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Thoughts On The Refractor?

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#51 Larry Geary

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Posted 07 August 2014 - 03:38 PM


The question was not meant to be rhetorical or to elicit a negative response. I am genuinely interested as to why people would prefer to see less detail rather than more.

 

 

It's a clear night, and I'm in the mood for some observing. I look at my 15" Dob, and I groan inwardly. "50 pound mirror box. Multiple trips outside. Assembly. Collimation. 2-3 hour cool-down time. Standing at the eyepiece. Resetting the equatorial table, and re-locating the object. A ladder. Up and down, and up, and down." I sigh, and take out a smaller scope - a 4" refractor, or maybe something even more convenient. It isn't that I prefer to see less detail, but that I am willing to make that trade, because if I didn't have that option, I might not go out at all.


Edited by Larry Geary, 07 August 2014 - 03:39 PM.

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#52 gnowellsct

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Posted 07 August 2014 - 04:04 PM

The question was not meant to be rhetorical or to elicit a negative response. I am genuinely interested as to why people would prefer to see less detail rather than more. Some of the responses have been thoughtful and I appreciate people taking the time to provide the reasoning for their preferences. I obviously view the hobby much different than some as i want the most detail from objects every time I get out. I am always looking for that wow factor. Even after I have seen an object several times I still get a sense of satisfaction from either seeing more detail or just the impressiveness of the object. Going down from the 10" for me feels like taking all the fun out...but that's just me...  

 

 

 

 

As I indicated I had similar sentiments using my C14 for around 5 years.  Eventually I began to "get it" with refractors.  One night I was looking at NGC 5053 which is a soft glob just offshore the more dramatic M53.  I enjoyed the view in my C14: just M53 all by itself.  Then NGC 5053 by itself.

 

Then I reduced to minimum magnification in the C14 (98x, 40 arc min field).  Still only NGC 5053.

 

Then I went to lower magnification in my 4" f/6.5.  Whoa!  There were *both* NGC 5053 and M53 in the field of view.  Then I kept reducing to 17x, still could seem them both easily.

 

Then I went over to my friend's dob which at the time had a 70mm.   We looked and you could see both.  With this field in my mind, I went back to my scope and looked through my 50mm finder.  To my astonishment, you could still see NGC 5053 as a slight depression in a field of stars in which M53 gleamed like a metropolis.  

 

You would not think of NGC 5053 as a finder object--most people stop at the Messiers--and you certainly would have missed it if the finder was all you had.  But working the sequence backwards made it very easy to detect, and was an object lesson in the different virtues of different apertures.

 

Your post focused on large objects but of greater interest is bringing in *paired* objects:  both sides of the Veil, both parts of the Double Cluster in Perseus, and more obscurely, NGC 6939 (open cluster) and NGC 6946 (galaxy) which straddle the boundary between Cygnus and Cepheus, and the aformentioned M53/N5053 are examples of notable pairs.  There are some glob pairs too.

 

A two inch widefield in the 30 to 40mm category (and preferably two) is fundamental to enjoying a telescope; if you put off the purchase when you finally get one you will say "I can't believe I watied this long to get a two inch wide field".  

 

Greg N


Edited by gnowellsct, 07 August 2014 - 04:05 PM.


#53 gnowellsct

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Posted 07 August 2014 - 04:14 PM

Incidentally, I say this a lot, but if you assume a 5mm night time exit pupil, which is typical for middle age, and consider the ratio of the jump from the naked eye to a four inch refractor, then to repeat the jump as a ratio from four inches would require an 80 inch telescope.  

 

Four inches is what opens the sky.  It gives you detail on Mars and Jupiter and Saturn, mind numbing numbers of craters on the moon, thousands of galaxies, hundreds of globs, double stars, and wide variety of dark and emission nebulae.  

 

And it would take 80 inches to do "that much again."

 

So, proud as we may be of our 5" and 14" and even 20" and 30" apertures, and keenly aware as we may be of the improvements in performance, the fact is that these are all improvements at the margin compared to what the first four inches of aperture do.   

 

Greg N


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#54 Sarkikos

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Posted 07 August 2014 - 05:08 PM

"It's a clear night, and I'm in the mood for some observing. I look at my 15" Dob, and I groan inwardly. "50 pound mirror box. Multiple trips outside. Assembly. Collimation. 2-3 hour cool-down time. Standing at the eyepiece. Resetting the equatorial table, and re-locating the object. A ladder. Up and down, and up, and down." I sigh, and take out a smaller scope - a 4" refractor, or maybe something even more convenient. It isn't that I prefer to see less detail, but that I am willing to make that trade, because if I didn't have that option, I might not go out at all."

 

Yes, indeed. For me that choice would be a C90, a C5 or a C80ED.  I have a C102 f/9.8 achromat, but that scope on my AT Voyager is even too much of a bother compared to those three smaller, even more convenient scopes. I can't remember the last time I brought out my 10" Dob here at home.  It's getting so that's too much trouble unless I'm driving to a dark site.

 

Mike


Edited by Sarkikos, 07 August 2014 - 05:14 PM.


#55 karstenkoch

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Posted 07 August 2014 - 05:21 PM

I am genuinely interested as to why people would prefer to see less detail rather than more.


The more you know about the night sky, the less you need to see to appreciate it. Some folks just want to have a relaxing evening under the stars without too much excess baggage. These folks occassionally like to see maximum detail, but there is usually someone nearby who has been terminally afflicted with aperture fever who is more than proud to show off the superiority of their equipment.
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#56 drollere

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Posted 07 August 2014 - 05:33 PM

Four inches is what opens the sky.  It gives you detail on Mars and Jupiter and Saturn, mind numbing numbers of craters on the moon, thousands of galaxies, hundreds of globs, double stars, and wide variety of dark and emission nebulae.  

 

And it would take 80 inches to do "that much again."

 

So, proud as we may be of our 5" and 14" and even 20" and 30" apertures, and keenly aware as we may be of the improvements in performance, the fact is that these are all improvements at the margin compared to what the first four inches of aperture do.

well, that's almost correct. back to my earlier post, here's a chart of the relative gains in light grasp and relative reduction in the resolution limit as aperture is increased from the minimum useable to the largest practicable (affordable, physically manageable, etc.).

 

resLG.gif

 

95% of the resolution benefit possible with the largest possible aperture is obtained with aperture that is only 20% of the maximum. in contrast, 95% of the light grasp benefit possible is only obtained with 98% of the largest possible aperture.

 

this concisely explains why folks interested in resolution -- double star astronomers, lunar and planetary astronomers, solar astronomers -- are perfectly content with relatively small aperture instruments that maximize luminance and frequency contrast. they are assisted by the triple inducements that their targets are inherently bright (too bright, in large apertures); the small apertures are relatively insensitive to atmospheric turbulence, yielding the maximum benefit of the resolution advantage; and the small aperture allows a relatively large range in focal ratios, emphasizing either wide field of view or increased magnification.

 

on the other hand, this also explains why folks interested in light grasp -- deep sky visual observers, almost exclusively -- push aperture limits as far as their pocketbooks or ability to move heavy equipment or stand on a ladder will allow. "aperture fever" is the result, which is facilitated by the very fast focal ratios that give large apertures a wide field of view and a bright image at the expense of magnification, which is less attractive due to the mirror and atmospheric turbulence that larger apertures can exacerbate.

 

astrophotographers are generally most interested in a flat, large image field in instruments of a size and weight that allows them to be accurately tracked or guided, which is for them also an inducement to smaller apertures: CCD sensitivity, image stacking, long exposure times can more than make up for the loss in light grasp.

 

this relationship is invariant whether you take the maximum practicable to be 100mm or 10 meters. to greg's suggested maximum of 30" (760mm), the 95% resolution instrument is 150mm.

 

instruments in the middle, between 8" to 12", try to have it both ways, and for many are successful compromises. but this obscures the fact that aperture is not a continuum of benefit but a rather sharply divided regime of competing benefits, and instruments larger or smaller than the compromise range emphasize one benefit over the other.



#57 gene 4181

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Posted 07 August 2014 - 07:40 PM

bruce , are you saying that a 150mm apeture will deliver 95% resolution? thanks,

#58 Jon Isaacs

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Posted 07 August 2014 - 09:48 PM

Bruce

 

 

What is the source of the graph and and what is the analysis that produced it.  Under is ideal conditions, if you double the aperture, the finest resoveable detail is 1/2 the size.  If the aperture of scope A is 20% of scope B, the scope B will resolve details 1/5 the size.. that seems inconsistant with the graph.

 

Jon



#59 samovu

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Posted 07 August 2014 - 10:27 PM

I think there is a place for both refractors and reflectors.. certainly M13 takes on a different character when viewed at 300x in a large scope.. but m13 is still enjoyable in a 4 inch..  there is no perfect scope but two or more well chosen scopes are superior, more capable, than any single scope.

 

I use refractors for low power wide field and easy look-easy setup.. Jupiter in a 4 inch apo..  definitely worthwhile.

 

Jon

 

+1

 

My first view of M13 at a semi dark site was breathtaking. It was through a Tak FS-78 but I believe it would have still been breathtaking through say an 80ED. But maybe just a bit more through the fluorite doublet :)

 

Clear views,

John



#60 BigC

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Posted 07 August 2014 - 11:44 PM

Under ideal sky or in space resolution  will increase along with aperture,no question.

 

I wonder if the graph is supposed to take into account the usual atmospheric disturbances that often limit seeing and usable resolution?

That is often said to make a 6" scope optimum on the idea 6" x 50x per inch =300x ,the "typical" maximum due to conditions.

 

Economically it has been aid that 80% of the benefits usually comes with only 20% of the effort/expense needed to achieve full/maximum benefits.I can't verify that but it seems more believable.That a scope costing 20% of the maximum would deliver 95% of the results wouldn't surprise. 

 

 

For myself there was a quite apparent difference in  the view comparing  a C5 and a C8,favoring the C8.

 

But I still enjoy using just about any size scope, provided it is stable and smooth mechanically and decent optically.There is a certain simplicity with a refractor;no spikes,no "hole in the middle"  if used in the day,freedom to use bigger variety of eyepieces,etc.

 

The scope used is directly related to the time available,and my mood and health at the time.

 

As the serious scientists are data analysts rather than observers it seems a serious amateur is one who has an observatory of at least several C14s on Paramounts doing automated imaging and spectroscopy.

 

Now serious observers who actually use their scope to LOOK at the beauty of the stars and objects no doubt comprise a larger group.

 

A parallel in photography:Ansel Adams pictures impress me more than the modern photoshopped ransom-note montages .



#61 Max Power

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Posted 08 August 2014 - 07:00 AM

When I lived way out in the country, a 4" scope was just fine.

But now near a city, a 4" is kind of disappointing.

The 8 or 10" gets a lot more use.



#62 Jon Isaacs

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Posted 08 August 2014 - 08:30 AM

 

I wonder if the graph is supposed to take into account the usual atmospheric disturbances that often limit seeing and usable resolution?

That is often said to make a 6" scope optimum on the idea 6" x 50x per inch =300x ,the "typical" maximum due to conditions.

 

The reality is that if the conditions are such that a 6 inch scope shows all it can show,  a decent 12 inch scope will show more. 

 

Jon



#63 De Lorme

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Posted 08 August 2014 - 10:15 PM

Jon, That's only part of the equation.  Remember dragging it out, cooling time, collimation and comfort is every bit as much of the equation as size. 

A PVS7 makes a telescope aperture 2.5X. While expensive to buy all at once making payment over a year and a half is doable.

Plus you don't have to buy anther mount. Some may say the green makes the view awful. Well I would tell them not to look at the back ground

which is green but to look at the stars that are white.  An image intensifier can't go through clouds but they do handle a hazy sky fairly well.

Would you rather see a haze,  a fuzzy haze, wait several days to a week for the skies to clear or see the object with a green back ground but pretty good

detail and a little noise?  When I had my 8079HP I was never able to use it in a clear sky. But it was far better then my 24mm ES eyepiece

in a bad sky. Which is why I have great expectations using my PVS7 in a clear sky if the rain ever stops and the clouds go away.

 

A 4" refractor =10", 5"=12.5", 6"=15". = Convenience LOL!  

 

De Lorme



#64 Jon Isaacs

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Posted 08 August 2014 - 10:42 PM

 

Jon, That's only part of the equation.  Remember dragging it out, cooling time, collimation and comfort is every bit as much of the equation as size. 

A PVS7 makes a telescope aperture 2.5X.

 

The PVS7 increases the effective light gather but not the effective aperture in terms of contrast and resolution.  The green, I have seen it, you can have it.. I find a 12.5 inch or 16 inch Dob more ergonomic than a 6 inch refractor.. 

 

There are many parts of the equation, I was discussing one.  I was merely pointing out the seeing permits a 6 inch to be optically limited, a 12 inch that is collimated, cooled etc, will show more.  The implication seemed to be that if the 6 inch can operate at 300x, then at 300x, it will show as much as a 12 inch at 300x but this is not the case.. 

 

Jon



#65 jrbarnett

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Posted 08 August 2014 - 11:13 PM

Inch for inch, refractors have the greatest light gathering capability.  They are far more efficient in the throughput department than any mirrored, obstructed scope.  The problem is, a really big refractor costs quite a bit more than a mirrored scope if like aperture.  As a rule of thumb, I'd say a 5.5" refractor is about as good on DSOs as an 8" mirrored scope.  So due to superior efficiency, you can get away with a smaller refractor for a given level of DSO performance.  Refractors over 6", though, are pretty hard to come by and not exactly easy to mount.

 

- Jim



#66 gnowellsct

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Posted 08 August 2014 - 11:17 PM

Well it's a digression but I've never understood the fascination for splittng stars in small (say five or six inch) refractors.  When you push the magnification the stars become dots.  I don't like looking at Airy disk dots.  

And the bigger scopes bring in more color.....

 

Anyhow I have very nice refractor but I'm just saying, it would not be my preferred instrument for splitting doubles.

 

GN



#67 Jon Isaacs

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Posted 08 August 2014 - 11:24 PM

Inch for inch, refractors have the greatest light gathering capability.  They are far more efficient in the throughput department than any mirrored, obstructed scope.  The problem is, a really big refractor costs quite a bit more than a mirrored scope if like aperture.  As a rule of thumb, I'd say a 5.5" refractor is about as good on DSOs as an 8" mirrored scope.  So due to superior efficiency, you can get away with a smaller refractor for a given level of DSO performance.  Refractors over 6", though, are pretty hard to come by and not exactly easy to mount.

 

- Jim

 

A throughput calculation for an 8 inch Newtonian with a 25% CO and 90% reflective mirrors says it would be equivalent to a perfect 7 inch scope. A 7 inch scope captures 62% more light than a 5.5 inch.  

 

Jon



#68 Eddgie

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Posted 09 August 2014 - 08:37 AM

Should I be looking at TAK's and the like for that visual wow experience, or just stick with the cheaper designs and get all that aperture?

Why are people so enamored with small refractors when they can't get that detail (visual)?

 

Thoughts?

 

My opinion....   No, don't bother with the Tak over a good ED scope for just visual use.   The vast majority of the benefit of triplet APOs or other advanced designs is only going to be realized when imaging.  For visual use, don't expect to see a wow or anything close to it vs using a similar sized ED doublet.   I have done side by side with many different APOs and ED scopes, and visually, the performance is far more similar than it is different.

 

I have tested ED and APO scopes side by side, and a well made ED scope can deliever pretty much the same observing experience.

 

I no longer recommend achromats though.   A smaller, lighter ED scope can deliver all of the performance of a larger, faster achromat.   Achromats greatly dilute the energy concentration that is possible with an ED or APO.     Just my opinion, but the more I used ED and APO type instruments, the more I realized how inferrior the achromats are.

 

I would recommend a good ED scope as a compliment to your larger reflector for the times when you want a nice, wide, coma free field.

 

Like you though, I find the views though my 12" dob to be far more rewarding than with my 110ED or 6" APO, and if it fits into the field, I would rather see it in the dob (which rolls out into the yard on a hand truck and can be deployed in two minutes flat!).


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#69 Daniel Mounsey

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Posted 09 August 2014 - 10:05 AM

this discussion reminds me of the guy who took a hammer back to the hardware store -- wanted his money back, he couldn't turn screws with it.

 

in general, the benefits of aperture work in opposite directions. as aperture increases, resolution increases, but the increased susceptibility of the larger aperture to atmospheric turbulence and cool down problems makes that increase difficult to exploit. at the same time, image brightness increases, and faint objects are easier to see and faint luminance contrasts emerge more clearly, which can be enjoyed at low magnifications where the effects of seeing are minimized.

 

i agree there is a significant difference between a 4" and a 12" aperture, but to get the same relative increase in image brightness you would need to step up to a 36" mirror. many reasons conspire to make ~8"-12" the sweet spot in amateur apertures.

 

to jon's comment ... i'm a double star astronomer first, but i think the benefits of resolution can be overstated. a 1" double is challenging in my 140mm TEC, and a 0.5" is challenging in my 250mm DK. but so what? there are plenty of both classes to be found in the sky, and there is nothing intrinsically more challenging in resolving a binary at the aperture limit in one aperture vs. another: the pleasure is the same. and i can more often exploit that 1" refractor than i can the 0.5" reflector, due to seeing, which is no small compensation.

 

Well stated.



#70 Daniel Mounsey

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Posted 09 August 2014 - 10:18 AM

Well it's a digression but I've never understood the fascination for splittng stars in small (say five or six inch) refractors.  When you push the magnification the stars become dots.  I don't like looking at Airy disk dots.  

And the bigger scopes bring in more color.....

 

Anyhow I have very nice refractor but I'm just saying, it would not be my preferred instrument for splitting doubles.

 

GN

 

 

:funny:



#71 Daniel Mounsey

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Posted 09 August 2014 - 10:25 AM

 

 

Like you though, I find the views though my 12" dob to be far more rewarding than with my 110ED or 6" APO, and if it fits into the field, I would rather see it in the dob (which rolls out into the yard on a hand truck and can be deployed in two minutes flat!).

 

 

 

This is merely and indication of the types of objects you are choosing to observe. Does everything look better in a 12" dob? I just want to understand the logic here. 


Edited by Daniel Mounsey, 09 August 2014 - 10:25 AM.


#72 BillP

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Posted 09 August 2014 - 11:50 AM

Well it's a digression but I've never understood the fascination for splittng stars in small (say five or six inch) refractors.  When you push the magnification the stars become dots.  I don't like looking at Airy disk dots.  

 

Too funny.  That's what I strive for as I can't stand it when stars are not those pretty little dots -- perfectly formed, sharp edges, nice and round, no wool! :lol:


Edited by BillP, 09 August 2014 - 11:51 AM.

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#73 Jon Isaacs

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Posted 09 August 2014 - 12:04 PM

 

Well it's a digression but I've never understood the fascination for splittng stars in small (say five or six inch) refractors.  When you push the magnification the stars become dots.  I don't like looking at Airy disk dots.  

 

Too funny.  That's what I strive for as I can't stand it when stars are not those pretty little dots -- perfectly formed, sharp edges, nice and round, no wool! :lol:

 

 

 

I also like those nice round dots.. to split the close ones, it's imperative.

 

Jon 



#74 BillP

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Posted 09 August 2014 - 12:07 PM

Hi All,

 

Admittedly my experience with the small refractor is limited, however I think I have just had an epiphany on these scopes. Although I affectionately call the refractor the 4x4 of scopes due to its ease of use and grab-n-go factor I am starting to think that it is not an ideal scope to separate stars on globular clusters and look for feint fuzzies. ...  I am starting to think if I want to have the impressive views where I can resolve the stars in clusters I am going to have to be at a 8" CAT min. Perhaps I will have to sell it and look for a new set up. I think I am still going to want to have an apo in my collection in the future but I am now skeptical that it's going to be the scope that gets used the most. ...

 

Thoughts?

 

For you...for now...I would agree that it will not get used the most.

 

At the current time in your observing career you are enthralled with globs and DSO...and you like seeing more and more stars in them.  So for that aspect of observing, aperture is what you need to focus on.  And FWIW, when I switch between my XT10 and 8" SCT for similar objects, the 8" is a disappointment.  So I would recommend a plan that includes something bigger than an 8" SCT. 

 

A point that Greg brought up though is that what is often not discussed is how our tastes change.  I have noticed this a lot about myself over the years related to observing.  So one needs to understand that there is no *final* setup, but just the setup that works for now as things will change.  One also has to realize that for targets like globs and faint fuzzies yes more aperture can be the ticket *IF* how you like to observe them is close up and personal.  But if you enjoy studying them from a further out perspective, and have come to enjoy that type of view, then the larger aperture smaller TFOV scopes can be limiting.  As example, M81 and M82 I view semi-often.  When I use the 10" I can make out nice details and structure, and of course mostly looking at them one at a time.  But the view I feel is most aesthetic and therefore the one I choose most often, is using a 6" refractor with a 30mm wide field eyepiece.  So it really does boil down to a person's taste at the time as to which type and aperture of scope is best suited to them.  Our tastes change over time...it's just part of life...and as they do our scopes need to change also.



#75 claytonjandl11

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Posted 09 August 2014 - 01:31 PM

After 15 yrs of being in this hobby and owning  reflectors from 8" up to 20", I've settled on refractors, 110mm, 140mm, and a 180mm. Why? Because it never ceases to amaze me at what I CAN see when knowing how to use these instruments properly. For DSO I'll use anywhere between 80x and 120X and rarely go over that, for planets 180x to 220x but if the seeing permits 300x plus. I have 5.2 mag skies on average where I live, and transperancy most of the time will be between above average to excelent. Seeing on the other hand will be between poor and average, a few times a year it might get above average to excelent. My 180mm can see detail up about 75% of what an 18" reflector can. My 140mm up to 75%-80% of a 12" reflector, and my 110mm about the same when compared to an 8" reflector. Now the images will be dimmer than the larger apetures, that being said most of the detail will be there razor sharp and crisp. Seldom have I seen views like this thur a reflector they tend to be noticably softer, I guess what I'm trying to say is that I'd rather give up a little bit of detail than be looking at a softer image. Plus with any of my refractors I can set them up and viewing in about 5 mins. Again this is my own experience with refractors and reflectors, others may have different ones.

 

Nick T.


Edited by claytonjandl11, 09 August 2014 - 01:32 PM.







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