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Equipment Discussions >> Refractors

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JJK
Carpal Tunnel


Reged: 04/28/08

Re: Using refractors for Deep Space Observing new [Re: saemark30]
      #5270197 - 06/13/12 08:13 PM

Quote:

Even for deep sky observing optical quality matters unless we are viewing wide angle objects at 30x.
Proper baffling is important and that is why refractors have an advantage over truss systems or reflective tubes.
But large dobs make up for that with lots of aperture.
Its amazing what a 10" or larger scope shows.
Even in the city Planetary Nebulas look good and OIII and UHC filters can work magic.
It would take a world class site to get the full resolution out of a 20" mirror.




I use my (80 mm to 180 mm) and A-P 10" Mak-Cass at home (mag 5.8 skies at best), but the 20" Obsession was really quite impressive under mag 6.4 skies in WV when they were transparent (most often the seeing was good but not world-class). I never worried about wringing out the full resolution of that scope. The scope's light grasp and excellent optics provided bright views with tight stars and colorful nebulae. It was a pleasure to use. I need to find a place to use the 25" scope that replaced the Obsession 20".


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JimP
Pooh-Bah


Reged: 04/22/03

Loc: South Carolina
Re: Using refractors for Deep Space Observing new [Re: JJK]
      #5270279 - 06/13/12 09:18 PM

Sounds fantastic JJK. And, of course, when you head out to the Dark Sky site you only take your big dob. That helps. You don't have to decide which scope to use or what type objects to view. I'd love a peak through that 25"! I'm sure it is fantastic!!

JimP


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Napersky
Carpal Tunnel
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Reged: 01/27/10

Loc: Chicagoland
What is inherently flawed with Reflectors and why new [Re: JimP]
      #5587367 - 12/24/12 03:06 PM

Mirrors. Every time light reflects it doubles the error of the wavefront.

We all know that a 1/8 PV surface error is 1/4 wavefront error effectively doubling all abberations and this is just the primary!

Now the 1/4th wave hits the secondary and doubles the error again to 1/2 wavefront error. If the scope is a straight thru Cassigrain then that's what you get 12",14", 16" highly abberated wavefront. If there is a diagonal involved double the error again.

This is why I do not so much like "Folded Refractors."

Mark

I am not an optical physicist so show me where I am wrong.


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JJK
Carpal Tunnel


Reged: 04/28/08

Re: Using refractors for Deep Space Observing new [Re: JimP]
      #5587497 - 12/24/12 04:43 PM

Quote:

Sounds fantastic JJK. And, of course, when you head out to the Dark Sky site you only take your big dob. That helps. You don't have to decide which scope to use or what type objects to view. I'd love a peak through that 25"! I'm sure it is fantastic!!

JimP




The 25" is very nice. It really needs to stay at a dark site though. Perhaps I can coax one of my friends with places in WV to house it for me (and them).

I looked through John Vogt's 32" Dob-Newt at the 2012 Black Forest Star Party. The views were stunning. The Swan looked like it was fluorescent.

I was outside of Brussels this past August. One of the professors I had dinner with knows of someone who is making a 1.1 m scope. I think it'll be housed in France. Now there's four excellent reasons to visit there (the art, wine, food, culture, and the 1.1 m)!


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aa6ww
scholastic sledgehammer
*****

Reged: 10/23/11

Loc: Sacramento, Calif.
Re: Using refractors for Deep Space Observing new [Re: JoeM101]
      #5590726 - 12/27/12 05:05 AM

Quote:

I have a question for all you refractor folks.. what would be the minimum aperture (for a refractor) to do decent DSO viewing?




I have plenty of fun using my TV-85 on deep space objects under dark skies. Lately however. I find my 6" F/5 Celestron Omni XLT is getting the most useage since its very compact and portable and 6" is no slouch!!

...Ralph


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aa6ww
scholastic sledgehammer
*****

Reged: 10/23/11

Loc: Sacramento, Calif.
Re: Using refractors for Deep Space Observing new [Re: mikey cee]
      #5590727 - 12/27/12 05:10 AM

Quote:

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




Every time I see this picture, the mushroom cloud just keeps expanding. Very Nice!!

...Ralph in Sacramento.


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aa6ww
scholastic sledgehammer
*****

Reged: 10/23/11

Loc: Sacramento, Calif.
Re: Using refractors for Deep Space Observing new [Re: Astrojensen]
      #5590730 - 12/27/12 05:19 AM

Quote:

Quote:

Aperture doesn't rule, looking up and observing does.....




Bravo!


Clear skies!
Thomas, Denmark





Triple Dog Bravo!!!

Ralph in Sacto.


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aa6ww
scholastic sledgehammer
*****

Reged: 10/23/11

Loc: Sacramento, Calif.
Re: Using refractors for Deep Space Observing new [Re: aa6ww]
      #5590734 - 12/27/12 05:27 AM

I'm noticing now, with winter here and temperatures dipping down into the 30's and below, (that's about as cold as it gets out here where I'm at in calif,) my SCT's sit tight in their trunks and my refractors become the scopes of choice to use. The portability and cool down factor plays into this mostly. I wonder if more refractors are sold in the winter time than reflectors or SCT?

I do more solar observing in the winter than spring and summer also, again with refractors, and even my smaller scopes like my TV-85 becomes less of a piggy back scope and more of a main scope when the weather is colder.

...Ralph in Sacto.


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Astrojensen
Post Laureate
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Reged: 10/05/08

Loc: Bornholm, Denmark
Re: What is inherently flawed with Reflectors and why new [Re: Napersky]
      #5590736 - 12/27/12 05:31 AM

Quote:

We all know that a 1/8 PV surface error is 1/4 wavefront error effectively doubling all abberations and this is just the primary!

Now the 1/4th wave hits the secondary and doubles the error again to 1/2 wavefront error.




If the secondary is of a higher accuracy than the wavefront that hits it, then the error of that wavefront won't be doubled, though it will still be affected. If both mirrors are 1/10th wave on the surface, then the final image will be 1/4th wave on the wavefront.

That is at least what my logic tells me.


Clear skies!
Thomas, Denmark


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Jon Isaacs
Postmaster
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: What is inherently flawed with Reflectors and why new [Re: Astrojensen]
      #5590768 - 12/27/12 06:47 AM

Quote:

Quote:

We all know that a 1/8 PV surface error is 1/4 wavefront error effectively doubling all abberations and this is just the primary!

Now the 1/4th wave hits the secondary and doubles the error again to 1/2 wavefront error.




If the secondary is of a higher accuracy than the wavefront that hits it, then the error of that wavefront won't be doubled, though it will still be affected. If both mirrors are 1/10th wave on the surface, then the final image will be 1/4th wave on the wavefront.

That is at least what my logic tells me.


Clear skies!
Thomas, Denmark




I think if both are 1/10th wave then the result would be 1/5th wave in one orientation and 1/7 wave in the other. This is because of the 45 degree angle on the diagonal.

Of course a refractor has at least 4 surfaces to figure. While the error for each surface is multiplied by n-1, usually a bit more than 0.5. Since there are a minimum of 4 surfaces this adds up to something greater than a factor of 2.

If high quality (or average quality) refractor optics were easily made, they would be cheap. The fact that a decent quality 8 inch achromatic objective from D&G costs about $2400 is indicative of the difficulty. An apochromatic objective is considerably more. For comparison, an 8 inch Zambuto mirror costs $990, a 14.5 inch costs about $2600...

Jon


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Asbytec
Guy in a furry hat
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Reged: 08/08/07

Loc: La Union, PI
Re: What is inherently flawed with Reflectors and why new [Re: Jon Isaacs]
      #5590790 - 12/27/12 07:46 AM

Sure, 1/8th on the primary surface is 1/4 P-V. Then, using a 1/8th diagonal might seem to add another 1/4 wave error to the wavefront making the wave 1/2 P-V before it strikes the eyepiece. Even in the simplest forms, the light must still transit 6 to 8 air to glass surfaces at the eyepiece. Then two more refractive surfaces in the eye. So, by the time the wave hits the retina, it appears to have more than 1 wave of aberration. No one sees this in any star test, which includes the eyepiece.

That level of aberration would make for a terrible view. And since this is not the case with almost every scope, there is a hole in this argument. I just cannot find a source to rebut it, yet.

The answer lies, I believe in the image forming optics as opposed to the flat diagonal. The waveform is made at the primary (reflectors have only one wave forming surface), that is where it varies from perfect reference sphere due to surface error. On the diagonal, the errors might be present, but they do not fundamentally reshape the wavefront. And if they do, it's likely a localized correction within the entire wavefront that may or may not be seen.

Accounting for the wavefront error between 4 to 6 optical surfaces of an APO, even at 1/20th wave smoothness, that would still add up to 4 to 6/20th's = 1/5th to 1/4th P-V error (depending on the type, coverage, and slope of the error, of course, but just assuming smooth SA.) Then that error travels through an eyepiece and they eye, too. So, if the math holds, even the best APO will be below 1/4 wave Raleigh limit.

Again, since this is not true, something is wrong with this argument. Jon may have hit on it above.

In any case, there is nothing wrong with a good parabola. In fact, it can produce the ideal spherical wavefront. Where as highly curved surfaces of APOs and my own Mak have a lot of higher order SA to deal with. Refractors deal with color on top of that.

There are no perfect scopes, just really darn good ones like a good quality refractor. However, reflectors probably have fewer inherent flaws in terms of correction.


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Jon Isaacs
Postmaster
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: What is inherently flawed with Reflectors and why new [Re: Asbytec]
      #5590980 - 12/27/12 10:57 AM

Quote:


Sure, 1/8th on the primary surface is 1/4 P-V.




I believe the precision of the mirror is typically reported as the wave front error rather than the surface error. Maybe Vla can help us here.

In terms of wave front error in a refractor, I believe Roland Christen uses a standard of 1/8-1/10 wave using an interferometer for his lenses.

The important wave errors are measured at the focal plane at the focal plane because the entire objective contributes to every point on the focal plane so the objective (and Newtonian secondary mirror) errors are summed at every point. The eyepiece only magnifies that image on a point by point basis, bringing your eye closer to the image, the eyepiece does not affect the overall wavefront error.

Jon


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doway1609
newbie


Reged: 05/17/10

Re: Using refractors for Deep Space Observing [Re: Jon Isaacs]
      #5603653 - 01/03/13 08:17 PM

"Aperture rules???" Well, that is IF you have all the time and money in the world plus a DARK SKY, then it can be so, but not for all folks as we know, I live in mag 3.5 skies and i had an 8inch dob but it was to cumbersome for me to drag it down 3 stairs and out the backyard, plus i had to use it MANUALLY in light polluted skies which is VERY inconvienient so i had to sell it, i have 3 scopes and the biggest is my 4" F7.7 refractor and its a GEM! Its has medium size objective lens, space objects are crisp and sharp especially moon, planets and star clusters, and its not very heavy so i can use it on my DS 2000 go-to mount and serves me very well! So more aperture is better but thats for a rabid deep sky fan that has lots of time and money on their hands plus scopes more than 8 or 10 inches dont work very well in light-polluted skies and here in Jersey transporting a massive scope to a dark site is a MAJOR adventure!

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Jon Isaacs
Postmaster
*****

Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Using refractors for Deep Space Observing new [Re: doway1609]
      #5604208 - 01/04/13 08:02 AM

Quote:

So more aperture is better but thats for a rabid deep sky fan that has lots of time and money on their hands plus scopes more than 8 or 10 inches dont work very well in light-polluted skies




I often use scopes 8, 10, 12.5 inches and even larger from my light polluted back yard... They definitely can out perform my 4 inch apo on the DSOs. Aperture is my friend when it comes to fighting light pollution...

Jon


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Scott BeithAdministrator
SRF
*****

Reged: 11/26/03

Loc: Frederick, MD
Re: Using refractors for Deep Space Observing new [Re: Jon Isaacs]
      #5604881 - 01/04/13 02:06 PM

While DSOs are not my primary point of interest, I do enjoy checking out the showpiece ones when I observe. Since all of my scopes are refractors, I guess I qualify for this thread by default.

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csrlice12
Postmaster
*****

Reged: 05/22/12

Loc: Denver, CO
Re: Using refractors for Deep Space Observing new [Re: Scott Beith]
      #5605155 - 01/04/13 04:29 PM

Dob folks use refractors for finding DSOs all the time....

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coutleef
Post Laureate
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Reged: 02/21/08

Loc: Saint-Donat, Quebec, Canada
Re: Using refractors for Deep Space Observing new [Re: csrlice12]
      #5605166 - 01/04/13 04:36 PM

wide DSOs benifit from a wide field refractor while faint DSOs benifit from aperture.

there is definitely a place for refractor for viewing DSOs


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Napersky
Carpal Tunnel
*****

Reged: 01/27/10

Loc: Chicagoland
Re: What is inherently flawed with Reflectors and why new [Re: Asbytec]
      #5606583 - 01/05/13 01:04 PM

Accounting for the wavefront error between 4 to 6 optical surfaces of an APO, even at 1/20th wave smoothness, that would still add up to 4 to 6/20th's = 1/5th to 1/4th P-V error (depending on the type, coverage, and slope of the error, of course, but just assuming smooth SA.) Then that error travels through an eyepiece and they eye, too. So, if the math holds, even the best APO will be below 1/4 wave Raleigh limit.

Again, since this is not true, something is wrong with this argument. Jon may have hit on it above.

In any case, there is nothing wrong with a good parabola. In fact, it can produce the ideal spherical wavefront. Where as highly curved surfaces of APOs and my own Mak have a lot of higher order SA to deal with. Refractors deal with color on top of that.

There are no perfect scopes, just really darn good ones like a good quality refractor. However, reflectors probably have fewer inherent flaws in terms of correction.






This doubling of the error only occurs with mirrors not with refractive surfaces. This might help explain why APOs produce such good images.

As far as the diagnonal, it's a mirror and should double the error but as Jensen mentioned if the diagonal were twice as good as the wavefront off the secondary then the error would be cancelled out. 1/4 wave off the secondary could be truely 1/4 wave to the eyepiece.

Mark


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galaxyman
Vendor - Have a Stellar Birthday
*****

Reged: 04/04/05

Loc: Limerick, Pa
Re: Using refractors for Deep Space Observing new [Re: coutleef]
      #5607548 - 01/05/13 11:38 PM

Quote:

wide DSOs benifit from a wide field refractor while faint DSOs benifit from aperture.

there is definitely a place for refractor for viewing DSOs




Yep, use all 3 of my refractors for DSO's both wide-field and high power. In fact much of our video series (Galaxy Log) has refractors being used for many observations, particularly with the use of small and medium size scopes.


Karl
E.O.H.

Chesmont Astronomical Society - www.chesmontastro.org
Galaxy Log - http://www.youtube.com/user/GalaxyLog4565?feature=mhee
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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