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Equipment Discussions >> Reflectors

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Bonco
Post Laureate
*****

Reged: 04/17/06

Loc: Florida
Re: Long focus Newtonian Vs refractor new [Re: ed_turco]
      #5762657 - 03/28/13 04:17 PM

Ed,
I doubt you'll have to eat your hat. I've owned an RV6 newt for years. Bought other scopes and it went into a closet even tho I knew it was a great performer. Several years ago I helped a neighbor get his RV6 properly aligned, he set it up next to my AP 6 inch f/8 refractor (Now considered semi APO) Well guess which one gave the most contrasty detailed views of Jupiter?...his RV6. So I sold my AP and pulled my RV6 out of the closet and restored it. Good luck on your project and I hope to learn the results. Also I have advocated that someone with the resources/knowledge could build a custom 6 inch f/8 Newt OTA that would rival the APO market at a fraction of the price. Maybe you're the man. I'll buy one.
Bill


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jpcannavo
scholastic sledgehammer
*****

Reged: 02/21/05

Loc: Ex NYCer, Now in Denver CO!
Re: Long focus Newtonian Vs refractor new [Re: Bonco]
      #5763137 - 03/28/13 08:37 PM

Quote:

Ed,
I doubt you'll have to eat your hat. I've owned an RV6 newt for years. Bought other scopes and it went into a closet even tho I knew it was a great performer. Several years ago I helped a neighbor get his RV6 properly aligned, he set it up next to my AP 6 inch f/8 refractor (Now considered semi APO) Well guess which one gave the most contrasty detailed views of Jupiter?...his RV6. So I sold my AP and pulled my RV6 out of the closet and restored it. Good luck on your project and I hope to learn the results. Also I have advocated that someone with the resources/knowledge could build a custom 6 inch f/8 Newt OTA that would rival the APO market at a fraction of the price. Maybe you're the man. I'll buy one.
Bill




Dittos on The RV-6!! One of the best planetary scopes of its aperture I have ever used - wonderful contrast! (It also trounces my 100mmED, as I suppose it should!)

As for central obstruction the definitive authority, for me, remains the awesome Vladimir Sacek!

Telescope Optics


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Mark Harry
Vendor
*****

Reged: 09/05/05

Loc: Northeast USA
Re: Long focus Newtonian Vs refractor new [Re: nirvanix]
      #5763815 - 03/29/13 08:10 AM

Quote:

I said it - it wasn't a keyboard study and it was a far more thorough analysis than anything I've ever seen posted on CN, but I'm not able to find the link. Understand it was based on the performance of the human optical system as well as the scope itself. My belief was based on that study, but if you believe something else that's fine. Personally I'm starting to think that people confirm their own beliefs more often than they realize.




*******
I have presented an observation of my own research, and related a bit about it.
" My belief was based on that study, ..."
Which means no research on yor part... ?

"I'm starting to think that people confirm their own beliefs more often than they realize."

I think it'd be good we participate without becoming sarchastic, please?
Would be much appreciated.
M.


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Peter Natscher
professor emeritus


Reged: 03/28/06

Loc: Central Coast California
Re: Long focus Newtonian Vs refractor new [Re: nirvanix]
      #5764081 - 03/29/13 09:54 AM

Remember: longer focal length has the advantage over faster systems with providing a more generous (wider) fine focus area at the eyepiece. I've observed with systems between F/14, F/8, F/4.3 and lately F/3.3, and the slower systems are more enjoyable to focus on double stars, the moon, and planets. At F/3.3 and faster, exact focus is had to find and changes dramatically with telescope thermal and atmospheric seeing issues that are always there. It takes a better quality night for a Newtonian to perform well against a APO. Some of us don't get many of the quality nights through the year for their Newt's to perform really well, especially at larger apertures.

Don't play down the thermal effects that Newt's always have. It's always an issue especially early on in the evening and during daytime observing. That issue alone is the biggest hurdle after the seeing conditions above and your own body heat. I use larger Dobs for most observing but find it refreshing to enjoy eyepiece views wit my AP 175 that don't show thermal effects and secondary contrast loss around planets along with vane spikes. Each system has its advantages.


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Peter Natscher
professor emeritus


Reged: 03/28/06

Loc: Central Coast California
Re: Long focus Newtonian Vs refractor new [Re: GeneT]
      #5764100 - 03/29/13 10:03 AM


As much as I like fiddling with my big Dob's, sometimes it's refreshing to set up a larger APO without needing to tweak anything. Just mount it and you're good to go. I like the variety in setup and use between these two types of telescope. At 63, it keeps my brain working.

Quote:

Both are great for planetary views. However, when you point a 6 inch refractor to a DSO, vs. a 12 inch or larger reflector, the increased light gathered of the reflector makes the question moot. What this question comes down to is--what objects do you like to view? If they line up the moon, planets, double/multiple stars, globulars and nebula/galaxies last, then the equation shifts.

I have followed this debate for years, and I come out in favor of mirror optics. People can differentiate and buy both. We don't have to limit ourselves to either a refractor or reflector.

Lastly, AP changes the equation further. I have seen some eye watering planetary and DSO photos taken with three and four inch refractors.




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Jon Isaacs
Postmaster
*****

Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Long focus Newtonian Vs refractor new [Re: Peter Natscher]
      #5764178 - 03/29/13 10:44 AM

Quote:

As much as I like fiddling with my big Dob's, sometimes it's refreshing to set up a larger APO without needing to tweak anything.




Indeed. But no scope is 100% immune to collimation and the need to fiddle. When that 175mm A-P refractor needs attention, it needs to go back east where Roland can fiddle with it.

Jon


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jgraham
Postmaster
*****

Reged: 12/02/04

Loc: Miami Valley Astronomical Soci...
Re: Long focus Newtonian Vs refractor new [Re: Jon Isaacs]
      #5764194 - 03/29/13 10:51 AM

My large refractor is by far the most comfortable and trouble-free visual scope in my collection. It holds collimation wonderfully well and it has so much back-focus that I can use two star diagonals giving a fully articulated eyepiece. I frequently use it while seated in a comfy desk chair next to a small table holding my charts and notebook.

Now all I need is a mild spring evening!


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ZuoZhao
sage


Reged: 10/16/12

Re: Long focus Newtonian Vs refractor new [Re: jpcannavo]
      #5764436 - 03/29/13 11:51 AM

I think the contrast important on DSO too. Then how about long focus Newtonian VS short focus Newtonian on DSO?

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CollinofAlabama
Carpal Tunnel
*****

Reged: 11/24/03

Loc: Lubbock, Texas, USA
Re: Long focus Newtonian Vs refractor new [Re: Jarad]
      #5764695 - 03/29/13 01:28 PM

Jarad's equation ...

Reflector Primary - Secondary = Refractor Primary Equivalent

... I think may need some adjusting. I don't have any empirical tests to verify this, but have noted Jon Isaacs many times writing that the real difference is more like

Reflector Primary - 1" = Refractor Primary Equivalent

I am thinking, to quantify Jon's assertion, perhaps the formula should look like this ...

Reflector Primary - (0.5 * Secondary) = Refractor Primary Equivalent

As one got above a 40% obstruction, this formula might not work so well. But I can't help but noticing that the Omni XLT 150, for example, has a 150mm objective with a 46.5mm CO. Now according to Jarad's formula, that should make the Omni XLT equivalent to a 103.5mm refractor for planetary performance. According to my reworked formula, the "Refractor Equivalent" of the Omni XLT should be a 126.75mm scope. Now, people may say this is wishful thinking, and I cannot say with any certainty that it isn't, but it would be nice to see how an Omni XLT 150 compares with 4" ED, 110mm ED, and 120mm ED scopes. I own neither the 110 nor 120mm ED scopes, though there's a STRONG temptation for me to buy the XLT and compare it to my 102mm ED scope. According to Jarad's formula, it should still surpass it, but really, just barely, and the planetary performance should really be about the same. Stay tuned, but anyone else wishing to evaluate my recalculation of that old Reflector ~ Refractor Equivalent Equivalence formula is more than welcome to educate me and the rest of the CN Brotherhood.

Oh, and for the record, the Omni XLT delivers the same number of photons to your retina as a 142.6mm refractor -- (Pi * r squared of the Primary) minus (Pi * r squared of the Central Obstruction). Obviously, those photons are goofed up a bit for human vision by the CO, but how much is the question.


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azure1961p
Postmaster
*****

Reged: 01/17/09

Loc: USA
Re: Long focus Newtonian Vs refractor new [Re: ZuoZhao]
      #5764877 - 03/29/13 02:31 PM

Quote:

I think the contrast important on DSO too. Then how about long focus Newtonian VS short focus Newtonian on DSO?




Not as apparent if apparent at all. Its a different level of resolution in seeing contrast when viewing skotopic versus photopic as on Jupiter for example. I highly doubt you'd ever see the difference on M51 or M33 - buiuttt - in terms of resolving stellar galactic nuclei it might work in your favor.

Open clusters and globular clusters are another matter. The small CO puts more light into the spurious disk do the rings are fainter which boosts contrast of dimmer stars. Its subtle but when you are at the limits there is no doubt a 20% CO. will show fainter cleaner stars than a 38% CO.
The less you spread out faint light in diffraction patterns the higher the apparent contrast against the night sky.

Pete


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Jon Isaacs
Postmaster
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Long focus Newtonian Vs refractor new [Re: azure1961p]
      #5765694 - 03/29/13 08:32 PM

Quote:

Quote:

I think the contrast important on DSO too. Then how about long focus Newtonian VS short focus Newtonian on DSO?




Not as apparent if apparent at all. Its a different level of resolution in seeing contrast when viewing skotopic versus photopic as on Jupiter for example. I highly doubt you'd ever see the difference on M51 or M33 - buiuttt - in terms of resolving stellar galactic nuclei it might work in your favor.

Open clusters and globular clusters are another matter. The small CO puts more light into the spurious disk do the rings are fainter which boosts contrast of dimmer stars. Its subtle but when you are at the limits there is no doubt a 20% CO. will show fainter cleaner stars than a 38% CO.
The less you spread out faint light in diffraction patterns the higher the apparent contrast against the night sky.

Pete






Contrast is important when viewing DSOs but a larger CO results in a fine scale loss of contrast, not overall contrast and at the low light levels, the eye cannot resolve those fine scale contrasts..

Jon


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Jeff Morgan
Postmaster
*****

Reged: 09/28/03

Loc: Prescott, AZ
Re: Long focus Newtonian Vs refractor new [Re: Peter Natscher]
      #5765858 - 03/29/13 10:28 PM

Quote:


As much as I like fiddling with my big Dob's, sometimes it's refreshing to set up a larger APO without needing to tweak anything. Just mount it and you're good to go. I like the variety in setup and use between these two types of telescope. At 63, it keeps my brain working.




I play in both leagues and can sympathize with you - to a point. Hauling out the 150+ pounds of equatorial mount and tripod (or pier) such a 7" refractor requires is most definitely not refreshing.

At least on the Dobsonian side of the field those big loads can be managed with wheelbarrow handles and don't have to be lifted to shoulder level, or higher.


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Peter Natscher
professor emeritus


Reged: 03/28/06

Loc: Central Coast California
Re: Long focus Newtonian Vs refractor new [Re: Jeff Morgan]
      #5765984 - 03/30/13 12:15 AM

Fortunately, I'm strong and energetic enough to spend the 15 minutes setting up the 50 lb. transportable pier, 35 lb. equatorial mount, three 18 lb counter weights, and a 43 lb. OTA. The pieces sit in my garage all day at near outside temperature and are hand-carried out onto my driveway 20 ft. distance. But once set up, there's no recollimating or equilibration time needed on this refractor system -- I can start observing right away at sunset or twilight. If I roll out my larger Dob, I have to recollimate and wait for cool down, about 30 minutes, and this doesn't settle until way after twilight as the air temp. keeps dropping and upsetting to primary mirror that's close to the pavement. Dob's need a cooler grassy field to sit on to escape this constant ground heat problem. Running Dob mirror fan's just blows more warmer ground air onto the primary extending the time to cool down. The APO sits way above the warmer air influence from the ground (at 6.5 ft. up) and escapes this eternal Dob problem. So, observing at home off of my driveway is more enjoyable using my refractor than my larger Dob. The Dob gets used away from home at darker sites. At home, the APO beats the larger Dob for immediate planetary-quality observing time!

Quote:

Quote:


As much as I like fiddling with my big Dob's, sometimes it's refreshing to set up a larger APO without needing to tweak anything. Just mount it and you're good to go. I like the variety in setup and use between these two types of telescope. At 63, it keeps my brain working.




I play in both leagues and can sympathize with you - to a point. Hauling out the 150+ pounds of equatorial mount and tripod (or pier) such a 7" refractor requires is most definitely not refreshing.

At least on the Dobsonian side of the field those big loads can be managed with wheelbarrow handles and don't have to be lifted to shoulder level, or higher.




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Peter Natscher
professor emeritus


Reged: 03/28/06

Loc: Central Coast California
Re: Long focus Newtonian Vs refractor new [Re: Jon Isaacs]
      #5766003 - 03/30/13 12:42 AM

Yeah, but I won't need to fiddle with it at every observing op and it's dependably consistent in performance. Very maintenance free. A Dob will require mirror recoating or eventual replacement, too.

Quote:

Quote:

As much as I like fiddling with my big Dob's, sometimes it's refreshing to set up a larger APO without needing to tweak anything.




Indeed. But no scope is 100% immune to collimation and the need to fiddle. When that 175mm A-P refractor needs attention, it needs to go back east where Roland can fiddle with it.

Jon




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azure1961p
Postmaster
*****

Reged: 01/17/09

Loc: USA
Re: Long focus Newtonian Vs refractor [Re: Peter Natscher]
      #5766007 - 03/30/13 12:45 AM

Nice dob Pete. I love the minimalist base the altitude bearings rest on - has this conceptual art feel about it.

Pete


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CollinofAlabama
Carpal Tunnel
*****

Reged: 11/24/03

Loc: Lubbock, Texas, USA
Re: Long focus Newtonian Vs refractor new [Re: timps]
      #5766095 - 03/30/13 02:57 AM

God! I shudder to think what a 6" apo costs! I don't even own a Synta 120mm ED largely because of the cost, and those are a bargain compared even to 4" TeleVue equipment. 6"? ¡Ay Caramba! Besides, I've seen for myself about 6 years ago a brand new Meade LB 8" throw up a better view of Saturn than an Orion 120mm ED scope.

Anyone who wants excellent views of anything, get an 8" F/6 dob. Best deal going for visual astronomy. Reasonably stable collimation, should cool down relatively fast (compared to 10" + mirrors) and will show WAY more than anybody's sub $2.5k refractor. Of course, if money's no object, perhaps taxes are too low on the wealthy


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jpcannavo
scholastic sledgehammer
*****

Reged: 02/21/05

Loc: Ex NYCer, Now in Denver CO!
Re: Long focus Newtonian Vs refractor new [Re: Peter Natscher]
      #5766196 - 03/30/13 06:10 AM

Quote:

Yeah, but I won't need to fiddle with it at every observing op and it's dependably consistent in performance. Very maintenance free. A Dob will require mirror recoating or eventual replacement, too.

Quote:

Quote:

As much as I like fiddling with my big Dob's, sometimes it's refreshing to set up a larger APO without needing to tweak anything.




Indeed. But no scope is 100% immune to collimation and the need to fiddle. When that 175mm A-P refractor needs attention, it needs to go back east where Roland can fiddle with it.

Jon







Sometimes (but not always) I see the whole refractor vs. reflector debate as really being about differences in disposition. Excuse the sloppy analogy to cars (of which I know little) but the Dob strikes me as being more like the tricked out hot rod, with the Apo being like the hit the road out of the factory BMW. Both may ultimately be comparable performers, but they capture very different sensibilities. The former is more of an ongoing work in progress, appealing more to the inveterate hands-on tinkerer, while the latter to the no fuss or muss, lets-get-going-and-perform type. Now surely this an idealized and over simplified generalization - and there are often elements of both orientations in many of us - but I do think it captures something.


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buddyjesus
Carpal Tunnel
*****

Reged: 07/07/10

Loc: Davison, Michigan
Re: Long focus Newtonian Vs refractor new [Re: jpcannavo]
      #5766201 - 03/30/13 06:27 AM

Long focus Newtonian Vs refractor

don't think you can go wrong either way. or both ways. or a cassegrain. or binos. They all are tools and have a place.

I am admittedly a refractor guy, but for how/what I observe, not having an apo doesn't really interfere. After I get a couple more cheap bits for the refractor, I plan on saving up for a Z12. They should complement each other well.


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Peter Natscher
professor emeritus


Reged: 03/28/06

Loc: Central Coast California
Re: Long focus Newtonian Vs refractor new [Re: jpcannavo]
      #5766660 - 03/30/13 11:52 AM

The way you might enjoy differences between your longer-focus Newtonian, larger Dob, or APO really depends on what location you're observing from.

Quote:

Quote:

Yeah, but I won't need to fiddle with it at every observing op and it's dependably consistent in performance. Very maintenance free. A Dob will require mirror recoating or eventual replacement, too.

Quote:

Quote:

As much as I like fiddling with my big Dob's, sometimes it's refreshing to set up a larger APO without needing to tweak anything.




Indeed. But no scope is 100% immune to collimation and the need to fiddle. When that 175mm A-P refractor needs attention, it needs to go back east where Roland can fiddle with it.

Jon







Sometimes (but not always) I see the whole refractor vs. reflector debate as really being about differences in disposition. Excuse the sloppy analogy to cars (of which I know little) but the Dob strikes me as being more like the tricked out hot rod, with the Apo being like the hit the road out of the factory BMW. Both may ultimately be comparable performers, but they capture very different sensibilities. The former is more of an ongoing work in progress, appealing more to the inveterate hands-on tinkerer, while the latter to the no fuss or muss, lets-get-going-and-perform type. Now surely this an idealized and over simplified generalization - and there are often elements of both orientations in many of us - but I do think it captures something.




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GeneT
Ely Kid
*****

Reged: 11/07/08

Loc: South Texas
Re: Long focus Newtonian Vs refractor new [Re: timps]
      #5767958 - 03/31/13 12:38 AM

An F9 mirror has the potential to perform well. However, my F5, 12.5 inch Portaball also gives excellent planetary views. A Zambuto mirror will serve you well!

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