133 replies to this topic

[jrcrilly]

All I know is, I will have ~ $500 to spend on a scope next month, and I'm getting a 10" dob, and, if I had $1000, I'd get a 12", if I had $2000 I'd get a 14", if I had enough for a folded 20"+ scope I'd get that. (don't want to climb ladders)

Hi, Curtiss.

Competition is a wonderful thing. It's only very recently that the numbers you list have become realistic - especially the $2000 14" truss dob. I saw one of those T-scopes recently and it looks like a heckuva well-built unit, by the way.

[erik]

HI DUDE, TELL ME ,EXACTLY HOW MANY PEOPLE OWN A 16 INCH REFRACTOR? THE BIGGEST ONES I SEE ON AS CONSISTANT BASIS ARE 6 OR 7 INCH IN APERTURE, AND ALMOST ANY NEWT 8 INCHES OR LARGER(AND THERES PLENTY OF THEM!)WILL STOMP ALL OVER THE MUCH MORE EXPENSIVE REFRACTOR.PERIOD. NO IFS, ANDS OR BUTS. SO NEWTS ARE MORE VERSATILE, BECAUSE WHILE YOUR LOOKING AT A FEATURELESS BLOB OF A GALAXY IN YOUR 4 INCH REFRACTOR, I'LL BE STUDYING INTRICATE DETAILS IN MY 8 INCH (AND SOON IN MY 16 INCH)NEWT.

[eric moerman]

Hey Dude, i think you scared everybody now.

[wes]

t has a lot to do with the view one gets! Refractors provide extremely nice and REFRACTOR-SHARP, (the views are called refractor-sharp for a reason), quite clear and Ultra-Contrasty detailed, (most detail for the apertures), views of the moon, planets, dso`s, etc., with PINPOINT STARS ACROSS THE ENTIRE FOV that look like diamonds on a velvetty black background.

As pointed out already, not one 6" newt including a lot with Zambuto, etc., optics could outperform my 5" Apo on planets. The Apo even showed more details in most dso`s within it`s reach, etc. In addition, many of these 6" newts couldn`t even outperform my 4" Apo on the moon, planets and details in most dso`s within it`s reach. Not only ALL this, but the contrast, sharpness, clearity, etc., of the views in the refractors were better also!

Then there`s the issue of seeing conditions. During bad seeing which occurs most of the time in most places, the views in a smaller refractor will have better contrast, sharpness, clearity, etc., while showing the same amount of planetary detail outside of those often ALL-TOO-RARE, often ALL-TOO-BRIEF moments of good seeing and ALL-TOO-RARE nights of good seeing. But get a refractor of the same aperture and it`ll stomp ALL over the newt in almost every way on such nights also!

[quote] Barry and John are the only two that seem to understand what I'm trying to say [/quote]

Hello Charles,

I understand what you`re trying to say also as I indicated earlier in this thread! For that reason, I intend for this to be my last post in this thread. Besides, it`s obvious that this thread can go back and forth like this forever if we choose for it to! I`m choosing for it not to. Now let`s see how many feel like they have to get the last word or post in. ;-)

Hope this Helps. :-)

Clear, STEADY, Skies!



[/quote]

Dude,

Now if no one responds to your last post with your claims
about refractors then will you not have the last word:smirk:

After reading all you posts I'm convinced I should sell my
14.5" Zambuto equipped Reflector and buy a 5" apo, NOT

Wes

[erik]

....and a couple of other points, "refractor sharp" probably means that your only using 100x magnification, because that little guy can't handle much more due to its limited aperture.and in a 16 inch newt(which are common and obtainable, unlike refractors)faint fuzzies are neither faint nor fuzzy, regardless of seeing, unlike a little refractor. even in my 8 inch, i can see real structure in the messier galaxies at my dark sky site. in my 4.7 inch refractor, at the same site, they' ARE faint and fuzzy, if i can see them at all. i know that's not a fair comparison, but you're saying that a 6 inch newt with zambuto optics can't keep up with your 5 inch APO?come on, my 8 inch newt with discovery mirrors(quite a few steps down from zambuto, i'm sure) blows away the views in any 5 or 6 inch refractor i've ever looked through. even when i had the chinese made orion mirrors in my scope, they couldn't touch it.the planets are tack sharp in my scope, with detail visible within the belts of jupiter. saturn is crisp, the cassini division is well defined and jet black. at least 5 of saturns moons are always visible(can you see 3 in the refractor?) and jupiters moons are disks, not pinpoints of light. there's no false color, chromatic abberation, or visible diffraction.anyway, this spirited debate is fun, but none of us are going to change our minds based on what we say.so to each their own. oh, by the way, sorry for the CAPS in my last post, i was on a computer at work that didn't print it out in lower-case letters for some reason. don't want to make anyone mad or anything.........

[Trever]

Well I cannot speak for anyone here but me, but I actually had the pleasure of comparing views through my 6 inch refractor and a 17 inch truss dob. I compared M31 , M42 and the planet Saturn through both. My refractor gave a much more pleasant view of the DSO's providing a black background and as much if not more detail than the Dob. M42 looked better than I had ever seen it including any reflector I have seen so far. The planet Saturn was pretty much the same in both scopes except the false color was present in mine. I did not have a Minus V filter at the time but do now. I would imagine that the refractors are much more forgiving in light polluted skies and bad seeing that a Dob so that may be why i had such a superior view. Plus after watching this guy haul this large dob out there made me appreciate my scope...

[jrcrilly]

Hmm, maybe it was more pleasent because it was from your scope? Unless his dob wasn't colliminated or cooled down. That's really strange.

I can see his point - especially if it was a very fast Dob (which many 17" and larger instruments are). Star fields tend to be prettier in refractors than in reflectors with large obstructions because of the darker background and pinpoint stars. On the other hand, the two deep sky objects mentioned would both benefit greatly from the larger aperture; I'd expect that the views would be more pleasing to me in the larger scope.

Therefore it is neccesary to have at least one of each.

I'm with you!

[wes]

Interesting. What would be a safe focal ratio in a reflector? And while I'm at it, is there a general size in dobsonions where increasing the apurture will only prove noticeably better on very rare occasions? Or where the inability to use low magnifications becomes a problem?

Hi Daniel,

With a Newtonian Reflector anything faster than F/6 will
require a Paracor coma corrector to correct the coma.
The Paracor will correct practically all the coma down to
F/4 , my scope is an F/4.3 and with the Paracor I have
pin point stars even close to the edge .
IMO the greatest problem with a fast reflector is that they
require very exact collimation , the margin of collimation error is much smaller in an F/4.3 than an F/7, it's also more difficult to figure a fast mirror but it can be done, Zambuto is renowned for
this .

Thanks,

Wes

[jrcrilly]

Interesting. What would be a safe focal ratio in a reflector?

For best contrast, it's nice if the central obstruction can be held below 20". At faster focal ratios this is difficult if it's also desired to maintain good overall field illumination. Most common Newts of F/5 or below have obstructions greater than 20% by diameter. That's why the "planet killer" Newts are so long - they keep the focal ratio low so they can have very small secondaries.

There are other issues with focal ratio as well; I see Wes has addressed those.

I have found that I can always results from an increase in aperture of 25% or so; smaller increments make smaller differences.

[wilash]

One last thing, if I were to consider getting into astrophotography I would prolly save the money and buy a APO when I could afford it vs buying an SCT...If your thinking about picture taking a APO is pretty hard to beat.

LivingNDixie

I have just two words to add - Takahashi Epsilon

[Jarad]

John -

It is only hard to keep the secondary small in small fast scopes. As the scopes get bigger, it is easier. The reason is that the focuser height becomes smaller compared to the scope size, so you have an effectively super-low profile focuser. For example, I had an 18" f4.2 dob with a 3.1" secondary - that's 17.2%. A 6" f4.2 couldn't do that. The reason is that the 18" had a 20" UTA (adding 1" to the light path) and a 1.6" focuser, plus about 0.25" extra travel, for a total of 2.85" past the edge of the mirror. The 6" scope can get away with a 7" tube (adding 0.5"), but will still have the 1.6" for a low-profile focuser (or more if it's not low-profile), and the 0.25" for travel, for 2.35". But 2.35" is a lot more proportionately to a 6" than the 2.85" for the 18".

If you look at the big scope makers (Starmaster, Obsession, etc.), you will see that the secondary size drops as a percentage of the diameter as the diameter goes up, even though the bigger scopes tend to have faster f-ratios. It is because of this effect. In small scopes (say less than 10" or so), it is hard to get below f5 without going above 20% secondary size, and without causing vignetting. No problem at all for big ones.

As to the example of the 17" dob above, if it performed that poorly, it either had bad optics, wasn't collimated or wasn't cooled down. Using cheap eyepieces in a fast scope will cause poor performance too (an erfle or SWA or similar in a f4.5 scope will have UGLY seagulls over most of the field due to astigmatism of the eyepiece).

Jarad

[wilash]

I hate to say this, but you guys could split hairs on a bald head. The ONLY reason we buy the types of telescopes we do is because we think they are cool. (Someone had to say it.) What really tickels me is that general MTF curves are presented as some kind of proof of image quality. All an MTF curve can do is give a relative plot of a systems resolution and contrast. But what MTF curves cannot tell you is what that image will look like. Image quality is a subject ive human response (as has been very well established, but not pointed out).

[erik]

hi guys, i also find it hard to believe that deep sky views would be better in a 6 inch refractor than a 17 inch dob.the dob should collect, what, 450% more light?the only explanation for that would be that it wasn't cooled down, the collimation was way off, or the viewers perception was biased towards his own scope(which we're probably all guilty of). planetary views, i suspect would be a tougher call, many people believe that since we're limited by the atmosphere rather than the seeing, that any increases in aperture over 10 inches doesn't signifigantly improve planetary viewing. although i don't think that that's quite true, sometimes a smaller aperture does seem to give a more pleasant view, even though the same amount of detail can be seen in both scopes(and during moments of good seeing, the large scope will see more.the image in the larger scope will also always appear bigger.and, because the big dob usually takes higher powers to take advantage of the moments of good seeing, someone that casually took a quick glance into the scope might a turbulent, boiling view and think that the refractor showed a better view.i always try to push the magnification on the planets to take advantage of moments of good seeing. and if the seeing is really bad, i can use less power (or stop the scope down for a more asthetic view)

[jrcrilly]

John -

It is only hard to keep the secondary small in small fast scopes. As the scopes get bigger, it is easier. The reason is that the focuser height becomes smaller compared to the scope size, so you have an effectively super-low profile focuser. For example, I had an 18" f4.2 dob with a 3.1" secondary - that's 17.2%.

Thanks, Jarad.

I didn't think of it that way. I'll comment that my previous fast dobs (16" F/4.5 and 17.5" F/3.8) both had secondaries larger than you describe (and over 20% by diameter); perhaps they were being very conservative about field illumination.

[Jarad]

Well, f3.8 is pretty fast.... Playing with Newt 2.0, I get the following:

Assuming a 19.5" tube for the 17.5" mirror, and a 1.6" low-profile focuser with 0.25" extra travel, I get that with a 3.1" secondary you'd have a tiny spot of 100% illumination, but 75% out to 1.6" (1.39 degrees). With a 3.5", you'd have a 0.5" 100% illuminated field (0.43 degrees), and 75% would be past the edge of the 2" focuser. So the 3.5" would probably be the choice to go with, which is right at 20%.

For the 16" f4.5, assuming an 18" tube and the same focuser, etc., I get that a 2.6" diagonal would have no 100% field, although the 75% would cover 1.35", and just over 1 degree. A 3.1" diagonal gives a 100% zone of 0.59" or 0.47 degrees, with 75% past the edge of the focuser again. The 3.1" would be 19.3% obstruction.

So these two scopes seem to kind of fall in between the commonly available diagonal sizes. The smaller size is just a bit too small, and the next size up is a bit of overkill, but probably the better choice or you give up having any decent 100% zone. They still squeak in at or just under 20% though.

But if you tried a 6" f3.8, for example, in a 7" tube and the same 1.6" low profile focuser (just a 1.25" one, forget about trying for 2"...), you'd need a 1.5" secondary to get a 0.07" 100% zone, and 0.8" 75% zone. That's 25% obstruction already, and to get a decent 100% zone the next step is 1.83" secondary, for 30.5%....

Jarad

[wes]

Hi Dude,

I think we ought to give you some kind of award for the post
with the most quotes in it:roflmao: , I though this thread
was about to end until your last post , yes Dude I think
most everyone would agree that inch for inch Refractors
are the superior design , there now do you feel better

Wes

[erik]

hi dude, i was surprised to see you post again on this thread, i thought you had said that you finished with this...oh well, my mistake, i wont quote you directly like seem to enjoy doing so much.back to our friendly debate, isn't it strange that the biggest refractor in the WHOLE WORLD is only 40 inches, (and even at 40 inches, it's had problems with the lenses sagging from all that weight)and many amateurs have newts aproaching that size?that brings me to one of my my biggest points,how many people can afford a 16 inch refractor?? if you can,that's great, you must be doing very well for yourself, i can't (not that i'd buy a 16 inch refractor if i could) and i don't know many people that can.sure, observatories have them but most amateurs do not.i said about 10 posts ago that with equal size, the edge goes to the refractor due to the newts secondary obstruction.BUT,add the diameter of the secondary obstruction, and the newt will show as good or better views every time(providing both scopes are in perfect working order). if refractors are so great, why don't we compare them with newts of the same price? so, i'll bring my 8 inch tube that i have about $700 into, and you bring...... oh, i'm sorry, no one makes an APO for $700.well, you could use an achromat, but then anything brighter than 2nd magnitude would have a big purple halo around it. you could use an ED-80(an excellent scope) but since i'd have 5 inches on you, ithe newt would probably win.the fact is,newts offer the best views of deep sky or solar system objects, when price(or portability in bigger apertures) is any factor, and a lot af people, including me, think that newts are better, period.BECAUSE THEY ARE!

[pete]

Well, I just want to say I found this very informative, and extremely entertaining. I believe this is somehow the way this discussion would have gone if Gali and Isaac were on this forum making their case. Just when you think it’s over, along comes Will with an off axis version of Isaac’s scope. I would like to add if you look thru one of Gali’s scopes today you would see it as he did 400 years ago. The same unfortunately can not be said for newts, because mirror coatings deteriorate over time. A refractor, coupled with a prism diagonal, eliminates optics deteriorating over time. Clearly this is one example of advantages of a refractor over reflector. I mention on behalf of William, who by now like the surface coating on his scope has deteriorated to dust, the Herschelian design eliminates the central obstruction argument against reflectors. Saving the best for last reflectors suffer no false color. However I do believe if Erik (Isaac) was given a 6” APO, his reflector would collect dust. I just do not take my newt out as often as I used to. Dude (Gali) on the other hand, if given a 17.5” inch newt would proudly bust out the big gun after showing everyone the night sky thru his refractor. Pete

[Garfield]

At the risk of beating a dead horse, my experience is that an APO refractor delivers consistently asthetically pleasing results every time, with detail limited only by atmospheric conditions and aperture. A high-quality, well designed newt can also deliver esthetically pleasing views, with detail limited only by atmospheric conditions and aperture. Everything else being equal, quality aperture wins, every time. Not surprising that (last time I checked anyway) Thomas Back's personal scope for planetary viewing is....wait for it.... a 20" Starmaster. Nuff said?

Gary in Ontario

[wilash]

One optical design is not "superior" over any other. It really does not matter if you form an image with lenses or mirrors. The question is how good the optics are. There is really no advantage between telescopes in a specfic price range, just different compromises.

An issue that has not been discused are the configurations of reflectors. There are many flavors. What I wish is that more companies would follow Takahashi's lead with bringing the reflector into the 21st century. The Epsilon and Mewlon are fine telescopes. And I really like the BRC-250 and FRC-300. The refractor design is limited by aperture - prices just get silly over 6" or 7" because it is difficult to make good lenses. The reflector clearly wins at large apertures. (Hopefully we have gotten beyond this silly business about seeing.)

I'm very impressed with some of the things amateurs have done with flexing mirrors to acheive better curves. And I think if some enterprising amateurs applied computer designs to mirror making there could be some very interesting results.

Also I think the refractor manufactures could get out of the 19th century as well. I think the doublet is passed its prime. Hell, the triplet is almost a dinosaur in the photographic industry. Here again the Japanese have been leaders - Takahashi FSQ-106 and the Pentax line of refractors. But I think that may also be because of the conservative nature of amateurs - more glass, less light (a very archaic notion).

[erik]

hi pete, at the risk of continuing this thread until the sun turns into a red giant and vaporizes all of us, i must add that while you are correct in saying that mirror coatings can deteriorate over time, i've never had it happen in the 20 years that i've owned reflectors. i know that they eventually do need recoating, maybe we should start a thread about the life expectancy of mirror coatings.however, as i mentioned in an earlier posting, i've also read about flourite being unstable and deteriorating over time. also, some adhesives that are used to glue lens elements together have been known to have problems over time. i think ,obviously, it depends on the climate and the conditions that the scope is exposed to. i think that if the scope is stored in a dry enviroment, and taken outside only when viewing, either scope design will last indefinitely. if it's left in a damp shed, than obviously, it will have more problems, regardless of whether it's a refractor, reflector ,or SCT.also, i think in a lot of ways, newt manufacturers have brought them into the 21st century. the quality of the mirrors is generally much better these days, and look at all the GPS and go-to mounts that all available now.and with CCD's anyone can get into astrophotography.