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Equipment Discussions >> Refractors

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Jon_Doh
scholastic sledgehammer


Reged: 09/16/11

Loc: On a receiver's back
Re: Using refractors for Deep Space Observing new [Re: Sasa]
      #5203544 - 05/03/12 11:07 AM

Jake, does your 8" Meade pick up anything that you can't see with the 6" refractor?

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stevetaylor199
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Reged: 09/21/11

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Re: Using refractors for Deep Space Observing new [Re: Jon Isaacs]
      #5203715 - 05/03/12 12:40 PM

Quote:

Quote:

The refractor dilemma: dim but perfect versus bright but flawed.

Both scenarios have merit, of course, but there's something beyond compare about having a large (>5") premium refractor out under a truly dark (NELM at zenith of 7+) sky. I actually appreciate my aperture here in the 'burbs more than I do in the boons.

- Jim




Brightness of an extended object is not dependent upon aperture, it's a function of exit pupil... Telescopes are like camera lenses, choose the right one for the particular job at hand. A 4 inch F/5.4 is just about right for the California Nebula, not so good for 14th magnitude galaxies or globular clusters. The reverse is true of large reflectors.

Jon Isaacs




Interesting. Isn't it also true, though, that exit pupil is controlled by the EP focal length? Can you explain a little more about how aperture and focal length have a role in exit pupil, and what size pupil you'd recommend for extended objects?


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Eddgie
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Re: Using refractors for Deep Space Observing new [Re: stevetaylor199]
      #5203814 - 05/03/12 01:34 PM

First, I keep reading that refractors have better contrast.

This is simply untrue in most cases. If a reflector has more clear aperture (primary diameter minus secondary diameter) it will have as good or better contrast than a smaller unobstructed scope with less clear aperture.

As for exit pupil, what Jon says is 100% true, but the story is more complex.

Let's say for example that you use two telescopes... One is a 10" reflector, and one is a 5" refractor.

If you use a 2mm exit pupil in the 10", you will need an eyepeice that gives 127x.

If want the same 2mm exit pupil in the 5", the magnification will have to be about 63.5x.

What this means is that to get the same size exit pupil in these scopes, the image scale will be twice as large in the scope with twice the aperture!

Or, put another way, I could use the bigger scope at 63.5x and get the same exact image scale, but the exit pupil would be twice as large, so the object would appear much brighter in the larger aperture at the same image scale.

As for contrast, people say that objects stand out against the background better. This is not at all how contrast transfer works. The ratio between an extended object and the average background sky will be more or less the same in an obstructed scope as it will in an unobstructed scope.

The difference is how the light from an extended object is focused within the boundry of the extended object. In other words, it is about how the delicate and faint structure in a galaxy or nebula will be presented. The scope with more clear aperture will do a better job of presenting the details of the structure of the object. This is what contrast transfer is about... How the details within the boundry of an extended target are formed. The telescope with the most clear aperture will do a better job of concentrating the light from the fine details within the structure into the right place so that more and more of these structures become visible.

In the example above, if you pointed both telescopes at the Ring Nebula, and you se a 2mm Exit pupil, the 10" scope sould show the Ring Nebula at twice the scale, making the structure easier to resolve visuaally (and one of the primary functions of a telescope is to increase image scale) and assuming that the 10" scope had more clear aperture than the 5" telescope, not only would the detail be presented much larger for the same exit pupil, it would also be presented with better contrast. You would see more structure within the ring.

And this is why larger apertures are generally better for deep space observing thatn smaller ones.

I personally gave up on small scopes for deep spece. Having owned maybe 6 or 7 4" refractors, as beautiful as the presentation of stars were in the field of view, I just could not get much detail out of them except on stellar targets (clusters, doubles).

Even my 6" APO fails to come close to rendering the detail and structure in extended targets that my 14" reflector does.

In general terms, any deep sky object that will fit into the field of my 14" scope shows far more datial than it does in my 6" refractor.

I love my 6" refractor. It is the best wide field scope I have ever ever ever used. And during the summer or winter, cruising the Mikly Way with it is an absolute delight.

But for deep sky, it simply cannot perform up to the level that I personally require. If I want to study objects in detail, I want a large aperture. Small scopes seem to just not have the light gatbering, image scale, contrast, and resolution that is required for anyting but "Showcase" objects.

But this is just me, and this is the "Refractors are manna from heaven" forum, so I expect that of the posters here, I will be in a tiny minority.

And honestly, if someone is happy with the amount of detail they can see in a 66mm refractor, then who is to say they are wrong to prefer it? Not me!

But seeing much structure in the ring nebula with a 66mm, 100mm, or even 150mm refractor is not quite as easy as with a 10" to 15" reflector.

Everyone should use what they like, but make no mistake... Aperture, and lots of it, is the key to seeing more DSOs, and more structure in DSOs.


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aa6ww
scholastic sledgehammer
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Reged: 10/23/11

Loc: Sacramento, Calif.
Re: Using refractors for Deep Space Observing new [Re: northernontario]
      #5204061 - 05/03/12 03:33 PM

Quote:

On the brighter galaxies (M51,81,82,85 for example), my 6" f/5.9 does very well. I've had good success in Virgo.

The Veil is also very nice. Open clusters are crazy good, but Globulars are a bit tough.

M42 is also very nice with a refractor. I love looking at doubles but I guess they don't count as DSOs.

But in the end...I am addicted to my Dob.

jake




Don't forget Markarians chain for how a big fast refactor can capture the entire chain in one field of view. I was with a group of people last year and brought out my 180mm F/6 APM refractor and they questioned why I'd want such a big refractor when their SCT's and Newts give them more aperture. Once they saw Markarians chain under those dark skies, and I compared it to the photos I had on my red screen laptop at the time, they were just mesmerized. Seeing the entire chain in one field of view and making out the "Eye's" in the chain, and counting the entire string, had them inviting me to do more observing with them with my big APM cannon. It's all about matching your scope and eyepieces to the object your observing, to get the most out of this hobby.

...Ralph


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russell23
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Reged: 05/31/09

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Re: Using refractors for Deep Space Observing new [Re: aa6ww]
      #5204788 - 05/03/12 11:31 PM

Quote:

Do many out here use refractors for deep space observing?




Yep - That is primarily what I do with my Vixen 5.5" refractor. For me - in addition to the great views I like the no-fuss aspect of refractors. Easy to set-up, quick cool down times, comfortable to sit at the end of the OTA, wide field views, pinpoint stars, inky black sky background.

I find I spend more time at the eyepiece with a refractor. When I had a large dob I spent more time trying to locate fainter objects that in the end appeared pretty much the same in the dob as brighter objects that were easier to find in the smaller refractor. After I realized that I stopped spending so much time with my nose in the charts and have spent more time just enjoying the view in the eyepiece.

Dave


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galaxyman
Vendor - Have a Stellar Birthday
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Reged: 04/04/05

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Re: Using refractors for Deep Space Observing new [Re: Eddgie]
      #5204847 - 05/04/12 12:35 AM

Quote:

First, I keep reading that refractors have better contrast.

This is simply untrue in most cases. If a reflector has more clear aperture (primary diameter minus secondary diameter) it will have as good or better contrast than a smaller unobstructed scope with less clear aperture.

As for exit pupil, what Jon says is 100% true, but the story is more complex.

Let's say for example that you use two telescopes... One is a 10" reflector, and one is a 5" refractor.

If you use a 2mm exit pupil in the 10", you will need an eyepeice that gives 127x.

If want the same 2mm exit pupil in the 5", the magnification will have to be about 63.5x.

What this means is that to get the same size exit pupil in these scopes, the image scale will be twice as large in the scope with twice the aperture!

Or, put another way, I could use the bigger scope at 63.5x and get the same exact image scale, but the exit pupil would be twice as large, so the object would appear much brighter in the larger aperture at the same image scale.

As for contrast, people say that objects stand out against the background better. This is not at all how contrast transfer works. The ratio between an extended object and the average background sky will be more or less the same in an obstructed scope as it will in an unobstructed scope.

The difference is how the light from an extended object is focused within the boundry of the extended object. In other words, it is about how the delicate and faint structure in a galaxy or nebula will be presented. The scope with more clear aperture will do a better job of presenting the details of the structure of the object. This is what contrast transfer is about... How the details within the boundry of an extended target are formed. The telescope with the most clear aperture will do a better job of concentrating the light from the fine details within the structure into the right place so that more and more of these structures become visible.

In the example above, if you pointed both telescopes at the Ring Nebula, and you se a 2mm Exit pupil, the 10" scope sould show the Ring Nebula at twice the scale, making the structure easier to resolve visuaally (and one of the primary functions of a telescope is to increase image scale) and assuming that the 10" scope had more clear aperture than the 5" telescope, not only would the detail be presented much larger for the same exit pupil, it would also be presented with better contrast. You would see more structure within the ring.

And this is why larger apertures are generally better for deep space observing thatn smaller ones.

I personally gave up on small scopes for deep spece. Having owned maybe 6 or 7 4" refractors, as beautiful as the presentation of stars were in the field of view, I just could not get much detail out of them except on stellar targets (clusters, doubles).

Even my 6" APO fails to come close to rendering the detail and structure in extended targets that my 14" reflector does.

In general terms, any deep sky object that will fit into the field of my 14" scope shows far more datial than it does in my 6" refractor.

I love my 6" refractor. It is the best wide field scope I have ever ever ever used. And during the summer or winter, cruising the Mikly Way with it is an absolute delight.

But for deep sky, it simply cannot perform up to the level that I personally require. If I want to study objects in detail, I want a large aperture. Small scopes seem to just not have the light gatbering, image scale, contrast, and resolution that is required for anyting but "Showcase" objects.

But this is just me, and this is the "Refractors are manna from heaven" forum, so I expect that of the posters here, I will be in a tiny minority.

And honestly, if someone is happy with the amount of detail they can see in a 66mm refractor, then who is to say they are wrong to prefer it? Not me!

But seeing much structure in the ring nebula with a 66mm, 100mm, or even 150mm refractor is not quite as easy as with a 10" to 15" reflector.

Everyone should use what they like, but make no mistake... Aperture, and lots of it, is the key to seeing more DSOs, and more structure in DSOs.




Interesting Eddgie, for I agree for the most part, for have seen and compared two large scopes (8" refractor & 22" dob) of their respective type side by side in my write-up (and drawings) here a couple years ago (Clash of the Titans 1 & 2). Beyond that I have observed using the 8" refractor with a number of club mates using large dobs up to 25".

Using quality eyepieces in both scopes and similar magnifications, it was both a fun and interesting night.

The conclusion was of course the 22" showed the brighter more detailed image, but the big refractor the "prettier" view. The overall image quality in crispness and darker sky background (which is pleasing to the eye) was in the refractors favor like I said at similar magnifications.

Seeing galaxies in the 14th to 15th mag. range from a good dark site is a testament of what a good large refractor can do.

So Ralph's post here about using refractors for deep space is nice to see, and has substance.

An example can be M-42, and though I've seen this wonderful object through many different scopes over the past 40 plus years, the best overall view I've seen was two years ago on top of Dolly Sods mountain in West Virginia with the 8" refractor. The detail (from obvious to delicate) and color was wonderful. A Hi-Def view for sure.




Karl
E.O.H.

Chesmont Astronomical Society - www.chesmontastro.org
Deep Space with Refractors - http://tech.groups.yahoo.com/group/deepspacewithrefractors
Telekit (Swayze optics) 22" F/4.5 Dob
Homemade (Parks Optics) 12.5" F/4.8 Dob
TMB/APM 8" f/9 Refractor”The Beast”. One great DEEP SKY achro
ES 6" f/6.5 achro. Good one
Celestron Omni XLT 102 refractor.
Celestron 10x60mm Binos


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Messyone
professor emeritus


Reged: 05/02/12

Loc: Down Under
Re: Using refractors for Deep Space Observing [Re: galaxyman]
      #5205317 - 05/04/12 10:26 AM

I love my 6" refractor for DSO's so much so I mounted a 4" f5 on it as a wide field viewer... don't know which one to look through sometimes! Also acts as a brilliant counterweight to the heavy objective end. A 16" Dob would be great too but am still thinking about that one. I am a refractor type will have to admit Matt

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stevetaylor199
sage
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Reged: 09/21/11

Loc: SE Wisconsin
Re: Using refractors for Deep Space Observing [Re: Eddgie]
      #5205690 - 05/04/12 02:07 PM

Says Eddgie:

Quote:


I love my 6" refractor. It is the best wide field scope I have ever ever ever used. And during the summer or winter, cruising the Milky Way with it is an absolute delight.





Interesting. I have a 6" f8 refractor and an 8" f6 reflector (with about a 25% central obstruction). They both have focal lengths of 1200mm. Will they perform equally in this sweeping application? What subtle differences should I notice? (They're both relatively new scopes and haven't seen dark summer skies yet.)


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Napersky
Carpal Tunnel
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Reged: 01/27/10

Loc: Chicagoland
Re: Using refractors for Deep Space Observing [Re: jrbarnett]
      #5260456 - 06/07/12 04:22 PM

Jim,

I echo your opinon. The fact is an almost perfect refractor will outperform a larger "Abberation-Limited" Dob or SCT every-time. The optical flaws of mass produced mirror scopes create the "mushy", cotton balls, and fuzies. Often a superior quality handmade refractor like an AP or Stellarvue will show DS0 detail those larger mirror scopes cannot.

Also it is much harder to test properly especially in the figuring process large uncoated mirrors with interferometry.

Mark


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Eric Gage
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Reged: 12/13/05

Re: Using refractors for Deep Space Observing [Re: Napersky]
      #5261049 - 06/07/12 10:56 PM

Quote:

Jim,

I echo your opinon. The fact is an almost perfect refractor will outperform a larger "Abberation-Limited" Dob or SCT every-time. The optical flaws of mass produced mirror scopes create the "mushy", cotton balls, and fuzies. Often a superior quality handmade refractor like an AP or Stellarvue will show DS0 detail those larger mirror scopes cannot.




Piffle. I love a good refractor as much as the next person (my personal favs were the NP127 and the TOA130), but my medium sized dob (15") blows them away on DSO's. You have to catch the photons before you can see them.


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maknewtnut
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Reged: 10/08/06

Re: Using refractors for Deep Space Observing [Re: Napersky]
      #5261056 - 06/07/12 11:01 PM

Quote:

Jim,

I echo your opinon. The fact is an almost perfect refractor will outperform a larger "Abberation-Limited" Dob or SCT every-time. The optical flaws of mass produced mirror scopes create the "mushy", cotton balls, and fuzies. Often a superior quality handmade refractor like an AP or Stellarvue will show DS0 detail those larger mirror scopes cannot.

Also it is much harder to test properly especially in the figuring process large uncoated mirrors with interferometry.

Mark




The term you might have been trying to refer to is diffraction limited.

To refer to an A-P as a handmade telescope can be argued as accurate. Stellarvue telescopes (at least most, and certainly most all in recent years) use optics made in China, which are also used by several other makers. They are most definitely not hand crafted in any manner that might be considered akin to A-P and TEC.

Your comment about testing uncoated reflective surfaces might be construed as misleading as many superior quality mirrors are effectively tested both during and after figuring.

Edited by maknewtnut (06/07/12 11:19 PM)


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mikey cee
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Reged: 01/18/07

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Re: Using refractors for Deep Space Observing [Re: Eric Gage]
      #5261068 - 06/07/12 11:07 PM Attachment (32 downloads)

All you really need is a 10" F/11 R30 with a little wide field help from a 6" F/8. Which deals with the photons in a way dobs can't.....sharp and to the point! Mike

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maknewtnut
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Reged: 10/08/06

Re: Using refractors for Deep Space Observing [Re: mikey cee]
      #5261102 - 06/07/12 11:22 PM

Perhaps you've never used a high quality Newtonian, or any reflecting or compound telescope for that matter. A well made design with superb optics can and will deliver pinpoint star images, even with that dreaded central obstruction....and all w/o chromatic aberration.

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JimP
Pooh-Bah


Reged: 04/22/03

Loc: South Carolina
Re: Using refractors for Deep Space Observing [Re: maknewtnut]
      #5261434 - 06/08/12 07:17 AM

Well, my answer to the original question, and avoiding the refractor vs larger dobsonian (Why is it always larger and not the same?) discussion, is yes absolutely I do. If you have a 6" scope there is a 10" which can gather more light. If you have a 10" there is a 20" which will gather more light. If you have a 20" there is a 30" which can gather more light... and on and on. Where do you stop? I stop with the refractors I have and love so much. My experience is that a guy with a 6" refractor will be found observing Messier or NGC galaxies while the guy down the road, with his 24"Newtonian, is observing very faint Abel galaxies. The interesting thing to me is that the Abel galaxies in the 24" appear no brighter or more detailed than the galaxies the guy with the 6" is observing!! And, Both observers are very happy!
And, Yes, double stars are indeed Deep Sky objects. In fact, they are my Favorite Deep Sky objects and I love to observe them with my refractors.

best,

JimP


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watcher
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Re: Using refractors for Deep Space Observing [Re: mikey cee]
      #5261480 - 06/08/12 08:09 AM

All you really need? I'm hoping to get by with a little less than that without going to one of those backwards mirror thingies with the spiked stars.

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galaxyman
Vendor - Have a Stellar Birthday
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Reged: 04/04/05

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Re: Using refractors for Deep Space Observing [Re: JimP]
      #5261567 - 06/08/12 09:27 AM

Quote:

Well, my answer to the original question, and avoiding the refractor vs larger dobsonian (Why is it always larger and not the same?) discussion, is yes absolutely I do. If you have a 6" scope there is a 10" which can gather more light. If you have a 10" there is a 20" which will gather more light. If you have a 20" there is a 30" which can gather more light... and on and on. Where do you stop? I stop with the refractors I have and love so much. My experience is that a guy with a 6" refractor will be found observing Messier or NGC galaxies while the guy down the road, with his 24"Newtonian, is observing very faint Abel galaxies. The interesting thing to me is that the Abel galaxies in the 24" appear no brighter or more detailed than the galaxies the guy with the 6" is observing!! And, Both observers are very happy!
And, Yes, double stars are indeed Deep Sky objects. In fact, they are my Favorite Deep Sky objects and I love to observe them with my refractors.

best,

JimP




Bingo Jim

Probably more then most, I've used or pushed my refractors on DSO's, and at the same time own two wonderful larger dobs (12.5" & 22").

I've had the 8" refractor next to the 22" dob, and I enjoyed the DSO views in both of course. The big dob shows the very faint stuff, and great detail on the bright stuff. The 8" refractor can go faint, but the views overall are just shall we say "prettier". M-42 is a great example.

I also like it that once in a while of observing in a comfortable seat instead of going up and down a 6' stepladder.

I've always said.

"It's not what the scope can't see, but what it can see"

In the case of my 3 refractors it's just beautiful images at the eyepiece.


Karl
E.O.H.



Chesmont Astronomical Society - www.chesmontastro.org
Deep Space with Refractors - http://tech.groups.yahoo.com/group/deepspacewithrefractors
Telekit (Swayze optics) 22" F/4.5 Dob
Homemade (Parks Optics) 12.5" F/4.8 Dob
TMB/APM 8" f/9 Refractor”The Beast”. One great DEEP SKY achro
ES 6" f/6.5 achro. Good one
Celestron Omni XLT 102 refractor.
Celestron 10x60mm Binos


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jmiele
Patron Saint?
*****

Reged: 12/04/10

Re: Using refractors for Deep Space Observing [Re: JimP]
      #5261696 - 06/08/12 10:48 AM

Quote:

Well, my answer to the original question, and avoiding the refractor vs larger dobsonian (Why is it always larger and not the same?) discussion, is yes absolutely I do. If you have a 6" scope there is a 10" which can gather more light. If you have a 10" there is a 20" which will gather more light. If you have a 20" there is a 30" which can gather more light... and on and on. Where do you stop? I stop with the refractors I have and love so much. My experience is that a guy with a 6" refractor will be found observing Messier or NGC galaxies while the guy down the road, with his 24"Newtonian, is observing very faint Abel galaxies. The interesting thing to me is that the Abel galaxies in the 24" appear no brighter or more detailed than the galaxies the guy with the 6" is observing!! And, Both observers are very happy!
And, Yes, double stars are indeed Deep Sky objects. In fact, they are my Favorite Deep Sky objects and I love to observe them with my refractors.

best,

JimP





+1 Jim, Well said...

It comes down to the desire of many to have a "one size fits all" strategy. That's not to say that folks using an instrument best suited for a given task or object list, can't learn to push the limits by improving their observing skills.
Even the environment you observe in greatly affects an instruments peak performance/purpose.

You either have a focus and area of interest and purchase and instrument to best meet your requirements given your observing site, or you have an instrument and learn too love what it can do for you.

Joe


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CounterWeight
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Reged: 10/05/08

Loc: Palo alto, CA.
Re: Using refractors for Deep Space Observing [Re: galaxyman]
      #5261806 - 06/08/12 12:02 PM

Last night before the clouds rolled in I took a quick look at Saturn at at about 160x in my TEC160... still and sharp, the atm was co-operating unlike the night before (also clouded over). Popped in the high power and at 320x it was perfect. Sharp / Crisp, 3-D and all the other colorful terms. Sure, getting into tiny exit pupil but I've no big issue with floaters yet for my age. Racing the clouds I popped in a lower wf eye piece and set for M13. Stepped up to about 160x and was nice (though I remeber far better in my big mirrors), lastly swung over to M82 and went back to about 160x again, the fuzzy band again sharp and clear (don't mean for that to sound the contradiction it might sound like). I've been doing this each night there are no clouds to get a perspective on the 3 and comparing largely to only my differing sky conditions. (I've sent my CCD to SBIG for a '~yearly one PM' so imaging not an option until it returns) This before jumping off to anything else if sky conditions allow.

Hard to forget the stunningly beautiful and impressive internal lit chandalier of diamonds (M13) easily seen in my old (well cooled and collimated) C-11 and the disturbed look of M82. I still see these objects in my refractor, and at times I'm still amazed at what I can only call the 'quality' of the view. It's no where near as bright and seems (by comparison to much larger A) only there is not as much to see, but what I see is memorable in it's own way. But 6.5" is still only that, and I guess if you are comfortable with the limitations of any apeture this small or smaller a refractor can and does allow for a lot of enjoyment at the eye piece.

Without reservation though, after a few years with the 160, I can in all honesty admit it's limitations compared to larger apeture scopes as far as what is possible and where. For DSO, I'd say there is a boundry at about 8" no matter the scope type or f/l (though I've never owned an 8" refractor) where the catalog of what is enjoyed and how changes significantly.

Here's the rub when I read these threads. For any object or group of them, it's possible to make statements one way or the other as to if or how might be shown to best advantage. All the optical quality things matter, and there are many to talk about. Over the years I've developed my 'standard candles' of different type objects that I compare in any scope. I think it's an important journey to make, and that you don't need to go up to 30" to get an idea, maybe just 12" would do? No need to be theoretic of abstract or pit one object against another as there will be truth all over the sky, each one it's own. And no surprise that a ~6" refractor can do things a 12" mirror cannot - and things a 12" mirror can do a 6" refractor cannot hope to accomplish (no matter how dark the skies or how much you pay)

IMHO it's not what each type scope can do and what it might be optimal for, it's more what as Eddgie put out about what the apeture can do with the very non abstract or theoretic human eyeball. My own take away is more along the lines of the catalog of potentially enjoyable objects increases with apeture more than the catalog of objects I loose because of FOV. I think the Messier Catalog makes for good standard clandles over time with different apetures as so may are at least detectable in even the smallest scopes. Absolutely under truly dark skies there is much more to talk about with anything, but not many folks I know that are observing day to day have them. Apeture is more a friend on objects outside our solar system, and a great ally in and going outside our galaxy. If it' lensed or reflected they all work, whatever is within your means and ability to use. It's the tradoffs between the objects, views, and apetures that matter - and there's enough to spend a lifetime on at almost any budget.


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Astrojensen
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Reged: 10/05/08

Loc: Bornholm, Denmark
Re: Using refractors for Deep Space Observing [Re: CounterWeight]
      #5262104 - 06/08/12 03:24 PM

Sometimes, it's just about using what one has and loves and feels comfortable with and stop dreaming about castles in the air.



It may not be the finest, most expensive, hand-crafted refractor in the world, but it's mine, it has very good optics and is a pleasure to use. If that is not reason enough to use it on any clear night, I don't know what is.


Clear skies!
Thomas, Denmark


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saemark30
Pooh-Bah


Reged: 02/21/12

Re: Using refractors for Deep Space Observing [Re: Sasa]
      #5262157 - 06/08/12 04:03 PM

Inch per inch refractors are king.
But dollars to donuts Newtonians offer more aperture.

A 10" mirror will perform better than an 8" achromat; and there is maybe only 1 vendor of 8" APOS and they are expensive.


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