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Equipment Discussions >> Refractors

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hfjacinto
I think he's got it!
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Reged: 01/12/09

Loc: Land of clouds and LP
Re: Using refractors for Deep Space Observing new [Re: Javier]
      #5266955 - 06/11/12 07:19 PM

Quote:

JimP... in Helder's defense I don't think he was calling me stupid. I believe he was referring to the other blanket statement that many people make that refractors are simply better for any and all objects (which I don't agree with either). I've known Helder for about three plus years and other than while poking fun at each other I've never heard him insult anyone or me.

Jav




Jav nailed it, I never called anyone stupid, what is stupid is the argument that refractors are the best scope. All scopes do things well, don't demean other scopes because they are not refractors.


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Ira
Carpal Tunnel
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Reged: 08/22/10

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Re: Using refractors for Deep Space Observing new [Re: hfjacinto]
      #5267041 - 06/11/12 08:39 PM

Don't you think it's stupid to refer to SCTs as "Fast Food" instruments. Really, that's just ridiculous.

/Ira


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hfjacinto
I think he's got it!
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Reged: 01/12/09

Loc: Land of clouds and LP
Re: Using refractors for Deep Space Observing new [Re: Ira]
      #5267167 - 06/11/12 09:55 PM

Quote:

Don't you think it's stupid to refer to SCTs as "Fast Food" instruments. Really, that's just ridiculous.

/Ira




I think SCTs are excellent scopes. Just repeating what some one else posted. http://www.cloudynights.com/ubbthreads/showthreaded.php/Cat/0/Number/5265894/page/0/view/collapsed/sb/5/o/all/vc/1

I agree the comment was ridiculous, that's why I reposted it. I can't believe no one else caught it.


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cheapersleeper
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Reged: 01/22/10

Loc: Sachse TX
Re: Using refractors for Deep Space Observing new [Re: hfjacinto]
      #5267199 - 06/11/12 10:15 PM

I saw it and typed out a response but removed it because I felt it would escalate the nastiness.

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JJK
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Reged: 04/28/08

Re: Using refractors for Deep Space Observing new [Re: Jon Isaacs]
      #5267265 - 06/11/12 10:44 PM

Quote:

Quote:

ontrast than a smaller unobstructed scope with less clear aperture.

SNIP

Eddgie, there's no doubt that a much larger reflector (Newt,SCT) will collect more light than a smaller refractor, and thus will bemore useful for detecting fainter DSOs. However, if a refractor and compound reflector have the same clear aperture, the refractor will exhibit better contrast on extended objects (especially assuming we're comparing a top-notch refractor against a fastfood SCT). The SCT's modulation transfer function is compromised because its secondary transfers some of the light energy from the central portion of the Airy disk to the outer rings. The only thing that artifact aids is the separation of double stars of nearly thesamemagnitude. It cannot enhance the contrast of extended objects or the visual separation of double stars of much different magnitude.




Inch for inch, the refractor does have the advantage for a number of reasons. But in general refractors are small, cats and Newts are large so the fact that the CO moves energy from the central disk to the rings is less important than the fact the smaller aperture increases the size of the disk. So, if one is comparing a 5 inch refractor to a 10 inch SCT, the rings are brighter but the first ring fits inside the disk of the 5 inch.

This is relevant for planets and bright objects, with dsos it is unlikely that the eye resolving details on this fine scale. But if it is, the MTF scaled to include aperture would give the contrast advantage to the 10 inch.

Jon




Jon, in my Quote reply, I mentioned OTAs of the same clear aperture, not a comparison of a 5" refractor and a 10" SCT.

However, theoretical considerations aside, based on my experience, I would prefer to use my 130 mm Zeiss APQ than a C11 to view the Moon or planets. In the former, the views are etched (e.g., Lunar shadows are jet black) and the contrast is simply stunning.

I'm not anti-compound telescopes. I own and use several, and I look forward to the day when I can also use a cherry picked C14.


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JJK
Carpal Tunnel


Reged: 04/28/08

Re: Using refractors for Deep Space Observing new [Re: Ira]
      #5267281 - 06/11/12 10:52 PM

Quote:

Don't you think it's stupid to refer to SCTs as "Fast Food" instruments. Really, that's just ridiculous.

/Ira




Ira, I used the term "fast food" SCTs. It wasn't meant to be ridiculous. I meant it to reflect how common (i.e., readily available) some scope types are. I'm sorry if you found the term offensive. As I mentioned here and elsewhere, I look forward to the day I can add a C14 to the arsenal.


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JJK
Carpal Tunnel


Reged: 04/28/08

Re: Using refractors for Deep Space Observing new [Re: hfjacinto]
      #5267330 - 06/11/12 11:35 PM

Quote:

Quote:

Don't you think it's stupid to refer to SCTs as "Fast Food" instruments. Really, that's just ridiculous.

/Ira




I think SCTs are excellent scopes. Just repeating what some one else posted. http://www.cloudynights.com/ubbthreads/showthreaded.php/Cat/0/Number/5265894/page/0/view/collapsed/sb/5/o/all/vc/1

I agree the comment was ridiculous, that's why I reposted it. I can't believe no one else caught it.




You misinterpreted my comment. IIRC, the term "fast food" OTAs may have originated with Roland Christen in a discussion about how his Mak- Cass performed (thermally?) compared to COMMONLY AVAILABLE SCTs. That was the spirit in which I used it in this thread. It was not intended to suggest that SCTs (or any other scopes) are not worthy of consideration. I have several Mak-Cass OTAs (Meade-Wegat 125mm & A-P's 10"), and hope to get a C14.

Should I misinterpret your comment that "SCTs are excellent telescopes" as a suggestion that refractors aren't? Of course not.

If you have an issue with something I or anyone else makes, why not simply ask for clarification? With this relatively casual medium, it's easy to be less than perfectly clear or to misinterpret someone's intention.


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

Loc: Limerick, Pa
Re: Using refractors for Deep Space Observing new [Re: JoeM101]
      #5267395 - 06/12/12 01:03 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?




Joe

That's an interesting question.

Last dark window we (some CAS members) were observing at a good dark site. We had 18", 22", and 25" dobs, and I had my 8" refractor. A non-member who we see occasionally had his new 3.5" APO.

Though the much bigger scopes were providing terrific views, this fellow and his smaller refractor had a number of great DSO views, that many would find very appealing.

I also own a 4" f/10 refractor, which I've grown quite fond of, though it's by far my smallest scope.

One night some months back I was out at a local dark site with the 4". The views here again were quite appealing. In fact I kind of made a mistake the following evening to the wife when I said "You know, I could be quite happy with the 4" refractor for my observing (DSO only) if that's all I could either afford or manage".

Thankfully she's a smart gal, and knew what I meant. I hope

So, this could be a very subjective question, but I think from good dark skies a good 4" refractor is a quality instrument for DSO observing. As I mentioned before that the late Walter Scott Houston of S&T (Deep Sky Wonders column) wrote a lot about using his 4" refractor on a number of objects.


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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Astrojensen
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Reged: 10/05/08

Loc: Bornholm, Denmark
Re: Using refractors for Deep Space Observing new [Re: JoeM101]
      #5267466 - 06/12/12 02:49 AM

Quote:

piggyback solution, refractor on reflector action!




Naughty! What if they crossbreed? What will that turn into? Mak-newton? Mak-cass? SCT?




Clear skies!
Thomas, Denmark


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Astrojensen
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Reged: 10/05/08

Loc: Bornholm, Denmark
Re: Using refractors for Deep Space Observing new [Re: JJK]
      #5267467 - 06/12/12 02:53 AM

Quote:

I used the term "fast food" SCTs. It wasn't meant to be ridiculous.




Don't we all like to get a burger with fries once in a while?

Got a very nice C8 myself.


Clear skies!
Thomas, Denmark


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JJK
Carpal Tunnel


Reged: 04/28/08

Re: Using refractors for Deep Space Observing new [Re: Astrojensen]
      #5267600 - 06/12/12 07:47 AM

Quote:

Quote:

I used the term "fast food" SCTs. It wasn't meant to be ridiculous.




Don't we all like to get a burger with fries once in a while?

Got a very nice C8 myself.


Clear skies!
Thomas, Denmark




Only if the burger is from Mak-Cass Donald's? ~\8^)


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Jim Curry
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Reged: 10/29/07

Loc: STL
Re: Using refractors for Deep Space Observing new [Re: Astrojensen]
      #5267714 - 06/12/12 09:20 AM

>Ultimately it's the "object" which determines what kind of telescope the observer should be using. Beginners...<

While it appears that most folks on these forums are using multiple scopes for their various astro endevours I'd wager a chocolate chip cookie that: 1. A beginner will be thrilled exploring any/all objects in the night sky with anything from 4" on up, the only limitation being portability in and out of the house or apartment; and 2. Moving away from this forum I'll bet most folks only own one scope which obviously is satisfactory for use on any object.

Now, once experience is gained and there's a few nickels in the pocket...the sky's the limit, so to speak.

Jim


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Jon Isaacs
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Using refractors for Deep Space Observing new [Re: JJK]
      #5267802 - 06/12/12 10:14 AM

Quote:

Jon, in my Quote reply, I mentioned OTAs of the same clear aperture, not a comparison of a 5" refractor and a 10" SCT.




I was aware of your stipulation and pointed out that I was in agreement that inch for inch a refractor will provide better contrast.

But many seem to assume that refractors have better contrast regardless of aperture and that is a mistaken assumption, the effect of a central obstruction is small in comparison to the effect of the outer obstruction. (aperture)

From where I sit, I look at Newtonians as providing the optimal larger aperture high performance views. One can buy near perfect optics and build scopes that are actively cooled and free from thermal issues and yet remain manageable and portable.

Jon


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PhilCo126
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Reged: 01/14/05

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Re: Using refractors for Deep Space Observing new [Re: Jon Isaacs]
      #5267949 - 06/12/12 11:53 AM

Loose from the basic beginners' rule (deepsky = reflector), one needs at least a 152 mm refractor to get a good look at DSO.
Therefor I wonder why it's so hard to find a COTS larger diameter achromatic refractor ( e.g. short tube 175 mm )
Scopes larger than 180 mm are "Special order" so out of most of us' league


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Jon Isaacs
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Reged: 06/16/04

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Re: Using refractors for Deep Space Observing new [Re: PhilCo126]
      #5267970 - 06/12/12 12:02 PM

Quote:


Loose from the basic beginners' rule (deepsky = reflector), one needs at least a 152 mm refractor to get a good look at DSO.




I am one those folks who just enjoys the particular scope I happen to be using... I don't "need" anything.. My 60mm F/7 or mJoe's 72mm refractor is sufficient aperture for many objects, my 25 inch is not sufficient for many objects..

Jon


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JoeM101
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Reged: 04/09/12

Loc: 45.66086, -73.54702
Re: Using refractors for Deep Space Observing new [Re: Jon Isaacs]
      #5267979 - 06/12/12 12:12 PM

Quote:

Quote:


Loose from the basic beginners' rule (deepsky = reflector), one needs at least a 152 mm refractor to get a good look at DSO.




I am one those folks who just enjoys the particular scope I happen to be using... I don't "need" anything.. My 60mm F/7 or mJoe's 72mm refractor is sufficient aperture for many objects, my 25 inch is not sufficient for many objects..

Jon




I agree with Jon, my 72 is awesome for most viewing! though i would love to be able to pick up a 130mm TMB someday.. maybe i play the lottery


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saemark30
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Reged: 02/21/12

Re: Using refractors for Deep Space Observing new [Re: JoeM101]
      #5268126 - 06/12/12 01:44 PM

It depends on the sky brightness.
A 5" refractor at a Bortle 1 location would show more than a 16" in mag 5 skies, I would think.
But for GC its all about aperture. You can use high mags to darken the sky but the stars are just points and stays bright.
An 80mm refractor can never outresolve a 10" Newtonian on GC like M13, M5, etc.


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PhilCo126
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Reged: 01/14/05

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Re: Using refractors for Deep Space Observing new [Re: saemark30]
      #5268173 - 06/12/12 02:20 PM

(Apo-)Refractors smaller than 150 mm are very suitable for DSO astrophotography but I don't know how M31 would look like in a 70 mm refractor for visual observing

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JoeM101
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Reged: 04/09/12

Loc: 45.66086, -73.54702
Re: Using refractors for Deep Space Observing new [Re: PhilCo126]
      #5268204 - 06/12/12 02:42 PM

Quote:

(Apo-)Refractors smaller than 150 mm are very suitable for DSO astrophotography but I don't know how M31 would look like in a 70 mm refractor for visual observing




not very good if anything


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Alan French
Night Owl
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Reged: 01/28/05

Loc: Upstate NY
Re: Using refractors for Deep Space Observing new [Re: JoeM101]
      #5268246 - 06/12/12 03:06 PM

Quote:

Quote:

(Apo-)Refractors smaller than 150 mm are very suitable for DSO astrophotography but I don't know how M31 would look like in a 70 mm refractor for visual observing




not very good if anything




Under dark skies, M31 spans 3 degrees. What do you need to see a field large enough the show it all, surrounded by dark sky? The answer is definitely not a lot of aperture and the resulting long focal length.

Clear skies, Alan


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