefully match magnifications as close as possible too).  I can then find stuff close to the "margins" of visibility in the 140ED and then switch to the 130SS and as I suspected, they were not visible in the 130SS or at best fleeting.

 

But I had to really work to see the differences and like others have said, the differences between the 130SS and my 152mm triplets are more obvious to me.

 

However, there is an obvious difference between your GTX and the 140mm TEC and that is the price.  Not subtle, particularly on the used market for a 140ED.

 

Jeff


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#35 daquad

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Posted 05 June 2017 - 10:59 AM

 

Great article, Bill!

 

On the other hand, some of us stressing the very slight improvement at the eyepiece when going from 130mm to 140mm, or from 130mm to 160mm have written many excellent posts about how to tweak out the very last detail using our small to medium sized refractors; e.g., by using the appropriate eyepiece or diagonal, or training our eyes by sketching, etc.. If every last detail is what we seek, than a 20% or more increase in light gathering and resolving power should make a noticeable difference at the eyepiece...

 

Oddly, I would not think so.  When one optimizes their eyepieces, they are typically working at refining their optical chain as much as possible.  And when that is done the aperture stays fixed of course and you start working on upgrading other components like the diagonal, baffling, mount.  And in these optimizations, the expectations is small incremental improvements, and those expected improvements are typically more about quality of the view, scatter reduction, achieving a starker background sky, improving perceived visual contrast, improving perceived color saturation/vividness, etc.  So not so much looking for light gathering gains when optimizing eyepieces IMO.

 

But moving aperture classes is a whole other exercise.  With this, the expectation changes and one typically looks for the next major level of performance and wants more major changes than one can get from optimization of an exiting optical chain.

 

FWIW, I've used masks on my scopes to determine how I personally react to different percentage changes in light gathering.  Anything less than a 40% increase I find underwhelming.  Above 40% is where for me the differences "start" to show some.  But that is just the start.  Given that extra costs and ergonomic burdens that comes with larger aperture, for me I just do not consider that the aperture gains outweigh the ergonomic/thermal/cost losses unless that light gathering gain is around 250%, and I feel that at the 400% level is where the "WoWs" start happening.

 

I'm with you, Bill.  As I said in my earlier post, a difference of at least one magnitude (factor of 2.512 or 250%) will provide significantly brighter views.  If one's small scope is an 80 mm, then upgrading is not that demanding on mount and pocketbook.  But with the "small" scope being 5+ inches, one must think very hard about upgrading to a larger refractor if looking for brighter DSO images.  That is why my "upgraded" scope is a 10" Newt.



#36 Markab

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Posted 05 June 2017 - 11:02 AM

If only going 10mm (all of 1cm lol), it's not noticeable.

 

Personally, I can't see the hooplah everyone raves about going from a 4" to a 5", but there ya go - some think it is, some don't.

 

Lewis, FWIW, I completely agree with you, after owning quite a few premium 4" and 5" refractors. There is a difference, but in many cases--not all--it is subtle. I think part of the reason you hear so much raving between these two sizes in particular is self-interest. A 5" is really about the upper limit that many people can comfortable afford, mount--and--is readily available. There is a certain smugness to that in comparison to the "lowly" 4"...which itself was considered a "medium sized" refractor two decades ago [an article from an early 1990s Astronomy magazine comes to mind that reviewed "medium size" refractors--all around 4"--including a Genesis, SPC-102F, AP, and I believe the blue 94mm Brandon Christian-sourced triplet]. Now 4" is considered small and 5" is medium. 6" is large in refractorland. If we do end up getting more supply of 6" refractors on the market, I would not be surprised if the supposed big gap in improvement shifts to 5" to 6" rather than the currently held 4" to 5".

 

That being said, I'd really love to own a 7" or 6" APO if I had an outside observatory...physics would suggest the views should be considerably better than an AP130, although at a significant portability penalty.


Edited by Markab, 05 June 2017 - 11:06 AM.

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#37 Scott99

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Posted 05 June 2017 - 11:42 AM

re: upgrading, I always end up with the largest possible aperture I can use on a given mount & tripod.  If I knew my mount for the 130 would handle a 140 I would eventually upgrade.   that's how I view it, rather than figuring out how much each millimeter will show, I assume the larger one is always better.


Edited by Scott99, 05 June 2017 - 11:42 AM.


#38 Astronought

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Posted 05 June 2017 - 11:56 AM

My FS102 is going nowhere though at 4" it just can't hit those little details.
Not being related to Bill Gates, I solved this problem by getting an MN190. waytogo.gif 
Refractor heresy - next year a 10" Newt!



#39 Jon Isaacs

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Posted 05 June 2017 - 12:02 PM

Is there a simple formula for determining the increase in detail visible as aperture increases? Just like we have for determining brightness?

 

You know that the size of the Airy disk is inversely proportional to the aperture.  If the optics are of equal quality, then the fine scale contrast and resolution are also inversely proportional.. 

 

Bigger scope = finer scale contrast, better resolution..  

 

Jon


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#40 BillP

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Posted 05 June 2017 - 12:03 PM

I'm with you, Bill.  As I said in my earlier post, a difference of at least one magnitude (factor of 2.512 or 250%) will provide significantly brighter views.  If one's small scope is an 80 mm, then upgrading is not that demanding on mount and pocketbook.  But with the "small" scope being 5+ inches, one must think very hard about upgrading to a larger refractor if looking for brighter DSO images.  That is why my "upgraded" scope is a 10" Newt.

 

 

If I had a nice 130mm as my only refractor and was looking for an upgraded visual experience, and I had the cash for something like a TEC180, I think I would be looking at a premium Newt or MCT instead.  Let the 130 do what only it does best, and look for a compliment instead of an upgrade.

 

So let's see...a TEC180 goes for around $20k I believe, so what could I get some premium maker like Teeter or someone to make me a custom 14" f/4.  I choose not to go over 14" because then I can still get 1.5deg TFOV out of it with something like a 21 Ethos.  I personally do not like being limited to less than 1.5deg TFOV from a scope.  At f/4 it will really be a small scope too.  Could easily deck that thing out with all the accessories and a complete set of eyepieces for less than the TEC180 cost.  So would enjoy what refractors can offer with the 130 and then enjoy in a small package what a 14" mirror can offer for fainter fare.


Edited by BillP, 05 June 2017 - 12:04 PM.

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#41 elwaine

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Posted 05 June 2017 - 12:08 PM

 

Great article, Bill!

 

On the other hand, some of us stressing the very slight improvement at the eyepiece when going from 130mm to 140mm, or from 130mm to 160mm have written many excellent posts about how to tweak out the very last detail using our small to medium sized refractors; e.g., by using the appropriate eyepiece or diagonal, or training our eyes by sketching, etc.. If every last detail is what we seek, than a 20% or more increase in light gathering and resolving power should make a noticeable difference at the eyepiece...

 

Oddly, I would not think so.  When one optimizes their eyepieces, they are typically working at refining their optical chain as much as possible.  And when that is done the aperture stays fixed of course and you start working on upgrading other components like the diagonal, baffling, mount.  And in these optimizations, the expectations is small incremental improvements, and those expected improvements are typically more about quality of the view, scatter reduction, achieving a starker background sky, improving perceived visual contrast, improving perceived color saturation/vividness, etc.  So not so much looking for light gathering gains when optimizing eyepieces IMO.

 

But moving aperture classes is a whole other exercise.  With this, the expectation changes and one typically looks for the next major level of performance and wants more major changes than one can get from optimization of an exiting optical chain.

 

FWIW, I've used masks on my scopes to determine how I personally react to different percentage changes in light gathering.  Anything less than a 40% increase I find underwhelming.  Above 40% is where for me the differences "start" to show some.  But that is just the start.  Given that extra costs and ergonomic burdens that comes with larger aperture, for me I just do not consider that the aperture gains outweigh the ergonomic/thermal/cost losses unless that light gathering gain is around 250%, and I feel that at the 400% level is where the "WoWs" start happening.

 

I agree with your points. But I stand by my statement that "If every last detail is what we seek, than a 20% or more increase in light gathering and resolving power should make a noticeable difference at the eyepiece...." It's not just the increase in light and the ability to use a larger exit pupil at a given magnification. Angular resolution also increases with aperture. However, a "noticeable" increase is not synonymous with a meaningful increase. The later is in the eyes of the beholder. When one factors in the extra cost, convenience of use, cool down, etc., the marginal gain in details hardly seems worthwhile... to me.

 

The cost to upgrade an eyepiece, or a diagonal, or flocking, etc., is quite small in comparison to the cost of upgrading to a larger refractor. So the cost to benefit ratio doesn't make much sense (to me) to do the later. But that's my opinion. The OP might conclude that the extra cost, etc., is worthwhile.



#42 SteveC

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Posted 05 June 2017 - 12:08 PM

 

I think you'd need to go 185 to notice any WORTHWHILE difference.

 

200mm if you want it to be a WoW

 

My chuckle of the day.



#43 MJB87

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Posted 05 June 2017 - 12:28 PM

I'm tempted.  Really tempted.  The CFF 185mm f/6.8 is calling my name. I love my GTX but suffer from focal length fever after having given up my 2700mm f/7 setup (14" EdgeHD with 0.7x focal reducer).  Besides, my mount needs more of a workout than it gets swinging around the GTX.

 

Someone stop me before I sin again...


Edited by MJB87, 05 June 2017 - 12:40 PM.


#44 Jeff B

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Posted 05 June 2017 - 12:32 PM

You are forgiven. angel2.gif meditation.gif praying.gif



#45 Jon_Doh

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Posted 05 June 2017 - 12:35 PM

 

I'm with you, Bill.  As I said in my earlier post, a difference of at least one magnitude (factor of 2.512 or 250%) will provide significantly brighter views.  If one's small scope is an 80 mm, then upgrading is not that demanding on mount and pocketbook.  But with the "small" scope being 5+ inches, one must think very hard about upgrading to a larger refractor if looking for brighter DSO images.  That is why my "upgraded" scope is a 10" Newt.

 

 

If I had a nice 130mm as my only refractor and was looking for an upgraded visual experience, and I had the cash for something like a TEC180, I think I would be looking at a premium Newt or MCT instead.  Let the 130 do what only it does best, and look for a compliment instead of an upgrade.

 

So let's see...a TEC180 goes for around $20k I believe, so what could I get some premium maker like Teeter or someone to make me a custom 14" f/4.  I choose not to go over 14" because then I can still get 1.5deg TFOV out of it with something like a 21 Ethos.  I personally do not like being limited to less than 1.5deg TFOV from a scope.  At f/4 it will really be a small scope too.  Could easily deck that thing out with all the accessories and a complete set of eyepieces for less than the TEC180 cost.  So would enjoy what refractors can offer with the 130 and then enjoy in a small package what a 14" mirror can offer for fainter fare.

 

If I had a nice 130 and money sitting around to spend on a 20K TEC 180 I'd buy a high quality mass produced 12 or 14 inch dob and use the rest of the money to build a nice observatory  :)


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#46 Scott99

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Posted 05 June 2017 - 03:15 PM

I'm tempted.  Really tempted.  The CFF 185mm f/6.8 is calling my name. I love my GTX but suffer from focal length fever after having given up my 2700mm f/7 setup (14" EdgeHD with 0.7x focal reducer).  Besides, my mount needs more of a workout than it gets swinging around the GTX.

 

Someone stop me before I sin again...

this is what I was talking about!  there is a large disparity between your mount and telescope size, an upgrade is inevitable.......

 

>>>Rich Neck Observatory (ROR)

    Astro-Physics 1100GTO
    Astro-Physics 130mm Starfire GTX



#47 BillP

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Posted 05 June 2017 - 03:23 PM

If I had a nice 130 and money sitting around to spend on a 20K TEC 180 I'd buy a high quality mass produced 12 or 14 inch dob and use the rest of the money to build a nice observatory  smile.gif

 

Excellent suggestion as well.  That is a lot of cash for an upgrade...so careful attention to be paid as to where is best to spend it.  Indeed for all that money one could upgrade to a 140, really good mount on permanent pier in an observatory.  So lots of options to think about rather than just all in the scope.


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#48 BillP

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Posted 05 June 2017 - 03:29 PM

I agree with your points. But I stand by my statement that "If every last detail is what we seek, than a 20% or more increase in light gathering and resolving power should make a noticeable difference at the eyepiece...."

 

 

Very true.  I guess just depends where one want the focus point to be at the moment.  One thing that always makes me chuckle when folks scoff at optimizing to best eyepieces and to focus on increasing aperture instead is that it sort of leaves a current problem and lets it propagate to a new scope.  Optimized components like eyepieces and diagonals seem to hold their stature for decades really.  So the scopes/objectives really come and go over time, often moving up in aperture.  However, the optimized accessories like the diagonals and eyepieces really stay around as the scopes change.  So it that light they are a bit more important because they will bring their magic to any aperture one gets.


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#49 Gofr

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Posted 05 June 2017 - 04:00 PM

My FS102 is going nowhere though at 4" it just can't hit those little details.
Not being related to Bill Gates, I solved this problem by getting an MN190. waytogo.gif 
Refractor heresy - next year a 10" Newt!

Don't feel bad. Despite my love for refractors, I had to eventually give in to mirrors also in order to feed my aperture fever. Though I avoided the newt and went with a CAT and got my C9. (I......just don't favour newts despite all their strengths.) Hey, at least it uses one lens and a rear port for viewing still. lol


Edited by Gofr, 05 June 2017 - 04:01 PM.


#50 donadani

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Posted 05 June 2017 - 04:34 PM

yea sometimes (like this evening) I ask myself too: "why you silly id... buy all this expensive apo stuff???" ;) 

 

Cloudy for the whole day - this evening reported to be one of the most interesting of the year - Io & Europa before Jupiter - together with their shadows and crowned by the GRS. Before 1,5h I saw a break in the clouds and to be fast I just grabed my 8" GSO Dobs and took it out. After 30min cooling another break off - I could see so much details in the bands, GRS shining in a bright red, both shadows, Europa just beginning to leave the planets disc and - man - I could swear to see a round structure in the band that could have been Io... - best experience for a looong time! :)

 

So go get a 8" f6 Dobs - Houston out ;)


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