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Equipment Discussions >> Reflectors

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City Kid
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Reged: 05/06/09

Loc: Northern Indiana
Re: Does your reflector give "refractor like" images? new [Re: Darren Drake]
      #5775240 - 04/03/13 10:58 AM

Quote:

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.



Now that would be a fun thread to follow!


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dscarpa
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Reged: 03/15/08

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Re: Does your reflector give "refractor like" images? new [Re: jnewton]
      #5775263 - 04/03/13 11:08 AM

I looked at Jupiter and DSOs through Jon's 12.5" F/4 Discovery with Paracorr and found the view refractor like. I don't usually notice the coma in my regular C9.25 but the using it the next night there it was. David

Edited by dscarpa (04/03/13 04:09 PM)


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nirvanix
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Reged: 06/07/07

Loc: Saskatoon, SK
Re: Does your reflector give "refractor like" images? new [Re: jpcannavo]
      #5775336 - 04/03/13 11:29 AM

Quote:


But assuming the best possible newtonian optics - excellent substrate surface, clean, and highest quality coating - do this:
Buy a large dob, say 16+ inches, with premium optics. Build a 6 inch off-axis mask, and knock yourself out enjoying extra-diffraction-free (Mak-Newt like? ) views of planets and bright stars when the mood hits you. And when you've had enough, crank up the aperture and resolution and do some serious observing!
Joe




So very well said Joe. I have a 4" aperture mask that when properly positioned on my 10" dob allows the light to fall uninterrupted by spider or CO onto a good mirror, turning my newt into a chromatic error free 4" f/12.5 that surpasses the most expensive 4" refractor in performance. I use if for double star splits mostly as it removes the vane scatter that gets in the way on tight doubles.


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GOLGO13
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Reged: 11/05/05

Loc: St. Louis area
Re: Does your reflector give "refractor like" images? new [Re: jnewton]
      #5775353 - 04/03/13 11:34 AM

I like both scope designs. If my time is limited, I am a big fan of a 3-4 inch apo refractor. Very short cool down, super sharp images, grab and go portability. They are not cheap, but they are very nice.

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 think the harder call is if I could only have one scope what would it be. Of the scopes I have it would be the 4 inch refractor. If I had a better newt like a high quality truss dob, I'd probably choose the newt. However, there is a point to where the size of the newt is too big for me. My opinion is a 15 inch F4.5 is as big as I would go. I'd probably prefer a 12.5 inch F5. Gonna have to save up for some time to get that in the future.


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GeneT
Ely Kid
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Reged: 11/07/08

Loc: South Texas
Re: Does your reflector give "refractor like" images? new [Re: jgraham]
      #5775380 - 04/03/13 11:46 AM

Quote:

Hmmmm, sorry, my experience has been no. My "big" refractor is a 6" f/8 achro while my reflector cut a broad swath from 4.5" f/4 to 16.5" f/6.5, Newtonians, MCTs, and SCTs. 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.
I luv my reflectors, but my biggo refractor is a special piece o'kit.




My point is not which give the better images. My point is that the different light characteristics of various types of telescopes will produce different flavors of images. The quality of an image is a different issue than the flavor. I like both Whopper and McDonald's quarter pounder cheese burgers. They may be of the same quality, but their flavoring characteristics are different.


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GeneT
Ely Kid
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Reged: 11/07/08

Loc: South Texas
Re: Does your reflector give "refractor like" images? new [Re: Darren Drake]
      #5775382 - 04/03/13 11:47 AM

Quote:

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.




Do it!


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kfrederick
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Reged: 02/01/08

Re: Does your reflector give "refractor like" images? new [Re: GeneT]
      #5775411 - 04/03/13 12:01 PM

Funny how people who have nothing in there lives perfect sure worry about the stars to be perfect.Bet most do not get there eyes checked or blood presure. but the stars need to be sharp . Guess I am one of those .Ha Ha

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Darren Drake
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Re: Does your reflector give "refractor like" images? new [Re: GeneT]
      #5775448 - 04/03/13 12:26 PM

Quote:

Quote:

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.




Do it!




Ah I just started one about achros vs SCTs. That should be good for a while lol.


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Galicapernistein
super member


Reged: 09/24/07

Loc: Detroit Michigan
Re: Does your reflector give "refractor like" images? new [Re: kfrederick]
      #5775521 - 04/03/13 01:02 PM

Quote:

Funny how people who have nothing in there lives perfect sure worry about the stars to be perfect.Bet most do not get there eyes checked or blood presure. but the stars need to be sharp . Guess I am one of those .Ha Ha




My doctor told me I suffer from Dimimageitis, and that the only cure is a large dob.


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kfrederick
Carpal Tunnel


Reged: 02/01/08

Re: Does your reflector give "refractor like" images? new [Re: Galicapernistein]
      #5775573 - 04/03/13 01:27 PM

funny but true

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FirstSight
Duke of Deneb
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Reged: 12/26/05

Loc: Raleigh, NC
Re: Does your reflector give "refractor like" images? new [Re: GOLGO13]
      #5775781 - 04/03/13 03:07 PM

Quote:

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.


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Sean Puett
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Reged: 09/06/10

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Re: Does your reflector give "refractor like" images? new [Re: kfrederick]
      #5775884 - 04/03/13 04:04 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...


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nevy
professor emeritus
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Reged: 02/07/12

Loc: UK
Re: Does your reflector give "refractor like" images? new [Re: Sean Puett]
      #5776208 - 04/03/13 07:04 PM

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

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buddyjesus
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Reged: 07/07/10

Loc: Davison, Michigan
Re: Does your reflector give "refractor like" images? new [Re: azure1961p]
      #5776347 - 04/03/13 08:18 PM

Quote:

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.


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Thomas Karpf
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Reged: 02/09/09

Loc: Newington, CT
Re: Does your reflector give "refractor like" images? new [Re: Starman1]
      #5776578 - 04/03/13 09:55 PM

Quote:


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.


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azure1961p
Postmaster
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Reged: 01/17/09

Loc: USA
Re: Does your reflector give "refractor like" images? new [Re: David Pavlich]
      #5776627 - 04/03/13 10:15 PM

Quote:

Quote:

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.




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

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

Edited by azure1961p (04/03/13 10:22 PM)


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GOLGO13
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Reged: 11/05/05

Loc: St. Louis area
Re: Does your reflector give "refractor like" images? new [Re: Thomas Karpf]
      #5776721 - 04/03/13 11:07 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.


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Datapanic
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Reged: 10/17/09

Loc: Tucson, Arizona
Re: Does your reflector give "refractor like" images? new [Re: GOLGO13]
      #5776960 - 04/04/13 02:30 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...



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buddyjesus
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Reged: 07/07/10

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Re: Does your reflector give "refractor like" images? new [Re: Datapanic]
      #5777014 - 04/04/13 05:18 AM

haha

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jpcannavo
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Reged: 02/21/05

Loc: Long Island New York
Re: Does your reflector give "refractor like" images? new [Re: GOLGO13]
      #5777015 - 04/04/13 05:19 AM

Quote:

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.

Edited by jpcannavo (04/04/13 05:19 AM)


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