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Refractor VS Reflector

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#76 Mike B

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Posted 19 May 2006 - 03:33 PM

Hi Mark-

... I have to say that there are few instances where the trouble messing with the big scopes is really worth the effort, and truth be known, I'm willing to wager there is a tendency to get optical trains that as a rule are inferior to smaller aps. When the weight of just the glass alone warps the mirror into a potato chip, I think I'd rather put my efforts into something else that's more consistent and with higher quality.

Where described as "big scopes" & "potato chip" mirrors, what aperture *range* are you talking about? Assuming *decently* designed mirror cells & overall scope structures, where (aperture-wise) do these issues first BEGIN to come into play, & where do they become more significant, or problematic? From what you've already stated, i think i can safely infer we're talking beyond 8" mirrors... but HOW far? I understand these are necessarily your opinions on the matter, but i respect your experience there- as well as others who are much more familiar than i with this species of beast! :lol:
Thanks,
:cool: mike b

#77 Alan French

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Posted 19 May 2006 - 04:38 PM


* Alan, please keep it above the belt, sir. I'm not talking about monster dobs. I will say it again. Aperture has everything to do with resolution and what you can see. That's just physics. I believe in modest instruments all the way, and I don't believe in the monster ego dobs. An inexpensive 8" dob of modest aperture to today's standards will gather 4x the light of a teeny 4" apo refractor. For visual observing alone, the 8" makes a heck of a lot more sense financially as well as what you can see with it. It's just a wise purchase if you pit the two scopes together for example. Plus with the 8" dob, one can make good use of narrowband filters, and see many thousands more objects than the 4" refractor. I just consider the title of the thread and its really a no-brainer to me. Notice I am not insulting you in any way, sir. God bless.


Sorry to be a bit snippish. Since folks in the thread were throwing around all sorts of apertures, I thought you were in the big Dob camp.

Clear skies, Alan

#78 claytonjandl11

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Posted 19 May 2006 - 05:17 PM


Sorry but the original question (that I aksed concerning, what Al Nagler told me which is THE beginning thread) was not which is better, refractor or reflector. But at what point in appeture does a high quality Newt. reflector equal a refractor. It was a very simple question, and only a handfull of replies have answered it or attemped to answer it.
[SNIP]
Nick T.


Nick,

Did you ever look up the two part article "Rules of Thumb for Planetary Telescopes," by William Zmek in the July (page 91) and September (page 83), 1993, issues of Sky & Telescope.

It will very nicely answer your question.

Clear skies, Alan


Thanks Alan,

I will look it up, and sorry for the slow response, I've been alittle busy of late.

Nick T.

#79 Alan French

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Posted 19 May 2006 - 05:28 PM

Nick,

I think you will find it interesting. Another good article is "Secrets of Telescope Resolution" in the June, 2006, issue of Sky & Telescope.

Clear skies, Alan

#80 Tommy5

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Posted 19 May 2006 - 11:58 PM

To answer your question, I would quess that around 10/12" the reflector willl equal the refractor,too many things work against the refractor as aperture increases,light has to pass through the glass of a refractor and a 10" piece of glass has to be thick,three thick pieces of glass in an apo triplet, no glass is completely transperant some photons will be lost,also collimation becomes difficult in a large refractor, the heavy glass has to be supported only by the edges,money issues aside, the larger refractors are very impractical,that is one reason why they are rare nowadays.I think the Grissom vs. the apo story is very telling. :cool:

#81 Mark Harry

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Posted 20 May 2006 - 08:00 AM

Mike, I was referring to "potato chip" with big mirrors as being problematic, for users, as well as the fabricators. Does it make sense to polish and figure a mirror while it lays flat, and then prop it up on a test stand, generally that consists of 2 pegs displaced some distance apart on the bottom edge of the mirror? In my mind, that isn't conducive to accurate mirror production methods, or end product.
I don't work/test my mirrors in such a fashion, but try to support them the exact same way on the test rack as the glass was worked. So far, I have been able to work up to 13" mirrors accurately by the methods I use, so maybe I should say "anything 13 inches or under makes for accurate mirrors". (???) The breaking point may be at 13", or somewhere else, who knows. But I think any mirror that is supported differently under test is a mis- represented mirror, and the fabricator of mirrors in the 20-36" range should at least be aware of that. Again, my 2 cents worth.
Regards, Mark

#82 Alan French

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Posted 20 May 2006 - 08:41 AM

To answer your question, I would quess that around 10/12" the reflector willl equal the refractor,too many things work against the refractor as aperture increases,light has to pass through the glass of a refractor and a 10" piece of glass has to be thick,three thick pieces of glass in an apo triplet, no glass is completely transperant some photons will be lost,also collimation becomes difficult in a large refractor, the heavy glass has to be supported only by the edges,money issues aside, the larger refractors are very impractical,that is one reason why they are rare nowadays.I think the Grissom vs. the apo story is very telling. :cool:


Modern optical glass is extremely transparent, and I would not worry about tranmission issues in a 10 or 12 inch lens.

You can rumage around the glass catalog at
http://www.oharacorp...wf/catalog.html
to get some idea of actual transmission.

Clear skies, Alan

#83 claytonjandl11

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Posted 20 May 2006 - 01:29 PM

Nick,

I think you will find it interesting. Another good article is "Secrets of Telescope Resolution" in the June, 2006, issue of Sky & Telescope.

Clear skies, Alan


Hi Alan,

yes I read that one, very interesting indeed.

Nick T.

#84 V.A.

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Posted 20 May 2006 - 02:02 PM

a big reason for the asthetically more pleasing images in the refractor is the smaller aperture and longer focal ratio.
ever look through a top quality ,small aperture ,long focus newtonian? you'd be hard pressed to pick a winner.

#85 Daniel Mounsey

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Posted 21 May 2006 - 12:40 AM

Hi Tommy,
Thank you for the kind words regarding Grissom and Pons Newts. I should mention though that even though the glass is very thick in these three element apos, their aperture also increases, so everything is still some-what equal. I would have to say no, the thickness of the glass is not the issue regarding the actual light through-put. The problem with the thickness is that these heavy chunks of glass just take forever to stabalize, especially when they are exposed to the ambient temperature as it drops.

Refractors typically appear under corrected and as the optics stabalize, begin to reach proper optical correction as long as they are made well. Not all do and out of respect, I will keep the ones that are horrible private. Doublets and triplets behave differently. To this day, regardless of all the hype the people in these forums wish to believe or argue, the Tak FC and FS series still remain the finest apochromats ever produced in the world to date. Doublets behave better and reach proper optical correction faster. As far as Al Nagler's comment regarding the 9" refractor, he is incorrect. I've seen what good Newtonians are like, in fact I mentioned this to Pons and he laughed. He said, Daniel don't even waste your time trying to convince these people. It's all about understanding the Newtonian's strengths and weakness's and how to deal with them. So much can go wrong in Newtonians and until the public address's them and truly understands them, they'll always like their apos better and can you blame them?

To this day, I have friends who have literally spent thousands and thousands of dollars on large apos and even have the courage to call me on the phone and tell me a reflector couldn't touch them. I just say OK, because I know in my heart, they have never been exposed to a properly designed reflector. When their day comes as mine once did, then they'll understand, I wonte even have to tell them. To be quite honest, I actually get a kick out of their confidence regarding their belief's on apos only to have the last laugh when their jaw drop six feet under after having looked through a reflector like Pons and Grissom's and only then will they understand. I hate to sound like a fool, but its simply true and that's all there is to it. Pons is a 53 year veteran and is one of the most experienced observers in the world and he's got the best of everything to compare. He would tell anybody the same thing. If people would like an opportunity to see it for themselves, then they are welcome to have a look. We can put both scopes side by side on the same night and you guys be the judge.

In a nut shell, here is how I describe a perfect Newtonian. This is the recipe regardless of aperture.

1.) Take a thin (not full thickness!) 8" to 14.5" highly corrected, smooth F-6 and no faster primary. It must produce perfect diffraction patterns on both sides of focus. Have Bryan Greer at Protostar hand pick you a tiny secondary with a 12% to 15% obstruction, since you're only using the information from the central portion of the secondary anyway. Send both optics to Spectrum in Florida and have a standard IAD coating put on each,

2.) Get a three vane curved spider with 60 deg. arcs no thicker than a 32th of an inch or about the thickness of four or five sheets of paper.

3.) Get a slightly over sized sono tube or material that remains completely dormant with absolutely NO baffles and a proper mirror cell.

4.) Very important. Get a rear fan using rubber bands to hold it in place hitting air against the primary and no side fans. Case and point, the fan must keep the entire optical system flushed at all times! If you've ever seen Grissom's or Pons, this is exactly how they do it. This will kill the thermals so quickly, they will not have any time to hinder the image.

5.) This is my favorite part. Go to a good observing site where there's some laminar air flow and stable temperatures with an 8" F-6 designed in this manner and find an observer with a $15,000 6" apochromatic system and ask he or she if you can compare images and last, watch the expression on their face when they look through yours. This is actually a true story which happened to me at Charlton years ago. If the apo out-performs it, then something's probably wrong with your optics or setup. I'd like to see the look on Al's face and by the way, I actually like Al, he's a very cool guy, seriously, but this hobby can be amusing at times.

#86 Mark Harry

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Posted 21 May 2006 - 07:56 AM

My additions to Daniel's reply:

I use uninsulated aluminum tubes, with at least an extra inch all the way around the mirror. No fan. Keep the scope in the barn all day, where it's cool. No temp problem. Mount the focuser with the base on the INSIDE of the tube, making sure it does not protrude into the light path for low focuser height.(smaller secondary)
Primary mirror is figured with very thin cerium, or rouge, with impeccable indicated error.
A 6" F/8-10 will in all likelihood be the equivalent of the 6" apo, a 7" F/8 will surpass it.

I don't laugh to myself, for I feel if I was the APO owner, I wouldn't like to be in that situation. Noone would. It's a shame that misconceptions such as these exist. Unfortunately, the majority believes in them. That is somewhat annoying, personally. My opinion & thoughts. Mark

#87 Alan French

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Posted 21 May 2006 - 09:38 AM

Daniel,

A nice summary of getting the most out of a Newtonian. I would, however, argue that #2 is not necessary. While diffraction spikes are annoying, they do little damage to contrast. The curved spiders just spread the diffraction around.

As a data point, my wife has been taking her 10" f/6 to Winter Star Party recently. It has a very fine mirror by Dan Joyce, and provided absolutely superb views of Saturn this year. The views with a 3.5mm Nagler were stunning. (Loaned by Uncle Al, who also enjoyed the view.)

Clear skies, Alan

#88 Daniel Mounsey

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Posted 21 May 2006 - 10:21 AM

Hi Alan,
Thank you. Like everything in life though, it's all relative. I own scopes with both spider designs and this kind of curve produces the most beautiful images of planets. The guys and I don't like seeing spikes bleeding at the limbs but I do agree that on a good night a four vane is fine. It's simply nicer and cleaner in a good curve.

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#89 KaStern

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Posted 21 May 2006 - 07:30 PM

Hi Darren,

Instead it has in large part to do with something rarely discused; thermal currents. Refractors light path's are a ONE WAY trip down a protected tube which isn't exposed to outside air currents. Newt's light paths have to go through an unprotected path TWICE which doubles the problems and also often mixes with the observers body heat.
I've done many comparisons with equal aperture refractors vs reflectors and the result always shows the newt to have much more turbulence.



my experience confirms to what you point out :waytogo:

Kurt Schreckling who did post some of his results concerning
thermal issues of telescopes placed under night skies built
the new silvery tube of my 8"f/6 dobson.
It consists of lightweight composite material (1cm hard-foam
between two thin aluminium layers):

http://www.astrotref...e&TOPIC_ID=1656

The performance of this scope on planets raised dramatically
when compared with the original taiwanese dobson.
It could outperform a relatively mediocre 6" ED Refractor
on Jupiter in a night with very good atmospherical seeing.
Subtile Colour differences were more obvious and some structures
seemed to be more present.

The mirror ist good (interferometric tests say 0.91 due to a
very slight astigmatism) and the new smaller secondary is superb.

When observing I put a efectivly 30cm long lightweight dewcap
on the tube to prevent dew and my own thermal effects from
getting into the optical path.

I think it is really worth the efforts to optimise thermal
behavior of the scope.

Best regards, Karsten

#90 Tommy5

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Posted 21 May 2006 - 09:34 PM

Thanks Dan for your expansive reply,perhaps you are right and the thickness of large apo glass isn't an issue, I stand corrected,thank for your extensive info on making reflectors work better on planets, I suspect that thermal issues are underestimated as a cause of less than great images in reflectors folks will tend to blame seeing conditions when thermal currents are the culprit. :confused:

#91 Daniel Mounsey

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Posted 21 May 2006 - 11:40 PM

Any time Tom. I think you're on the right track.

#92 Alan French

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Posted 22 May 2006 - 07:25 AM

Daniel Mounsey wrote, in part, about improving Newtonians...
5.) This is my favorite part. Go to a good observing site where there's some laminar air flow and stable temperatures

Daniel,

This is where things go awry for many of us, and why many folks decide to get a modest APO instead of a Newtonian. I don't think there are any such locations in the northeast or in most of the northern U.S.

Most people I know who made the APO choice made it because they understand their local conditions, not because they have some misconception about what an APO can do.

Clear skies, Alan

#93 Bill McHale

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Posted 22 May 2006 - 03:00 PM

Ok, I was in training last week and not able to keep up on this thread. Been interesting catching up though.

Don't have much to add (at the moment) but I do need to slightly disagree with one thing Daniel said...


1.) Take a thin (not full thickness!) 8" to 14.5" highly corrected, smooth F-6 and no faster primary. It must produce perfect diffraction patterns on both sides of focus. Have Bryan Greer at Protostar hand pick you a tiny secondary with a 12% to 15% obstruction, since you're only using the information from the central portion of the secondary anyway. Send both optics to Spectrum in Florida and have a standard IAD coating put on each,


If you are at the smaller end of the scale (8-10") I would definitely go slightly larger than the diagonal. While there is improvment in downsizing the diagonal below 20% it is very slight. I personally think that an 8" f/8 or a 10" f/6 makes a grand all around instrument and I think it is worth it to have a larger fully illuminated field of view. I wouldn't go above 20% though.

#94 Starman1

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Posted 22 May 2006 - 06:24 PM

I personally think that an 8" f/8 or a 10" f/6 makes a grand all around instrument and I think it is worth it to have a larger fully illuminated field of view. I wouldn't go above 20% though.

That depends on what you want to illuminate. As the scope gets smaller, the distance from the secondary to the focal plane becomes a larger percentage of the focal length, and the secondary has to increase in order to provide the 70% edge-of-field illumination needed for general use. Yes, you can make the secondary smaller if you ONLY want to view planets, but even that doesn't work in shorter,smaller scopes.
This is another advantage of big dobs--they have a smaller secondary, percentage-wise.
While I agree the two scopes you pick could be good planetary instruments, arbitrarily choosing a small secondary is not wise. Choosing a secondary to provide proper illumination is.

#95 Alan French

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Posted 22 May 2006 - 06:49 PM

My first serious telescope was an 8" f/8, and it provided many years of enjoyment. I sold it because we have two 10" f/6 Newts, and they are also great scopes, and always had a little edge on the 8".

I would certainly agree that either would be a fine choice.

Clear skies, Alan

#96 Daniel Mounsey

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Posted 23 May 2006 - 03:03 AM

Don,
Good to hear from you. Thanks for sharing.

#97 Bill McHale

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Posted 23 May 2006 - 10:11 AM

I personally think that an 8" f/8 or a 10" f/6 makes a grand all around instrument and I think it is worth it to have a larger fully illuminated field of view. I wouldn't go above 20% though.

That depends on what you want to illuminate. As the scope gets smaller, the distance from the secondary to the focal plane becomes a larger percentage of the focal length, and the secondary has to increase in order to provide the 70% edge-of-field illumination needed for general use. Yes, you can make the secondary smaller if you ONLY want to view planets, but even that doesn't work in shorter,smaller scopes.
This is another advantage of big dobs--they have a smaller secondary, percentage-wise.
While I agree the two scopes you pick could be good planetary instruments, arbitrarily choosing a small secondary is not wise. Choosing a secondary to provide proper illumination is.


I agree that choosing proper illumination is important, but I really don't think it is an issue with a 20% diagonal on an 8" f/8 or a 10" f/6. I ran some numbers on Mel Bartels online diagonal calculator and going with a 1.52" Diagonal for the 8" f/8 and a 1.83" diagonal for the 10" f/6 yeilds fully illuminiated fields up to almost 0.4" off axis and fields with usable illumination up to between 0.6" and 0.7" off axis. Granted you may not get full use of your 35mm Panoptic or your 31mm Nagler, but those illumination levels should be find for any 1.25" eyepiece and for most 2" eyepieces as well.

Mind you I agree that this is not an issue with larger dobs... but I think everyone should probably have a 6"-10" scope in their inventory. That size is more than sufficient to do a night of serious observing, but small enough that (especially with Dobs) the scope will never be a reason not to observe.

#98 Starman1

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Posted 23 May 2006 - 10:50 AM

Bill,
Just so long as a diagonal calculator (like Bartels') is used, and the compromise is understood. The standard "rule of thumb" for lunar observation is 1/2" of 100% illumination (i.e. a radius of 6.4mm at 100%), and many planetary observers are also lunar afficionados. And the 1.83" diagonal works as a minimum size on that 10" f/6 for just that. It might be noted, though, that a 2.0" secondary still meets the 20% requirement, yet adequately illuminates the field stop on a 35 Panoptic. I'd feel comfortable with that as a general use size.
Yet another advantage of the longer focal length scopes.
[I presumed secondary to focus, on the 10", as 8.5", so I presume a short focuser is purposely chosen.]

#99 Bill McHale

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Posted 23 May 2006 - 11:59 AM

Bill,
Just so long as a diagonal calculator (like Bartels') is used, and the compromise is understood. The standard "rule of thumb" for lunar observation is 1/2" of 100% illumination (i.e. a radius of 6.4mm at 100%), and many planetary observers are also lunar afficionados. And the 1.83" diagonal works as a minimum size on that 10" f/6 for just that. It might be noted, though, that a 2.0" secondary still meets the 20% requirement, yet adequately illuminates the field stop on a 35 Panoptic. I'd feel comfortable with that as a general use size.
Yet another advantage of the longer focal length scopes.
[I presumed secondary to focus, on the 10", as 8.5", so I presume a short focuser is purposely chosen.]


Yes, a 2" would also meet the 20% requirement for a 10" but it is (for some reason) a less common size among vendors. I chose my sizes from protostar's listings since they have a good reputation. I also was assuming a focuser height of about 1.5" (moonlite makes a crayford that meets this criteria). A short focuser like that is going to require a longer tobe or a light shield to block stray light, but it should be adequate. In any case, the difference in illuminated field size between the 1.83" abd 2" is not huge.. about 0.1" off-axis for both the fully illuminated and the 70% illuminated zone. At that point I would say it probably should come down to price and quality in one's decision making.. at least in mine ;).

#100 Patrick

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Posted 23 May 2006 - 02:05 PM

In any case, the difference in illuminated field size between the 1.83" abd 2" is not huge.. about 0.1" off-axis for both the fully illuminated and the 70% illuminated zone.



I ran the numbers thru NEWT and I'm getting different results that what you've stated for some reason. With a 1.83" secondary, I'm getting a field diameter of 1.18" at 75% illumination and 0.44" at 100% illumination. With a 2.14" secondary, the numbers are 1.52" and 0.80" respectively. So, it appears to me that there is a substantial difference between the two secondary sizes as far as field illumination is concerned.

I have a 2.14" secondary on my 10" f/6, which is preferable to me because I enjoy deepsky wide field observing as well as planetary observing and am using at least one 2" eyepiece. The 2.14" secondary yields a CO of 21.4% which seems very acceptable to me. I've looked at a number of designs in NEWT and concluded that unless you don't care about edge illumination at all, it is pretty difficult to design a Newtonian with a really small secondary, at least in a smaller aperture scope with an f/ratio less than f/8.

Patrick


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