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Does your reflector give "refractor like" images?

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#51 FirstSight

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Posted 03 April 2013 - 02:07 PM

If I have a lot of time I like to setup both a refractor and my 10 inch dob. Dob for DSOs (and planets when seeing is good)...and refractors for wide fields and planets.


I agree that a 10" to 15" reflector and a really good 4" refractor are the perfectly complementary combination. I often set up my TV NP-101 alongside my 12" reflector, and bounce back and forth between them all night, sometimes comparing images but much of the time soaking in the panoramic widefield view in the NP-101 plus some 150x-ish view of planets, and individual DSOs in the 12" reflector.

#52 Sean Puett

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Posted 03 April 2013 - 03:04 PM

I would answer yes for the most part. Paracorr helps make my reflector coma free. Decent eyepieces don't introduce flaws of their own. As long as collimation is dead on, I would say it is very refractor like. It is NOT a refractor. It still shows spikes on the brightest stars. I find some beauty in the spikes anyway. There is no field curvature. My reflector has perfect color correction. There is no hint of false color, focused or not. My reflector has a big problem though showing so many extra stars. Globs look bright and contain individual stars, not like a grey smudge. Talk about diamonds, a nicely resolved glob under dark skies, that is diamonds on black velvet.
So, why do I still own and enjoy a refractor? 4.5*tfov and nice flat field. I use it to help locate dso that I am not sure how to find yet. Scanning the milky way with a 4"f5.4 refractor is one of the best ways to spend part of a summer night. When you start to get tired but, you are not yet resigned to the zero gravity chair and binoculars. Carefree pointing of the refractor I mentioned will help give you a second wind. Once that runs out it is time for the chair and binoculars before bed...

#53 nevy

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Posted 03 April 2013 - 06:04 PM

I had a refractor once , didn't like it , sold it , I like dobs.

#54 buddyjesus

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Posted 03 April 2013 - 07:18 PM

But then there's thermal issues. At worst the column of air in front of the mirror is a defocusing lens the refractor camp doesn't have issue with to the same degree to say nothing of the sheer thickness of them. I'd place thermals as a close second behind large CO contrast degredation.

Its almost silly when you consider the thermal issues a reflector has to endure...

Imagine an apo where an observing buddy is breathing on the side of your dewshield every time you look in the ocular. That's no different than what happens at the reflector focuser when observing.

Imagine you have a push-to refractor and every time you need to shift it you reach up with your hand on the dew shield edge to nudge it with all the thermals pouring off that - something I think we've all seen with reflectors - particularly in out if focus stars on collimating.

Then there's the length of a dew shade on an apo compared to the length of a truss or closed tube. It's simply more room to further create the lensing effect of this warmer air.

The sheer thickness of the glass is what Gary Seronik refrs to has the effect of having a heat battery. Even a very large apo at least has the glass in thinner sections*.

Anyway, having dealt with and been (finally) successful for the most part dealing with the defocusing effects of thermals Im a true believer in it being one if the serious potential detriments to reflecting systems when left unabated.

Pete
*theres been mention apos over 7" have very long cool down issues but this doesn't seem like it has to be if there is air between the objective elements why not a cell that blows air through these spaces and be done with it. A reflector should only have such an easy option.


I think you and I have the same solution in mind Pete. a small scope to complement a big one.

#55 Thomas Karpf

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Posted 03 April 2013 - 08:55 PM

My 12.5" always beats the 5" and 6" apos in the field, but that's not really a fair comparison. Alas, I have no 10-12" apo refractors handy to compare to it. But, for the price of my high-end 12.5" f/5 with Zambuto mirror, I could have bought a nice 4" apo on a cheap mount.


The fun part about any sort of comparison is the idea of 'apples to apples'.

By cost ($1000 max)? An 8-10" dobsonian vs a 4-5" achro on an inexpensive mount. Dobsonian wins hands down.

Aperture? 8" reflector vs 8" refractor, or 4" reflector vs 4" refractor. Refractor wins, but who the heck really wants to try and transport and mount an 8" refractor, or wants to use a 4" reflector?

Tube length? A 2' long SCT is a 12" diameter scope, which beats everything else.

#56 azure1961p

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Posted 03 April 2013 - 09:15 PM

If it was April 1st again I might have the courage to goto the refractor forum and start a "Does your refractor give reflector like images?" thread. :lol:


You would have probably gotten a Moderator Alert for stirring up trouble. :roflmao:

David


Yeah but it would've been funny till it got locked.

Edit: Ok I took the plunge and posted it in the refractor forum.



Pete

#57 GOLGO13

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Posted 03 April 2013 - 10:07 PM

I think one thing that is often overlooked is comparing apples to apples quality levels between reflectors and refractors. For instance, a lot of people seem to compare a 4 inch Televue or other top end company to an Orion 10 inch dob. I'm not saying there is anything wrong with an Orion 10 inch dob (love mine), but it's not a high end dobsonian. A better comparision would be a dob which has been optimized design wise and has a high quality mirror. You could even compare those in terms of cost. A high end 10 inch dob can cost as much as a high end 4 inch apo.

People really should look through as many scopes and designs as they can to appreciate what type of scope they would like. A friend of mine has a Maksutov Newtonian which provides absolutely awesome views. It's not your normal scope design talked about on these forums, but it's a great scope design. Aperture is great, but there is also something to be said for grab and go.

A lot of it comes down to what are you using the scope for. There are many objects where a small 4 inch APO would be the best scope for the job. There are many other objects (faint DSOs) where a very large newt would be the best option (especially for us folks with limited funds). A small refractor is a nice travel scope.

Personally I like all scope designs and wouldn't mind owning one of each. I have much more of an appreciation for refractors now than I did when I began observing. But I still understand the value of aperture. Best views of planets I have had were through a 30 inch F3.6ish dob in need of a new coating. We often debate aperture from 6 inches to 8 and from 8 inches to 10. The big differences are going from 8 to 16. That's when you really get an appreciation for aperture.

#58 Datapanic

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Posted 04 April 2013 - 01:30 AM

Talk about stirring up trouble! Those that like the Reflectors, Refractors, SCT's and Classics forums are going to catch on to the conspiracy rather quickly! But meanwhile...

:gotpopcorn:

#59 buddyjesus

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Posted 04 April 2013 - 04:18 AM

haha

#60 jpcannavo

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Posted 04 April 2013 - 04:19 AM

I think one thing that is often overlooked is comparing apples to apples quality levels between reflectors and refractors. For instance, a lot of people seem to compare a 4 inch Televue or other top end company to an Orion 10 inch dob. I'm not saying there is anything wrong with an Orion 10 inch dob (love mine), but it's not a high end dobsonian. A better comparision would be a dob which has been optimized design wise and has a high quality mirror. You could even compare those in terms of cost. A high end 10 inch dob can cost as much as a high end 4 inch apo.

People really should look through as many scopes and designs as they can to appreciate what type of scope they would like. A friend of mine has a Maksutov Newtonian which provides absolutely awesome views.


I think this is an excellent point. It's one thing to ask "does the typical reflector on the field match the typical refractor". Its quite another to ask how the best examples of each compare. I think this bears on the whole Mak-Newt performance thing as well. Setting aside the differences in thermal issues - height above ground, open vs. closed tube etc - there is no intrinsic reason why the on-axis performance (specifically the modulation transfer function) of a spherical mirror with a meniscus corrector should exceed that of a single paraboloid. Yes, central obstruction is often mentioned, i.e. the "tiny 20% obstruction". But lots of dobs have similarly "tiny" obstructions. What about spiders? Well, apart from the aesthetics of spikes, the impact on the MTF with thin veins is minimal. I think the real difference comes down to overall quality. these scopes are typically made - as are Apos - to a far higher standard than a similarly mass produced dob. And this is true in all respects: optical surfaces, tube baffling etc. And, in the case of spherical surfaces, high optical standards are more easily achieved.

#61 David Pavlich

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Posted 04 April 2013 - 05:07 PM

I think one thing that is often overlooked is comparing apples to apples quality levels between reflectors and refractors.


Very common. I always question when someone wants to compare a basically handcrafted refractor to an off the shelf Newt or SC and do it in an apples to apples context. And 9 times out of 10, it's comparing planetary views. That's fine if all you do is look at Jupiter and Saturn and the Moon, but that 10" off the shelf Newt is a big winner when you compare it to a 4" top shelf refractor when looking at NGC891. ;)

David

#62 Daniel Mounsey

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Posted 04 April 2013 - 09:55 PM

I am curious on this topic as I sometimes see that claim by scope makers. I am especially curious as to honest opinions regarding the <f/4 large dobs. We all know you get a brighter image and can see more, but does anyone feel that you get as crisp an image as an APO? :question:


In some cases they actually can, but if you want a view that's more often like a refractor, then you'll just need to get a refractor. BTW, that's the short version.

#63 Daniel Mounsey

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Posted 04 April 2013 - 10:14 PM

Nah, 12.5" will ALWAYS show more.




Oh really, how about Collinder 70 Don. :lol:

#64 okieav8r

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Posted 04 April 2013 - 10:23 PM

I think one thing that is often overlooked is comparing apples to apples quality levels between reflectors and refractors.


Very common. I always question when someone wants to compare a basically handcrafted refractor to an off the shelf Newt or SC and do it in an apples to apples context. And 9 times out of 10, it's comparing planetary views. That's fine if all you do is look at Jupiter and Saturn and the Moon, but that 10" off the shelf Newt is a big winner when you compare it to a 4" top shelf refractor when looking at NGC891. ;)

David


I agree David. Due to a reflector's low cost per inch of aperature, I'm going to employ the light gathering capability of a big newt every time when it comes to looking at faint objects like distant galaxies. Observing such objects are where big reflectors really shine. On the other hand, I love the views of open clusters and rich field views of the Milky Way that I get with my 4" refractor. Different uses for different scopes.

#65 Jon Isaacs

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Posted 04 April 2013 - 10:37 PM

The image through the unobstructed refractor is just so darned clean. You can also easily max-out the exit pupil without having to dodge around the shadow of the secondary.



It's F/8... if your NEwtonian were F/8...

Jon

#66 Starman1

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Posted 05 April 2013 - 12:20 AM


Nah, 12.5" will ALWAYS show more.




Oh really, how about Collinder 70 Don. :lol:

Or Kemble's Cascade, or Collinder 399 (The Coathanger), or..., or..., or...
So THAT'S why they made the NP101! :foreheadslap: :tonofbricks:

#67 Sean Puett

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Posted 05 April 2013 - 06:58 AM

NP 101 was made for guys with large reflectors. It is the perfect companion to a large reflector. It does well everything that a large reflector does not. The np127 is even better but, twice the price or so.

#68 azure1961p

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Posted 05 April 2013 - 07:02 AM

I agree . A refractor capable of low power wide fields is the perfect compliment to a large scope . In my case, large is 8" so 70mm fits the bill perfectly :whee: :whee: :whee: :whee:

Pete

#69 Ed D

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Posted 05 April 2013 - 08:40 AM

I couldn't agree more. In my case it's a 6" f/8 Dob :dob: and a 72mm f/6 refractor :refractor:, and throw in my 8x56 binos for good measure :watching:.

Ed D

#70 Mike B

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Posted 05 April 2013 - 09:42 AM

NP 101 was made for guys with large reflectors. It is the perfect companion to a large reflector.



In my case they're a 15" :dob: & a WO ZS66 'fractor :refractor:.
:grin:


Fine companions!

#71 bherv

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Posted 05 April 2013 - 11:31 AM

My reflectors: 16" f4.5, 8" f/6 and 5" f/5. My refractors: 4" f/9.8, 80mm f/5.6 and a handful of 60mm's.
Barry

#72 jgraham

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Posted 05 April 2013 - 11:55 AM

"It's F/8... if your NEwtonian were F/8..."

Hmmm, wellll, I've made dozens (yes dozens) of Newtonians from f/3.7 to f/10. My pair of biggos are a 16.5" f/6.5 and a 16" f/4.5. What I wasn't expecting from my 6" f/8 achro is how well it performs as an RFT. Prior to this my favorite was my trusty olde 6" f/4. The little f/4 is still one of my favorite compact scopes. I often sweep the sky with it craddled in my arms.

What a neat time to be an amateur astronomer!

#73 Sean Puett

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Posted 05 April 2013 - 02:39 PM



What a neat time to be an amateur astronomer! [/quote]

This is what I say. Has there ever been a time with so much affordable quality astronomy gear? No. Scopes that some of us complain about would have cost a fortune 100+ years ago. And before that, you had to be nobility to afford a telescope. Now we reflector vs refractor and apo vs achro to death. Sorry for sidetracking the thread but, we are the most fortunate amateurs and some of us do not realize it. :jump:






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