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Equipment Discussions >> Classic Telescopes

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Sasa
professor emeritus


Reged: 11/03/10

Loc: Ricany, Czech Republic
Re: Is bigger better? new [Re: DAVIDG]
      #6340761 - 01/28/14 11:30 AM

You are right Dave. Now I always try to perform star-test on newly required optics with green filter. One can definitely quickly recognize a lemon. In case of my current refractors it is splitting hairs. To my untrained eyes, AS80/1200 and AS110 are showing mild spherical aberration (I would say at the level of lambda/6-lambda/8, this is very hard for me to judge), Telementor (C63) is essentially perfect from this point of view but it seems to have very mild astigmatism. I don't recall any issues with my 250mm Newton but the first 150mm one was showing sometimes quite strange patterns with a bump at the edge of one side. I did not know too much about star-tests at that time, but now I interpret it as a sign of tube currents.

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Ziggy943
Post Laureate


Reged: 08/11/06

Loc: Utah
Re: Is bigger better? new [Re: Sasa]
      #6340859 - 01/28/14 12:24 PM

When you say "better", better at what?

Jon gives good answers to the unasked question.

So much depends on the condition of the telescope, not just the aperture. The lack of thermal equilibrium will destroy an image in any telescope. Bigger telescopes take longer to reach that condition.

I have set up the 9" Clark next to larger telescopes many times. Sometimes the Clark gives a better view, i.e., Jupiter, Moon, double stars and sometimes it doesn't.

I don't mind seeing something better than what my 9" can produce.


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bremms
Carpal Tunnel
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Reged: 08/31/12

Loc: SC
Re: Is bigger better? new [Re: DAVIDG]
      #6341262 - 01/28/14 03:19 PM

Dave said it well, A lot large optics are not that great. Most C8's are 1/2 wave or worse. I've had some fine mirrors and a couple of duds. It's simple plop a small refractor down and just get a good image. Not so with a 6-12.5" newt. They take a long time to reach equilibrium. My 6" " F8 Newt has a good mirror and ALWAYS shows more detail than a 60mm, 80mm or 100mm refractor unless the seeing is dreadful. It must be cooled down for 1-2 hours. My 4" Jaegers puts up a little fight, but the 6" reflector has a small CO and no CA.

Bottom line is, alot of larger scopes don't have great optics and/ or have cool down and tube current issues.


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terraclarke
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Reged: 05/29/12

Loc: Just South of the Mason-Dixon ...
Re: Is bigger better? new [Re: bremms]
      #6341290 - 01/28/14 03:32 PM

That's why I am looking forward to getting my 6 inch F10 built. I have a beautiful set of 1/10 wave optics, the primary mirror has a beautiful parabola, I have a very small, elliptical 1/10 wave matched flat, nice cell, spider, focuser, and finder, all waiting for a tube and rings. That will be my summer project.

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Bonco
Post Laureate
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Reged: 04/17/06

Loc: Florida
Re: Is bigger better? new [Re: starman876]
      #6341507 - 01/28/14 05:30 PM

Quote:

Good viewing of the planets is largely dependent on image scale. The longer the focal length along with larger optics is always a good winner. That is why so many F15 systems provide such excellent views of the planets. The C14 with it's 140 inch focal length is a good example. When you get a C14 with good smooth optics and it is dialed in correctly and the seeing is good the view is pretty good. However, no matter what you say a large refractor with good optics providing those pin point stars on a velvet back ground is about as close as to driving a 911 Porsche. There is no substitute.




Personally I don't think there is an answer to you original question. There are too many variables and qualifications. I remember a night where I had one of my best views of Jupiter thru a C14. Next in fairly dark skies, M33. Very dim view the galaxy which filled the eyepiece field even tho it was long focus. The contrast was not impressive. So I move to my 4 inch f/5 refractor and look at M33. Perfectly framed, spiral structure much more evident and contrast superb. Your question is like asking the better of apples and oranges. But I enjoy the thread.
Bill


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photiost
scholastic sledgehammer
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Reged: 12/14/06

Loc: Montreal, Canada
Re: Is bigger better? new [Re: Bonco]
      #6341529 - 01/28/14 05:45 PM

"When you say "better", better at what? "

Better contrast, better resolution and more surface details were visible.

We were also able to see 2 more of Saturns moons with the 12.5 Reflector.

The observation was made around 2am and these scopes had been setup from 8pm so they had all reached thermal equilibrium.



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Joe Cepleur
Carpal Tunnel


Reged: 03/18/10

Loc: Dark North Woods
Re: Is bigger better? new [Re: photiost]
      #6341600 - 01/28/14 06:24 PM

DaveG -- How would I begin to learn to test optics? I'd love to know how good or bad my scopes are! Maybe I should spare you the effort of answering, because I'm unlikely to take apart a scope to bench test it, although I am learning how to star test

Sasa -- Why do you star test with a green filter?

"Image Scale" -- Please could someone explain that? I think it means that, for the same aperture, the smaller field of view of a longer focal length means any given object fills more of the field of view, so more of the scope's resolution is naturally devoted to the object at the image plane. It's already bigger, so it looks bigger with less magnification from an eyepiece. This makes it resistant to becoming over-magnified. It could be (could it be?) that, for the same magnification, smaller aperture with longer focal length could thus get better resolution than bigger aperture with shorter focal length. If true, there should be equations for this.

As to what I buy: Aperture, schmaperture. If I can't lift it, I won't buy it!


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starman876
Postmaster
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Reged: 04/28/08

Loc: VA
Re: Is bigger better? new [Re: Joe Cepleur]
      #6341648 - 01/28/14 06:55 PM

http://starizona.com/acb/ccd/equipbasicsscale.aspx

here is a good explination


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starman876
Postmaster
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Reged: 04/28/08

Loc: VA
Re: Is bigger better? *DELETED* new [Re: Joe Cepleur]
      #6341653 - 01/28/14 06:58 PM

Post deleted by RLTYS

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TCW
Pooh-Bah


Reged: 11/05/13

Loc: The North 40
Re: Is bigger better? new [Re: starman876]
      #6341666 - 01/28/14 07:07 PM

It also depends on what you are observing. Planets or wide field deep space objects, double stars or small, faint fuzzies. All telescope designs have certain compromises and design limitations such as long or short focal length or chromatic aberration. I don't think there is such a thing as the perfect-all purpose scope.

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bremms
Carpal Tunnel
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Reged: 08/31/12

Loc: SC
Re: Is bigger better? new [Re: starman876]
      #6341678 - 01/28/14 07:12 PM

Yes.. for what objects? Planets? M33 is a large LSB object. Sky transparency and large exit pupil are the key. To me, it looks best in a 8-10 inch reflector at 5mm exit pupil. I was observing M1 a good bit lately. It is better a higher magnification from my light polluted back yard. It's barely visible in any thing under 4" and I wasn't able to see it in a good 60mm.

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actionhac
Post Laureate
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Reged: 08/09/08

Loc: Seattle
Re: Is bigger better? new [Re: bremms]
      #6341725 - 01/28/14 07:35 PM

Sometimes the telescope choice is out of our control like right now I switched to a dew resistant newt because of dew.
All forms of finders have been useless lately and my 6" MCT was shut down and then my Edmund 4"f15.
I brought out my RV-6 and I'm OK now except no finder.
I've had some very strange weather.
The RV-6 is stunningly good, funny how we forget. Actualy I knew the RV-6 is very good but I was using a $1000 MCT why bother with the old RV-6? Now I'm scared to bring the Mak back out I think the RV-6 will beat the expensive exotic.

Robert

Edited by actionhac (01/28/14 07:41 PM)


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starman876
Postmaster
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Reged: 04/28/08

Loc: VA
Re: Is bigger better? new [Re: actionhac]
      #6341740 - 01/28/14 07:43 PM

Nothing wrong with a RV6. Really good all around scopes.
Depending on what you are viewing determines the scope selection. However, throw in a high end 6" APO with a focal length of about F7 you have a really good all around scope that will provide detail that will require a lot more aperture in a reflector before you will be able to see the same detail. That is what I mean is bigger better. Also, that black velvet background that many of us love is so hard to obtain with a reflector. The Tak CN212 as a cassegrain provides that black velvet background. That is one fine reflector. That is the only reflector I have seen that in.


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wfj
Pooh-Bah
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Reged: 01/10/08

Loc: California, Santa Cruz County
Re: Is bigger better? new [Re: starman876]
      #6341886 - 01/28/14 09:06 PM

Quote:

However, throw in a high end 6" APO with a focal length of about F7 you have a really good all around scope that will provide detail that will require a lot more aperture in a reflector before you will be able to see the same detail.



Errm. If the CO is less than 20%, no difference on detail same size.
Scattering from the mirror coatings will limit the black background.

Best coatings on both, best baffling ... no diff.


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bremms
Carpal Tunnel
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Reged: 08/31/12

Loc: SC
Re: Is bigger better? new [Re: wfj]
      #6341931 - 01/28/14 09:33 PM

a 6" F8 or F10 Newt with a really good mirror and small secondary will surprise you. Really, I looked through a 6" F9 newt with a 1.25" Quartz diagonal an 1/12 wavefront mirror. It was a dead heat with a AP 6" F12. The AP was a tiny bit better. TINY. My 10" F6 kicked the 6" AP. You need to use a well collimated newt with a GOOD mirror. That AP was a fantastic scope, but I have to agree with WFJ. 6" APO will not best a very good quality 8"+ reflector. Don't compare an AP 6" to a Orion Dob. Compare it to a long focus Zambuto mirror scope.

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bremms
Carpal Tunnel
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Reged: 08/31/12

Loc: SC
Re: Is bigger better? new [Re: bremms]
      #6341937 - 01/28/14 09:37 PM

I will say, a 6" F7 AP would be a killer all around scope. It will outperform an average 8" or maybe a not so good 10" dob.
Nice wide fields.. No too hard to mount. Sure would trade my 6" F10 Jaegers for one.


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starman876
Postmaster
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Reged: 04/28/08

Loc: VA
Re: Is bigger better? new [Re: bremms]
      #6341959 - 01/28/14 09:53 PM

Well, I had a 10" Royce dob with a protostar secondary and tuned to perfection and it could not outdo my 6" AP. The dob was good but not that good. Maybe if you have an exceptional mirror it would be good contest. Even the Intes MN76 I had would not outdo the AP. I have not had any of the other scopes out long enough to do a fair comparison. The Quantum 8 and the Tak CN212 should be a good comparison to the AP. The portaball so far has given me the best view of Jupiter ever. Must have been a rare night of good seeing. I would love to see Jupiter in a 25" dob on a night of good seeing with a great mirror. Yes, bigger is better on nights of good seeing as long as the optics are also excellent to allow seeing that fine detail that only good optics will provide.

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Sasa
professor emeritus


Reged: 11/03/10

Loc: Ricany, Czech Republic
Re: Is bigger better? new [Re: Joe Cepleur]
      #6342309 - 01/29/14 03:25 AM

Quote:


Sasa -- Why do you star test with a green filter?





I'm not sure if one needs green filter for long achromats, probably not, for example star test of C63/840 lens looks good also in white light. But some of my former telescopes were pretty fast, like Stellarvue 80/480mm triplet or Vixen 130 ED SS. There is usually fair amount of spherochromatism in such fast refractors (dependence of spherical aberration on wave length). As a result, the defocused star pattern does not look like in textbooks and it is hard (at least for me) to judge the quality of the optics. With green filter you are checking the optics at the wave length at which the optics was optimized.


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Jon Isaacs
Postmaster
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Is bigger better? new [Re: Sasa]
      #6342358 - 01/29/14 04:13 AM

Quote:

Hello Jon, yes, I meant NGC5053, a globular close to M53. Sorry for the typo. I was chasing it for several nights with 250mm Newton but with no luck. To my surprise, I glimpsed it through ED100 just on the first try from the same location.

It is not important that it is globular. I meant it as a typical member of class of objects with low surface brightness. It was just a first one that came to my mind.




It's been a while since I looked at NGC-5053 but it's pretty easy to find since it's about 1 degree from M53. I have to think it was an issue with light from a street light because it should be easy pickings in the larger scope..

Jon


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Jon Isaacs
Postmaster
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Is bigger better? new [Re: Bonco]
      #6342365 - 01/29/14 04:26 AM

Quote:

Personally I don't think there is an answer to you original question. There are too many variables and qualifications. I remember a night where I had one of my best views of Jupiter thru a C14. Next in fairly dark skies, M33. Very dim view the galaxy which filled the eyepiece field even tho it was long focus. The contrast was not impressive. So I move to my 4 inch f/5 refractor and look at M33. Perfectly framed, spiral structure much more evident and contrast superb. Your question is like asking the better of apples and oranges. But I enjoy the thread.
Bill




That is basically an issue with a large aperture, slow scope. Celestron specs the C-14 at F/11 so the wides possible field of view is about 0.66 degrees. M-33 is about 0.7 degrees x 1.1 degrees, it over flows the eyepiece. But in a similar sized Newtonian working at F/5, it can be properly framed against the dark sky and it can be quite impressive. I like to wander around M33 with my 16 inch F/4.42 (F/5.07 with the Paracorr), there are some interesting details to be seen.

Jon


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